2014 Master Thesis
A study on silicide semiconductors for
high efficiency thin film
photovoltaic devices
Taichi Inamura
12M36055
Department of Electrical and Electronic Engineering
Tokyo Institute of Technology
Supervisor
Professor: Hiroshi Iwai
Associate Professor: Kuniyuki Kakushima
Abstract
Semiconductor silicides have been attracted as thin film solar cell
material candidates for next generations, owing to its appropriate bandgaps
with large absorption coefficient and abundance of resources. Especially,
BaSi2/-FeSi2 tandem thin silicide solar cells are expected to achieve
efficiency 40 %.
One of the issues in semiconductor silicides is that the carrier density is
still in the order of 1018 cm-3, which is three orders of magnitude high
considering sufficient depletion width to absorb sun light with absorption
coefficient of 105 cm-1 at 1.0 eV.
Although, composition control is reported to be the key to achieve
-FeSi2 with low carrier density, the origin of carrier has not yet been
clarified.
The purpose of this thesis is to investigate the optimal Fe/Si composition
for carrier density reduction and to clarify the origin of carriers. Moreover,
light response of -FeSi2 films has been characterized by fabricating
Schottky-type -FeSi2 solar cell.
-FeSi2 films have been deposited by multi-sputtering process, which
consists of cyclic deposition of Fe and Si layers in a multi-target sputtering
system with subsequent crystallization annealing. As the thickness of each
film can be well controlled by sputtering time, the composition of -FeSi2
films can be easily controlled. The origin or carriers has been characterized
through temperature dependent resistivity measurement to extract the
activation energy.
By changing the composition of -FeSi2, it has been found that Si-rich
condition with Si /Fe ratio of 2.25 has shown the largest resistivity of ~0.6
cm. Low temperature measurements have revealed four kinds of defect
levels, which can be categorized into two types; one related to composition of
-FeSi2 and the other related to crystalline defects. The former type with
deep activation energy has been assigned though measurements of samples
with different compositions, and the latter one with shallow activation
energy by crystallization annealing temperature. Based on the above
measurements a carrier density of 1016 cm-3 can be achieved at 90 K.
-FeSi2/p+-Si Schottky solar cell measured at 90 K has shown a photovoltaic
response with open circuit voltage of 40 mV, which is low considering the
bandgap of ideal -FeSi2.
Equivalent circuit modeling has revealed the presence of shunt
resistance which suggests semiconductor nature within the film.
In the near future, more fine composition ratio control, improvement of
crystalline quality and increase of shunt resistance is required for -FeSi2
thin film photovoltaic device.
A study on semiconductor silicides for high efficiency thin film photovoltaic devices
Contents
Chapter1 Introduction
1.1 Thin film solar cell ...................................................................................................... 2
1.2 Introduction of semiconductor silicides .................................................................... 3
1.3 Introduction of BaSi2 / -FeSi2 tandem solar cell ..................................................... 5
1.4 Issues in semiconductor silicides ............................................................................. 10
1.5 Reports on semiconductor silicides .......................................................................... 11
1.6 Purpose of this study ................................................................................................ 14
1.7 Outline of this thesis ................................................................................................ 15
Reference ............................................................................................................................. 17
Chapter 2 Fabrication and characterization
2.1 Fabrication procedure .............................................................................................. 20
2.2 Experimental details ................................................................................................ 21
2.2.1 SPM cleaning and HF treatment ..................................................................... 21
2.2.2 RF magnetron sputtering .................................................................................. 21
2.2.3 Photolithography and Metal etching ................................................................ 22
2.2.4 Lift-off process .................................................................................................... 23
2.2.5 Rapid thermal annealing (RTA) ....................................................................... 23
2.2.6 Vacuum evaporation for Al deposition.............................................................. 23
2.3 Characterization Method ......................................................................................... 25
2.2.1 Fourier transform infrared spectroscopy (FT-IR) ................................................ 25
2.2.2 Four-point probe method ....................................................................................... 26
2.2.3 Transmission Line Model (TLM) ........................................................................... 27
2.2.4 Van der Pauw method ....................................................................................... 30
Reference ............................................................................................................................. 32
Chapter 3 Formation of -FeSi2 3.1 Introduction .............................................................................................................. 34
3.2 Infrared absorption characteristics of -FeSi2 ....................................................... 35
3.3 X-ray diffraction pattern of -FeSi2......................................................................... 38
A study on semiconductor silicides for high efficiency thin film photovoltaic devices
3.4 Conclusion ................................................................................................................. 38
References ............................................................................................................................ 39
Chapter 4 Electrical characteristics of -FeSi2
4.1 Introduction .............................................................................................................. 42
4.2 Resistivity measurement by four-point method ..................................................... 43
4.2.1 Resistivity control by Si/Fe composition ratio ...................................................... 43
4.2.2 Extraction of defect level position ......................................................................... 45
4.2.3 Extraction of defect concentration ........................................................................ 46
4.2.4 Effect of sputtering atmosphere ............................................................................ 48
4.3 Resistivity measurement by TLM ........................................................................... 49
4.4 Carrier density measurement by van der Pauw .................................................... 50
4.5 Conclusion ................................................................................................................. 54
Reference ............................................................................................................................. 55
Chapter 5 Demonstration of -FeSi2 Schottky solar cell
5.1 Introduction .............................................................................................................. 57
5.2 J-V characteristics of -FeSi2/Si Schottky solar cell .............................................. 58
5.3 Equivalent circuit of solar cell ................................................................................. 59
5.4 Conclusion ..................................................................................................................... 62
Reference ............................................................................................................................. 63
Chapter 6 Conclusion
Chapter 1 Introduction
1
Chapter 1
Introduction
1.1 Thin film solar cell
1.2 Introduction of semiconductor silicides
1.3 Issues in semiconductor silicides
1.4 Reports on semiconductor silicides
1.5 The purpose of this study
References
Chapter 1 Introduction
2
1.1 Thin film solar cell
Solar cells have been expected as clean energy replaced to thermal
power generation used coal, petroleum oil and liquefied natural gas or
nuclear power generation and so on. However, a solar cell amounts to only
~0.2 % of total power generation energy supply so far (Figure1.1). The cost
per Kilowatt-Hour of solar cell is much higher than that of thermal or
nuclear power generation. Therefore, to spread the usage of solar cells more
widely and rapidly, not only the improvement of conversion efficiency but
also the decrease of the production cost is indispensable. Figure 1.2 is
classification of solar cells according to the used materials [1.1]. The
crystalline silicon (c-Si) accounts for 90 % or more in a solar cell market [1.2].
However, there is a problem of being approaching limit of conversion
efficiency caused by the band gap. The materials with a variety of band gap
are needed for further conversion efficiency improvement.
Figure 1.1 Distribution of consumption of primary energy with respect to
coal, nuclear, hydro and renewable energy resources
Renewable
PV
Wind
Biomass
Geothermal65.4%
Coal
Nuclear24.8%
Hydro7.8%
0.4%
0.4%
0.3%
1.0%
Chapter 1 Introduction
3
Figure 1.2 Classification of solar cell according to materials
Thin film solar cells have been focused as important photovoltaic cells
for future generations, owing to several advantages. One is that we can limit
usage and save resources thanks to thin film structure. Therefore, the cost
for manufacturing can be kept low. The other is that these have various
bandgap. Hence, higher conversion efficiency than crystalline Si are
expected by tandem structure.
1.2 Introduction of semiconductor silicides
Various thin films have been reported so far. Table 1.1 shows some
examples of it.
