LOW RESISTANCE CONTACTS TO N-TYPE GERMANIUM
A DISSERTATION
SUBMITTED TO THE DEPARTMENT OF ELECTRICAL ENGINEERING
AND THE COMMITTEE ON GRADUATE STUDIES
OF STANFORD UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
Jui-Yen Jason Lin
June 2013
http://creativecommons.org/licenses/by-nc/3.0/us/
This dissertation is online at: http://purl.stanford.edu/qr469kh6783
© 2013 by Jui-Yen Lin. All Rights Reserved.
Re-distributed by Stanford University under license with the author.
This work is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.
ii
I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.
Krishna Saraswat, Primary Adviser
I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.
James Harris
I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.
Yoshio Nishi
Approved for the Stanford University Committee on Graduate Studies.
Patricia J. Gumport, Vice Provost Graduate Education
This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file inUniversity Archives.
iii
iv
Abstract
The scaling of conventional silicon transistors faces several obstacles including the
need to introduce materials such as germanium which have higher carrier mobilities.
Several challenges need to be addressed in these new materials systems. In the case of
germanium, contact resistance to n-type Ge is particularly problematic. This thesis
presents two approaches to address this issue. The first is the use of metal-insulator-
semiconductor contacts, whereby inserting a thin insulator between the metal and
semiconductor can reduce the barrier height and reduce contact resistance. Tunneling
resistance and series resistance effects are studied experimentally and theoretically. The
second approach is the use of germanide contacts in conjunction with high doping
techniques. Low contact resistances are obtained due to the high dopant activation level
and dopant segregation effects.
v
Acknowledgements
I would like to thank my research advisor, Professor Krishna Saraswat, for all his
support and guidance over the last several years. His door was always open, and I have
received a tremendous amount of advice from him. I am especially grateful for his
encouragement when I changed research directions half way through my time at Stanford.
This thesis would not have been possible without his vast knowledge, patient mentoring,
and gentle encouragement.
I have been very fortunate to interact with several faculty members. Professor Yoshio
Nishi, Professor James Harris, and Professor Philip Wong not only served on my
committee but gave me very helpful suggestions during my research. In addition to
serving as my committee chair, Professor Paul McIntyre’s expertise in TiO2 was a
tremendous resource and I am very fortunate to have received his help.
This work was very much a collaborative effort. I wish to thank Dr. Arunanshu Roy,
not only for his simulation expertise, but also for being a fantastic colleague. I will
always cherish our debates regarding our simulated and experimental results. Also, Dr.
Bin Yang’s wealth of industrial experience was eye-opening, and I am still amazed at
how he was able to point me to very useful papers after every discussion. I am also very
fortunate to have collaborated with Suyog Gupta, whose energy and passion is as
impressive as his technical expertise.
A large portion of this work was done at the Stanford Nanofabrication Facility (SNF)
and the Stanford Nanocharacterization Laboratory (SNL), and it would not have been
possible without the staff’s support. I am especially grateful to J Provine for help with
ALD and Dr. James McVittie for help with anything plasma related.
vi
I would like to thank Gail Chun-Creech for very efficient administrative help ranging
from POs and reimbursements to room reservations and scheduling.
The Saraswat group members are definitely among the smartest and most dedicated
individuals I know, and I am truly honored to call them my colleagues and friends. Our
impromptu late night meetings in SNF are a prized memory, and I appreciate our mutual
encouragement to persevere after failures.
Finally, I am very thankful to have a loving family, encouraging and supporting me
every step of the way. With their unyielding integrity and hardworking spirit, my parents
and brother have always been my role models. I dedicate this thesis to them.
vii
Table of Contents
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Thesis Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Methods to Reduce Ge N-Type Contact Resistance . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Properties of the Ge Material System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Dopant Activation Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2.1 Laser Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2.2 Phosphorus & Antimony Coimplantation . . . . . . . . . . . . . . . . . . . . . . 12
2.2.3 Fluorine Vacancy Passivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3 MIS Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3.1 Fermi Level Pinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3.2 Fermi Level Depinning using Ultrathin Dielectrics . . . . . . . . . . . . . . 19
2.3.3 Resistance Due to Tunneling Through the Dielectric . . . . . . . . . . . . . 22
2.4 Chalcogen Passivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.5 Dopant Segregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
viii
3 TiO2 MIS Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.1 ALD Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2 Band Offsets to Germanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.3 MIS Contacts on Epitaxial Germanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.3.1 Barrier Height Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3.2 Effect of Metal Workfunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.3.3 Comparing MIS Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4 TiO2 MIS Contacts on n+ Germanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.4.1 Measurement Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.4.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.4.3 Series Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.5 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.5.1 Ge N-Channel MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.5.2 Metal Source/Drain Transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.5.3 Asymmetric Metal-Semiconductor-Metal Photodetectors . . . . . . . . . . 55
3.5.4 Spin Injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4 Physics of MIS Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.2 Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.2.1 Effect of Dipoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.3 Effect of Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.4 Effect of Series Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.5 Effect of High Semiconductor Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.6 MIS Contact Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.6.1 Oxygen-Deficient TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.6.2 Indium Tin Oxide (ITO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.6.3 Oxygen-Deficient ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.7 Scalability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
4.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
ix
5 Germanide Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
5.1 Nickel Germanide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
5.2 Formation of NiGe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.3 P and Sb Coimplantation with NiGe Contacts . . . . . . . . . . . . . . . . . . . . . . . . . 95
5.3.1 Contact Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5.3.2 Effect of Dopant Segregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.4 Chalcogen Segregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
6.1 Benchmarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
6.2 Contributions and Suggestions for Future Work . . . . . . . . . . . . . . . . . . . . . . 106
List of References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
x
List of Tables
1.1 Carrier mobilities for several semiconductors. . . . . . . . . . . . . . . . . . . . . . . . . . 1
2.1 Summary of MIS contact schemes in literature. . . . . . . . . . . . . . . . . . . . . . . . . 21
5.1 Summary of Electrical Measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.1 Summary of Selected Contact Schemes on N-Type Ge. . . . . . . . . . . . . . . . . . . 105
xi
List of Figures
1.1 (From [5]) For planar MOSFETs, parasitic resistance increases with scaling,
becoming a larger portion of the total device resistance. Beyond 20nm, the
parasitic resistance may even begin to dominate device characteristics. . . . . . . 2
1.2 The parasitic series resistance in a MOSFET can be divided into two main
components: contact resistance (Rc) and junction resistance (Rj). . . . . . . . . . . . 3
2.1 (From [6]) SIMS and SRP profiles show chemical and electrically active
dopant concentrations, respectively. P-type Ge doped with boron show
electrically active concentration above 1020
cm-3
. However, n-type Ge doped
with phosphorus show electrically active concentration of 2×1019
cm-3
, even
though the chemical concentration is higher. Similar results are obtained for
As and Sb n-type dopants in Ge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 (From [9]) Specific contact resistivity as a function of doping density and
electron barrier heights for n-type Si. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 (From [10]) At metal/semiconductor interfaces, the metal Fermi level lies
within a narrow range of energies within the semiconductor band gap due to
Fermi level pinning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4 (From [11]) The measured electron barrier height is not significantly
modulated by changing metal workfunction. Line (a) is the ideal (Schottky)
limit, showing considerable deviation from experimental results. . . . . . . . . . . . 9
2.5 (From [16]) The electrically active Sb profile shown by the SRP line indicates
n-type dopant activation beyond 1020
cm-3
in germanium using laser
annealing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.6 (From [16]) Specific contact resistivity of 7×10-7
Ωcm2 was reported using
laser annealing of Sb-implanted germanium. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.7 (From [17]) High dopant activation using coimplantation of P and Sb shown
by the SRP line. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.8 (From [21]) Enhanced phosphorus dopant activation due to coimplantation of
fluorine (blue line). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
xii
2.9 (From [22]) Fluorine can reduce the defects caused by Ge self-implantation
(red line to blue line). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.10 Typical distribution of acceptor-like and donor-like states within the
semiconductor band gap, with the charge neutrality level indicated. . . . . . . . . . 17
2.11 (From [27]) Charge transfer between semiconductor band gap states and metal
causes the metal EF to align with the charge neutrality level. . . . . . . . . . . . . . . 17
2.12 Strong metal Fermi level EFM pinning near the Ge valence band results in a
large specific contact resistivity to n-type Ge. . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.13 (From [30]) By inserting silicon nitride between metal and n-type Si, contact
resistance can be reduced. There is an optimal thickness before contact
resistance increases again due to tunneling resistance. . . . . . . . . . . . . . . . . . . . . 19
2.14 (From [30]) The inserted insulator reduces MIGS, allowing the metal Fermi
level to rise closer to the conduction band and reducing the effective barrier
height. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.15 (From [33]) Si3N4 MIS contacts on germanium. . . . . . . . . . . . . . . . . . . . . . . . . 21
2.16 Tradeoff between lower ΦBN and tunneling resistance results in an optimum
dielectric thickness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.17 (From [50]) Tunneling transport simulations on Al2O3 (left) and Si3N4 (right)
MIS contacts on n-type Ge. It is very difficult to achieve low contact
resistivity using these materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.18 Conduction band offset is an essential material selection criterion since
tunneling resistance in MIS contacts must be minimized. TiO2 is identified as
a promising material in this aspect. CBO data from [27]. . . . . . . . . . . . . . . . . . 24
2.19 (From [50]) TiO2 breaks the tradeoff between lower ΦBN and tunneling
resistance due to the low CBO to germanium. Even relatively thick TiO2 can
be used. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.20 (From [55]) On n-type Ge, rectifying behavior changes to ohmic behavior
after sulfur passivation, while the opposite happens on p-type Ge, indicating a
decrease in ΦBN and increase in ΦBP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.21 (From [58]) Increasing As dose decreases effective ΦBN, causing the n-type
Ge contact to become more ohmic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
xiii
3.1 SRPES spectra for (a) TiO2 and Ge valence bands, (b) Al2O3 and Ge valence
bands, and (c) Al 2p peak. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2 Band offsets of the Al2O3/Ge and TiO2/Ge systems. . . . . . . . . . . . . . . . . . . . . . 30
3.3 Schematic cross-section of MIS contacts fabricated on heteroepitaxially
deposited Ge on Si. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.4 SRP profile showing electrically active n-type doping concentration of in situ
doped epitaxially-grown Ge on Si. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.5 High-resolution TEM images of the TiO2 MIS contact. . . . . . . . . . . . . . . . . . . . 33
3.6 TiO2 MIS contacts on ~1018
cm-3
moderately-doped n- and p-type Ge. . . . . . . . 34
3.7 Al2O3 MIS contacts on ~1018
cm-3
moderately-doped n- and p-type Ge. . . . . . . 34
3.8 A decrease in ΦBN is accompanied by an increase in ΦBP. . . . . . . . . . . . . . . . . . 35
3.9 Richardson plot of select TiO2 MIS devices, showing a significantly reduced
extracted effective ΦBN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.10 Band diagrams of Al2O3 and TiO2 MIS contacts. While both can reduce the
effect electron barrier height, the lower tunnel resistance of TiO2 allows TiO2
MIS contacts to outperform those using Al2O3. . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.11 TiO2 MIS contacts using (a) Al and (b) Pt as the metal display similar I-V
characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.12 Relative specific contact resistivity of various MIS contacts. TiO2 MIS
contacts achieve about 1000× improvement while Al2O3 MIS contacts only
achieve roughly 10× improvement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.13 Schematic effect of CBO and ΦBN on specific contact resistivity. . . . . . . . . . . . 40
3.14 Schematic of a circular transmission line method test structure for measuring
ρC. The equivalent circuit is shown in (b). The use of 4 probes negates the
effect of the probe resistance RPROBE on the measurement. . . . . . . . . . . . . . . . . 42
3.15 Higher dose of 1.8×1015
cm-2
does not result in higher active dopant
concentration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.16 Schematic cross-section of the circular TLM structure used for ρC
measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
xiv
3.17 Measured ρC of Al2O3 and TiO2 MIS contacts on n+ Ge. Al2O3 MIS contacts
are immediately limited by tunneling resistance. TiO2 MIS contacts achieve
roughly 70× improvement in ρC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.18 The semiconductor doping affects the MIS contact characteristics. While
TiO2 does not add tunneling resistance, it can still limit the contact if the
resistance of TiO2 becomes similar to the resistance of the Schottky barrier for
highly doped substrates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.19 Schematic diagram of ρC versus interfacial layer thickness for different doping
levels, showing the effects of different resistances. The red double lines are
for Al2O3, while the blue single lines are for TiO2. The dotted lines are for the
Al2O3 tunneling resistance and the TiO2 series resistance, which represents a
lower bound on ρC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.20 TiO2 MIS contact ρC compared to TiO2 series resistance. . . . . . . . . . . . . . . . . . 50
3.21 Gate last process flow for Ge NMOSFET with TiO2 integrated on n+
source/drain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.22 ID-VD characteristics of gate last Ge NMOSFET with TiO2 MIS contacts. . . . . 52
3.23 Process flow and schematic cross section of gate first Ge NMOSFET
incorporating TiO2 MIS contacts and its ID-VD characteristics. . . . . . . . . . . . . . 53
3.24 (From [67]) Operating principle of a metal source/drain MOSFET. The p-
channel device is drawn here and compared with a conventional doped
source/drain MOSFET. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.25 (From [72]) Asymmetric barriers at the source and drain junctions of a MSM
photodetector can reduce dark current. However, the improvement is limited
due to metal Fermi level pinning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.26 Schematic of asymmetric MSM photodetector incorporating TiO2 MIS contact
and its accompanying dark current reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . 57
3.27 (From [75]) (a) Schematic of spin MOSFET. (b) Simulated ID-VD
characteristics for parallel and antiparallel source/drain ferromagnet
orientations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
xv
4.1 TiO2 MIS contacts also show a reduction in ΦBN on Si, GeSn, GaAs (from
[47]), and GaSb (from [48]), suggesting similar mechanisms as the TiO2/Ge
system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.2 Normalized C-V characteristics of SiO2 and TiO2/SiO2 capacitors with
different metals. There is a strong indication of metal Fermi level pinning at
the metal/TiO2 interface due to the lack of flatband voltage modulation. . . . . . 64
4.3 Flatband condition of the TiO2/SiO2 capacitor, showing the location of
possible dipoles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.4 Schematic band diagrams at flatband conditions showing band alignments of
TiO2 MIS contacts and conventional contacts, both with Fermi level pinning. . 68
4.5 Calculated ρC as a function of metal effective workfunction, indicating the
presence of a dipole at the metal/TiO2 interface. . . . . . . . . . . . . . . . . . . . . . . . . 69
4.6 (a) A difference in oxygen areal density can result in an interfacial dipole,
which shifts band alignments due to a rapid change in the vacuum level (E0).
(b) In the metal/TiO2 case, relaxation of oxygen atoms at the interface causes
a positive dipole and shifts the metal Fermi level (EFM) towards E0. Solid
lines indicate band alignments before oxygen transfer and dotted lines indicate
band alignments after oxygen transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.7 (From [85]) Reduction in specific contact resistivity can be caused by fixed
charge in the oxide. In this case, the effect of bulk (left) and interface (right)
fixed charge in Al2O3 MIS contacts is simulated. . . . . . . . . . . . . . . . . . . . . . . . . 72
4.8 TiO2 MIS contacts with Pt or Ti metal behave differently after 300°C FGA. . . 73
4.9 Simulated TiO2 MIS contact resistivity (a) with and (b) without series
resistance on 1019
cm-3
n-type Ge. Stars are effective metal workfunctions
inferred from experiments, with the dotted line as a guide. The specific
contact resistivity contours are labeled in units of Ωcm2. . . . . . . . . . . . . . . . . . . 75
4.10 Metal effective workfunction at the metal/TiO2 interface for various oxide
thicknesses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.11 Simulated ρC with and without series resistance for TiO2 MIS contacts on
1019
cm-3
n-type Ge. Effective metal workfunctions of Fig. 4.10 are assumed
for this calculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
xvi
4.12 (a) Simulated ρC for TiO2 MIS contacts with different effective workfunctions
(EWF) with and without series resistance (RS). (b) The improvement factor of
these MIS contacts by eliminating series resistance. . . . . . . . . . . . . . . . . . . . . . 78
4.13 Simulated TiO2 MIS contact resistivity (a) with and (b) without series
resistance on 1020
cm-3
n-type Ge. Stars are effective metal workfunctions
inferred from experiments, with the dotted line as a guide. The specific
contact resistivity contours are labeled in units of Ωcm2. . . . . . . . . . . . . . . . . . . 80
4.14 Simulated ρC with and without series resistance for TiO2 MIS contacts on
1020
cm-3
n-type Ge. Effective metal workfunctions of Fig. 4.10 are used. . . . . 81
4.15 (a) Simulated ρC for TiO2 MIS contacts with different semiconductor doping
with and without series resistance (RS). Effective metal workfunction was
taken to be 4.1eV. With series resistance, the 1019
/cm3 and 10
20/cm
3 lines are
nearly indistinguishable, implying that series resistance is dominating in this
case. (b) The improvement factor of these MIS contacts by eliminating series
resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.16 TiO2 resistivity can be reduced by annealing in FGA for 5 minutes, from
37.5Ωcm (as deposited) down over three orders of magnitude to 0.021Ωcm
(500°C). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.17 Simulated TiO2 MIS contact resistivity on (a) 1019
cm-3
and (b) 1020
cm-3
n+ Ge.
The specific contact resistivity contours are labeled in units of Ωcm2.
Different colors correspond to different TiO2 resistivity: blue (2.3Ωcm), green
(0.077Ωcm), and red (0.021Ωcm). The black line is the ideal ρC without
series resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.18 Electrical characteristics of ITO MIS contacts on ~1018
cm-3
n-type Ge.
Significant increase in current density is observed indicating a reduction in
ΦBN without introducing tunneling or series resistance. . . . . . . . . . . . . . . . . . . . 88
4.19 (From [94]) I-V characteristics of ZnO MIS contacts on n-type Ge. . . . . . . . . . 89
4.20 Simulated ρC as a function of n-type Ge doping level for a variety of effective
metal workfunctions using 1nm TiO2 MIS contacts. The dotted gray line is
for a metal/Ge contact, where the effective metal workfunction is pinned at
4.58eV. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
xvii
5.1 Temperature window for the low resistance phase of NiGe is between 300°C
and 450°C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
5.2 Schematic cross-section of TLM structures used to extract ρC. Germanide
contacts are shown on the left and conventional metal contacts are shown on
the right. NiGe formation consumes some germanium, resulting in a slightly
recessed contact. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.3 (From [86]) Electrically active n-type dopant profile as measured by SRP for
P only (blue) and P+Sb coimplant (red) samples. . . . . . . . . . . . . . . . . . . . . . . . 97
5.4 (From [86]) SIMS profile for NiGe contacts on P+Sb coimplanted samples. P
and Sb segregation at the NiGe/Ge interface can be seen, where the interface
was determined from the Ni and Ge concentrations. . . . . . . . . . . . . . . . . . . . . . 99
5.5 (From [86]) Simulated ρC versus n-type Ge doping concentration for various
barrier heights. NiGe contacts show an apparent 0.1eV reduction in ΦBN
compared to the pinned Al/Ti contacts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
5.6 (From [86]) Schematic diagram of (a) metal/Ge and (b) NiGe/Ge contacts.