Crystal line silicon
(c-Si)
monocrystalline / single crystalline
Polycrystalline / multi crystalline
amorphous silicon
(a-Si)Microcrystalline / m-Si
CIS (CIGS)
CdTe
GaAs
Dye sensitized
polymer
silicon base
compound base
organic based
solar cell
Chapter 1 Introduction
4
Table 1.1 Comparison of thin film solar cells
a-Si and CIGS are the leading thin film solar cells due to its high
absorption coefficient. Especially, a-Si solar cell could be applied to a liquid
crystal display production process. Therefore, a-Si solar cell was rapidly
developed and widely expanded in a world. Disadvantage of a- Si solar cell is
that theoretical conversion efficiency is near 20 % which is low for solar cells
for next generation solar cell. Furthermore, vacuum process such as plasma
CVD or sputtering is used in the fabrication process. Therefore, the process
cost is high. Similarly, CIGS also uses the vacuum process and the
production cost is high although conversion efficiency shows high value near
30 %. However, there is a problem of using Indium of rare metal. Also, a dye
sensitized solar cell or an organic thin film attracted recently attention as
future low cost solar cells. But the problem for organic materials is the light
degradation. Furthermore, conversion efficiency for this material is
insufficient to exceed 30 % or more. On the other hand, semiconductor
silicide materials are thought to be the powerful candidates to achieve low
cost and high conversion efficiency compared to above materials. Various
silicides have been reported to have bandgap. For example, the band gap of
BaSi2, -FeSi2 and Mg2Si are 1.4 eV, 0.8 eV and 0.75 eV respectively. In
addition, there are abundantly resources for these three silicides. Moreover,
the fact that BaSi2 and -FeSi2 are stable against light illumination is
reported. After all, in order to achieve above goal, semiconductor silicides
Bandgap
Eg (eV)
Transition
type
Absorption
coefficient
α (cm-1)
Resources DegradationEfficiency
(%)
a-Si 1.7 indirect 104 Excellent Good 20 [1.3]
CIGS 1.0~1.6 direct 105 Bad Excellent 29 [1.3]
Organic 1.0~ indirect 105 Good Bad 14 [1.3]
BaSi2 1.4 indirect 105~ Excellent Excellent [1.4] 32 (cal.)
FeSi2 0.8 direct 105~ Excellent Excellent [1.4] 24 (cal.)
Silicide Mg2Si 0.75 indirect 105~ Excellent No data 22 (cal.)
CrSi2 0.3 indirect 105 ~ Good No data 8 (cal.)
ReSi2 0.1 direct 104~ Bad No data 1 (cal.)
Chapter 1 Introduction
5
have been considered as suitable candidates.
1.3 Introduction of BaSi2 / -FeSi2 tandem solar cell
If BaSi2 and -FeSi2 are connected in series, tandem thin silicide film
solar cell can be realized (Figure 1.3).
Figure 1.3 Structure of BaSi2/-FeSi2 tandem solar cell
Indeed, we calculate the maximum efficiency of tandem solar cell. The
conversion efficiency, , of solar cell is calculated as the ratio between the
generated maximum power, Pm, generated by a solar cell and the incident
power, Pin. The incident power is equal to the irradiance of AM 1.5 spectrum,
normalized to 1000 W/m2. Therefore, the is given by
η =𝑃𝑖𝑛
𝑃𝑜𝑢𝑡=
𝐽𝑚𝑎𝑥𝑉𝑚𝑎𝑥
𝑃𝑖𝑛=
𝐽𝑠𝑐𝑉𝑜𝑐𝐹𝐹
𝑃𝑖𝑛 (1.1)
where Jmax is maximum current density, Vmax maximum voltage, Jsc short
circuit current density, Voc open circuit voltage, FF fill factor.
The irradiance of AM 1.5 spectrum can be calculated from the spectral
power density, P(), using the following equation:
𝑃𝑖𝑛 = ∫ 𝑃(𝜆)𝑑𝜆∞
0 (1.2)
BaSi2
-FeSi2
transmitted light
incident light
Chapter 1 Introduction
6
= ∫ 𝜙(𝜆)ℎ𝑐
𝜆𝑑𝜆
∞
0 (1.3)
where 𝜙(𝜆) is photon flux density, h Plank’s constant, c speed of light.
When we denote g as the wavelength of photons that corresponds to the
band gap energy of the absorber of the solar cell, only the photons with
energy higher than the bandgap are absorbed, it means photons with λ ≤ 𝜆𝑔.
The fraction of the incident power, pabs that is absorbed by a solar cell and
used for energy conversion is expressed as
𝑃𝑎𝑏𝑠 =∫ 𝜙(𝜆)
ℎ𝑐
𝜆
𝜆𝑔0
∫ 𝜙(𝜆)ℎ𝑐
𝜆
∞0
(1.4)
A part of the absorbed energy, the excess energy of photons, is lost due to
the thermalization of photo-generated electrons and holes in the absorber
material. The fraction of the absorbed energy that the solar cell can deliver
as useful energy, puse, is described by
𝑃𝑢𝑠𝑒 =𝐸𝑔 ∫ 𝜙(𝜆)𝑑𝜆
𝜆𝑔0
∫ 𝜙(𝜆)ℎ𝑐
𝜆𝑑𝜆
𝜆𝑔0
(1.5)
We can determine the conversion efficiency limited by the spectral
mismatch
η = 𝑃𝑎𝑏𝑠𝑃𝑢𝑠𝑒 =∫ 𝜙(𝜆)
ℎ𝑐
𝜆𝑑𝜆𝐸𝑔 ∫ 𝜙(𝜆)𝑑𝜆
𝜆𝑔0
𝜆𝑔0
∫ 𝜙(𝜆)ℎ𝑐
𝜆𝑑𝜆 ∫ 𝜙(𝜆)
ℎ𝑐
𝜆𝑑𝜆
𝜆𝑔0
∞0
(1.6)
Figure 1.4 illustrates the fraction of the AM 1.5 spectrum that can be
converted into a usable energy.
Chapter 1 Introduction
7
Figure 1.4 The fraction of AM 1.5 spectrum that can be converted into a
usable energy
In general, when light arrives on an interface between two media, a part
of the light is reflected from and the other part is transmitted through the
interface. This means that a part of the incident energy that can be
converted into a usable energy by the solar cell is lost by reflection. We shall
denote the total reflectance as R*, which can be considered as the effective
reflectance in the wavelength range of interest.
In most c-Si solar cells, one of the metal electrodes is placed on the front
side of the cell. The metal-covered area does not allow the light to enter the
solar cell because it totally reflects the light in wavelength range of interest.
When we denote the total area of the cell Atot and the cell area that is not
covered by the electrode Af, the active area of the cell is determined by the
ratio of Af / Atot. This ratio is called the coverage factor and determines the so
called shading losses. However, in the case of thin film solar cell, transparent
conductive film such as ITO is used for front side electrode. Therefore,
Af /A tot ~1.
When light penetrates into a material, it will be absorbed as it
propagates through the material. The absorption of light in the material
depends on its absorption coefficient. Incomplete absorption in the absorber
due to its limited thickness is an additional loss that lowers the efficiency of
Wavelength
Non absorption λg
Chapter 1 Introduction
8
the energy conversion. The incomplete absorption loss can be described by
the internal optical quantum efficiency, QEop.
Not all charge carriers that are generated in a solar cell are collected at
the electrodes. The photo-generated carriers are the excess carriers with
respect to the thermal equilibrium and are subjected to the recombination.
This can be expressed by the electrical quantum efficiency, QEel.
With these taken into consideration, the absolute external quantum
efficiency QE which is defined as the number of charge carriers collected per
incident photon at each wavelength can be approximated as
Q(E) = (1 − 𝑅∗)𝑄𝐸𝑜𝑝(𝜆)𝑄𝐸𝑒𝑙(𝜆) (1.7)
The maximum current density that the solar cell can deliver is
determined by the bandgap of the absorber layer that determines which
photons of the incident radiation can generate electron-hole pairs. The
maximum current density, Jmax is described as
𝐽𝑚𝑎𝑥 = 𝑞 ∫ 𝜙(𝜆)𝑑𝜆𝜆𝑔
0 (1.8)
The short-circuit current density is determined by the absolute external
quantum efficiency
𝐽𝑠𝑐 = 𝐽𝑚𝑎𝑥(1 − 𝑅∗)𝑄𝐸𝑜𝑝𝑄𝐸𝑒𝑙 (1.9)
= q(1 − 𝑅∗)𝑄𝐸𝑜𝑝𝑄𝐸𝑒𝑙 ∫ 𝜙(𝜆)𝑑𝜆𝜆𝑔
0 (1.10)
The open-circuit voltage depends on the saturation current and
short-circuit current density.
𝑉𝑜𝑐 =𝑘𝑇
𝑞𝑙𝑛 (
𝐽𝑠𝑐
𝐽0) (1.11)
where k is Boltzmann’s constant, Jo saturation current density. The
saturation current density depends on the recombination in the solar cell
that cannot be avoided and is referred to as the fundamental recombination
which is determined by the voltage factor, qVoc/Eg.
Chapter 1 Introduction
9
The maximum power generated by a solar cell is dependent on the fill
factor, FF. In case of a solar cell that behaves as an ideal diode only direct
recombination occurs and the maximum FF is a function of Voc.