NiGe contacts with dopant segregation introduce a small dipole which shifts
the germanide Fermi level towards the germanium conduction band. . . . . . . . 101
5.7 Effect of sulfur segregation to the NiGe/Ge interface causes a reduction in
effective ΦBN. With increased levels of sulfur, the Schottky diode reverse
current increases and becomes more ohmic. . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
1
Chapter 1
Introduction
1.1 Motivation
The silicon metal-oxide-semiconductor field-effect transistor (MOSFET) has been the
building block of integrated circuits (IC) for several decades. Historically, transistor
performance was improved simply by scaling the transistor dimensions by 0.7× in every
technology node, in what has become known as Moore’s Law [1]. However, the
performance gains from classical scaling have diminished for two main reasons. First,
simply decreasing the channel length no longer improved MOSFET drive current, and
second, the effect of parasitics became proportionally greater. To address the first issue,
it has become necessary to introduce other performance boosters such as strain [2-4]
since the 90nm node. Alternative channel materials have also generated significant
interest due to the higher carrier mobilities compared to silicon. Table 1.1 lists electron
and hole mobilities for several semiconductors in bulk material.
Table 1.1: Carrier mobilities for several semiconductors
Mobility
(cm2V
-1s
-1)
Si Ge GaAs In0.53Ga0.47As InAs GaSb
Electron 1400 3900 8500 12,000 40,000 3000
Hole 450 1900 400 300 500 1000
Germanium is particularly attractive because its electron and hole mobilities are both
higher than silicon. Unlike As-based III-V semiconductors with its high electron
mobility but low hole mobility, the use of Ge allows for complementary metal-oxide-
2
semiconductor (CMOS) transistors in one material system, greatly simplifying the
manufacturing process. Furthermore, since Ge is a group IV element like Si, it is
completely compatible with current manufacturing facilities as it does not act as a dopant
impurity. It is therefore critical to study all aspects of germanium transistor processes
and device characteristics.
As mentioned earlier, parasitic series resistance forms an increasingly larger portion
of the total device resistance, leading to lower MOSFET drive current. Channel
resistance decreases due to shorter channel lengths, but parasitic resistance actually
increases (Fig. 1.1). As a result, in highly scaled devices parasitic resistance may begin
to dominate, leading to significant reduction in transistor drive current.
Figure 1.1: (From [5]) For planar MOSFETs, parasitic resistance increases with scaling,
becoming a larger portion of the total device resistance. Beyond 20nm, the parasitic
resistance may even begin to dominate device characteristics.
While Ge p-channel MOSFETs (PMOSFETs) have been demonstrated with superior
performance, n-channel MOSFETs (NMOSFETs) have suffered from poor drive current.
One reason for this is the particularly high parasitic resistances in Ge NMOSFETs. In
3
general, parasitic resistance can be divided into the contact resistance between the metal
and semiconductor and the junction resistance of the doped semiconductor source/drain
regions (Fig. 1.2). The metal Fermi level (EF) at metal/Ge interfaces lies close to the Ge
valence band (EV), leading to a large barrier to n-type Ge and therefore a high contact
resistance. Furthermore, n-type dopants in Ge generally have electrically active
concentrations in the low 1019
cm-3
range, which results in a high junction resistance.
These parasitics must be reduced in order to improve Ge NMOSFET performance. This
thesis focuses on n-type Ge contact resistance reduction.
Rc
Rj
GateMetal
Rparasitic = Rc + Rj
Figure 1.2: The parasitic series resistance in a MOSFET can be divided into two main
components: contact resistance (Rc) and junction resistance (Rj).
1.2 Thesis Organization
This thesis addresses the critical issue of high contact resistance to n-type germanium,
especially in the context of germanium high-speed logic MOSFETs. In Chapter 2, the
current state-of-the-art in Ge n-type contacts are reviewed. Methods include high dopant
activation techniques, metal-insulator-semiconductor (MIS) contacts, chalcogen
passivation, and dopant segregation. Chapter 3 introduces the use of the TiO2 dielectric
in MIS contacts. The use of TiO2 resulted in a significant reduction in tunneling
4
resistance compared to traditional MIS contacts. Chapter 4 explores the physics of
contact resistance reduction in MIS contacts. Dielectric series resistance was shown to
affect MIS contact performance. Chapter 5 discusses nickel germanide contacts,
including the use of co-doping and chalcogen/dopant segregation. Finally, Chapter 6
summarizes the main contributions of this work and proposes some future work.
5
Chapter 2
Methods to Reduce Ge N-Type Contact Resistance
As mentioned in Section 1.1, germanium has the potential to replace silicon as the
channel material due to its high electron and hole mobilities. However, one of the most
pressing issues is the need to reduce the Ge parasitic resistances.
2.1 Properties of the Ge Material System
In order to reduce junction resistance it is necessary to dope the source and drain
regions as heavily as possible. In general, the junction resistance will be related to the
sheet resistance Rsh of the doped region; for a uniform n-type region, it can be expressed
as:
jDn
shxNq
R
1 (2.1)
where q is the electronic charge, μn is the electron mobility, ND is the doping
concentration, and xj is the junction depth. Since the junction depth xj is typically subject
to electrostatic constraints for controlling short-channel effects, the only process
parameter is the doping concentration, ND. For Si, doping levels in excess of 1020
cm-3
are
easily achieved. In germanium, however, electrically active n-type dopant concentrations
are limited to the low 1019
cm-3
range [6]. Fig. 2.1 shows the difficulty in completely
activating n-type dopants in Ge. Chemical concentration was measured using secondary
ion mass spectroscopy (SIMS) while the electrically active concentration was measured
6
using the spreading resistance profiling (SRP) technique. Although the solid solubility of
phosphorus is above 1020
cm-3
, the electrically active concentration after rapid thermal
annealing (RTA) is only about 2×1019
cm-3
. Similarly low levels of n-type doping are
achieved using arsenic and antimony.
Figure 2.1: (From [6]) SIMS and SRP profiles show chemical and electrically active
dopant concentrations, respectively. P-type Ge doped with boron show electrically active
concentration above 1020
cm-3
. However, n-type Ge doped with phosphorus show
electrically active concentration of 2×1019
cm-3
, even though the chemical concentration is
higher. Similar results are obtained for As and Sb n-type dopants in Ge.
This problem is further exacerbated by the fact that n-type dopants diffuse quite
rapidly in germanium. The fundamental reason behind the low dopant activation and fast
diffusion is the fact that defects in germanium are dominated by vacancies and behave
electrically as p-type dopants. It has been determined that donor diffusion was greatly
enhanced by vacancies [7]. Furthermore, these vacancies tend to form donor-vacancy
complexes which are more energetically favorable compared to unbound vacancies [8].
The n-type dopants are effectively compensated by the p-type vacancies by the formation
of these complexes.
7
Figure 2.2: (From [9]) Specific contact resistivity as a function of doping density and
electron barrier heights for n-type Si.
In addition to the high sheet resistance at the source/drain junctions, the low
electrically active n-type dopant concentration also leads to a contact resistance between
the metal and n-type Ge. Contact resistance Rc can be related to process parameters:
AR cc / (2.2)
N
mSBc
*2exp
(2.3)
Here, A is the contact area, ρc is the specific contact resistivity, ΦB is the barrier height
from the metal to the semiconductor majority band, ℏ is the reduced Planck’s constant, εs
is the semiconductor permittivity, m* is the carrier effective mass, and N is the
semiconductor majority carrier density at the surface. The inability to achieve high n-
type doping in germanium has a deleterious effect on contact resistance. In order to get
8
appreciable current flow at the metal-semiconductor contact, the barrier must be
sufficiently thin to allow for carrier tunneling, which is achieved by increasing doping.
To get a specific contact resistivity below 10-7
Ωcm2 which is required for scaled devices,
Fig. 2.2 predicts that a doping density greater than 1020
cm-3
is needed [9]. This is a
significant challenge since germanium n-type doping is typically limited to low 1019
cm-3
as previously discussed.
The barrier height ΦB can also be reduced in order to reduce contact resistance, as
shown in Fig. 2.2. Ideally, for an n-type contact, the electron barrier height ΦBN can be
modulated by changing the metal:
SMBN (2.4)
where ϕM and χS are the metal workfunction and semiconductor electron affinity,
respectively. Unfortunately, the metal Fermi level (EF) is typically pinned within the
semiconductor band gap, a phenomenon known as Fermi level pinning. For Si, the
pinning location is about one-third of the band gap above EV, while in Ge it is pinned
about 0.08eV above EV [10-11] as shown in Fig. 2.3.
Figure 2.3: (From [10]) At metal/semiconductor interfaces, the metal Fermi level lies
within a narrow range of energies within the semiconductor band gap due to Fermi level
pinning.
9
This leaves a large electron barrier of about 0.58eV for n-type Ge. Fermi level
pinning is much stronger in the Ge case due to the smaller band gap and higher dielectric
constant compared to Si. Fig. 2.4 compares the measured electron barrier heights at
metal/n-Ge contacts for a variety of metals compared to the ideal case (i.e. the Schottky
limit) given by Equation 2.4.
Figure 2.4: (From [11]) The measured electron barrier height is not significantly
modulated by changing metal workfunction. Line (a) is the ideal (Schottky) limit,
showing considerable deviation from experimental results.
With conventional ion implantation and RTP, contact resistance to n-type Ge is very
high at around 10-4
Ωcm2 [12, 13]. It is worth mentioning at this time that contact
resistance is expected to be the least scalable among the components of parasitic
resistance. By the 32nm node, contact resistance may contribute close to 70% of the total
parasitic series resistance [14]. The Fermi level pinning problem will be further
discussed in Section 2.3. Nevertheless, it is clear that the Schottky barrier must be
reduced in order to achieve low contact resistance.
10
2.2 Dopant Activation Techniques
This section reviews some methods that have been used to increase the n-type dopant
concentration beyond the conventional low 1019
cm-3
levels.
2.2.1 Laser Annealing
Laser annealing for dopant activation is a natural evolution of the annealing
techniques. In order to reduce dopant diffusion, a shorter anneal time is desirable.
Furnace anneal times were in the minutes regime, which gave way to rapid thermal
processing (RTP) in the seconds regime. More advanced techniques such as flash and
spike annealing could achieve annealing times in the millisecond regime. However, for
scaled technologies, this would still result in significantly diffused source/drain regions.
Recently, there has been considerable effort to use laser annealing to achieve very intense,
microsecond-length optical pulses. Because of the very short pulse duration, dopants
may be supersaturated in substitutional sites [15], leading to very high electrically active
doping concentrations. However, it is still unclear whether these metastable conditions
can be maintained during subsequent device fabrication thermal steps and product
reliability.
The ability to achieve high doping is a great incentive to use laser annealing on
germanium as in the work of Thareja et al. [16] and Martens et al. [12]. In the work by
Thareja et al. [16], laser annealing was used on Sb-implanted germanium. Antimony was
chosen because it is a heavy atom and could have a shallow implant at reasonable
energies. Over 1020
cm-3
dopant activation was achieved using a laser fluence of 0.7Jcm-2
as shown in Fig. 2.5. The laser fluence must be chosen carefully. With a lower laser
11
fluence there is incomplete melting of the surface germanium layer, resulting in
recrystallization with defects. Because of the high doping, this also resulted in a low
specific contact resistivity of 7×10-7
Ωcm2 as shown in Fig. 2.6, which was the lowest
reported value at the time.
0 500 1000 1500 200010
18
1019
1020
1021
1022
Co
nc
en
tra
tio
n (
cm
-3)
Depth (A)
SIMS - as implanted
SIMS - 0.4 J/cm2
SIMS - 0.2 J/cm2
SIMS - 0.7 J/cm2
SRP - 0.7 J/cm2
Sb - 10keV, 5 x 1015 cm-2
Figure 2.5: (From [16]) The electrically active Sb profile shown by the SRP line
indicates n-type dopant activation beyond 1020
cm-3
in germanium using laser annealing.
Figure 2.6: (From [16]) Specific contact resistivity of 7×10-7
Ωcm2 was reported using
laser annealing of Sb-implanted germanium.
12
In the work of Martens et al. [12], laser annealing was also used on arsenic-implanted
germanium. After ion implant and laser activation, Ni was deposited and annealed to
form NiGe. The achieved dopant activation levels was not shown, however the contact
resistance was reduced to 2.5×10-6
Ωcm2. This reduction in the contact resistance was
partially attributed to the dopant snowplow effect by NiGe formation. Since the
formation of NiGe consumes the top layer of Ge, the arsenic dopant atoms pile up at the
NiGe/Ge interface. This causes the effective dopant concentration at the contact to be
higher. Also, there is recent work that suggests dopant segregation at these interfaces
may reduce the Schottky barrier height. This effect will be discussed in more detail in
Section 2.5.
2.2.2 Phosphorus & Antimony Coimplantation
With a single dopant species the doping concentration is limited to the low 1019
cm-3
range using RTA. However, Kim et al. [17] from IBM were able to achieve 1020
cm-3
doping concentration using both phosphorus and antimony together in conjunction with
standard RTA at 500°C. A phosphorus implant is followed by a shallow antimony
implant, and if the energies and doses are chosen carefully, high activation can be
achieved, as shown in Fig. 2.7.
13
Figure 2.7: (From [17]) High dopant activation using coimplantation of P and Sb shown
by the SRP line.
While it is not yet clear why this approach works, Kim et al. [17] speculates that the
small substitutional phosphorus atoms induce a local tensile strain on the germanium
lattice. To compensate, the larger antimony atom is used to apply local compressive
strain. As a result the germanium lattice is less disrupted by the presence of dopant
atoms. This theory is given credence by the fact that coimplantation using phosphorus
and arsenic was not able to enhance dopant activation [18] suggesting that a large donor
atom such as Sb is required. Using Al (95nm)/Ti (5nm) contacts, specific contact
resistivity of 8×10-7
Ωcm2 was obtained [19], which is comparable to the values obtained
using laser annealing of Sb. This method is particularly attractive since this approach is
quite simple to implement and does not require the expensive laser anneal step, using a
simple RTP system for the annealing. Furthermore, the semiconductor system is in
thermal equilibrium, which avoids the issues of metastability and its associated process
integration and reliability challenges. This method will be further exploited in Chapter 5.
14
2.2.3 Fluorine Vacancy Passivation
In Section 2.1, it was pointed out that the electrically p-type vacancy defects in
germanium were responsible for the deactivation of n-type dopants. It was recently
theorized [20] using density functional theory (DFT) that fluorine can form fluorine-
vacancy clusters. These clusters are more energetically favorable than both unbound
vacancies as well as donor-vacancy clusters, thereby decreasing the amount of dopant
deactivation. Experimental work by Jung et al. [21] showed that co-implantation of
phosphorus and fluorine indeed enhanced dopant activation slightly to around 1020
cm-3
,
although the surface was still limited to about 3×1019
cm-3
, as shown in Fig. 2.8. This
may be due to the fact that there are more defects closer to the surface due to the
disruption in the crystal lattice.
Figure 2.8: (From [21]) Enhanced phosphorus dopant activation due to coimplantation
of fluorine (blue line).
In order to further study the effect of fluorine, Jung et al. [22] also self-implanted Ge
in order to artificially introduce defects. Since these defects are electrically active, they
15
can be measured using SRP as p-type conductivity. Fig. 2.9 is the SRP plot showing a
large number of defects with the Ge self-implant (red line). Upon an appropriate fluorine
implant and annealing the defects are significantly reduced (light blue line), almost to the
level of the sample without the self-implant (yellow line). This experiment shows the
efficacy of fluorine in passivating the bulk defects in germanium; when phosphorus and
fluorine are coimplanted, the resulting dopant levels are higher than with phosphorus
alone.
Figure 2.9: (From [22]) Fluorine can reduce the defects caused by Ge self-implantation
(red line to blue line).
This method is attractive since fluorine can also be incorporated in high-k/Ge gate
stacks to improve capacitance-voltage (C-V) frequency dispersion, interface state density
(Dit), and gate leakage [23, 24].
2.3 MIS Contacts
Section 2.2 discussed various methods to increase the n-type doping in Ge. In this
section, the focus is on metal-insulator-semiconductor (MIS) contacts, which aim to
16
reduce the electron barrier height from the metal to the semiconductor. The main idea is
to unpin the metal Fermi level by inserting an ultrathin dielectric between the metal and
semiconductor.
2.3.1 Fermi Level Pinning
When a metal is placed in contact with a semiconductor, a Schottky barrier is formed.
In the ideal theory by Schottky and Mott, the n-type barrier height is predicted by
Equation 2.4. Using a metal with a large workfunction would generate a large barrier,
and the metal-semiconductor contact would be rectifying. Conversely, using a metal with
a small workfunction would create a small or even negative barrier, and the contact
would be ohmic. However, at any semiconductor surface, as in a metal-semiconductor
contact, the crystal lattice is severely disrupted, giving rise to surface states that occupy
energies within the semiconductor band gap. Furthermore, the presence of the metal also
creates metal-induced gap states (MIGS) within the semiconductor band gap [25, 26, 10].
MIGS is the result of metal electron wave functions that penetrate into the semiconductor,
resulting in gap states. These interface states give rise to an apparent pinning of the metal
Fermi level at metal-semiconductor contacts.
These states are characterized as donor-like or acceptor-like depending on their
charge. Donor-like states typically occupy the lower portion of the band gap whereas
acceptor-like states typically occupy the upper portion as depicted in Fig. 2.10. The
energy where there is an equal density of acceptor-like and donor-like states is called the
charge neutrality level (ECNL). Donor-like states above the Fermi level are empty and
therefore positively charged; acceptor-like states below the Fermi level are filled and
17
therefore negatively charged. If the metal EF is aligned with the semiconductor ECNL, the
positive charges from empty donor-like states is canceled out by negative charges from
filled acceptor-like states. The interface is therefore charge neutral.