By combining the previous equation, the conversion efficiency can be
written as
η =∫ 𝜙(𝜆)
ℎ𝑐
𝜆𝑑𝜆
𝜆𝑔0
∫ 𝜙(𝜆)ℎ𝑐
𝜆𝑑𝜆
∞0
𝐸𝑔 ∫ 𝜙(𝜆)𝑑𝜆𝜆𝑔
0
∫ 𝜙(𝜆)ℎ𝑐
𝜆𝑑𝜆
𝜆𝑔0
(1 − 𝑅∗)𝑄𝐸𝑜𝑝𝑄𝐸𝑒𝑙𝑞𝑉𝑜𝑐
𝐸𝑔𝐹𝐹 (1.12)
For simplicity, we assume that total reflectance is 0 %, quantum
efficiency is 100 %, and FF is 1. In the case of tandem solar cell, current is
limited by smaller value in each cell and voltage is sum of each cell because
they are connected in series.
Figure 1.5 shows the calculated efficiency of tandem solar cell. Assuming
1.4 eV of BaSi2 with 0.8 eV of -FeSi2, efficiency over 40 % could be expected.
Figure 1.6 shows I-V characteristics of BaSi2/-FeSi2 tandem solar cell. From
this, the open circuit voltage could exceed 1.5 V with short circuit current of
2.5 A/cm2. Therefore, tandem silicide solar cell could be expected as a new
thin film solar cell.
Figure 1.5 Conversion efficiency of tandem solar cell
45%40%
35%30%
Top : 1.4eV
Bottom : 0.8eV
top cell bandgap (eV)
bottom
ce
ll b
an
dg
ap
(eV
)
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.00.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
Chapter 1 Introduction
10
Figure 1.6 I-V Characterization of BaSi2/-FeSi2 tandem solar cell
1.4 Issues in semiconductor silicides
Thin film solar cells have been investigated for next generation energy
discussed in previous section. Especially, semiconductor silicides such as
BaSi2 and -FeSi2 are expected as material of tandem solar cell thanks to
some advantages. However, there are some challenges in semiconductor
silicides. One is high carrier density [1.5]. BaSi2 and -FeSi2 have large
absorption coefficient of over 105 cm-1. Therefore, the requirement for silicide
solar cell is that it should have depletion width of several hundred
nanometers to absorb most of the light from sun. Figure 1.7 shows depletion
layer width dependence of -FeSi2 on carrier density. This figure reveals that
carrier densities of 1015 ~1016 cm-3 are required. Therefore, low carrier
density of the film is required for thin film silicide solar cell.
0 0.5 1.0 1.5 2.0-0.5-3.0
-2.0
-1.0
0
1.0
2.0×102
cu
rre
nt d
en
sity(
A/m
2)
voltage (V)
Voc
Jsc
Chapter 1 Introduction
11
Figure 1.7 Target value of carrier density of -FeSi2 for sufficient absorption
of the light from sun
1.5 Reports on semiconductor silicides
Lots of works have reports on silicide formations. Such as ion
implantation of Fe into Si with annealing, reactive epitaxy method, MOCVD
have been used to form -FeSi2 [1.6, 1.7, 1.8]. As for film quality, there are a
lot of researches on carrier density of -FeSi2 formed by MBE. The carrier
density of the formed films is reported to depend on Si and Fe composition
ratio. In the case of Figure 1.8 (a), when the composition ratio is 1 to 2, the
type of semiconductor becomes n-type and below 1.8, -FeSi2 are p-type
semiconductor. On the other hand, in the case of Figure 1.8 (b), when the
composition ratio is 1 to 2, the type becomes p-type. Therefore, composition
control is the key to achieve -FeSi2 with low carrier density. However, the
origin of carrier has not yet been clarified.
1014 1015 1016 1017 1018 1019 10201
10
102
103
Carrier density (cm-3)
De
ple
tio
n -
laye
r w
idth
(n
m)
the area around hereCurrent statusTarget
Chapter 1 Introduction
12
Figure 1.8 Carrier density dependence of -FeSi2 on Si/Fe composition ratio
(a) Ref.[1.9], (b) Ref.[1.10]
K. Takakura et al.,
Jpn. J. Appl. Phys ,
Vol. 39, 790 (2000)
1.85
2.6
(a)
(b)
Chapter 1 Introduction
13
There have been limited reports on solar cells fabricated using -FeSi2.
The highest efficiency reported is 3.7 % obtained on a crystalline n-type
-FeSi2 film epitaxially grown on p-Si(111) (Figure 1.8) [1.11]. Minimizing
the ~40 % visible light reflectance from -FeSi2 with antireflection coating
may help to improve the efficiency up to ~6 % [1.12]. However, this value is
still much lower than the theoretical value. As it was previously mentioned,
it could be a possible cause that depletion layer isn’t sufficiently spread for
absorption from sun.
Figure 1.9 Schematic device structure of a thin-film n--FeSi2/p-Si
heterojunction solar cell (a) and a typical I-V characteristic curve under
sunlight of air mass 1.5, 100mW/cm2 illumination (b)
Chapter 1 Introduction
14
1.6 Purpose of this study
As mentioned in previous section, semiconductor silicides have been
attracted as new generation thin film photovoltaic materials. Based on the
above introduction, the purpose of this study is to develop a process to realize
fine control of Fe/Si composition ratio, to investigate the optimal Fe/Si
composition for carrier density reduction and to clarify the origin of carriers.
In this process, just Fe and Si layer are sputtered in situ, followed by proper
annealing as shown in Figure 1.10. Moreover, light response of -FeSi2 films
has been characterized by fabricating Schottky-type -FeSi2 soalr cell.
Figure 1.10 Schematic illustration of multi-sputtering process. A set of Fe/Si,
with arbitrary composition ratio, is cyclically stacked, followed by proper
annealing to form -FeSi2
FeSi
・・
・annealing FeSi2
Si substrate Si substrate
1 set
Chapter 1 Introduction
15
1.7 Outline of this thesis
Figure 1.11 shows the contents of this thesis. This thesis is consisted of 6
parts.
In chapter 1, the introduction of this thesis is stated.
In chapter 2, the fabrication process of devices and electrical
characterization are explained.
In chapter 3, -FeSi2 formed by our multi-stacked process is confirmed.
It becomes obvious that -FeSi2 with bandgap of 0.8 eV is formed.
In chapter 4, electrical characteristics of -FeSi2 are examined.
Resistivity of -FeSi2 is dependent on composition ratio. Moreover, four kinds
of activation energy for carriers can be extracted.
In chapter 5, -FeSi2 Schottky solar cell was demonstrated. It is testified
that -FeSi2 has ability of solar energy conversion.
Finally, chapter 6 summarizes this study.
Chapter 1 Introduction
16
Figure 1.11 Contents of this thesis
Chapter 1
Chapter 2
-FeSi2 Formation Electrical characteristics of -FeSi2
Demonstration of -FeSi2 Schottkysolar cell
Conclusion
Introduction
Fabrication and characterization
Chapter 3 Chapter 4
Chapter 5
Chapter 6
Chapter 1 Introduction
17
Reference
[1.1] AIST, Research Center for Photovoltaic Technologies “Classification of
solar cell”
[1.2] IEA, “Energy Technology Perspective, Pathway to a Clean Energy System” (2012)
[1.3] E. Arvizu, World Future Energy Summit, NREL (2013)
[1.4] T. Suemasu, New Technology Presentation Meeting at University of
Tsukuba (2012)
[1.5] T. Suemasu, K. Takakura, C. Li, Y. Ozawa, Y. Kumagai, F. Hasegawa,
“Epitaxial growth of semiconducting -FeSi2 and its application to
ligh-emitting diodes”, Thin Solid Films, 461,209-218 (2004)
[1.6] M. Sugiyama, Y. Maeda, “Microstructure characterization of ion-beam
synthesized -FeSi2 phase by transmission electron microscopy”, Thin Solid
Films, 381, 256 (2001)
[1.7] T. Suemasu, T. Fujii, M. Tanaka, K. Takakura, Y. Iikura, F. Hasegawa,
Jpn. J. Appl. Phys, 36, 3620 (1997)
[1.8] M. Suzuno, K. Akutsu, H. Kawakami, K. Akiyama, T. Suemasu,
“Metalorganic chemical vapor deposition of -FeSi2 on -FeSi2 seed crystals
formed on Si substrates”, Thin Solid Fioms, 519, 24, 8473-8476 (2011)
[1.9] K.Takakura, T. Suemasu, Y. Ikura, F. Hasegawa, “ Control of the
Conduction Type of Nondoped High Mobility -FeSi2 Films Grown from Si/Fe
Multilayers by Change of Si/Fe Ratios”, Jpn. J. Appl. Phys, 39, 787-791
(2000)
[1.10] N. Seki, K. Takakura, T. Suemasu, F. Hasegawa, “Conduction type and
defect levels of -FeSi2 films grown by MBE with different Si/Fe ratios”,
Materials Science in Semiconductor Processing, 6, 5-6, 307-309 (2003)
[1.11] Z. Liu, S. Wang, N. Otogawa, Y. Suzuki, M. Osamura, Y. Fukuzawa, T.
Ootsuka, Y. Nakayama, H. Tonoue, Y. Makita, “ A thin-film solar cell of
high-quality -FeSi2/Si heterojunction prepared by sputtering”, Solar Energy
Chapter 1 Introduction
18
Materials&Solar Cells, 90, 276-282 (2006)
[1.12] Y. Makita, T. Ootsuka, Y. Fukuzawa, N. Otogwa, H. Abe, L. Zhengxin,
Y. Nakayama, “-FeSi2 as Kankyo (environmentally friendly) semiconductor
for solar cells in the space application”, Proc. SPIE, 6197, 61970O (2006)
Chapter 2 Fabrication and characterization
19
Chapter 2
Fabrication and characterization
2.1 Fabrication procedure
2.2 Experimental details
2.3 Characterization Method
2.4
References
Chapter 2 Fabrication and characterization
20
2.1 Fabrication procedure
Figure 2.1 shows fabrication procedure of the sample for -FeSi2
Schottky solar cell. The sample was fabricated on n-type (100)-oriented Si
substrate. The substrate impurity concentration is 3×1015 cm-3. To determine
the diode area, 400-nm-thick thermal SiO2 was formed. It was patterned by
photolithography and etched by buffered hydrofluoric acid (BHF). After SPM
cleaning and HF treatment, thin thermal SiO2 (2~3 nm) was formed because
of protection Si surface from resist and developers. Lift-off pattering and HF
treatment due to removing SiO2 for protection formed by thermal oxidation
were performed. FeSi2 was deposited by RF sputtering. After that, lift-off due
to removing FeSi2 which exists at excess area was performed. Rapid thermal
annealing (RTA) in F.G. (N2:97%, H2:3%) ambient was performed due to
silicidation. Finally, an Al film was formed as a backside electrode by
thermal evaporation. Finally,
Figure 2.1 Fabrication procedure of -FeSi2/Si heterojunction solar cell
p-Si(100) Sub (~1015 cm-3)
SPM and HF cleaning
Deposition by RF sputtering (silicide semiconductor)
Lift-off
Backside Al contact
Silicidation by RTA in F.G.
Diode patterning
BHF etching of SiO2
Thermal oxidation for isolation
Chapter 2 Fabrication and characterization
21
2.2 Experimental details
2.2.1 SPM cleaning and HF treatment
Various contaminations such as particles and organic substances are
produced during semiconductor manufacturing process. They become a cause
of false operation. Therefore, surface treatment and cleaning are important
and unavoidable during device fabrication. SPM cleaning is one of the
effective cleaning methods. Hydrogen peroxide solution (H2O2) and sulfuric
acid (H2SO4) (H2O2 : H2SO4=1 : 4) are used as cleaning liquid. The substrates
were dipped in this liquid which is kept at 150 oC for 5 minutes. Because of
its oxidizability, particles and organic substance are oxidized and separated
from the surface of Si substrate. Then, the samples were rinsed in DI water.
After that, they were dipped in hydrofluoric acid (1% HF) for 1 min to remove
native and oxidized SiO2 during SPM.
2.2.2 RF magnetron sputtering
-FeSi2 is deposited by radio frequency (RF) magnetron sputtering with
Ar gas. An RF with 13.56 MHz is applied between substrate side and target
side. Because of difference of mass, Ar ions and electrons are separated. A
magnet is set underneath the target, so that the plasma damage is
minimized. Electrons run through the circuit from substrate side to target
side, because substrate side is subjected to be conductive and target side is
subjected to be insulated. Then, target side is negatively biased and Ar ions
hit the target.
Chapter 2 Fabrication and characterization
22
Figure 2.2 Schematic illustration of RF magnetron sputtering
2.2.3 Photolithography and Metal etching
The photolithography process during the device fabrication was utilized.
First of all, a thin uniform positive photoresist layer of S1805 was coated on
the samples by spin coating followed by baking at 115 oC for 5 minutes on a
hot plate. Next, the samples were aligned and exposed through e-beam
patterned hard-mask with high-intensity ultraviolet (UV) light at 405 nm
wavelength. For positive resists, exposure to the UV light changes the
chemical structure of the resist so that it becomes more soluble in the
developer. Exposed samples were developed by the specified developer
(NMD-3). The exposed resist is washed away by the developer solution,
leaving windows of the bare underlying material. Therefore, contains an
exact copy of the pattern which is to remain on the wafer. Post baking was
done at 130 oC for 10 minutes.
Ar+
Substrate
Plasma
Target
~
Permanent magnet
e-Ar+
e-
Chapter 2 Fabrication and characterization
23
2.2.4 Lift-off process
Lift-off is the process which selectively removes deposited films.
Following photolithography and deposition, resists and deposited films
which exist on excess area are left by ultrasonic cleaning with acetone.
2.2.5 Rapid thermal annealing (RTA)
Rapid thermal annealing (RTA) is performed for silicidation. In this
study, QHC-P610CP (ULVAC RIKO Co. Ltd) is used as RTA equipment. The
annealing was performed by six infrared lamps surrounding the sample
stage made of carbon coated SiC. The heating temperature was controlled by
thermocouple feedback. Heating chamber is filled with F.G. to interfere with
oxidation. In this study, the time of elevated temperature is 30 seconds and
the time of annealing is 5 minutes.
2.2.6 Vacuum evaporation for Al deposition
Al for backside electrodes is deposited by vacuum evaporation in vacuum
chamber at a base pressure up to 1.0×10-3 Pa. Al source is set on tungsten
(W) boat and heated up to boiling point of Al by joule heating. However,
melting point of W is higher than boiling point of Al, W boat doesn’t melt.
Chamber pressure was kept under 4×10-3 Pa. Figure 2.3 shows illustration of
Al deposition.
Chapter 2 Fabrication and characterization
24
Figure 2.3 Schematic illustration of vacuum evaporation
Figure 2.4 Schematic illustration of -FeSi2 Schottky solar cell fabrication
process
W boat
sample
Quartz thickness monitor
Al source
Large current
Al
Al
Al
Al
~ 10-3 Pa
p+-Si(100)
SiO2 SiO2 400 nm
p+-Si(100)
SiO2 SiO2SiO2
p+-Si(100)
SiO2 SiO2
p+-Si(100)
SiO2 SiO2
p+-Si(100)
SiO2 SiO2
semiconductor silicide
p+-Si(100)
SiO2 SiO2
p+-Si(100)
SiO2 SiO2
Al
Resist
SPM and HF cleaning
Diode patterning
BHF etching of SiO2
Thermal oxidation to protect Si surface
Lift-off patterning
HF treatment
Deposition by RF sputtering in Ar
Lift-off by ultrasonic cleaning with acetone
Backside Al contact by vacuum evaporation
Chapter 2 Fabrication and characterization
25
2.3 Characterization Method
2.2.1 Fourier transform infrared spectroscopy (FT-IR)
In general, Fourier transform infrared spectroscopy (FT-IR) is used to
examine how molecules are formed by observing an infrared spectrum
originated from molecular vibration. However, we use this method to extract
the bandgap.
We measure absorption characteristics by transmission method. Figure
2.5 shows illustration of this method. Light is detected through air, sample
and stage. We measured the background absorption to remove the excess
absorption of air and stage before samples are measured.
Figure 2.5 FT-IR transmission method
The measured spectrum divided from background gives a transmittance.
Absorption coefficient is defined as
α = −1
𝑑𝑙𝑛 (
𝐼
𝐼0) =
1
𝑑𝑙𝑛 (
1
𝑇) (2.1)
where I0 and I are intensity of light before and after incidence on a sample, d
is thickness of a sample, T is transmittance.