Acceptor-like
Donor-like
EC
EV
Ele
ctro
n E
ner
gy
ECNL
Figure 2.10: Typical distribution of acceptor-like and donor-like states within the
semiconductor band gap, with the charge neutrality level indicated.
In the ideal Schottky model, the metal EF may lie above or below the ECNL at
equilibrium. In the presence of gap states, however, the metal EF tends to line up with
ECNL. Suppose the metal EF lies below ECNL, as in Fig. 2.11, electrons would be
transferred from the semiconductor to the metal, which has the effect of raising the metal
EF towards ECNL [27]. The opposite happens when the metal EF lies above ECNL.
Figure 2.11: (From [27]) Charge transfer between semiconductor band gap states and
metal causes the metal EF to align with the charge neutrality level.
18
The degree to which the metal EF is pinned is known as the pinning factor S [28]. In
the presence of these interface states, the effective metal workfunction ϕM,eff can be
written as:
CNLMCNLeffM S , (2.5)
where ϕM is the metal workfunction and ϕCNL is the charge neutrality level referenced to
the vacuum level. If S = 0 (Bardeen limit), the contact is completely pinned; if S = 1
(Schottky limit), the contact is ideal with no pinning. For Ge, pinning factor S ranges
from 0.02 to 0.05 [10, 11] indicating very strong pinning. The charge neutrality level lies
about 0.03eV to 0.09eV above the valence band [11, 29]. These two facts combined give
rise to the very large electron Schottky barrier at metal contacts to n-type Ge, as depicted
in Fig. 2.12. Because of this, there is a strong motivation to use techniques for depinning
the Fermi level on n-type Ge.
EFM pinned near Ge Ev
Large ΦBN
ρc > 10-4 Ωcm2
Metal N-Type Ge
Figure 2.12: Strong metal Fermi level EFM pinning near the Ge valence band results in
a large specific contact resistivity to n-type Ge.
19
2.3.2 Fermi Level Depinning using Ultrathin Dielectrics
The method of depinning the Fermi level by inserting an ultrathin dielectric between
the metal and semiconductor to form an MIS contact was first demonstrated by Connelly
et al. [30] in 2004. In that work, Si3N4 was used to depin the Fermi level on silicon
substrates to reduce specific contact resistivity, as shown in Fig. 2.13. By inserting a
large band gap material (in this case, Si3N4) between the metal and semiconductor, the
metal electron wave function penetration is reduced, leading to a reduction in MIGS.
Fermi level pinning is then partially mitigated, as depicted schematically in Fig. 2.14.
Figure 2.13: (From [30]) By inserting silicon nitride between metal and n-type Si,
contact resistance can be reduced. There is an optimal thickness before contact resistance
increases again due to tunneling resistance.
Figure 2.14: (From [30]) The inserted insulator reduces MIGS, allowing the metal
Fermi level to rise closer to the conduction band and reducing the effective barrier height.
20
Another explanation for these results involves electronic dipoles, which can be
formed at any interface. In a MIS contact, there are metal-insulator and insulator-
semiconductor interfaces. Additionally, there may be an insulator-insulator dipole if two
different dielectrics are used or if there is a native oxide. According to the bond
polarization theory, the metal-semiconductor interface will have an electronic dipole
created by interfacial energy relaxation [31, 32]. This interfacial dipole alters the
effective metal workfunction and manifests as Fermi level pinning. By inserting a
dielectric as in a MIS contact, this dipole is replaced by other dipoles caused by the new
interfaces. If these dipoles have the correct polarity, they will effectively shift the metal
workfunction towards EC, thereby causing a decrease in contact resistance with the
insertion of an insulator.
An ultrathin layer of silicon nitride can also be used in Ge MIS contacts for Fermi
level depinning [33]. In Fig. 2.15, as the thickness of Si3N4 is increased for n-Ge, the
current-voltage (I-V) characteristics change from rectifying to ohmic, indicating a
decrease in the effective electron barrier height. Beyond the optimum thickness of 2nm,
the current drops due to the added resistance due to tunneling through the dielectric. For
p-Ge, the current simply drops due to both a higher hole Schottky barrier (ΦBP) as well as
added tunneling resistance. These results are typical of depinning experiments using MIS
contacts. Table 2.1 summarizes the experimental work on MIS contacts in the literature,
including our own work and the subject of this thesis. A wide variety of dielectrics can
depin the Fermi level on both group-IV and III-V semiconductors, making this a
21
powerful technique. For bilayer dielectric stacks, the dielectric closest to the metal is
written first.
Figure 2.15: (From [33]) Si3N4 MIS contacts on germanium.
Table 2.1: Summary of MIS Contact Schemes in Literature
Semiconductor Dielectric References
Si Si3N4
AlOx
LaOx/SiO2, AlOx/SiO2
TiO2
Connelly, 2004 [30]
Coss, 2011 [34]
Coss, 2009 [35]
This work (Section 4.1)
Ge Si3N4
Ge3N4
GeOx
GeOx, AlxOy
Al2O3
MgO
Y2O3
TiO2
Kobayashi, 2009 [33]
Lieten, 2008 [36]
Takahashi, 2007 [37]
Nishimura, 2008 [38]
Zhou, 2008 [39]
Lee, 2010 [40]; Zhou, 2010 [41]
Li, 2012 [42]
This work [43-45]
Ge1-xSnx TiO2 This work (Section 4.1)
GaAs Si3N4, Al2O3
HfO2, ZrO2, TiO2
Al2O3/TiO2
Hu, 2010 [46]
Hu, 2011 [47]
Hu, 2011 [47]
In1-xGaxAs Al2O3 Hu, 2010 [46]
GaSb TiO2 Yuan, 2011 [48]
22
2.3.3 Resistance Due to Tunneling Through the Dielectric
Although MIS contacts can reduce the effective barrier height, the inserted dielectric
adds a tunneling resistance to the overall contact resistance. This results in a tradeoff as
depicted schematically in Fig. 2.16.
Con
tact
Res
ista
nce
Dielectric Thickness
Figure 2.16: Tradeoff between lower ΦBN and tunneling resistance results in an
optimum dielectric thickness.
We studied this tradeoff in more detail using tunneling transport simulations.
Assuming parabolic bands and Fermi-Dirac statistics, the Tsu-Esaki tunneling model [49]
gives the current density through a tunneling contact as:
Max
Min
E
EZZZTOT dN
h
qmJ
3
*4 (2.6)
kT
kTkTN
ZFS
ZFM
Z
exp1
exp1
ln (2.7)
23
where m* is the density of states effective mass, q is the electronic charge, h is Planck’s
constant, η is the tunneling probability, εZ is the longitudinal electron energy, k is the
Boltzmann constant, T is the absolute temperature, and εFM and εFS are the metal
and semiconductor Fermi levels respectively. The tunneling probability η is calculated
by discretizing the insulator and semiconductor depletion region and using the transfer
matrix formalism. More details can be found in Roy et al. [50] and his thesis [51]. The
specific contact resistivity can then be calculated based on its definition:
0
V
CJ
V (2.8)
where V is the small applied voltage, and J is the calculated tunneling current. Fig. 2.17
shows the simulation results for Al2O3 and Si3N4 MIS contacts on 1019
cm-3
doped n-Ge.
Figure 2.17: (From [50]) Tunneling transport simulations on Al2O3 (left) and Si3N4
(right) MIS contacts on n-type Ge. It is very difficult to achieve low contact resistivity
using these materials.
The implication of these simulated results is that it is very difficult to achieve low
specific contact resistivity (<10-7
Ωcm2). MIS contacts using these materials would need
to fully depin the metal Fermi level (S about 1) with only 0.5nm of dielectric or thinner.
Beyond this thickness, the tunneling resistance dominates and the advantages of a lower
ΦBN are lost. One way to break this tradeoff is to use a dielectric with a lower conduction
24
band offset (CBO). Fig. 2.18 shows the CBO of common dielectrics, and TiO2 emerges
as a strong contender due to its roughly zero CBO. The simulation results shown in Fig.
2.19 confirm this intuition. As long as sufficient depinning is achieved (S > 0.8), it
should be possible to achieve less than 10-7
Ωcm2 specific contact resistivity. Even up to
2nm of TiO2 could theoretically be used since there is no tunnel barrier. We study TiO2
MIS contacts experimentally in Chapters 3 and 4.
SiO2 Al2O3 Si3N4 La2O3 Y2O3 HfO2 ZrO2 Ta2O5 TiO2
TiO2 promising material due to low CBO
Figure 2.18: Conduction band offset is an essential material selection criterion since
tunneling resistance in MIS contacts must be minimized. TiO2 is identified as a
promising material in this aspect. CBO data from [27].
Figure 2.19: (From [50]) TiO2 breaks the tradeoff between lower ΦBN and tunneling
resistance due to the low CBO to germanium. Even relatively thick TiO2 can be used.
25
2.4 Chalcogen Passivation
Chalcogens are the elements in column 16 of the periodic table and include oxygen,
sulfur, selenium, and tellurium. Semiconductor surfaces will normally oxidize in ambient
conditions in order to minimize surface energy. The surface may also relax by
reconstructing the surface, although this likely results in bonds that differ significantly
from the bulk. Furthermore, reconstructed surfaces may still have dangling bonds.
Sulfur and selenium passivation of semiconductor surfaces was proposed by E. Kaxiras
[52] to prevent surface reconstruction and restore the ideal bulk lattice geometry. Using a
monolayer of Se, for example, it is possible to reduce the electron Schottky barrier from
the pinned value of 0.4eV to 0.08eV [53]. It is likely that the pinning effect is decreased
due to the decrease in surface states as a result of this surface passivation. A similar
effect is seen on Ge [54, 55], as seen in Fig. 2.20.
Figure 2.20: (From [55]) On n-type Ge, rectifying behavior changes to ohmic behavior
after sulfur passivation, while the opposite happens on p-type Ge, indicating a decrease in
ΦBN and increase in ΦBP.
26
Chalcogen passivation can also be accomplished by using germanides. A shallow
implant of the chalcogen species is first done, followed by the reactive metal (e.g. Ni)
deposition and annealing to form germanide. During the growth of the germanide, the
top layer of the germanium substrate is consumed. The chalcogen species will
preferentially segregate at the germanide/germanium interface. This method has been
demonstrated using sulfur and nickel germanide to partially depin the Fermi level [56].
2.5 Dopant Segregation
In the previous section, it was noted that during germanidation, chalcogen atoms can
segregate to the germanide/germanium interface. If dopants are present in the
germanium, they will also segregate at the interface due to the snowplow effect. In this
technique, care must be taken to ensure that the dopant depth is shallow (i.e. less than the
Schottky contact depletion region) in order to retain the Schottky junction characteristics;
otherwise pn-junction characteristics would show. There is still considerable controversy
over the mechanism responsible for the apparent reduction in barrier height using dopant
segregation. One theory is the charged substitutional dopants at the interface form an
interfacial dipole with the charged interface states [57]. The result is an apparent shift in
metal Fermi level by an amount equal to the interfacial dipole. Both phosphorus and
arsenic have been used [58] with NiGe in order to reduce the effective electron barrier
height, as shown in Fig. 2.21. Increasing the As dose in this case causes the NiGe/n-Ge
contact to become more ohmic, indicating a decrease in the effective electron barrier
height.
27
Figure 2.21: (From [58]) Increasing As dose decreases effective ΦBN, causing the n-
type Ge contact to become more ohmic.
2.6 Summary
High semiconductor doping and low barrier height between the metal and
semiconductor are required to achieve good contacts. In the case of n-type Ge, however,
the semiconductor doping is limited to the low 1019
cm-3
regime. Furthermore, because of
strong Fermi level pinning, the metal Fermi level is pinned close to the Ge valence band
at metal/Ge contacts. These two issues result in a high n-type Ge contact resistance.
Several methods to increase the n-type doping concentration above 1020
cm-3
were
reviewed, including laser annealing, phosphorus and antimony coimplantation, and
fluorine vacancy passivation. MIS contacts, chalcogen passivation, and dopant
segregation were reviewed as possible techniques for reducing metal/n-Ge barrier heights.
Chapters 3 and 4 discuss low resistance MIS contacts on n-type Ge in detail. Chapter 5
discusses the use of germanides with phosphorus and antimony coimplantation for low
contact resistance.
28
Chapter 3
TiO2 MIS Contacts
In Section 2.3.3, we predicted that TiO2 may be a good material for MIS contacts
based on its low conduction band offset (CBO) to germanium. This chapter will discuss
our experimental work on TiO2 MIS contacts as well as some applications of this
technique.
3.1 ALD Deposition
TiO2 can be deposited by a variety of methods, including reactive sputtering and
atomic layer deposition (ALD). The work described herein is based on the ALD method.
In ALD, two precursors are sequentially pulsed repeatedly, with a purge step in between,
resulting in highly conformal and uniform films. For TiO2,
tetrakis(dimethylamido)titanium (TDMA-Ti), [(CH3)2N]4Ti, and water, H2O, were the
two precursors. One pulse of TDMA-Ti and one pulse of water comprise one cycle. In
our experiments TDMA-Ti was always pulsed first to minimize interfacial layer
formation. The deposition rate was about 0.4Å/cycle, and was uniform over the
temperature range of 150°C to 250°C, which was the temperature limit of the system (a
Cambridge NanoTech Savannah ALD system). Again, in order to minimize interfacial
layer growth, the lowest temperature of 150°C was used unless otherwise stated. Atomic
force microscopy (AFM) of the films revealed very uniform deposition, with a root-
29
mean-square (RMS) roughness about 0.36nm, which is only slightly higher than a
pristine Si prime wafer.
Al2O3 MIS contacts were also fabricated as control samples. Al2O3 was deposited at
200°C using the precursors trimethylaluminum (TMA) and water, again with TMA first.
The deposition rate was about 1Å/cycle. This allows us to compare a low CBO material
(i.e. TiO2) with a high CBO material (i.e. Al2O3) in the context of MIS contacts.
3.2 Band Offsets to Germanium
Valence band offsets (VBO) were first measured on ALD-deposited TiO2 and Al2O3
on Ge using synchrotron radiation photoelectron spectroscopy (SRPES). Low energy (hν
= 120eV - 160eV) photons from the Stanford Synchrotron Radiation Lightsource beam
line 8-1 allows for precise measurement of the VBO by taking the difference between the
valence band spectrum of TiO2 or Al2O3 and comparing it to the valence band spectrum
of bulk Ge. The Ge 3d peak was used for alignment. Fig. 3.1(a) shows the valence band
spectra of TiO2 on Ge and bulk Ge, indicating a VBO of 2.9eV. The bandgap of
amorphous TiO2 has been reported in the 3.3eV – 3.5eV range [59], while the crystalline
phases of TiO2 tend to have smaller band gaps [60]. We expect our ALD-deposited TiO2
films to be amorphous; since we used the band gap data only to calculate CBO, the larger
band gap was used since it represents the worst case and leads to a larger CBO. This
gives a CBO of TiO2/Ge between -0.26eV and -0.06eV. This barrier is essentially zero,
confirming the possibility of using TiO2 MIS contacts for minimizing tunneling
resistance. Fig. 3.1(b) shows the valence band spectra of Al2O3 on Ge and bulk Ge,
indicating a VBO of 3.7eV. The ALD Al2O3 band gap was found to be 6.1eV as
30
determined from the energy loss spectrum of the Al 2p peak as shown in Fig. 3.1(c),
which is consistent with published results for ALD Al2O3 [61]. This gives an Al2O3/Ge
CBO of about 1.7eV. Note that we could not use this technique to determine the band
gap of our ALD-deposited TiO2 because the Ti 3p peak was too wide and therefore
masked the energy loss onset. The VBO and CBO data for these two MIS systems is
summarized in Fig. 3.2.
Figure 3.1: SRPES spectra for (a) TiO2 and Ge valence bands, (b) Al2O3 and Ge valence
bands, and (c) Al 2p peak.
Al2O3
TiO2
Ge Ge6.1eV 0.66eV 0.66eV
3.7eV
1.7eV
2.9eV3.3eV - 3.5eV
0.06eV - 0.26eV
Figure 3.2: Band offsets of the Al2O3/Ge and TiO2/Ge systems.
31
Based on the band diagrams in Fig. 3.2, it is clear that TiO2 MIS contacts should be
able to outperform MIS contacts using higher CBO materials such as Al2O3 because of
significantly lower tunneling resistance. However, because TiO2 is a narrower band gap
material, it may not be as effective as wider band gap materials in blocking metal electron
wave function penetration and the formation of MIGS. Therefore, the next section looks
into whether the insertion of TiO2 can actually reduce ΦBN.
3.3 MIS Contacts on Epitaxial Germanium
TiO2 MIS contacts were fabricated on epitaxially-grown germanium on silicon
substrate, as depicted in Fig. 3.3. Ge doped with an electrically active concentration
about 1018
cm-3
was heteroepitaxially deposited on Si wafers. SRP profile of the n-type
doping is shown in Fig. 3.4. This level of doping was chosen to make the changes in
barrier heights more apparent. Results from higher doping levels that mimic n+
source/drain regions are described in Section 3.4.
SiO2 SiO2
Si (500μm)
400nm TiO2 or Al2O3
Al (300nm)
Epitaxial moderately-doped Ge (1.5μm)
Al (100nm)/Ti (10nm) or Al (100nm)
or Pt (50nm)
Figure 3.3: Schematic cross-section of MIS contacts fabricated on heteroepitaxially
deposited Ge on Si.
32
Figure 3.4: SRP profile showing electrically active n-type doping concentration of in
situ doped epitaxially-grown Ge on Si.
N-type Ge was deposited on n-type Si and p-type Ge was deposited on p-type Si to
prevent the formation of a pn-junction. 400nm SiO2 was then deposited by chemical
vapor deposition (CVD) at 400°C for device isolation. Standard lithography techniques
were used to pattern contact holes in the SiO2. ALD TiO2 (at 150°C) or Al2O3 (at 200°C)
films were deposited; different insulator thicknesses were achieved by using a different
number of ALD cycles. Some samples were fabricated without a dielectric layer in order
to observe the contact characteristics of metal directly on Ge. Before ALD, three cycles
of dilute HF (2%) and H2O cleaning followed by a dilute HCl dip were done to remove
native germanium oxides. Metal was then deposited by e-beam evaporation onto the
sample service. As shown in Fig. 3.3, Al/Ti, Ti, and Pt metallization were used to
examine the effect of different metal workfunctions. Standard lithography techniques
were again used to pattern the metal pads. An argon sputter etch was performed using
the metal as the hard mask to completely etch the exposed TiO2. Due to the low band
gap of TiO2, electrical conduction can sometimes be observed in TiO2 films. The as-
33
deposited TiO2 exhibited a very high resistivity of about 0.4Ωm. Nevertheless, this argon
sputter etch ensures complete isolation between adjacent devices. Finally, 300nm of
aluminum was deposited on the sample backside to get a good backside contact. High-
resolution transmission electron microscopy (TEM) images of the metal-insulator-
semiconductor interfaces are shown in Fig. 3.5. Good uniformity of the TiO2 was
observed.