Light source
Detector
stagesample
Chapter 2 Fabrication and characterization
26
For direct transition semiconductor, absorption coefficient is given by
𝛼𝑑𝑖𝑟 =𝐴√ℏ𝜔−𝐸𝑔
ℏ𝜔 (2.2)
(𝛼𝑑𝑖𝑟ℏ𝜔)2 = 𝐴2(ℏ𝜔 − 𝐸𝑔) (2.3)
where ℏis photon energy, and Eg is bandgap.
Therefore, the bandgap is extracted by measuring absorption coefficient.
2.2.2 Four-point probe method
The most common method for measuring resistivity is the four-point
probe method [2.1, 2.2]. A small contact current is passed through the outer
two probes and the voltage is measured between the inner two probes. For a
thin wafer with thickness W which is much less than either the length a or
the width d (Figure 2.5), the sheet resistance Rsh is given by
𝑅𝑠ℎ =𝑉
𝐼𝐶𝐹 (2.4)
in units of W/sq., where CF is the correction factor.
The resistivity is then
ρ = 𝑅𝑠ℎ𝑊 (2.5)
In the limit of d ≫ S, where S is the probe spacing, the correction factor
CF becomes /ln2 (=4.54).
Chapter 2 Fabrication and characterization
27
Figure 2.5 Current flow and voltage measurement of four-probe method
2.2.3 Transmission Line Model (TLM)
The transmission line model (TLM) is a well-known classical method for
measuring the sheet and contact resistance [2.3, 2.4]. This method
determines the specific contact resistivity which is not the resistance of the
metal-semiconductor interface alone, but it is practical quantity describing
the real contact.
When current flows from the semiconductor to metal, it encounters the
resistances c and Rsh in Figure 2.7, choosing the path of least resistance.
The potential distribution under the contact is determined by both c and Rsh
according to [2.5]
V(x) =𝐼√𝑅𝑠ℎ𝜌𝑐
𝑍
cosh[(𝐿−𝑥)/𝐿𝑇]
sinh(𝐿/𝐿𝑇) (2.6)
where L is the contact length, Z the contact width, and I the current flowing
into the contact.
The “1/e” distance of the voltage curve is defined as the transfer length
LT given by
𝐿𝑇 = √𝜌𝑐/𝑅𝑠ℎ (2.7)
a
d
W
V
S
Chapter 2 Fabrication and characterization
28
This length can be thought of as that distance over which most of the
current transfers from semiconductor into the metal or from the metal into
the semiconductor.
Figure 2.7 Current transfer from semiconductor to metal represented by the
arrows. The semiconductor/metal contact is represented by the c-Rsh
equivalent circuit with the current choosing the path of least resistance.
Figure 2.8 shows TLM test structure. When the voltage is measured in
the ladder structure between contacts 1 and 4, for example, the current flow
may be perturbed by contacts 2 and 3. The effect of contacts 2 and 3 depends
on the transfer length LT and the contact length L. For L ≪ 𝐿𝑇, the current
does not penetrate appreciably into the contact metal and contacts 2 and 3
have no effect on the measurement. For L ≫ 𝐿𝑇, the current does flow into
the metal and the contact can be thought of as two contacts, each of length LT,
joined by a metallic conductor [2.6].
Chapter 2 Fabrication and characterization
29
Figure 2.8 Basic structure of TLM
For contacts with L ≥ 1.5𝐿𝑇 and for a front contact resistance
measurement of the structure in Figure 2.8, the total resistance RT between
any two contacts is
𝑅𝑇 =𝑅𝑠ℎ𝑑
𝑍+ 2𝑅𝑐 (2.8)
The total resistance is measured for various contact spacing and plotted
versus d as illustrated in Figure2.9. Three parameters can be extracted from
such a plot. The slope Δ𝑅𝑇/Δd = 𝑅𝑠ℎ/𝑍 leads to the sheet resistance Rsh with
the contact width Z independently measured. The intercept at d=0 is RT=2Rc
giving the contact resistance Rc.
Figure 2.9 A plot of total resistance as a function of contact spacing
1 2 3 4 5
Z W
L d1 d2 d3 d4
0 d
RT
2Rc
Slope = Rsh/Z
Chapter 2 Fabrication and characterization
30
2.2.4 Van der Pauw method
Van der Pauw method is a useful method to measure the resistivity,
carrier density, and mobility. Figure 2.10 shows general geometry for Van der
Pauw method. Required condition for this method is that the contacts are at
the circumference of the sample and are sufficiently small, the sample is
uniformly thick, and does not contain isolated holes.
Figure 2.10 Basic structure of Van der Pauw Hall sample
The sample of Figure 2.10, the resistivity is given by [2.7]
ρ =𝜋𝑡
𝑙𝑛2
𝑅12,34+𝑅23,41
2𝐹 (2.9)
where R12,34 = V34/I. The current I enters the sample through contact 1 and
leaves through contact 2 and V34 = V3-V4 is the voltage between contacts 3
and 4. R23,41 is similarly defined. Current enters the sample through two
adjacent terminals and the voltage is measured across the other two
adjacent terminals. F is a function of the ratio Rr = R12,34/R23,41 only,
satisfying the relation
𝑅𝑟−1
𝑅𝑟+1=
𝐹
𝑙𝑛2𝑎𝑟𝑐𝑜𝑠ℎ (
𝑒𝑥𝑝(ln 2/𝐹)
2) (2.10)
For symmetric samples (circles or squares), F=1.
The hole and electron densities are given by
1 2
34
Chapter 2 Fabrication and characterization
31
p =1
𝑞𝑅𝐻; 𝑛 = −
1
𝑞𝑅𝐻 (2.11)
where RH is Hall coefficient and is defined as
𝑅𝐻 =𝑑𝑉𝐻
𝐵𝐼=
𝑡Δ𝑅24,13
𝐵 (2.12)
Δ𝑅24,13 = |𝑉13−𝑉13
0
𝐼13| (2.13)
where VH is hall voltage, t is sample thickness, B is magnetic field applied in
a direction perpendicular to the sample. V13 and V013 are the voltage between
contacts 1 and 4 with and without a magnetic field.
The mobility m is defined as the product of the Hall coefficient and
conductivity
μ = |𝑅𝐻|𝜎 =|𝑅𝐻|
𝜌 (2.14)
where is conductivity, is resistivity.
Chapter 2 Fabrication and characterization
32
Reference
[2.1] W.E. Beadle, J.C. C. Tsai, R.D. Plummer, Eds., “Quick Reference
Manual for Silicon Integrated Circuit Technology”, Wiley, New York (1985)
[2.2] F.M. Smits, “Measurement of Sheet Resistivities with the Four-Point
Probe”, Bell Syst. Tech. J., 37, 711 (1958)
[2.3] H. Murrman, D. Widmann, “Current crowding on metal contacs to
planar devices”, Electron Devices, IEEE Transactions on, 16, 12, 1022-1024
(1969)
[2.4] W. Shockley, “Research and investigation of inverse epitaxial uhf power
transistors”, Technical Documentary Report AT TDR, 64-207, AF Avionics
Laboratory, Research and Technology Division, Air Force Systems Command,
Wright-Patterson AFB (1969)
[2.5] H.H. Berger, “Models for contacts to planar devices”, Solid State
Electronics, 15, 145-158 (1972)
[2.6] L.K. Mak, C.M. Rogers, D.C. Northrop, “Specific Contact Resistance
Measurement on Semiconductor”, J. Phys. E: Sci. Instr. 22, 317-321 (1989)
[2.7] L.J. van der Pauw, “A Method of Measuring Specific Resistivity and
Hall Effect of Discs of Arbitrary Shape”, Phill. Tech. Rep., 13, 1-9 (1958)
Chapter 3 Formation of -FeSi2
33
Chapter 3
Formation of -FeSi2
3.1 Introduction
3.2 Infrared absorption characteristics of -FeSi2
3.3 X-ray diffraction pattern of -FeSi2
3.4 Conclusion
References
Chapter 3 Formation of -FeSi2
34
3.1 Introduction
As described in the chapter 1, semiconductor -FeSi2 has attracted much
attention in recent years because of its prominent photovoltaic properties
[3.1, 3.2, 3.3]. For -FeSi2 formation process, lots of works have reports. For
example, MOCVD [3.4], EB (Electron Beam) vapor [3.5], ion implantation of
Fe into Si with annealing [3.6], RDE (Reactive Deposition Epitaxy) [3.7], and
sputtering by FeSi2 target [3.8] have been used to form -FeSi2. For film
quality, there are a lot of reports about the samples formed by MBE
(Molecular Beam Epitaxy) [3.9, 3.10]. They report that the carrier density of
the formed films depends on Si and Fe composition ratio. Therefore, to
realize fine control of Fe/Si composition ratio is the key for reducing carrier
density. However, it is difficult to control Fe/Si composition ratio. Based on
the above introduction, we propose a simple multi sputtering process to form
-FeSi2 to realize fine control of Fe/Si composition ratio. The process is
simple, just multi-stacking Fe and Si layer in situ sputtering with proper
annealing. As the thickness of each film can be well controlled by sputtering
time, the composition of -FeSi2 films can be easily controlled. The purpose of
this chapter is to confirm formation of -FeSi2 by multi-stacking process.
Figure 3.1 Schematic illustration of multi-stacking process
FeSi
・・
・
annealing FeSi2
Si substrate Si substrate
1 set
Chapter 3 Formation of -FeSi2
35
3.2 Infrared absorption characteristics of -FeSi2
Figure 3.2 shows infrared absorption characteristics of -FeSi2. For the
sample (a) which is reactively formed by 20 nm-thick Fe, conspicuous
absorption cannot be observed. On the other hand, for the sample (b) with
Fe/Si stacked sputtered process and sample (c) with FeSi2 target, increase in
absorption edge around 6400 cm-1 in the spectrum can be observed while
increasing the annealing temperature from 600 to 900 oC. For the samples
(b) and (c) annealed at 1000 oC, absorption could not be confirmed. -FeSi2 is
reported to transform phase (-FeSi2) which has metal nature at high
annealing temperature [3.11]. Therefore, these samples annealed at 1000 oC
could not be -FeSi2 but -FeSi2.
We calculate difference absorption spectrum between the sample with 20
nm- thick and with 10 nm-thick.
𝐼20(𝑘) − 𝐼10(𝑘) = 𝐼𝐹𝑒𝑆𝑖2(k) (3.1)
where I20(k) and I10(k) are the spectrum intensity of the sample with
20-nm-thick and 10-nm-thick respectively.
Figure 3.3 shows the subtracted absorption spectrum ((a) is Fe/Si
stacked sample, (b) is FeSi2 target sample). Increase in the absorption edge
around 6400 cm-1 in the spectrum can be more clearly observed.
Chapter 3 Formation of -FeSi2
36
Figure 3.2 Infrared absorption characteristics of -FeSi2
(a) Fe mono layer (b) Fe/Si stacked (c) FeSi2 target
Figure 3.3 Difference absorption spectrum as a difference between the
sample with 20 nm- thick and with 10 nm-thick
(a) Fe/Si stacked (b) FeSi2 target
Wavenumber (cm-1)
4000500060007000
as depo
400℃
500℃
600℃
700℃
800℃
900℃
1000℃
Ab
so
rba
nce
(a
.u)
No
absorption edge
Wavenumber (cm-1)
α-FeSi2
4000500060007000
as depo
400℃
500℃
600℃
700℃
800℃
900℃
1000℃
absorption edge
as depo
400℃
500℃
600℃
700℃
800℃
900℃
1000℃
Wavenumber (cm-1)
4000500060007000
α-FeSi2
absorption edge
(a) (b) (c)
500250045006500
Wavenumber (cm-1)
Ab
so
rba
nce
(a
.u)
Wavenumber (cm-1)
as depo
400℃
500℃
600℃
700℃
800℃
900℃
as depo
400℃
500℃
600℃
700℃
800℃
900℃
500250045006500
6400 cm-1
6400 cm-1
(a) (b)
FeSi2 – 10 nm
Si substrate
FeSi2 – 20 nm
Si substrate
FeSi2 – 10 nm
Chapter 3 Formation of -FeSi2
37
-FeSi2 is reported to be direct transition semiconductor [3.12] and its
absorption coefficient is given by
𝛼𝑑𝑖𝑟 =𝐴√ℏ𝜔−𝐸𝑔
ℏ𝜔 (3.2)
(𝛼𝑑𝑖𝑟ℏ𝜔)2 = 𝐴2(ℏ𝜔 − 𝐸𝑔) (3.3)
where ℏis photon energy, Eg is bandgap. Therefore, the bandgap is
extracted by measuring absorption coefficient. Figure 3.4 shows magnified
absorption edge of samples annealed at 600 oC, where band gap of 0.8 eV can
be extracted.
Figure 3.4 Bandgap calculation of -FeSi2 (a) Fe/Si stacked (b) FeSi2 target
(600 oC, 5min
annealing in N2)
0
1
2
3
4
5
6
7
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Photon energy (eV)
Eg=0.8eV
×
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Photon energy (eV)
0
1
2
3
4
5
6
7
(600 oC, 5min
annealing in N2)
Eg=0.8eV
×
Fe/Si stacked FeSi2 target
(a) (b)
Chapter 3 Formation of -FeSi2
38
3.3 X-ray diffraction pattern of -FeSi2
Figure 3.5 shows θ-2θ x-ray diffraction (XRD) pattern of the sample
annealed at 600 oC. Strong peaks related to -FeSi2 can be observed.
Figure 3.5 X-ray diffraction pattern of -FeSi2
3.4 Conclusion
In order to achieve the fine control of Fe and Si composition ratio, we
propose multi-sputtering process. The formation of -FeSi2 formed by our
process was confirmed by FT-IR and XRD measurement. From FT-IR result
of the films formed by Fe/Si stacking and FeSi2 target, absorption edge of 0.8
eV was obtained. And also, from XRD, strong peaks related to -FeSi2 were
observed. Therefore, we can conclude that -FeSi2 can be formed with our
multi-sputtering process.
2θ (deg)
10 20 30 40 50 60 70 80 90 100
(600℃ 5min
annealing in N2)
Inte
nsity (
a.u
) FeS
i 2(2
02)/
(220)
FeS
i 2 (4
22)
FeS
i 2 (004)/
(040)
FeS
i 2 (133)
orthorhombus
Chapter 3 Formation of -FeSi2
39
References
[3.1] Z. X. Liu, M. Watanabe, M. Hanabusa, “Electrical and photovoltaic
properties of iron-silicide/silicon heterostructure formed by pulsed laser
deposition”, Thin Solid Films, 381, 262, (2001)
[3.2] Y. Fukuzawa, T. Ootsuka, N. Otogawa, H, Abe, Y. Nakayama, Y. Makita,
Proc. SPIE, 6197, 61970N (2006)
[3.3] M. Shaban, K. Nomoto, S. Izumi, T. Yoshitake, IEEE Electron Device
Lett, 31, 1428 (2010)
[3.4] M. Suzuno, K. Akutsu, H. Kawakami, K. Akiyama, T. Suemasu,
“Metalorganic chemical vapor deposition of -FeSi2 on -FeSi2 seed crystals
formed on Si substrates”, Thin Solid Fioms, 519, 24, 8473-8476 (2011)
[3.5] D.H. Tassis, C.L. Mitsas, T.T. Zorba, M. Angelekeris, C.A. Dimitriadis,
O. Valassiades, Di.I. Siapkas, G. Kiriakidis, Semicond. “Optical and
electrical characterization of high quality -FeSi2 thin films grown by solid
phase epitaxy”, Applied Surface Science, 102, 178-183 (1996)
[3.6] M. Sugiyama, Y. Maeda, “Microstructure characterization of ion-beam
synthesized -FeSi2 phase by transmission electron microscopy”, Thin Solid
Films, 381, 256 (2001)
[3.7] M. Tanaka, Y. Kumagaya, T. Suemasu, F. Hasegawa, “Formation of
-FeSi2 Layers on Si(001) Substrates”, Jpn. J. Appl. Phys, 36, 3620-3624
(1997)
[3.8] D. Tan, C.T. Chua, G.K. Dalapati, D.Chi, “Effect of Al incorporation on
the crystallization kinetics of amorphous FeSi2 into poly -FeSi film on
Chapter 3 Formation of -FeSi2
40
SiO2/Si(100) substrate”, Thin Solid Films, 98, 013507 (2011)
[3.9] T. Suemasu, K. Takakura, C. Li, Y. Ozawa, Y. Kumagai, F. Hasegawa,
“Epitaxial growth of semiconducting -FeSi2 and its application to
light-emitting diodes”, Thin Solid Films, 461, 209-218 (2004)
[3.10] N. Seki, K. Takakura, T. Suemasu, F. Hasegawa, “Conduction type and
defect levels of -FeSi2 films grown by MBE with different Si/Fe ratios”,
Materials Science in Semiconductor Processing, 6, 5-6, 307-309 (2003)
[3.11] F.X. Zhang, S. Saxena, “Phase stability and thermal expansion
property of FeSi2”, Scripta Materialia, 54, 1375-1377 (2006)
[3.12] L. Wang, C. Lin, X. Chen, S. Zou, L. Qin, H. Shi, W.Z, Shen, M. Osling,
“A clarification of optical transition of -FeSi2 film”, Solid State
Communications, 97, 5, 385-388 (1996)
Chapter 4 Electrical characteristics of -FeSi2
41
Chapter 4
Electrical characteristics of -FeSi2
4.1 Introduction
4.2 Resistivity measurement by four-point method
4.3 Resistivity measurement by TLM
4.4 Carrier density measurement by van der Pauw
4.5 Conclusion
References
Chapter 4 Electrical characteristics of -FeSi2
42
4.1 Introduction
One of the issues of -FeSi2 is that sufficient depletion width cannot be
formed because carrier density of -FeSi2 is too high [4.1]. Moreover, the
origin of carriers has not yet been clarified. In this chapter, we discuss the
origin of carriers.