Ti
TiO2
Ge
Ti
TiO2
Ge
12nm
(a) (b)
Figure 3.5: High-resolution TEM images of the TiO2 MIS contact.
The current-voltage (I-V) characteristics of TiO2 and Al2O3 MIS contacts are shown
in Fig. 3.6 and Fig. 3.7, respectively. Measurements were carried out with the sample
backside as the second contact. For TiO2 MIS contacts on n-Ge (Fig. 3.6(a)), the I-V
characteristics start out Schottky-like (0nm TiO2 case), indicating a high ΦBN. With
4.6nm and 5.8nm TiO2 inserted, however, the reverse bias current increases, and the I-V
becomes more ohmic-like. By 8.8nm of TiO2, the I-V is essentially symmetric and the
current levels are also more than three orders of magnitude higher, indicating a very low
ΦBN, which we later verified using temperature-dependent I-V measurements. The fact
that the 5.8nm and 8.8nm samples look similar is an indication that thicker TiO2 does not
34
increase tunneling resistance significantly, as was predicted by the CBO estimation using
SRPES. For p-type Ge (Fig. 3.6(b)), the opposite behavior is seen. In the 0nm TiO2 case,
the I-V appears ohmic even at this moderate doping since ΦBP is very close to zero due to
Fermi level pinning. With the insertion of 8.8nm of TiO2, the metal Fermi level moves
towards the conduction band and the current drops significantly as a result of a large ΦBP
(Fig. 3.8). As expected, the I-V characteristics becomes rectifying due to the large hole
barrier.
(b) Al/Ti/TiO2/p-Ge(a) Al/Ti/TiO2/n-Ge
Figure 3.6: TiO2 MIS contacts on ~1018
cm-3
moderately-doped n- and p-type Ge.
(b) Al/Al2O3/p-Ge(a) Al/Al2O3/n-Ge
Figure 3.7: Al2O3 MIS contacts on ~1018
cm-3
moderately-doped n- and p-type Ge.
35
Higher ΦBP
Lower ΦBN
Figure 3.8: A decrease in ΦBN is accompanied by an increase in ΦBP.
TiO2 MIS contacts behave quite differently from Al2O3 MIS contacts, which are
shown in Fig. 3.7. In Fig. 3.7(a) on n-Ge using Al2O3, there is a slight increase in current
with 1nm Al2O3 at low biases, which can be attributed to a reduction in ΦBN. However,
with 2nm Al2O3 or more, the resistance due to tunneling through Al2O3 starts to become
dominant and the current drops markedly. For p-type Ge, Fig. 3.7(b) shows a significant
decrease in current levels as a result of increasing Al2O3 thickness. This is due to both
the added tunneling resistance as well as an increase in ΦBP. These results are in line
with results in the literature using high CBO materials.
3.3.1 Barrier Height Measurements
Temperature-dependent I-V measurements were made between 78K and 260K for the
5.8nm and 8.8nm TiO2 devices on n-type Ge in order to extract the effective Schottky
barrier height. Note that the barrier height extracted using the following method is the
electrically equivalent barrier height of a metal-semiconductor junction, but it may or
may not be the physical barrier height in the metal-insulator-semiconductor junction.
36
Starting with the Schottky contact equation:
1expexp2*
kT
qV
kT
qTAJ BN (3.1)
where A* is the Richardson constant, T is temperature in Kelvins, q is the electronic
charge, k is the Boltzmann constant, and V is the applied voltage, we can approximate the
reverse saturation current J0 as:
kT
qTAJ BNexp2*
0 (3.2)
By rearranging terms and taking the natural logarithm, we obtain:
kT
qA
T
J BN
*
2
0 lnln (3.3)
Therefore, in a plot of ln(J0/T2) versus 1/T, the slope will be equal to –qΦBN/k, from
which ΦBN can be extracted. This type of plot is known as a Richardson plot and is
shown in Fig. 3.9 for select TiO2 MIS contacts.
5.8nm TiO2
ΦBN = 104meV
8.8nm TiO2
ΦBN = 65meV
Figure 3.9: Richardson plot of select TiO2 MIS devices, showing a significantly reduced
extracted effective ΦBN.
37
The extracted ΦBN was 0.104eV for the 5.8nm TiO2 device and 0.065eV for the
8.8nm TiO2 device. This is a substantial reduction in ΦBN from the 0nm case, where ΦBN
≈ 0.58eV [10].
With this information along with the band offset measurements through SRPES (Fig.
3.2), band diagrams of Al2O3 and TiO2 MIS contacts can be drawn, as in Fig. 3.10. Both
Al2O3 and TiO2 MIS contacts can reduce the effective barrier height for electrons.
However, the large CBO of Al2O3 presents a large tunnel barrier; the added tunneling
resistance therefore limits the performance of the contact. In the case of TiO2 MIS
contacts, however, there is much lower tunneling resistance, which allows TiO2 MIS
contacts to significantly outperform those using Al2O3.
Al/Al2O3/n-Ge Al/Ti/TiO2/n-Ge
Figure 3.10: Band diagrams of Al2O3 and TiO2 MIS contacts. While both can reduce
the effect electron barrier height, the lower tunnel resistance of TiO2 allows TiO2 MIS
contacts to outperform those using Al2O3.
38
3.3.2 Effect of Metal Workfunction
The effect of metal workfunction was also studied by fabricating TiO2 MIS contacts
with aluminum (small workfunction) and platinum (large workfunction) metals, in
addition to the devices presented earlier, which used titanium. The fabrication process
was identical, except for the metal used. The I-V characteristics of these devices on n-
type Ge are shown in Fig. 3.11. In general, the characteristics are quite similar. The
pinned contact without TiO2 shows low current levels; increasing the thickness of the
TiO2 increases the current densities by about 1000× at low applied biases. Again, there is
minimal added tunneling resistance, which allows for thick (up to 8.8nm) of TiO2 to be
used.
(b) Pt/TiO2/n-Ge(a) Al/TiO2/n-Ge
Figure 3.11: TiO2 MIS contacts using (a) Al and (b) Pt as the metal display similar I-V
characteristics.
It would appear that a similar reduction in ΦBN can be achieved, irrespective of the
metal used. However, this is actually an unexpected result. In the case of platinum metal,
with its large workfunction (~5.5eV), if complete unpinning was achieved by inserting
TiO2, the electron barrier height should further increase leading to an increase in contact
39
resistance. However, the opposite was observed. By inserting TiO2 between Pt and n-Ge,
the contact resistance actually decreased as witnessed by the increase in current densities
in Fig. 3.11(b). Based on this, we conclude that, rather than Fermi level unpinning, what
actually occurs is the metal Fermi level shifting towards the conduction band. This
strongly favors some sort of dipole mechanism, with the amount of Fermi level shifting
corresponding to the magnitude of that dipole. This idea will be explored further in
Chapter 4.
3.3.3 Comparing MIS Contacts
In order to compare Al2O3 and TiO2 MIS contacts, Fig. 3.12 plots the relative specific
contact resistivity as a function of oxide thickness. The TiO2 devices show a reduction in
ρC of about 1000× till about 5.8nm TiO2. With thicker TiO2, the contact resistivity
remains roughly constant with no significant tunneling resistance. In contrast, the Al2O3
devices show only a small initial decrease in ρC up to 1nm. Tunneling resistance
dominates with thicker Al2O3 and ρC increases very rapidly. This analysis clearly
illustrates the advantage of using a low CBO material such as TiO2 in MIS contacts.
These results are summarized schematically in Fig. 3.13, showing the tradeoff
between lower ΦBN and added resistance. Many dielectrics can be used in MIS contacts
to decrease ΦBN. However, the ρC of a MIS contact with a high CBO material would
increase exponentially after the optimum point due to tunneling. A low CBO material
would not have the exponential degradation of tunneling-limited contacts. In a very large
scale integration (VLSI) application, it would be difficult to achieve the optimum Al2O3
thickness uniformly since the current depends exponentially on thickness. The use of
40
TiO2, for example, relaxes the uniformity requirements since a wide range of thicknesses
still yielded a lower ρC, potentially giving a wider oxide thickness process window for
easier integration on a 300mm wafer.
1000x reduction
Figure 3.12: Relative specific contact resistivity of various MIS contacts. TiO2 MIS
contacts achieve about 1000× improvement while Al2O3 MIS contacts only achieve
roughly 10× improvement.
Spec
ific
Co
nta
ct R
esis
tivi
ty
Thickness of Interfacial Layer
Low CBO material adds less resistance
High CBO material adds tunneling resistance
Reduction in ΦBN
Figure 3.13: Schematic effect of CBO and ΦBN on specific contact resistivity.
41
3.4 TiO2 MIS Contacts on n+ Germanium
In the previous section, we established the advantages of using TiO2 as the dielectric
material in MIS contacts. In this section, we use TiO2 MIS contacts on heavily doped n+
Ge to assess the applicability of this technique in reducing contact resistance to the n+
source/drain regions of Ge NMOSFETs. Furthermore, prior work on MIS contacts has
typically focused on ΦBN reduction on lightly doped substrates, as we did in the previous
section. The minimum ρC that can be achieved at higher substrate doping has not been
investigated.
3.4.1 Measurement Structure
Measurements of specific contact resistivity (ρC) were made using the circular
transfer length method (cTLM) and a 4-point probe setup, as shown in Fig. 3.14. This
method uses small circular contacts of radius L and a large outer contact with variable
gap spacing d between the two. In our structures, the circular contacts had a 50μm radius,
and were separated by 4, 6, 8, 10, 14, 20, 24, and 30μm from the outer contact. The
change in total resistance RTOTAL as a function of gap spacing can be used to extract ρC
[62]. If L >> d, then
CLdL
RR T
shTOTAL 2
2
(3.4)
L
d
d
LC 1ln (3.5)
where Rsh is the sheet resistance of the underlying doped layer (i.e. n+ Ge), LT is the
transfer length, and C is a correction factor due to the circular geometry. When this
correction factor is used, RTOTAL will be a linear function of gap spacing d. The transfer
42
length is an important parameter for a metal-semiconductor contact; it represents the
distance over which most (i.e. about 63%) of the current transfers from the
semiconductor into the metal and vice versa. When RTOTAL is plotted versus gap spacing
d, the slope yields Rsh and the y-intercept yields LT. From here, specific contact
resistivity can be estimated by:
2
TshC LR (3.6)
The use of circular structures as opposed to linear contacts makes these structures self-
isolating and can be fabricated in a single lithography step as described later, greatly
simplifying the process flow.
1 2 3 4
I GND
V2 V3
RPROBE RPROBE
RGe
RCRC
V2 V3
I GND
(a) (b)
dL
Figure 3.14: Schematic of a circular transmission line method test structure for
measuring ρC. The equivalent circuit is shown in (b). The use of 4 probes negates the
effect of the probe resistance RPROBE on the measurement.
Because very small contact resistances are measured, the probe resistance can alter
the measurement considerably. The measured total resistance was on the order of 1Ω for
the smallest gap spacing. The probe resistance of the Cascade measurement station at
Stanford University was measured to be about 2Ω, so it is necessary to use a 4-probe
measurement as shown in Fig. 3.14. Current is forced through the contact between
43
probes 1 and 4, while the voltage was measured at probes 2 and 3. Total resistance is
then:
I
VVRTOTAL
32 (3.7)
Care must be taken when using this method in order to ensure that each metal contact is
an equipotential surface.
Fabrication of these cTLM structures started with an epitaxial germanium wafer.
Undoped Ge heteroepitaxially deposited on Si served as the starting material. Although
no p-type dopants were used during the Ge growth, these undoped Ge samples were
lightly p-type due to the electrically p-type defects. SRP analysis of these samples gave
about 1014
cm-3
p-type carrier concentration. To prepare the samples for ion implantation,
20nm of plasma enhanced chemical vapor deposition (PECVD) SiO2 was deposited.
Phosphorus implantation at 90keV with 6×1014
cm-2
dose was used, followed by rapid
thermal annealing at 500ºC for 10 seconds. The resulting substrate had a surface
concentration of approximately 3×1019
cm-3
as measured by SRP. As discussed in
Chapter 2, this represents the highest achievable electrically active n-type dopant
concentration in Ge using a single implant and RTA. To confirm this, SRP analysis was
also done using samples that received a higher dose (i.e. 1.8×1015
cm-2
); despite triple the
dose, the active phosphorus concentration did not increase, as shown in Fig. 3.15. The
lower dose of 6×1014
cm-2
was used for the following experiments since it would cause
less damage to the substrate.
44
Figure 3.15: Higher dose of 1.8×1015
cm-2
does not result in higher active dopant
concentration.
After ion implantation and dopant activation, the PECVD SiO2 was stripped off using
dilute HF. The Ge surface was cleaned and ALD at 150ºC was used to deposit TiO2 or
Al2O3. Metal was deposited by e-beam evaporation and patterned using standard
lithography techniques. For TiO2 devices, Al (100nm)/Ti (10nm) was used, while Al
(100nm) was used for the Al2O3 devices. Ti on TiO2 and Al on Al2O3 were chosen to
eliminate the formation of another dielectric through the reaction between the metal and
the primary dielectric; for example, Al on TiO2 could form an Al2Ox/TiOx bilayer. The
finished cross-section schematic is shown in Fig. 3.16.
P-type Si (500μm)
Highly-doped n+ Ge (junction at 250nm)
Epitaxy P-type Ge (1.2μm)
ALD Oxide
Metallization(Al or Al/Ti)
Figure 3.16: Schematic cross-section of the circular TLM structure used for ρC
measurement.
45
3.4.2 Results and Discussion
70x reduction
Figure 3.17: Measured ρC of Al2O3 and TiO2 MIS contacts on n+ Ge. Al2O3 MIS
contacts are immediately limited by tunneling resistance. TiO2 MIS contacts achieve
roughly 70× improvement in ρC.
Measured specific contact resistivities of Al2O3 and TiO2 MIS contacts on heavily
doped n+ Ge are shown in Fig. 3.17 as a function of oxide thickness. Without any
interfacial oxide, ρC ≈ 10-4
Ωcm2, which is similar to results reported by other groups for a
single implant followed by RTA [12, 13]. With Al2O3 MIS contacts, no improvement in
ρC was observed despite the reduction in ΦBN since the contacts are immediately limited
by tunneling resistance even for very thin Al2O3. On the other hand, MIS contacts with
1nm TiO2 show a 70× reduction in ρC, down to about 1.3×10-6
Ωcm2. We believe that this
represented the lowest reported ρC for a MIS contact on n+ Ge at the time this work was
done [44, 45]. This significant reduction in ρC is achieved because the TiO2 interfacial
layer can reduce ΦBN without introducing tunneling resistance. With thicker TiO2,
however, ρC begins to increase again, which was not observed with TiO2 MIS contacts on
lighter doped Ge substrates. This increase is attributed to the series resistance of TiO2
46
due to the TiO2 material resistivity. It is distinct from the tunneling resistance that is
typically observed since TiO2 and Ge have a roughly zero CBO. Furthermore, the
increase in ρC for the TiO2 device is much gentler than that of the Al2O3 device,
suggesting a different mechanism. This is the first time that this has been experimentally
observed since the tunneling resistance from high-CBO materials used in previous reports
masks the impact of this series resistance. This newly identified effect acts as an
additional tradeoff mechanism and will be discussed further in the next subsection.
The reason why series resistance affects MIS contacts on heavily doped substrates but
not more lightly doped ones is explained with the help of Fig. 3.18. For a lower doping,
the Schottky barrier is quite thick so the material resistance of the TiO2 itself is not
significant, even if relatively thick TiO2 is used. However, at higher doping, the Schottky
barrier is thin and tunneling through the barrier is relatively easy, so its associated
resistance is smaller. In this case, the TiO2 material resistance may become comparable
and therefore significant to the overall resistance.
Large SB resistanceTiO2 resistance not significant
Smaller SB resistance
TiO2 resistance becomes significant
Lower Doping
Higher Doping
Metal
n-Ge
TiO2
Metal
n-Ge
TiO2
RTiO2 << RSB RTiO2 ≈ RSB
+ - + -
(a) (b)
Figure 3.18: The semiconductor doping affects the MIS contact characteristics. While
TiO2 does not add tunneling resistance, it can still limit the contact if the resistance of
TiO2 becomes similar to the resistance of the Schottky barrier for highly doped substrates.
47
Fig. 3.19 schematically summarizes the results of Al2O3 and TiO2 MIS contacts on
moderately and heavily doped Ge. The dotted lines represent the Al2O3 tunneling
resistance and the TiO2 series resistance, which set the lower bound on achievable ρC.
Tunneling resistance has a much stronger dependence on thickness, resulting in a much
steeper line for Al2O3, which translates to a very severe tradeoff between ΦBN reduction
and added resistance. This can be seen by the fact that a ρC reduction was still possible at
light or moderate substrate doping but becomes impossible at high substrate doping since
ρC immediately becomes limited by the tunneling resistance even with very thin
interfacial layers. The TiO2 series resistance is a much gentler tradeoff. At low or
moderate doping, the interfacial layer series resistance effect is not seen since it is much
lower than the resistance associated with the Schottky barrier. At high doping, however,
the Schottky barrier width is much thinner, and its associated resistance is dramatically
reduced to the point where the interfacial layer series resistance is now comparable and
can limit the contact. Therefore ρC reduction is only possible with thin and low band
offset interfacial layers. It should be noted that, while this analysis was done for the case
of Al2O3 and TiO2 on Ge, these tradeoffs should hold for a wide variety of interfacial
layers and substrates. This provides an opportunity to improve MIS contacts by using
less resistive interfacial layers, which will be discussed in Chapter 4. It is possible that,
with only 1nm of TiO2 available to us in light of this series resistance tradeoff, it may not
be sufficient to achieve adequate reduction in ΦBN. The use of a thicker, but more
conductive interfacial layer, may result in a lower ΦBN and consequently a lower ρC.