The energy level of the carriers and its concentration can be extracted by
measuring the temperature dependent resistivity. The resistivity and
carrier density n are given by
ρ =1
𝑞𝑛𝜇 (4.1)
n = √𝑁𝑐𝑁𝑑𝑒−
𝐸𝑐−𝐸𝑑2𝑘𝑇 = √𝑁𝑐𝑁𝑑𝑒
−𝐸𝑎
2𝑘𝑇 (4.2)
where q is electronic charge, m mobility, Nc effective density of state of
conduction band, Nd donor impurity density, k Boltzmann’s constant, T
absolute temperature, and Ea activation energy.
Therefore, ln is expressed by
𝑙𝑛𝜌 =𝑞𝐸𝑎
2𝑘
1
𝑇− (𝑙𝑛𝑞 +
1
2𝑙𝑛𝑁𝑐𝑁𝑑 + 𝑙𝑛𝜇) (4.3)
Figure 4.1 shows an example of resistivity on inverse absolute
temperature (Arrhenius’ plot). The slope of the resistivity corresponds to the
activation energy and the intercept to y-axis is the relative concentration of
the level. Therefore, we can speculate the origin of defect and its
concentration by changing the composition or annealing condition.
Chapter 4 Electrical characteristics of -FeSi2
43
Figure 4.1 Extraction of defect level position and concentration
4.2 Resistivity measurement by four-point method
4.2.1 Resistivity control by Si/Fe composition ratio
Table 4.1 shows prepared samples whose thickness of Si layer was
changed from Fe-rich condition to Si-rich to confirm compositional control of
-FeSi2. The film resistivity was measured by four point probe method.
10-1
1
10R
esis
tivity (
・cm
)
1000/T (K-1)102 4 6 8 12 14 16 18 20 22
1
2
Arrhenius’ plot
slope : activation energy
intercept : relative defect concentration (under constant m)
Chapter 4 Electrical characteristics of -FeSi2
44
Table 4.1 Compositional control of -FeSi2
Figure 4.2 shows resistivity control of the films after annealing at 800 oC.
By changing the composition ratio, it was confirmed that the resistivity is
dependent on composition ratio. When Fe and Si ratio is 1 to 2.25, a largest
resistivity of -FeSi2 has been obtained.
Figure 4.2 Resistivity dependence of -FeSi2 on Si/Fe composition ratio
sampleFe thickness
(nm)
Si thickness
(nm)
sets
(number of)
Si/Fe
composition
A 2.0 5.1 10 1.50
B 2.0 6.8 10 2.00
C 2.0 7.6 10 2.25
D 2.0 8.5 10 2.50
Assumed Si/Fe atomic ratio1.0 1.5 2.0 2.5 3.0
Re
sis
tivity(・cm
)
10-1
1
1:1.50
1:2.00
1:2.25
1:2.50
A
B
C
D
(800℃ 5min
annealing in F.G.)
Chapter 4 Electrical characteristics of -FeSi2
45
4.2.2 Extraction of defect level position
The energy level of the carriers and its concentration can be extracted by
measuring the temperature dependent resistivity. Figure 4.3 shows
resistivity change of -FeSi2 film with composition ratio of 2.25, annealed at
800 oC. By fitting resistivity change on temperature, we can extract 4 levels;
shallow level of 13 and 51 meV and 0.11 and 0.2 eV.
Figure 4.3 Resistivity dependence of -FeSi2 on temperature
Theoretical calculation has revealed that the lowest position of the
defective levels of Si vacancy exist ~ 0.2 eV higher from the top of the valence
band, while the highest position of the defective levels of Fe vacancy exist ~
0.3 eV lower from the bottom of the conduction band [4.2]. On the other hand,
the formation energy is given by [4.3, 4.4]
Δ𝐸𝑓𝑜𝑟𝑚𝑎𝑡𝑖𝑜𝑛 = [𝐸𝑇(𝑑𝑒𝑓𝑒𝑐𝑡𝑖𝑣𝑒) + 𝜇𝑥] − 𝐸𝑇(𝑝𝑒𝑟𝑓𝑒𝑐𝑡) (4.4)
where ET (defective) is total energy of the defective unit cell with a vacancy of
Si or Fe, mx (x=Si, Fe) is the chemical potential of Si or Fe, ET (perfect) is the
total energy of the perfect crystal. The chemical potentials can be varied
within a range limited by the three constraints:
1000/T (K-1)102 4 6 8 12 14 16 18 20 22
10-1
1
10
Resis
tivity (
・cm
)
Ea2 = 110 meV
Ea3 = 51 meV
Ea4 = 13 meV
Ea1 = 200 meV
Fe : Si = 1 : 2.25
800 oC, 5min annealing in F.G.
Ea4
Ec
EvEa1 Ea2 Ea3
defect activation energy
Ea1 200 meV
Ea2 110 meV
Ea3 51 meV
Ea4 13 meV
Chapter 4 Electrical characteristics of -FeSi2
46
𝜇𝑆𝑖 ≤ 𝜇𝑆𝑖(𝑏𝑢𝑙𝑘) (4.5)
𝜇𝐹𝑒 ≤ 𝜇𝐹𝑒(𝑏𝑢𝑙𝑘) (4.6)
2𝜇𝑆𝑖 + 𝜇𝐹𝑒 = 𝜇𝐹𝑒𝑆𝑖2(𝑏𝑢𝑙𝑘) (4.7)
where 𝜇𝐹𝑒𝑆𝑖2(bulk), the chemical potential of the bulk FeSi2, is a constant
value calculated as the total energy per FeSi2 unit formula.
The formation energies were calculated under the two extreme
conditions, the Si-rich limit [𝜇𝐹𝑒 = 𝜇𝐹𝑒𝑆𝑖2(𝑏𝑢𝑙𝑘) − 2𝜇𝑆𝑖(𝑏𝑢𝑙𝑘) 𝑎𝑛𝑑 𝜇𝑆𝑖 = 𝜇𝑆𝑖(𝑏𝑢𝑙𝑘)]
and Fe-rich limit [𝜇𝑆𝑖 = 1/2(𝜇𝐹𝑒𝑆𝑖2(𝑏𝑢𝑙𝑘) − 𝜇𝐹𝑒(𝑏𝑢𝑙𝑘) 𝑎𝑛𝑑 𝜇𝐹𝑒 = 𝜇𝐹𝑒(𝑏𝑢𝑙𝑘))] . The
formation energies of the native point defects depend largely on the atomic
chemical potentials of Fe and Si. The silicon vacancy exhibits the lowest
formation energy at the Fe-rich limit and the iron vacancy exhibits the
lowest formation energy at the Si-rich limit. At the Si-rich limit, the
calculated Δ𝐸𝑓𝑜𝑟𝑚𝑎𝑡𝑖𝑜𝑛 values are 2.258, 2.249, 3.133 and 2.180 eV for
removing SiⅠ, SiⅡ, FeⅠ, and FeⅡ from the perfect crystal, respectively. At the
Fe-rich limit, the calculated Δ𝐸𝑓𝑜𝑟𝑚𝑎𝑡𝑖𝑜𝑛 values are 1.172, 1.163, 5.306 and
4.353 eV for removing SiⅠ, SiⅡ, FeⅠand FeⅡ from the perfect crystal,
respectively. The formation energies of Si vacancy in -FeSi2 are smaller
than those of Fe vacancy except near the region of the Si-rich limit, which
implies the formation of Si vacancy in -FeSi2 can be more favorable than the
formation of Fe vacancy. Therefore, the observed defect level of 0.2 eV can be
thought to be originated from Si vacancy.