48
Interfacial Layer Thickness
Al2O3 tunneling resistance
TiO2 series resistance
Spec
ific
Co
nta
ct R
esis
tivi
ty
MIS on moderately doped Ge
MIS on heavilydoped n+ Ge
Figure 3.19: Schematic diagram of ρC versus interfacial layer thickness for different
doping levels, showing the effects of different resistances. The red double lines are for
Al2O3, while the blue single lines are for TiO2. The dotted lines are for the Al2O3
tunneling resistance and the TiO2 series resistance, which represents a lower bound on ρC.
The minimum achieved ρC (1.3×10-6
Ωcm2) is in the same range as those in recently
demonstrated contact schemes including laser dopant annealing (7×10-7
Ωcm2 [16] and
2.5×10-6
Ωcm2 [12]), P and Sb coimplantation (8×10
-7Ωcm
2 [19]), and Si passivation
(1.4×10-6
Ωcm2 [12]). Most of these approaches aim to increase semiconductor doping (>
1020
cm-3
) in order to thin the Schottky barrier and increase the tunneling current.
However, ΦBN remains high in these approaches. MIS contacts are unique in that they
aim to reduce ΦBN and actually achieve a similar ρC despite a doping level about ten
times lower. It is of course desirable to combine high doping techniques with a reduced
ΦBN to meet the needs of VLSI contacts.
49
3.4.3 Series Resistance
In the previous section, a new tradeoff mechanism was identified, namely the
interfacial layer series resistance. Since this kind of resistance results from carrier
scattering (electron scattering in TiO2), we first calculate the mean scattering length.
Using the following relations:
q
kTD (3.8)
*m
q (3.9)
DL (3.10)
where D is the diffusion coefficient, k is the Boltzmann’s constant, T is the temperature, q
is the electronic charge, μ is the carrier mobility, m* is the carrier effective mass, τ is the
mean scattering time, and L is the mean scattering length. From literature [63, 64], TiO2
has an electron mobility on the order of 2cm2/Vs. Its effective mass ranges from (2 –
4)m0 to (10 – 16)m0 [63], where m0 is the electron rest mass, depending on crystal axis.
Since our deposited TiO2 was not annealed, we believe it to be amorphous, so we assume
m* ≈ 8m0. Using these values, we obtain a mean scattering length of approximately
0.2nm. Since we are only interested in the order of magnitude, the exact value is not
important. This length scale is approximately the lattice constant of anatase and rutile
TiO2, indicating scattering at essentially every atom. This short mean scattering length
implies that electrons do indeed get scattered even in thin TiO2 layers, so it is meaningful
to assign a series resistance to TiO2 for these structures.
Fig. 3.20 shows the specific contact resistivity as a function of oxide thickness. The
green line is the resistance of TiO2 itself based on our measured bulk material resistivity
50
of about 37.5Ωcm. The use of TiO2 MIS contacts allows us to approach the TiO2 series
resistance lower limit. With the exception of the Ti/1nm TiO2/n+ Ge case, all data points
lie close to, but above, the TiO2 series resistance line. In the 1nm TiO2 case, the Ti metal
pad likely reacted with the very thin deposited TiO2, and may have converted a
significant amount to a reduced TiO2-x form, which is known to be more conductive than
stoichiometric TiO2 [65]. With thicker TiO2, the Ti metal pad cannot significantly alter
the entire thickness of the ALD-deposited TiO2, so its resistivity remains unchanged.
Figure 3.20: TiO2 MIS contact ρC compared to TiO2 series resistance.
This is strong evidence that these TiO2 MIS contacts eventually get limited by the
TiO2 series resistance and it is the reason why ρC increases after 1nm TiO2. In MIS
contacts, it is advantageous to choose, not only a material with low band offsets, but also
a conductive material. It is therefore a bit of a misnomer to call these metal-insulator-
semiconductor contacts, since the “insulator” portion needs to be as conductive as
possible. We propose to use conductive oxides such as indium tin oxide (ITO) to get
around this tradeoff mechanism. Chapter 4 will examine some of these issues further.
51
3.5 Applications
In this section we discuss some possible applications of TiO2 MIS contacts, and also
MIS contacts in general.
3.5.1 Ge N-Channel MOSFET
The need to reduce contact resistance in germanium NMOSFETs was described in
Chapter 1. TiO2 MIS contacts to the n+ source/drain regions were integrated in a gate last
process flow, described schematically in Fig. 3.21.
P-type Si (500μm)
n+ n+
Epitaxy p-type Ge (1.5μm)
SiO2 SiO2SiO2
TiO2
Al/Ti
Pt Gate
Al2O3 Gate Ox
P-type Si (500μm)
n+ n+
Epitaxy p-type Ge (1.5μm)
SiO2 SiO2
TiO2
Al/Ti
(1) Implant P for S/D through patterned SiO2, RTA at 500 C for 10s
P-type Si (500μm)
n+ n+
Epitaxy p-type Ge (1.5μm)
SiO2 SiO2SiO2
20nm SiO2 cap
(2) Remove 20nm SiO2 cap, ALD TiO2, Al/Ti deposition and patterning
(3) Remove TiO2 and SiO2 from gate region(4) ALD Al2O3 (11nm), Pt gate by liftoff
Figure 3.21: Gate last process flow for Ge NMOSFET with TiO2 integrated on n+
source/drain.
52
The starting substrate was again a heteroepitaxially grown Ge on Si wafer, with no
intentional doping which resulted in a lightly p-type layer. Thick SiO2 was deposited by
PECVD, and subsequently patterned for the source/drain contact areas. In these areas,
20nm PECVD SiO2 was deposited in preparation for ion implantation, which was
phosphorus at 6×1014
cm-2
at 90keV. The sample was then annealed at 500°C for 10
seconds, after which the 20nm SiO2 cap was removed. The surface was cleaned in
preparation for ALD TiO2 and Al (100nm)/Ti (10nm) metal deposition. One sample did
not get TiO2 to provide a control. The metal was patterned into contacts. The TiO2 and
SiO2 in the gate region were selectively removed by hydrofluoric acid. Finally, the gate
oxide (11nm ALD Al2O3) and gate metal (platinum) were deposited and patterned using
photoresist liftoff.
Fig. 3.22 shows the ID-VD characteristics of a 2μm-channel device. TiO2 MIS
contacts can be successfully integrated on the source/drain regions of a Ge NMOSFET.
Well-behaved ID-VD characteristics were obtained. There was no significant difference
between the samples with and without TiO2. This is attributed to the fact that, at 2μm,
the channel lengths are still too large, so the contact resistance is much smaller than the
channel resistance. Nevertheless, at scaled CMOS dimensions, reduced contact
resistance would result in significantly higher current.
Figure 3.22: ID-VD characteristics of gate last Ge NMOSFET with TiO2 MIS contacts.
53
Germanium transistors using a gate first process were also fabricated, as shown in Fig.
3.23. In these samples bulk p-type Ge was used. Al2O3 oxide with a GeO2 interfacial
layer by ozone post-oxidation was used as the gate oxide, which provided excellent
interface passivation with Dit in the low 1011
cm-2
range [66]. This is a vast improvement
over our earlier gate last process using Al2O3 only, which resulted in Dit in the 1012
cm-2
to 1013
cm-2
range. Immediately after gate oxide deposition, TiN and W was sputtered to
form the gate metal. After gate metal etch, the samples were implanted with phosphorus
(1.5×1015
cm-2
and 25keV). Annealing was done at 400°C for 5 minutes. Using the
source/drain photoresist pattern, the GeO2/Al2O3 bilayer gate oxide was etched away,
followed by ALD TiO2 deposition and Al (100nm)/Ti (10nm) deposition and liftoff.
Again, control samples without TiO2 were fabricated. Although there is significant
improvement in current compared to the gate last process flow, the total resistance of the
3μm gate length device is still too large, and therefore no difference was observed
between the device with and without TiO2 MIS contacts. In order to accurately
characterize the effect of MIS contacts, it is necessary to use short channel transistors.
W/TiN Gate
GeO2/Al2O3 Gate Ox
n+ n+
P-type Ge
TiO2
Al/Ti
Lg = 3μm
0V
1V
VGS = 2V
(1) GeO2/Al2O3 gate oxide by ALD(2) W/TiN by sputtering and patterned(3) Implant P, S/D lithography(4) Etch and clean S/D contact area(5) ALD TiO2, PVD Al/Ti and liftoff
Figure 3.23: Process flow and schematic cross section of gate first Ge NMOSFET
incorporating TiO2 MIS contacts and its ID-VD characteristics.
54
3.5.2 Metal Source/Drain Transistors
One way of reducing parasitic resistance is to use metal source/drains instead of using
doped semiconductors. The key different is the use of a Schottky junction instead of a pn
junction at the source-channel and channel-drain junctions, which gives rise to its name
Schottky barrier MOSFETs (SB-MOSFET). The operating principle is described in Fig.
3.24 for a p-channel device. For a n-channel device, the electron barrier must be kept
small to allow for high currents to flow in the on-state. Furthermore, a small electron
barrier leads to a large hole barrier which reduces leakage in the off-state.
Figure 3.24: (From [67]) Operating principle of a metal source/drain MOSFET. The p-
channel device is drawn here and compared with a conventional doped source/drain
MOSFET.
From theoretical calculations, it is estimated that barrier heights need to be below
0.1eV in order for SB-MOSFETs to outperform conventional MOSFETs [68, 69]. In Ge,
55
Fermi level pinning near the valence band means that this is easily accomplished for p-
channel devices [70, 71]. However, n-channel SB-MOSFETs require a significantly
lower ΦBN. There have been attempts to use GeO2 [37] and Si3N4 [33] MIS contacts to
depin the metal Fermi level and thereby reduce ΦBN. Although working MOSFETs were
successfully fabricated, performance was not good. This may be due to the large band
offsets of these materials. Although both GeO2 and Si3N4 MIS contacts demonstrate
Fermi level depinning characteristics on lightly-doped substrates, the inverted channel
region in the on-state has a very high charge density and therefore acts similar to a
highly-doped substrate. As we noted in Section 3.4, at higher doping levels, the
resistance (both tunneling resistance and series resistance) of the interfacial layers
become increasingly important. To get high current injection from the metal source into
the inversion layer in the on-state, a low band offset material needs to be used. This
means TiO2 or perhaps even ITO MIS contacts may be good choices for SB-MOSFETs,
and should be explored further.
3.5.3 Asymmetric Metal-Semiconductor-Metal Photodetectors
Metal-Semiconductor-Metal (MSM) photodetectors are made from two back-to-back
Schottky junctions. Photogenerated carriers within the depletion region can be collected
as photocurrent. Compared to PIN photodetectors, which use a p-type, intrinsic, and n-
type region, MSM photodetectors have the advantage of fast operation and ease of
integration. However, they suffer from higher dark currents compared to their PIN
counterparts. One way to decrease the dark current is to use asymmetric metal contacts
with different barrier heights, as depicted schematically in Fig. 3.25.
56
Figure 3.25: (From [72]) Asymmetric barriers at the source and drain junctions of a
MSM photodetector can reduce dark current. However, the improvement is limited due
to metal Fermi level pinning.
The dark current in MSM photodetectors can be divided into two categories. The
first category is carrier injection over the Schottky barrier (I1 and I2 in Fig. 3.25), and the
second is carrier generation in the semiconductor (I3 and I4 in Fig. 3.25). The latter
component is generally low and can be minimized by using high quality, low defect
density substrates. Carrier injection over the Schottky barriers is largely responsible for
the high dark current in MSM photodetectors. If both contacts have metal Fermi levels
close to the valence band, then one side will have a high barrier but the other side will
have a low barrier. If one can use a metal at the drain side with its Fermi level close to
the conduction band, and apply a bias with the appropriate polarity, dark current can be
reduced due to the decrease in I2 (Fig. 3.25). Only about 10× reduction in dark current
was observed by using different metals in germanium MSM photodetectors [72] because
Fermi level pinning prevents such a large hole barrier height from forming, as is
necessary to reduce I2.
57
Other approaches have been attempted in order to decrease the electron barrier height
selectively at one contact. Sulfur [73] and dopant [74] segregation during nickel
germanide formation have both been used for this application, and significant reduction
in dark current was observed (up to 1000×). MIS contacts can also be used to tune the
barrier height selectively at one side of the MSM photodetector, as shown in Fig. 3.26.
P-type Ge
SiO2 > 600x
Al/TiTiO2
Ni/Ti
Figure 3.26: Schematic of asymmetric MSM photodetector incorporating TiO2 MIS
contact and its accompanying dark current reduction.
In this process, a bulk p-type Ge wafer was used in order to minimize bulk defects
which may lead to dark current. PECVD SiO2 was used as isolation and one contact was
first patterned and etched. ALD TiO2 and Al/Ti was deposited and patterned as one
contact. The other contact was then opened and Ni/Ti was then deposited and patterned
using liftoff. In both contacts, titanium is the contacting metal; Ni was used only for ease
of liftoff. The asymmetry comes from one contact having TiO2 and consequently a lower
ΦBN, while the other contact is metal directly on semiconductor resulting in a pinned
contact and a high ΦBN. In this experiment, 8.8nm of TiO2 was used. Fig. 3.26 also
shows the dark I-V characteristics. With a positive voltage bias on the TiO2 MIS contact,
the dark current was reduced by over 600× compared to the symmetric MSM
58
photodetector as a result of a reduction in thermionic emission current I2. This very large
reduction in dark current addresses one of the major shortcomings of MSM
photodetectors.
3.5.4 Spin Injection
One exciting application of MIS contacts is in the emerging field of spintronics. By
using electron spin in addition to its charge, spintronics offer several potential benefits,
including lower power operation and nonvolatility. One recent proposal is the spin
MOSFET [75], shown in Fig. 3.27.
Figure 3.27: (From [75]) (a) Schematic of spin MOSFET. (b) Simulated ID-VD
characteristics for parallel and antiparallel source/drain ferromagnet orientations.
These devices resemble a conventional MOSFET except that a ferromagnetic
source/drain is used. The ferromagnetic source injects electrons with one spin, i.e. spin
polarized current. The drain then detects electrons with a spin that is parallel to its own
polarization. If both the source and drain are polarized in the same orientation (parallel),
a large current can flow; conversely, if they are antiparallel, only a small current can flow.
In this way, the device can be turned into a low resistance state if not in use, thereby
reducing standby power. Also, the orientations of the source/drain ferromagnets can be
59
used as a nonvolatile memory element. However, spin injection from a metallic
ferromagnet into a semiconductor is challenging because of the conductivity mismatch
problem [76]. To solve this, an insulator is inserted between the ferromagnet and
semiconductor, which is precisely a MIS contact. The insulator used should be spin-
selective, and is typically either MgO or Al2O3. Both of these materials have been well-
studied as a MIS contact [39-41].
Although the focus of this thesis is on extending the CMOS roadmap through the
reduction in contact resistance, there is considerable potential of MIS contacts in beyond
CMOS applications.
3.6 Summary
TiO2 was verified to have a low conduction band offset to germanium using SRPES.
As a result, TiO2 MIS contacts achieved a roughly 1000× reduction in ρC on ~1018
cm-3
n-
type Ge substrates. On more heavily-doped substrates (i.e. ~1019
cm-3
n-type Ge), TiO2
MIS contacts reduced ρC from ~10-4
Ωcm2 to 1.3×10
-6Ωcm
2, representing a 70× reduction.
The low conduction band offset is the primary reason why TiO2 MIS contacts
significantly outperform conventional Al2O3 MIS contacts due to the reduction in
tunneling resistance. In addition to the resistance due to tunneling through the dielectric,
the series resistance of the interfacial layer was identified as a new tradeoff mechanism.
Pinning at the metal/TiO2 interface was also observed which suggested a shift in the
metal Fermi level, as opposed to Fermi level unpinning. Both the series resistance
tradeoff and the shift in Fermi level will be discussed in more detail in Chapter 4. Finally,
several possible applications of MIS contacts were discussed, including CMOS
60
transistors using conventional doped source/drain as well as metallic source/drain,
optoelectronic devices such as MSM photodetectors, and beyond CMOS devices for spin
injection.
61
Chapter 4
Physics of MIS Contacts
4.1 Motivation
Given the many possible applications of MIS contacts, it is imperative that its
underlying mechanisms and operation be studied in more detail. We focus on the
TiO2/Ge system, but the physics should be applicable to many semiconductors. TiO2
MIS contacts have demonstrated a decrease in ΦBN on Si, Ge, GeSn, GaAs [47], and
GaSb [48], as shown in Fig. 4.1.
Si GeSn
GaAs GaSb
Figure 4.1: TiO2 MIS contacts also show a reduction in ΦBN on Si, GeSn, GaAs (from
[47]), and GaSb (from [48]), suggesting similar mechanisms as the TiO2/Ge system.
62
As future MIS contacts will need to minimize tunneling and series resistance in order
to achieve low contact resistivity, the TiO2/Ge system is an excellent one to study due to
its low tunneling resistance and tunable series resistance via oxygen vacancies. In this
chapter we will first examine the mechanisms responsible for ΦBN reduction in TiO2 MIS
contacts. The series resistance tradeoff will be examined more closely. Finally, we will
explore the scalability of MIS contacts.
4.2 Theory of Operation
In Section 2.3 the two main theories of MIS contacts were outlined, namely the
reduction in MIGS and the interfacial dipole theory. If the effects were purely MIGS,
unpinning the metal Fermi level would cause different behavior depending on the metal
bulk workfunction. Smaller workfunction metals would result in a larger reduction in
ΦBN; metals with very large workfunctions should increase contact resistivity. However,
this was not observed, as was noted in Sections 3.3.2 and 3.3.3. As Fig. 3.12 shows, Ti,
Al, and Pt metal TiO2 MIS contacts all improved ρC. Considering only the effect of
MIGS reduction, the Pt case should not improve ρC because of its large workfucnction. It
is evident that other effects are also at work. Because the TiO2 contacts with different
metals all improve by roughly the same magnitude (roughly 1000× decrease in ρC) in
each case, this strongly suggests a shifting of the metal Fermi level, rather than unpinning.
If the metal Fermi level is shifted from its pinned location near the charge neutrality level
by a similar amount for each metal, then the resultant effective ΦBN and measured ρC
would be similar in each case.
63
This shifting of the metal Fermi level strongly suggests a dipole-type mechanism. A
dipole is essentially a large change in electrical potential over a short distance. In
general, a dipole may form at any interface between two dissimilar materials. In a TiO2
MIS contact, the dipole may form at the metal-insulator or insulator-semiconductor
interface. In the presence of a native oxide, there is potentially a dipole at the insulator-
native oxide interface as well. A dipole at the metal-insulator interface will manifest as
an apparent change in the metal workfunction, by an amount equal to the magnitude of
the dipole. A dipole at the insulator-semiconductor interface will show as a change in the
height of the insulator tunnel barrier. A dipole at the insulator-native oxide interface
would again alter the effective height of the insulator tunnel barrier; furthermore, the
overall shape of the tunnel barrier may change which would impact the tunneling
probability. As long as the dipole polarity is oriented correctly, any of these dipoles may
give rise to a lower effective ΦBN and hence a lower ρC.
To explore the idea further, two types of metal-oxide-semiconductor (MOS)
capacitors were fabricated. The results of the capacitance-voltage (C-V) measurements
are shown in Fig. 4.2. The solid symbols show metal/SiO2/Si capacitors and the open
symbols show metal/TiO2/SiO2/Si capacitors. Lightly-doped p-type Si was used in all
cases. SiO2 was grown by thermal oxidation to ensure a good Si/SiO2 interface with
minimal defects. TiO2 was deposited on top of the SiO2 by ALD. In the SiO2 capacitors,
the flatband voltage (VFB) can be modulated by the metal, indicating no Fermi level
pinning. In the TiO2/SiO2 capacitors, however, there is very little VFB modulation with
large differences in these metal’s workfunctions, indicating significant Fermi level
pinning in these device stacks.
64
Indication of EF pinning at metal/TiO2
interface
Figure 4.2: Normalized C-V characteristics of SiO2 and TiO2/SiO2 capacitors with
different metals. There is a strong indication of metal Fermi level pinning at the
metal/TiO2 interface due to the lack of flatband voltage modulation.
Al, Ti, PtMetal
TiO2 SiO2 Si
+ - + - + -VD1 VD2 VD3
(b)
Figure 4.3: Flatband condition of the TiO2/SiO2 capacitor, showing the location of
possible dipoles.
65
To examine this TiO2/SiO2 stack in more detail, the band diagram in Fig. 4.3 is used.
In general each interface may have a dipole, whose magnitude is denoted by VD1, VD2,
and VD3. We can write the flatband voltage VFB as:
MDSMDDDFB VKVVVV 1321 (4.1)
where ϕM and ϕS are the metal and semiconductor workfunctions, respectively, and K is a
constant. This is essentially the standard flatband voltage equation modified by the VDx
terms, which represent the potential dropped by the three possible dipoles. We make the
reasonable assumption that the interface dipoles and its magnitude are determined only
by the two materials at that interface. For the three TiO2/SiO2 capacitors corresponding
to the three different gate metals, VD2, VD3, and ϕS remain unchanged and can be lumped
into the constant K. Based on the C-V data in Fig. 4.2, the VFB’s are very similar for all
three TiO2/SiO2 capacitors, meaning that VD1 depends mostly on ϕM. This implies the
existence of a dipole at the metal/TiO2 interface, whose magnitude VD1 depends on the
metal workfunction. To state this in another way, the metal workfunction influences the
dipole magnitude to exhibit a similar VFB for the overall system. The metal Fermi level is,
in effect, pinned at the same energy, independent of the metal workfunction. This
pinning at the metal/TiO2 interface is likely a result of MIGS [96], and will be further
discussed later in this section. The reduction in ΦBN, which was both experimentally
measured through temperature-dependent I-V measurements and inferred from a
reduction in ρC, indicates that the pinning dipoles at the metal/TiO2 interface shift the
metal Fermi level from near the Ge valence band edge toward the conduction band.
Essentially we have traded one pinned contact for another one, with the fortunate
difference that the TiO2 MIS contact pins at an energy level favorable for n-type contacts.
66
This is not to say that MIGS does not play a role. Separating the metal from the
semiconductor may indeed reduce MIGS at the interface, since the electron wave
functions from the metal decays exponentially within the insulator. Any reduction in the
interfacial states has the effect of reducing the pinning effect, as noted by Cowley and
Sze [77]:
S
SDit
1101.1 13 (4.2)
where Dit is the interface state density in states/(cm2-eV) and S is the pinning factor. The
pinning factor S is defined as:
M
BNS
(4.3)
where ΦBN is the barrier height and ϕM is the metal workfunction. A pinning factor of 1
indicates ideal, Schottky behavior since the barrier height exactly tracks the metal
workfunction, as follows from Equation 2.4. A pinning factor of 0 indicates complete
pinning, meaning the barrier height does not change at all with different metal
workfunctions.
Going back to Equation 4.2, it is clear that a large Dit leads to a small S, or in other
words, a strongly pinned contact. Conversely, a reduction in MIGS leading to a lower Dit
would suggest a larger pinning factor S, indicating a more ideal contact. From these
arguments, the reduction in MIGS would alleviate some symptoms of metal Fermi level
pinning. However, by itself, it cannot explain all the experimental data presented here in
this thesis and in the literature. A dipole at the metal/TiO2 interface is needed to fully
account for the reduction in ΦBN as well as the observed behavior with different metals in
TiO2 MIS contacts. This dipole may be formed by a combination of surface
67
reconstruction/relaxation and charge transfer at the interface, and will be further
discussed in Section 4.2.1.
The observation of pinning at the metal/TiO2 interface is actually not an unexpected
one. Although we have referred to TiO2 as an insulator in keeping with the metal-
insulator-semiconductor terminology established in the literature, TiO2 is typically
classified as a semiconductor, albeit one with a relatively large band gap (3.3eV – 3.5eV).
Its conductivity can change by orders of magnitude by intentional doping of oxygen
vacancies, as alluded to earlier in Chapter 3 and will be further explored in Section 4.6.
These oxygen vacancies acts as electron donors and result in n-type TiO2 with carrier
concentrations that can reach 1019
cm-3
or more. It is not strange, then, to expect Fermi
level pinning at the metal/TiO2 interface. In fact, we may expect very strong pinning at
this interface, based on the formula for pinning factor S, as proposed by Mönch [78]:
211.01
1
S (4.4)
where ε∞ is the electronic part of the dielectric constant of the semiconductor. The
relative dielectric constant of TiO2 can be very high, especially the crystalline phases
which can be up to 80 [79]. For TiO2, ε∞ was determined to be 7.8 [27], which is
significantly higher than other dielectrics. Using that value of ε∞, the pinning factor S for
TiO2 would be a very low 0.18 based on Equation 4.4. This supports our earlier
speculation of MIGS at the metal/TiO2 interface. Because of the low pinning factor S, we
should therefore expect strong pinning at the metal/TiO2 interface. It is simply fortunate
that the pinning location is a favorable one for n-type Ge contacts, as depicted
schematically in Fig. 4.4.
68
TiO2
Ge GeEFM EFM
TiO2 MIS Contact Metal/n-Ge Contact
Figure 4.4: Schematic band diagrams at flatband conditions showing band alignments of
TiO2 MIS contacts and conventional contacts, both with Fermi level pinning.
4.2.1 Effect of Dipoles
The presence of dipoles at the metal/TiO2 interface causes an apparent shift in the
metal Fermi level. Fig. 4.5 shows calculated ρC as a function of the metal effective
workfunction (EWF), assuming 1nm of TiO2 and 1019
cm-3
n-type Ge. It was calculated
using the same theoretical framework discussed in Section 2.3.3. Referring back to Fig.
3.20, the Al/TiO2/n+ Ge line shows a specific contact resistivity of about 10
-5Ωcm
2 at
1nm TiO2, and based on this we expect an effective metal workfunction of about 4.32eV
at the metal/TiO2 interface with 1nm of TiO2. This represents a shift of about 0.26eV
from the pinned case where the metal effective workfunction is about 4.58eV. This shift
can be attributed partially to a dipole at the metal/TiO2 interface. The Al/TiO2/n+ Ge line
was used instead of the one with Ti metal in order to avoid complications due to the
increased TiO2 conductivity.
69
1nm TiO2
Figure 4.5: Calculated ρC as a function of metal effective workfunction, indicating the
presence of a dipole at the metal/TiO2 interface.
It can be seen that even a relatively small dipole can significantly reduce ρC due to the
exponential dependence between them as seen in Fig. 4.5. A larger dipole at the
metal/TiO2 interface causes a greater apparent shift in the metal Fermi level towards the
conduction band (assuming a positive dipole), and this results in a greater reduction in
ΦBN and ρC.
The exact origin of these dipoles is still controversial. It was proposed by Kita and
Toriumi [80] that a difference in the areal density of oxygen atoms in the two materials
determines the magnitude and direction of the interfacial dipole. Oxygen ions would
move from the higher density material to the opposite side of the interface, creating a
dipole as depicted in Fig. 4.6(a). This causes an apparent shift in band alignments since
the vacuum level changes very rapidly. It has been noted that the two dipoles at the
metal-insulator and insulator-semiconductor interfaces need to be different in order to
result in a net dipole in the MIS structure [81], which is then responsible for shifting the
70
metal Fermi level. It was suggested [81] that this could be accomplished by using an
insulator that acts as a diffusion barrier to oxygen. In this way, a different oxygen areal
density could exist at the two sides of the insulator, allowing a net dipole to form. In
reality, oxygen ions do not move, but rather the exact interfacial bonding structure
between the metal and the oxide results in a relaxed position of the metal and the oxygen
atoms. Their position likely determines the dipole magnitude and direction [82, 83].
Although the exact interfacial bonding structure is beyond the scope of this thesis, the
estimated dipole of up to 0.26eV is a reasonable one, as it is certainly true that there
exists a large difference in oxygen areal density between the metal contact and TiO2.
Furthermore, the magnitude falls within the range of reported VFB shifts thought to be
caused by dipoles [80]. Fig. 4.6(b) draws the dipole schematically at the metal/TiO2
interface, showing the formation of a positive dipole. The oxygen areal density theory
therefore correctly predicts the polarity of the dipole needed for ΦBN reduction, and is a
possible origin of the dipole at the metal/TiO2 interface.
TiO2
EFM
Lower Oxygen Areal Density
Higher Oxygen Areal Density
- +O-
E0
O-
- +
E0(a) (b)
Figure 4.6: (a) A difference in oxygen areal density can result in an interfacial dipole,
which shifts band alignments due to a rapid change in the vacuum level (E0). (b) In the
metal/TiO2 case, relaxation of oxygen atoms at the interface causes a positive dipole and
shifts the metal Fermi level (EFM) towards E0. Solid lines indicate band alignments
before oxygen transfer and dotted lines indicate band alignments after oxygen transfer.
71
4.3 Effect of Annealing
It has been observed that forming gas annealing (FGA) of certain MIS contacts
resulted in a significantly increased ρC, it was suggested that fixed charge or interface
charge in the oxide may be responsible for the correct operation of MIS contacts [84].
The MIS contacts used in that work were Al/Al2O3/n-GaAs devices. By annealing these
contacts, the fixed and interface charges were reduced, which rendered the MIS contacts
ineffective. The resulting ρC was worse than the control case of metal directly on
semiconductor.
Bulk or interface charge in the insulator causes a shift in the bands. If the overall net
charge is positive, the potential drop across the insulator would result in a lower ΦBN.
We studied this idea theoretically using the framework established in Section 2.3.3.
MIGS and interfacial dipoles were assumed to be zero in order to isolate the effect of
positive fixed charge. More details can be found in the publication by Roy et al. [85].
Fig. 4.7 shows the simulation result using Al2O3 on 1019
cm-3
n-type Ge MIS contacts
assuming different bulk and interface fixed charge. In general, the fixed charge required
is quite large. For bulk fixed charge, 3×1020
cm-3
is required for noticeable reduction in
ρC. At 1nm, this translates to 3×1013
cm-2
areal charge density. Similarly, an interface
fixed charge of 3×1013
cm-2
is required for ρC reduction. Such values of fixed charge are
about one to two orders of magnitude larger than typical fixed charge densities in high-k
gate stacks. Furthermore, the effect of fixed charge on ρC appears to be quite sensitive to
the charge densities. Even a slight 3× reduction in fixed charge densities (i.e. 3×1020
cm-3
to 1×1020
cm-3
for bulk charge or 3×1013
cm-2
to 1×1013
cm-2
for interface charge) negates
72
the effectiveness of MIS contacts, resulting in no ρC reduction as seen in Fig. 4.7. The
unreasonably large charge densities required point to the fact that fixed charge likely does
not play a major role in MIS contacts.
Figure 4.7: (From [85]) Reduction in specific contact resistivity can be caused by fixed
charge in the oxide. In this case, the effect of bulk (left) and interface (right) fixed charge
in Al2O3 MIS contacts is simulated.
However, the fact that FGA renders some MIS contacts ineffective is an important
observation. Fig. 4.8 shows the relative ρC for TiO2 MIS contacts before and after FGA
at 300°C. The closed symbols denote Ti/TiO2/n-Ge contacts, showing a reduction in ρC
with the introduction of as deposited TiO2. However, with FGA, these contacts revert
back to the original, pinned value for ρC. In contrast, the open symbols show Pt/TiO2/n-
Ge contacts, again showing a reduction in ρC with the introduction of as deposited TiO2.
In this case, however, forming gas annealing does not seem to significantly alter ρC.
Indeed, the MIS contact continues to perform well and a reduced value of ρC is
maintained. The only difference between the two sets of data is the metal used (either Ti
or Pt). The vastly different behavior can be explained by considering the metal/TiO2
interface, which is critical to the correct operation of TiO2 MIS contacts. Because of the
high reactivity of titanium, the Ti metal reacts with TiO2 to form a significant interfacial
73
layer of TiO2-x at such a high temperature. The interface is significantly altered and the
interfacial dipole is not maintained during this thermal treatment. For the case with Pt
metal, because of the thermal stability of platinum, the interfacial dipole at the Pt/TiO2
interface is maintained.
Figure 4.8: TiO2 MIS contacts with Pt or Ti metal behave differently after 300°C FGA.
The fact that some MIS contacts are rendered ineffective after FGA is not due to a
reduction in fixed charge, but rather a response of the interfacial dipoles to the applied
temperature. It is therefore critical to preserve the interface properties in order to retain
the dipole and the associated benefits to ρC.
4.4 Effect of Series Resistance
Series resistance was identified as one of the limiters to MIS contact performance. In
this section we examine this more closely using the simulation framework established
earlier. However, since the tunneling model we used does not take into account
74
scattering events, which is the origin of the series resistance, we cannot directly include
this in our calculations. Instead, we simply add the contact resistance calculated using
the Tsu-Esaki model described in Chapter 2 (from here on denoted by ρC,Tsu-Esaki to
distinguish it from the total specific contact resistivity, ρC) to the resistance calculated
from the bulk material resistivity values:
tIEsakiTsuCC , (4.5)
where ρI is the insulator resistivity and t is its thickness. While this method of estimation
may not give the exact values, the overall trend should hold. Furthermore, this method is
significantly faster than more accurate methods, such as those using the non-equilibrium
Green’s function (NEGF) formalism. Also, in Section 3.4.3 we calculated the mean
scattering length for carriers in TiO2 to be less than the typical thicknesses used,
suggesting that it is indeed appropriate to estimate its resistivity using its bulk values. In
particular, we believe ballistic transport through the TiO2 layer is unlikely given the short
scattering length. Since we are mainly interested in drawing conclusions from the overall
trend, this simple model should suffice.
Using Equation (4.5) we simulate TiO2 MIS contacts on 1019
cm-3
n-type Ge with and
without series resistance as shown in Fig. 4.9. For the plot with series resistance, TiO2
resistivity ρI was taken to be about 37.5Ωcm which was experimentally measured. In
both parts of Fig. 4.9, the vertical axis is the effective metal workfunction in eV at the
metal/TiO2 interface. A larger dipole results in a lower effective workfunction because of
an apparent shift in the metal Fermi level towards the conduction band, resulting in a
smaller electron barrier. Contours of constant ρC are plotted against TiO2 thickness. Red
stars are experimental data points for Al/TiO2/n+ Ge contacts, with the dotted line as a
75
guide for the eye. Green stars have the same position as red stars, but they are used in the
plot without series resistance since those points are not directly based on experimental
data.
10-6
10-1
10-2
10-3
10-4
10-5(a)
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
(b)
Figure 4.9: Simulated TiO2 MIS contact resistivity (a) with and (b) without series
resistance on 1019
cm-3
n-type Ge. Stars are effective metal workfunctions inferred from
experiments, with the dotted line as a guide. The specific contact resistivity contours are
labeled in units of Ωcm2.
76
Note that the results correctly predict a ρC approximately 10-4
to 10-3
Ωcm2 for a
pinned metal/n-Ge contact. From Fig. 4.9(a), it can be seen that the effective metal
workfunction quickly drops at small TiO2 thicknesses but saturates at around 4.28eV
value corresponding to a roughly 0.3eV shift from the pinned energy of 4.58eV.
The effective metal workfunction versus oxide thickness is plotted in Fig. 4.10. At
0nm, the metal Fermi level is still at the pinned energy. The effective workfunction
drops quickly going to 1nm of TiO2, and then saturates at that value. From this, we can
establish that about 1–2nm of TiO2 is needed to create the dipole at the metal/TiO2
interface.
Figure 4.10: Metal effective workfunction at the metal/TiO2 interface for various oxide
thicknesses.
From Fig. 4.9 we can make some important observations. There is a large qualitative
difference between the results with and without series resistance. The biggest difference
occurs at low ρC (or low effective workfunctions), suggesting that series resistance will
be extremely important in highly scaled, low resistance contacts. With series resistance
77
included, it is essentially impossible to obtain a ρC below 10-6
Ωcm2 even with very low
values of ΦBN. This is because the carriers must still drift through the oxide and may be
scattered. However, if series resistance can be controlled, we see that it is possible to
reach very small ρC as long as ΦBN is sufficiently low. This provides a great incentive to
reduce the TiO2 resistivity, as will be discussed in Section 4.6. Even with the effective
workfunctions that we have achieved using TiO2 MIS contacts (i.e. saturating at about
4.28eV as shown in Fig. 4.10), there is an improvement if series resistance can be
minimized, as shown in Fig. 4.11.
Figure 4.11: Simulated ρC with and without series resistance for TiO2 MIS contacts on
1019
cm-3
n-type Ge. Effective metal workfunctions of Fig. 4.10 are assumed for this
calculation.
Although the improvement is slight for this case, the potential improvement is much
greater if higher dipole magnitudes can be achieved. Fig. 4.12(a) plots the simulated ρC
of two MIS contacts, with effective workfunctions of 4.28eV (smaller dipole) and 4.1eV
(larger dipole). Note that the plot only starts at 0.5nm thickness since some oxide is
78
needed to set up the interfacial dipole. There are two important observations. With high
series resistance, there is very little difference between the 4.28eV and 4.1eV MIS
contact. This implies that the contact is series resistance limited, rather than barrier
height limited. Since we believe we have achieved a 4.28eV effective workfunction, we
conclude that we are beginning to enter the series resistance limited regime. Therefore,
even if greater depinning can somehow be achieved, there will not be an accompanying
decrease in ρC if highly resistive TiO2 continues to be used.
Figure 4.12: (a) Simulated ρC for TiO2 MIS contacts with different effective
workfunctions (EWF) with and without series resistance (RS). (b) The improvement
factor of these MIS contacts by eliminating series resistance.
The second observation is the possibility of significant enhancement at lower
effective metal workfunctions. With a 4.28eV EWF, series resistance only slightly limits
the contact. With a 4.1eV EWF, however, the contact is severely series resistance limited.
Eliminating this series resistance results in a large reduction in ρC. This improvement
factor is plotted in Fig. 4.12(b). For a contact with a 4.1eV EWF, the potential gains can
be over two orders of magnitude. Furthermore, if such a low ΦBN can be achieved
79
without significant series resistance, it appears that ρC in the low to mid 10-8
Ωcm2 range
can be achieved.
Series resistance is potentially a very serious performance limiter in MIS contacts,
especially for ones that have achieved very low effective barrier heights. Some ideas for
possible solutions are presented in Section 4.6.
4.5 Effect of High Semiconductor Doping
As mentioned in Chapter 2, some methods of dopant activation can increase the
electrically active n-type concentration to 1020
cm-3
and above. This section examines
theoretically the effect of higher semiconductor doping on MIS contacts.
Fig. 4.13 shows the simulated ρC of TiO2 MIS contacts on 1020
cm-3
n-type Ge with
and without series resistance. TiO2 resistivity was taken to be 37.5Ωcm. Again, the
expected dipole magnitudes are denoted by green stars with the dotted line as a guide.
Note that the model correctly predicts the ρC of metal directly on 1020
cm-3
n+ Ge to be
about 3.3×10-7
Ωcm2. This is similar to reported results at this high level of doping, such
as those using laser annealing (7×10-7
Ωcm2 [16] and 2.5×10
-6Ωcm
2 [12]) or
phosphorus/antimony coimplantation (8×10-7
Ωcm2 [19] and 2.1×10
-6Ωcm
2 [86]). The
slightly lower ρC is likely due to the idealities of the simulation and the lack of process
integration issues.
80
10-3
10-4
10-510-6(a)
10-3
10-4
10-5
10-6
10-7
10-8
(b)
Figure 4.13: Simulated TiO2 MIS contact resistivity (a) with and (b) without series
resistance on 1020
cm-3
n-type Ge. Stars are effective metal workfunctions inferred from
experiments, with the dotted line as a guide. The specific contact resistivity contours are
labeled in units of Ωcm2.
In comparing the two parts of Fig. 4.13, the effect of series resistance becomes very
apparent at high dipole magnitudes (low effective metal workfunctions). In comparing
81
Fig 4.9(a) to Fig. 4.13(a) (i.e. 1019
cm-3
and 1020
cm-3
doping), series resistance begins to
limit the contact at relatively higher ΦBN for higher doping concentrations. This is
expected because the resistance associated with the Schottky barrier is smaller for the
higher doping, so it is more sensitive to the resistance added by the insulator. This
implies that series resistance will become a greater issue if MIS contacts are applied on
more highly doped substrates. It will be essentially impossible to achieve ρC below mid
10-7
Ωcm2 because the series resistance effect would require a large dipole with less than
0.3nm of TiO2 to achieve lower ρC. In contrast, if series resistance is removed,
significantly lower ρC becomes possible even with the thicker TiO2 needed to set up the
interfacial dipole.
We can use these simulated ρC values to estimate how TiO2 MIS contacts would
perform on 1020
cm-3
n+ Ge by extracting the ρC at the estimated dipole magnitudes, which
is shown in Fig. 4.14. Without TiO2, the contact is in the mid 10-7
Ωcm2 range as
expected. With series resistance factored in, it is not possible to get an improvement
because series resistance immediately limits the MIS contact. If we were able to
eliminate the series resistance of TiO2, it would be possible to achieve a reduction in ρC.
Figure 4.14: Simulated ρC with and without series resistance for TiO2 MIS contacts on
1020
cm-3
n-type Ge. Effective metal workfunctions of Fig. 4.10 are used.
82
It should be noted that the performance gained by eliminating series resistance is
greater for 1020
cm-3
doping (Fig. 4.14), when compared to lower doping (Fig. 4.11). This
is examined in Fig. 4.15, which compares the effect of series resistance on 1019
cm-3
and
1020
cm-3
n-type Ge. In both cases, a 4.1eV EWF is assumed. With high series resistance,
both doping levels actually yield similar ρC because the MIS contact is limited by the
insulator series resistance which is unaffected by semiconductor doping, rather than the
Schottky barrier which can be modulated by semiconductor doping. However, by taking
away series resistance, the ρC drops to a lower value for the higher doping.
Figure 4.15: (a) Simulated ρC for TiO2 MIS contacts with different semiconductor
doping with and without series resistance (RS). Effective metal workfunction was taken
to be 4.1eV. With series resistance, the 1019
/cm3 and 10
20/cm
3 lines are nearly
indistinguishable, implying that series resistance is dominating in this case. (b) The
improvement factor of these MIS contacts by eliminating series resistance.
It is predicted that series resistance is a greater problem at higher doping, and can
limit MIS contacts even at moderate dipole magnitudes. However, this presents an
opportunity; the improvement by reducing series resistance is greater for higher doped
83
substrates, as seen in Fig. 4.15(b). This potentially allows for low ρC to be achieved. In
fact, assuming a 4.1eV EWF on 1020
cm-3
n-type Ge without insulator series resistance, a
ρC below 10-8
Ωcm2 is possible. This provides a great incentive to reduce the resistivity of
the insulator layer in MIS contacts.
4.6 MIS Contact Design
We briefly summarize the key tradeoffs that occur in MIS contacts. The first is the
tunneling resistance tradeoff which occurs when electrons must tunnel through the
insulator layer. As we have shown theoretically and experimentally, this tradeoff
mechanism can be seen even at relatively lighter substrate doping levels. It can be
partially mitigated by using low CBO materials such as TiO2. These kinds of MIS
contacts perform significantly better than those with high CBO materials. Furthermore,
they are more scalable to higher substrate doping levels; however, at higher substrate
doping and its accompanying lower ρC, an additional tradeoff mechanism appears in the
form of insulator series resistance. Since electrons must drift through the insulator
through the MIS contact, it is advantageous to use insulators that have a low resistivity.
Therefore, in order to achieve low ρC MIS contacts, it is necessary to use materials with
low resistivity and a low CBO. In the next few sections, we discuss some ideas for
materials that might satisfy these requirements.
4.6.1 Oxygen-Deficient TiO2
The as-deposited resistivity of the ALD TiO2 used in this work was measured to be
about 37.5Ωcm. One way to lower the TiO2 resistivity is to reduce it chemically to form
84
oxygen-deficient TiO2-x [65]. We achieved this through the use of forming gas annealing
(FGA) for 5 minutes at various temperatures. To measure TiO2 resistivity, 9nm of ALD
TiO2 was first deposited on oxidized Si wafers with very thick SiO2 to confine electrical
conductivity to the TiO2 layer. The optional FGA is done at this point. Metal contacts
were then deposited and patterned into circular TLM structures. Since the total resistance
as a function of gap spacing is measured, the sheet resistance can be extracted. The
resulting TiO2 resistivity is plotted in Fig. 4.16.
Figure 4.16: TiO2 resistivity can be reduced by annealing in FGA for 5 minutes, from
37.5Ωcm (as deposited) down over three orders of magnitude to 0.021Ωcm (500°C).
Without any annealing, TiO2 has a high resistivity of about 37.5Ωcm. However, even
a low temperature 200°C 5-minute FGA reduces the resistivity by over ten times, down
to 2.3Ωcm. With a 500°C 5-minute FGA, TiO2 resistivity is reduced by almost 2000×
down to 0.021Ωcm. Because forming gas is a reducing agent, oxygen is stripped from
TiO2. The resulting oxygen vacancies act as electrical donors [65]. As the temperature
85
of the FGA is increased, the TiO2 becomes increasingly oxygen-deficient leading to
higher doping levels. The higher carrier density then leads to a reduction in resistivity.
Such a large reduction in insulator resistivity could significantly reduce the
deleterious effects of insulator series resistance. We first study the effect of different
insulator resistivity theoretically, using the same methodology described in Section 4.4.
The results are plotted in Fig. 4.17 for both 1019
cm-3
and 1020
cm-3
n+ Ge.
In Fig. 4.17, the solid black line represents TiO2 MIS contacts without series
resistance, while each of the colored dotted lines represent TiO2 MIS contacts with
different TiO2 resistivities. The blue, green and red dotted lines correspond to a 200°C
anneal (2.3Ωcm), 350°C anneal (0.077Ωcm), and 500°C anneal (0.021Ωcm) respectively.
At relatively high ρC, all the lines lay on top of each other, indicating little effect of series
resistance. At low ρC, however, the lines start to diverge once the contact resistance
becomes dominated by insulator resistivity. Lowering the TiO2 resistivity causes the
contours to more closely resemble those without series resistance. The red dotted line,
corresponding to a 500°C FGA TiO2, shows little divergence from the black line
representing no series resistance, even for a ρC as low as 10-8
Ωcm2. This suggests that a
TiO2 resistivity below 0.021Ωcm may be sufficient, and this can be achieved by
annealing in forming gas at 500°C.
86
10-6
10-8
(a)
10-4
10-7
10-6
10-8
(b)
10-4
10-7
Figure 4.17: Simulated TiO2 MIS contact resistivity on (a) 1019
cm-3
and (b) 1020
cm-3
n+
Ge. The specific contact resistivity contours are labeled in units of Ωcm2. Different
colors correspond to different TiO2 resistivity: blue (2.3Ωcm), green (0.077Ωcm), and red
(0.021Ωcm). The black line is the ideal ρC without series resistance.
We attempted to fabricate oxygen-deficient TiO2-x MIS contacts on n+ Ge by inserting
an extra RTA step immediately after ALD TiO2 and before the metal contact deposition.
87
However, the limitations of the RTA system at the Stanford Nanofabrication Facility
(SNF) prevented a controlled experiment. The RTA system did not have a load lock, and
consequently, there was oxygen present in the chamber during the anneal, which we
believe resulted in the growth of an interfacial layer of GeOx. Because of this, a ρC
reduction could not be observed due to the relatively large CBO of GeOx. Nevertheless,
the simulations show that if sufficiently oxygen-deficient TiO2-x can be used, the
insulator series resistance problem can be largely mitigated.
4.6.2 Indium Tin Oxide (ITO)
Indium tin oxide (ITO) is a commonly used transparent conductive oxide (TCO) for
solar cell applications. In this application, they are used in place of metal wiring and
contacts since metal would block sunlight from reaching the active regions of the solar
cell. ITO is a mixture of In2O3 and SnO2, with typical SnO2 concentrations up to 10%.
Like TiO2, ITO is a wide band gap semiconductor which can be doped n-type by oxygen
vacancies. SnO2 is the source of these oxygen vacancies in ITO and thus increasing SnO2
increases the doping level; however, too much SnO2 causes the electron mobility to drop.
In this way, its resistivity can be controlled and can be in the low 10-4
Ωcm range [92],
well below what is needed to mitigate series resistance effects in MIS contacts. ITO can
be deposited by physical vapor deposition techniques including sputtering, as well as
ALD [93] for better thickness control.
Because of their high conductivity, we explored the use of ITO in MIS contacts. ITO
MIS contacts were fabricated in the same way as TiO2 MIS contacts, except the ALD
88
TiO2 step is replaced by sputtered ITO. Fig. 4.18 plots the I-V characteristics of ITO MIS
contacts on moderately doped (~1018
cm-3
) n-type Ge.
Figure 4.18: Electrical characteristics of ITO MIS contacts on ~1018
cm-3
n-type Ge.
Significant increase in current density is observed indicating a reduction in ΦBN without
introducing tunneling or series resistance.
As with the case of TiO2 MIS contacts, there is a significant increase in current levels
with the use of ITO MIS contacts, indicating that ITO can also effectively reduce the
electron barrier height. Furthermore, very thick ITO can be used (up to 13nm), indicating
the minimization of tunneling resistance. Because of the high conductivity of ITO, these
MIS contacts should not suffer from series resistance effects. Future work is needed to
apply ITO MIS contacts on heavily doped semiconductors; with its low tunneling
resistance and material resistivity, it has the potential to achieve low ρC values.
4.6.3 Oxygen-Deficient ZnO
Recently, P. P. Manik et al. [94] fabricated ZnO MIS contacts on n-type Ge. As seen
in Fig. 4.19, the use of ZnO can reduce ΦBN, causing the current levels to increase as
89
expected. However, with an anneal, the current level increases further. This can be
explained by the doping behavior of ZnO, which is again a wide band gap semiconductor
doped n-type by oxygen vacancies. Because titanium was used as the metal contact, a
thermal treatment causes oxygen from the ZnO to migrate towards titanium to minimize
energy, resulting in oxygen vacancies in the ZnO layer. This oxygen-deficient ZnO is
now more conductive and also further minimizes tunneling resistance, resulting in a
higher current level.
Figure 4.19: (From [94]) I-V characteristics of ZnO MIS contacts on n-type Ge.
Using this technique on n+ Ge with a doping level of 2.5×10
19cm
-3, a very low ρC of
about 1.4×10-7
Ωcm2 was achieved. To our knowledge, this is the lowest reported ρC on
n-type Ge to date.
4.7 Scalability
In this section we discuss the scalability of MIS contacts, using the TiO2 system as an
example. Fig. 4.20 plots the simulated ρC as a function of germanium substrate n-type
doping density for a variety of effective metal workfunctions using 1nm TiO2 MIS
90
contacts. The thickness of 1nm was chosen since a certain thickness is required to
properly set up the interfacial dipole. Different thicknesses will only change the results
slightly. Series resistance was neglected since it is assumed this issue can be properly
mitigated through the use of more conductive materials. The simulated ρC of pinned
metal/n-Ge contacts is also shown, and agrees well with experimental data.
Figure 4.20: Simulated ρC as a function of n-type Ge doping level for a variety of
effective metal workfunctions using 1nm TiO2 MIS contacts. The dotted gray line is for
a metal/Ge contact, where the effective metal workfunction is pinned at 4.58eV.
At lower doping levels even a slight decrease in metal effective workfunction from
4.58eV to 4.5eV is enough for this MIS contact to outperform conventional contacts. At
a doping level of 1020
cm-3
, a reduction from 4.58eV to about 4.35eV is required. These
effective workfunctions have already been achieved using TiO2 MIS contacts, which is a
positive sign. Furthermore, the model predicts that at 1020
cm-3
doping, an effective
workfunction of 4.3eV would achieve a low ρC of about 10-7
Ωcm2. To achieve ρC below
10-8
Ωcm2 an effective workfunction of about 4.1eV would be required. These EWF
values can be relaxed somewhat if even higher doping can be achieved. It appears that
91
MIS contacts are very scalable and capable of achieving very low ρC at high doping
levels.
It is interesting that the lowest ρC on n-type Ge to date was achieved using MIS
contacts (i.e. ZnO MIS contacts [94]). Specific contact resistivities may improve further
when MIS contacts are applied together with high doping techniques. To successfully
integrate these types of contacts on more heavily doped substrates, the physical
mechanisms behind the operation of MIS contacts should be well studied in order to
create interfacial dipoles with larger magnitudes. Also, the fundamental tradeoffs should
be well understood, including the effects of tunneling resistance and insulator series
resistance in order to achieve low values of ρC.
4.8 Summary
Both MIGS and dipoles appear to play important roles in the operation of MIS
contacts. MIGS at the metal/TiO2 interface result in strong pinning due to the high
dielectric constant of TiO2. A dipole at that interface, likely resulting from surface
reconstruction or charge transfer, shifts the metal Fermi level towards the conduction
band, making it favorable for n-type contacts. Our experiments with annealing TiO2 MIS
contacts suggest that the metal/TiO2 interface is critical for the correct operation of the
MIS structure, rather than charges in the oxide.
The series resistance effect was also studied theoretically, and it was found to be a
potential performance limiter, especially in the case of the low barrier height, high doping
regime. In order to achieve low ρC values, it will be necessary to reduce the interfacial
layer resistivity, which can be achieved using oxygen-deficient TiO2-x, ITO, or oxygen-
92
deficient ZnO. If series resistance can be controlled, MIS contacts appear to be scalable
to very low ρC values.
93
Chapter 5
Germanide Contacts
5.1 Nickel Germanide
Germanium forms a germanide with many metals, including nickel, cobalt, titanium,
platinum, and platinum [87]. While other metals also form germanides, their formation
temperatures are very high or they did not yield a low resistivity phase. This renders
them unsuitable for integration in source/drain contacts since a high temperature anneal
may affect the shallow junction dopant profiles; high resistivity is also deleterious since
they would increase the transistor access resistance. Among the candidate metals, nickel
is a good choice because it forms nickel monogermanide (NiGe) at a relatively low
temperature and the resulting NiGe exhibits low resistivities, in the range of 22μΩcm
[87].
Because of its advantages, nickel germanide has been widely used in n+ Ge contacts.
Using phosphorus implant and 500°C anneal to achieve 3–6×1019
cm-3
doping
concentration, NiGe contacts achieved 3.46×10-6
Ωcm2 in the work by Shayesteh et al.
[88]. Arsenic implant was also used in that work but the resulting ρC was not as good.
The work by Gallacher et al. [89] achieved a very low ρC of (2.3±1.8)×10-7
Ωcm2 using
NiGe contacts. This very low ρC was attributed to annealing temperature optimization to
obtain the nickel monogermanide phase. Another explanation could be the fact that the
phosphorus doping was achieved via in situ doping during the Ge epitaxial growth rather
than by ion implantation; this results in lower defects (i.e. p-type vacancies) in the
94
germanium layer. Furthermore, as discussed in Chapter 2, nickel germanides have been
used to segregate dopants or chalcogens at the germanide/germanium interface, resulting
in a lower effective barrier height.
In the next few sections, we discuss the formation of NiGe and apply it to n+ Ge
contacts to yield very low specific contact resistivities.
5.2 Formation of NiGe
Nickel germanide can be formed by depositing a thin (typically 10nm – 30nm) nickel
on germanium, and annealing. During the annealing process, germanium is consumed to
form nickel germanide. The amount of nickel consumed depends on the annealing
temperature since the nickel germanide between the nickel and germanium slows down
the reaction rate. To a large extent, the annealing temperature also controls the
germanide phase. Fig. 5.1 plots the measured sheet resistance of a thin nickel germanide
film, which was grown from 20nm of deposited Ni and annealed at various temperatures
for 30 seconds. Sheet resistance was measured using a standard automated 4-point probe
setup. Although the underlying germanium may also conduct some current, because
highly-resistive undoped germanium wafers were used, its contribution to the sheet
resistance would be low. The measured sheet resistance is dominated by the nickel
germanide layer. The lowest resistance was found to be from films grown between
300°C and 450°C, indicating a wide and relatively low temperature window. Although
we did not confirm this, we believe this temperature window to give nickel
monogermanide (i.e. NiGe) since it is the low resistance phase. Furthermore, this
temperature window is similar to the temperatures used in literature [87-90].
95
Temperature Window: 300°C - 450°C
Figure 5.1: Temperature window for the low resistance phase of NiGe is between 300°C
and 450°C.
5.3 P and Sb Coimplantation with NiGe Contacts
In Chapter 2 it was mentioned that phosphorus and antimony coimplantation could
achieve higher n-type dopant concentration (over 1020
cm-3
) compared to implantation
with a single dopant species. In this section, we combine this technique with NiGe
contacts and achieve very low specific contact resistivities.
As before, circular TLM structures were used to obtain ρC. The starting material was
undoped germanium heteroepitaxially deposited on silicon. A thin (5nm – 10nm) SiO2
cap was deposited by PECVD before ion implantation. Phosphorus at 90keV energy and
6×1014
cm-2
dose and antimony at 65keV energy and 6×1014
cm-2
was used. Control
samples only received the phosphorus ion implant. Dopant activation was done at 500°C
for 10 seconds in nitrogen. Photoresist with the circular TLM structure was patterned
and hard baked for liftoff. The SiO2 was etched away using dilute HF immediately prior
to deposition of 20nm Ni, 10nm Ti, and 100nm Al. After metal liftoff and photoresist
removal, the NiGe was formed using RTA at 350°C for 30 seconds in nitrogen. Non-
96
germanide contacts were also fabricated for comparison. In these samples, metal
deposition consisted of 10nm Ti followed by 100nm Al. Metal liftoff and photoresist
removal were done as in the germanide samples, and the RTA step is skipped. A
schematic of the cross-section is shown in Fig. 5.2. Note that the germanide samples
have a recessed contact because of the germanium consumption during the germanidation
process. Although we assumed that 20nm of Ni was totally consumed to form about
50nm of NiGe, this was not confirmed. If we had unreacted Ni it would simply form part
of the contact pad and should not affect the electrical results.
P-type Si (500μm)
Highly-doped n+ Ge
Epitaxy P-type Ge (2μm)
SiO2 SiO2
100nm Al10nm Ti
50nm NiGe
P-type Si (500μm)
Highly-doped n+ Ge
Epitaxy P-type Ge (2μm)
SiO2 SiO2
100nm Al10nm Ti
Figure 5.2: Schematic cross-section of TLM structures used to extract ρC. Germanide
contacts are shown on the left and conventional metal contacts are shown on the right.
NiGe formation consumes some germanium, resulting in a slightly recessed contact.
Note that it is important to cap the Ni with other metals without a vacuum break
during this process to prevent oxidation. If this was not done, the ρC was typically higher.
Also, it is important to use a thick metal pad (at least 100nm of Al); if the pad resistance
is too large, the metal is no longer an equipotential surface resulting in erroneous ρC
values extracted using the TLM technique. Also, if the pad resistance was too high, the
measured resistance would depend on the exact measurement probe tip position. Finally,
a low germanidation temperature was chosen (350°C) since it is low enough to prevent
97
significant dopant diffusion, but still high enough to reside within the temperature
window for NiGe formation.
After this process, the electrically active n-type dopant concentration near the surface
is about 7×1019
cm-3
for the coimplantated samples as shown in Fig. 5.3. This is
significantly higher than the phosphorus only samples (about 2×1019
cm-3
) although not as
high as the reported activation levels in the original work [17] which reached over
1020
cm-3
. One possible reason for this lower dopant level is the fact that epitaxial
germanium substrates were used instead of bulk substrates; the slightly higher defect
density could decrease the activation levels. Secondly, this process is very sensitive to
the SiO2 cap layer thickness and doping conditions. Some further optimization in these
process parameters should push the dopant activation level to 1020
cm-3
or higher.
Figure 5.3: (From [86]) Electrically active n-type dopant profile as measured by SRP
for P only (blue) and P+Sb coimplant (red) samples.
98
5.3.1 Contact Resistance
The electrical data from the circular TLM measurement is summarized in Table 5.1
for all four combinations of samples (2 metallization and 2 doping schemes). For
phosphorus only implant with conventional metallization (i.e. Al/Ti), the specific contact
resistivity was about 4.1×10-5
Ωcm2, which is around the expected value. By going to the
P+Sb coimplanted sample with conventional metallization, the specific contact resistivity
drops to 2.1×10-6
Ωcm2. This large reduction in ρC is due to the increase in dopant
activation, which can be seen by the reduction in the n+ layer sheet resistance (45.6Ω
down to 34.7Ω for the coimplanted sample). The sheet resistance was extracted using the
circular TLM measurement, and it agrees well with SRP data.
Table 5.1: Summary of Electrical Measurements
Contact Scheme Specific Contact
Resistivity (Ωcm2)
Sheet Resistance
(Ω)
Al/Ti on P only 4.1×10-5
45.6
Al/Ti on P+Sb 2.1 ×10-6
34.7
NiGe on P only 2.0×10-6
46.2
NiGe on P+Sb 5.5×10-7
36.5
The NiGe contacts behave slightly differently. With the same doping conditions,
NiGe contacts outperform conventional metal contacts. With phosphorus only, going
from Al/Ti to NiGe contacts reduced ρC from 4.1×10-5
Ωcm2 to 2.0×10
-6Ωcm
2. With
P+Sb coimplantation, going from Al/Ti to NiGe contacts reduced ρC from 2.1×10-6
Ωcm2
to 5.5×10-7
Ωcm2. It should be noted that the underlying n
+ Ge layer sheet resistances
stayed the same between the Al/Ti contacts and the NiGe contacts, indicating that the
dopant activation level is similar. Because of this, it can be inferred that the use of NiGe
contacts seems to alter the effective electron barrier height, which will be discussed in the
next subsection. The use of NiGe contacts on P+Sb coimplanted n+ Ge yielded the
99
lowest ρC at the time the work was done [86], and may be suitable for integration onto
source/drain regions of Ge NMOSFETs.
5.3.2 Effect of Dopant Segregation
The fact that NiGe contacts show lower ρC than their Al/Ti counterparts can be
explained by considering the effect of dopant segregation. As discussed in Chapter 2,
dopant segregation at germanide/germanium interfaces can reduce the effective barrier
height. Fig. 5.4 shows the secondary ion mass spectroscopy (SIMS) profile for the NiGe
contacts on P+Sb coimplanted samples.
Figure 5.4: (From [86]) SIMS profile for NiGe contacts on P+Sb coimplanted samples.
P and Sb segregation at the NiGe/Ge interface can be seen, where the interface was
determined from the Ni and Ge concentrations.
The NiGe/Ge interface is approximately 50μm deep and was estimated from the Ni
and Ge profiles. The P and Sb concentrations inside the nickel germanide are
significantly reduced compared to Fig. 5.3 due to the snowplow effect. They are pushed
towards the interface during the germanidation process. As seen in Fig. 5.4, there is a
100
significant peak in dopant concentrations at the NiGe/Ge interface, especially for the Sb
species due to its larger mass.
Specific contact resistivity was simulated versus doping concentration for a variety of
barrier heights as shown in Fig. 5.5. The same tunneling matrix formalism introduced in
Section 2.3.3 was used for these calculations. For conventional metallization (Al/Ti), the
data points for P only (at about 2×1019
cm-3
doping) and P+Sb (at about 7×1019
cm-3
doping) lie on the line for a 0.55eV barrier, which is close to the expected value due to
Fermi level pinning. However, for the lower ρC contacts using NiGe, the two data points
lie on the line for a roughly 0.44eV barrier, indicating an apparent ~0.1eV barrier height
reduction.
Figure 5.5: (From [86]) Simulated ρC versus n-type Ge doping concentration for various
barrier heights. NiGe contacts show an apparent 0.1eV reduction in ΦBN compared to the
pinned Al/Ti contacts.
Even though the doping concentration is similar for Al/Ti or NiGe contacts using
P+Sb coimplantation, the lower ρC achieved by the NiGe contacts can be partially
attributed to the apparent reduction in ΦBN due to a small dipole at the NiGe/Ge interface.
101
The large concentration of donors on the germanium side of the interface leads to positive
ions, which sets up the dipole as shown schematically in Fig. 5.6. This shifts the vacuum
level higher on the NiGe side, which effectively shift the NiGe Fermi level towards the
conduction band.
Figure 5.6: (From [86]) Schematic diagram of (a) metal/Ge and (b) NiGe/Ge contacts.
NiGe contacts with dopant segregation introduce a small dipole which shifts the
germanide Fermi level towards the germanium conduction band.
Combined with a high active doping concentration using P+Sb coimplantation, this
reduction in ΦBN produces a very low ρC of 5.5×10-7
Ωcm2. One simple way to improve
this ρC even further is to optimize the P and Sb implantations to achieve over 1020
cm-3
active doping versus the roughly 7×1019
cm-3
used in this work. However, the ρC of
5.5×10-7
Ωcm2 is already lower than that achieved using techniques such as laser dopant
annealing (7×10-7
Ωcm2 [16] and 2.5×10
-6Ωcm
2 [12]), P and Sb coimplantation with
conventional metallization (8×10-7
Ωcm2 [19]), and Si passivation (1.4×10
-6Ωcm
2 [12]).
The low temperature requirement of the NiGe formation process also makes this contact
scheme more easily integrated into a CMOS process flow.
102
5.4 Chalcogen Segregation
As discussed in Chapter 2, chalcogens can be used to depin the metal Fermi level at
metal/semiconductor interfaces. In particular, sulfur and selenium have produced good
depinning characteristics on germanium. In this section we briefly describe the use of
sulfur segregation in conjunction with NiGe.
The starting substrate was a lightly n-type doped bulk Ge wafer. After substrate
cleaning and native oxide removal, 20nm of Ni was deposited onto the sample surface.
Sulfur was then ion implanted at 10keV energy and 1×1015
cm-2
dose into the thin Ni
layer. RTA was then performed at various temperatures to both form the NiGe as well as
segregate the sulfur to the NiGe/Ge interface in one step. Additional metal was then
deposited and patterned into pads for electrical measurements. There are alternative
ways to achieve sulfur segregation using germanides. For example, it is possible to do a
shallow implant of sulfur directly into the germanium followed by Ni deposition and
germanidation. Another method is to form the NiGe first, followed by sulfur ion
implantation and a second anneal to drive the sulfur to the NiGe/Ge interface. In this
section, however, we focus on the method of implanting sulfur into Ni, followed by RTA.
The electrical I-V characteristics of these NiGe/Ge Schottky diodes with sulfur
segregation are shown in Fig. 5.7. The control sample (black line) did not receive a
sulfur implant, but the RTA was still performed at 300°C for 30 seconds to form the
NiGe. Going from the control sample to the sample with sulfur annealed at 300°C (red
line) provided no benefits, indicating that the thermal budget was not enough to segregate
the sulfur to the interface. However, at 500°C there is a significant increase in the diode
reverse current, indicating a decrease in ΦBN. Note that 500°C is approximately the
103
upper limit of the NiGe formation temperature window, so it would not be advisable to
go above this temperature. Indeed, at 600°C, the current starts to drop again likely as a
result of the degradation of the NiGe layer.
Figure 5.7: Effect of sulfur segregation to the NiGe/Ge interface causes a reduction in
effective ΦBN. With increased levels of sulfur, the Schottky diode reverse current
increases and becomes more ohmic.
It can be seen that a higher thermal budget allows for more segregation and is
favorable in terms of a lower ΦBN. However, the thermal stability of the germanide
places a limit on allowed temperatures. It may be worthwhile to explore alternative
germanides which can withstand higher temperatures, such as nickel-alloy germanides,
which can potentially have a higher temperature window if refractory materials such as
Ti or Pt are incorporated into the NiGe. Another approach is to use an ammonium
fluoride pretreatment [91] which can extend the NiGe temperature window up to 600°C.
The use of sulfur and other chalcogens opens up further possibilities for germanide
contacts. Although dopant segregation in P+Sb coimplanted samples with NiGe contacts
exhibited a roughly 0.1eV ΦBN reduction, it may be possible to achieve further ΦBN
104
reduction if chalcogens are also incorporated at the interface. This would increase
process complexity in that three ion implants would need to be performed and optimized,
but the result could be a significantly lower ρC.
5.5 Summary
Using phosphorus and antimony coimplantation to achieve high n-type dopant
activation in Ge together with NiGe resulted in very low ρC of 5.5×10-7
Ωcm2. On
similarly-doped substrates, NiGe contacts achieved a lower ρC compared to standard
metallization. This was attributed to the effect of dopant (i.e. Sb) segregation at the
NiGe/Ge interface which reduced the effective barrier height by about 0.1eV. Sulfur
segregation using NiGe was also found to reduce the effective barrier height. This opens
up the possibility to achieve low ρC using germanide contacts by incorporating both
dopant and chalcogen segregation to lower the barrier height.
105
Chapter 6
Conclusions
6.1 Benchmarks
Several n-type Ge contacts are summarized in Table 6.1.
Table 6.1: Summary of Selected Contact Schemes on N-Type Ge
Method References Substrate
Doping (cm-3
)
Specific Contact
Resistivity (Ωcm2)
Conventional Contacts
P with Ti/Al [12, 13] 2-3×1019
~10-4
MIS Schemes
TiO2 MIS Contacts [44, 45] 2-3×1019
1.3×10-6
ZnO MIS Contacts [94] 2.5×1019
1.4×10-7
Doping Schemes
P+Sb with Ti/Al [86]
[19]
7×1019
1×1020
2.1×10-6
8×10-7
P+Sb with NiGe [86] 7×1019
5.5×10-7
P with F Defect Passivation [21] 1×1020
~10-6
Sb with Laser Annealing [16]
[12]
1×1020
1×1020
7×10-7
2.5×10-6
Other
Si0.8Ge0.2 Contacts [95] 1×1020
~1×10-6
Si Passivation [12] 1×1020
1.4×10-6
Conventional contacts on a phosphorus doped substrate yields a relatively high ρC of
around 10-4
Ωcm2. This is due to the Fermi level pinning problem and the low electrically
active doping concentration (low 1019
cm-3
range). To address the Fermi level pinning
problem, MIS schemes have been used, including those using TiO2 and ZnO, both of
which achieve a lower ρC without increasing the doping level. Doping can also be
increased to reduce ρC, including the use of P and Sb coimplantation, fluorine passivation,
and laser annealing. Finally, the use of silicon, either in the form in a SiGe alloy or a thin
106
Si layer on top of the Ge contact, have been used to reduce ρC. The improvement here is
due to the higher doping concentration present in Si or SiGe compared to Ge.
6.2 Contributions and Suggestions for Future Work
This thesis has focused on two methods for reducing n-type Ge contact resistance.
The first method is the use of MIS contacts. Although tunneling resistance in these
structures was already previously identified, using tunneling transport simulations we
concluded that low ρC can only be achieved with low band offset interfacial layers. TiO2
was identified as a candidate material due to its roughly zero CBO to Ge. Experiments
confirmed the significant reduction in tunneling resistance using TiO2 MIS contacts. On
n+ Ge with low 10
19cm
-3 doping, TiO2 MIS contacts achieved a ρC of 1.3×10
-6Ωcm
2,
which significantly outperformed conventional MIS contacts using high CBO materials.
Series resistance of the insulator layer was identified as a new tradeoff mechanism which
can limit ρC in the low ρC and/or high doping regime. Dipoles at the metal/TiO2 interface
were identified as being partially responsible for the ΦBN reduction as the dipole gives
rise to a lower effective metal workfunction. In particular, charges in the insulator do not
seem to play a major role.
There are several avenues which can be pursued. TiO2 MIS contacts can be applied
on a wider variety of semiconductors which have an electron affinity close to that of TiO2
(~4eV) since this gives rise to a low CBO. More generally, the fundamental principle to
minimize tunneling resistance can be used for evaluating any MIS contact scheme,
including the case for p-type contacts. For p-type MIS contacts, valence band offset
would need to be minimized. In order to improve contact performance, MIS contacts
107
should be used in conjunction with high doping techniques since both high doping and
low barrier heights are needed for very low ρC. Oxygen-deficient TiO2 or ITO should be
explored in the context of MIS contacts since insulator series resistance will become very
important under these conditions. Efforts should also go towards achieving higher dipole
magnitudes. One way is to introduce additional dipoles by using a bilayer insulator
approach as in [47], except low CBO materials should be used throughout the entire
contact. Another method is to combine chalcogen passivation with MIS contacts; for
example, MIS contacts can be applied to sulfur-passivated germanium substrates. This
may have the potential to achieve even lower ΦBN values if the two techniques create an
additive effect. Finally, MIS contacts should be applied on short channel MOSFETs in
order to clearly see the current improvement due to parasitic resistance reduction.
Alternative transistors such as metal source/drain transistors incorporating MIS contacts
should be explored further as these have the potential to significantly reduce parasitic
resistance below that of conventional doped source/drain transistors.
The second method is the use of germanide contacts. In particular, we studied nickel
germanide and applied it to a highly doped substrate using the phosphorus and antimony
coimplantation technique. By doing so, a very low ρC of 5.5×10-7
Ωcm2 was achieved.
Dopant segregation due to the NiGe formation process was found to give a small
reduction in ΦBN, leading to the low value of ρC obtained.
In terms of future work, one very exciting possibility is the use of chalcogens. By
segregating chalcogens such as sulfur or selenium at the germanide/germanium interface,
a potentially lower ΦBN could be achieved compared to dopant segregation alone. The
experiment should also be performed on bulk germanium wafers with carefully optimized
108
implant conditions; the lower defect densities should lead to over 1020
cm-3
active doping
concentration (versus the 7×1019
cm-3
used in this work) and an even lower ρC. Again,
this method of contacting germanium should be applied on the short channel Ge
MOSFETs to observe the increase in transistor performance.
109
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