4.2.3 Extraction of defect concentration
Figure 4.4 shows Arrhenius’s plot dependency of -FeSi2 on annealing
temperature. As the annealing temperature increases, the intercept of
Arrhenius’s plot increases keeping its slope constant. This indicates that the
concentration of each defect reduces by annealing process. However, the
increased amounts of intercept vary by each level.
Chapter 4 Electrical characteristics of -FeSi2
47
Figure 4.4 Change of Arrhenius’s plot by annealing
Indeed, Figure 4.5 shows changes of each defect concentration. The
defect level of 0.2 eV and 0.11 eV showed slight reduction by 23 % with
higher annealing temperature, while shallow level showed large reduction
by 63 and 96 % for 51 and 13 meV, respectively. As all the films have the
same composition ratio, the first two levels are estimated to be related to
composition of -FeSi2. On the other hand, the shallow levels can be thought
to be related to crystalline defects, which can be recovered by higher
temperature annealing.
2 4 6 8 10 12 14 16 18 20 22
1000/T (K-1)
10-1
1
10
102
103
Re
sis
tivity (
・cm
)
Ea2 = 110 meV
Ea3 = 51 meV
Ea4 = 13 meV
Ea1 = 200 meV
High
Low
annealing
temp
Fe : Si = 1 : 2.25
Chapter 4 Electrical characteristics of -FeSi2
48
Figure 4.5 Reduction of defect concentration by annealing
4.2.4 Effects of sputtering atmosphere
To extract the effect of sputtering atmosphere, we deposit -FeSi2 films
in various atmosphere; (i) Ar (40 sccm), (ii) Ar (40sccm) + O2 (0.17 sccm), (iii)
Ar + N2 (0.15 sccm), (iv) Kr (7 sccm). Figure 4.6 shows oxygen, nitrogen and
krypton effects on resistivity of -FeSi2. To introduce either oxygen or
nitrogen causes the resistivity of -FeSi2 to be decreased by two orders of
magnitude. These results suggest that residual oxygen or nitrogen species
are one possible cause of high carrier density of -FeSi2. On the other hand,
the resistivity of -FeSi2 showed two orders of magnitude higher values by
changing sputtering gas from Ar to Kr. As for Si, it is reported that
sputtering rate of Kr is lower than Ar [4.5]. Moreover, recoil particle is
provided and dense film is formed by Kr sputtering [4.6]. It could be one of
the key to reduce residual gas in the film for low carrier density of -FeSi2.
750 800 825 8500
0.2
0.4
0.6
0.8
1
Re
lative
de
fect co
nce
ntr
atio
n
Annealing temperature (oC)
a1a2a3a4
-23%
-63%
-96%
Chapter 4 Electrical characteristics of -FeSi2
49
Figure 4.6 Sputtering gas effects on resistivity of -FeSi2
4.3 Resistivity measurement by TLM
In this section, the resistivity of -FeSi2 films was measured by TLM
method. Figure 4.7 shows the total resistance RT at 2 V as a function of
contact spacing d. A linear approximation of the measured resistance (black
line) is then extrapolated. From the slope and intercepts to y-axis of this
line, sheet resistance Rsh (≈ 3.7×105 /sq.) and contact resistivity c (≈ 63.5
・cm) were extracted. This result of sheet resistance matches that measured
by previous four-point method. Therefore, contact resistivity can be ignored
in the result measured by four-point method.
Ar only Ar + O2 Ar + N2 Kr
1
10-1
10-2
10-3
10-4
10
102
Re
sis
tivity (
・cm
)
Sputtering atmosphere
800 oC, 5min
annealing in F.G.
Chapter 4 Electrical characteristics of -FeSi2
50
Figure 4.7 Total resistance as function of contact spacing
4.4 Carrier density measurement by van der Pauw
In this section, the carrier density and resistivity of the -FeSi2 films were
evaluated by Hall measurement using the van der Paw method. The applied
magnetic field was 0.3 T. Figure 4.8 shows typical hall voltage. The hall
coefficient RH is defined as [4.7]
𝑅𝐻 =𝑑𝑉𝐻
𝐵𝐼 (4.8)
We measured hall voltage by changing the direction of the magnetic field
and calculated hall coefficient. From hall coefficient, it is confirmed that the
formed -FeSi2 is n-type and the carrier density of -FeSi2 is 7.66×1018 cm-2.
-FeSi2
d
20 40 60 80 10000
2
4
6
10
12
14
16To
tal r
esis
tan
ce @
2V
(
)
Contact spacing (mm)
800 oC, 5min annealing in F.G.
Thickness : 80 nm
Chapter 4 Electrical characteristics of -FeSi2
51
Figure 4.8 Typical I-V characteristics for carrier density measurement using
van der Pauw
By using same patterned sample, we can measure resistivity too. Figure
4.9 shows typical I-V characteristics for resistivity measurement. Current
enters the sample through two adjacent terminals and the voltage is
measured across the other two adjacent terminals. Resistivity of 3.45 ・cm
was extracted by measuring I-V characteristics for four times as it was
mentioned in chapter 2.
0 1 2 3-1-2-3Current (mA)
0
2
4
6
8
-2
-4
-6
-8
Hal
l vo
ltag
e (m
V)
800 oC, 5minannealing in F.G.
Magnetic field : 0.3T
Chapter 4 Electrical characteristics of -FeSi2
52
Figure 4.9 Typical I-V characteristics for resistivity measurement using van
der paw
The mobility m is defined by
μ =|𝑅𝐻|
𝜌 (4.9)
From above results, electron mobility of -FeSi2 is 23.6 cm2/Vs.
Figure 4.10 shows resistivity of -FeSi2 dependency on film thickness.
Resistivity ranges from 3.45 to 16.8 ・cm in all samples. Figure 4.11 shows
carrier density and mobility of -FeSi2 dependency on film thickness. Carrier
density is reduced with thicker film thickness, while mobility is increased.
0 1 2 3 4 5-1-2-3-4-5-6
-4
-2
0
2
4
6
Current (mA)
Volta
ge (
V)
800 oC, 5minannealing in F.G.
×10-1
Chapter 4 Electrical characteristics of -FeSi2
53
Figure 4.10 Resistivity of -FeSi2 dependence on film thickness
Figure 4.11 Carrier density and mobility of -FeSi2 dependence on film
thickness
0 100 200 300 400 500 600
Film thickness (nm)
0
4
8
12
16
Res
isti
vity
(
・cm
)
80 nm
300 nm
500 nm
0 100 200 300 400 500 600
Film thickness (nm)
1017
1018
1019
Car
rier
den
sity
(cm
-2)
20
25
30
35
40
Mo
bili
ty (
cm2/V
s)
80 nm
300 nm
500 nm
mobility
carrier density
Chapter 4 Electrical characteristics of -FeSi2
54
4.5 Conclusion
In order to confirm compositional control of -FeSi2, we changed the
thickness of Si and Fe layer and measured the film resistivity by four point
probe method. It is revealed that the resistivity is dependent on composition
ratio. With Si and Fe ratio of 2.25, a largest resistivity of -FeSi2 has been
obtained. From temperature dependency, four kinds of defect levels have
been extracted. Moreover, it is detected that they can be categorized into two
types; one related to composition of -FeSi2 and the other related to
crystalline defects. For sputtering atmosphere, oxygen and nitrogen caused
decrease of resistivity of -FeSi2. Therefore, to control the composition ratio
and reduce residual gases in the film are the key to achieve low carrier
density of -FeSi2.
The resistivity of -FeSi2 was measured by not only four-point method
but also TLM method. The results measured by four-point method and TLM
match. Therefore, it is no problem to discuss resistivity of films measured by
four-point method.
Carrier density and mobility were evaluated by Hall measurement using
the van der Paw method. Carrier density and mobility of -FeSi2 with 80 nm
thickness were 7.66× 1018 cm-2 and 23.6 cm2/Vs respectively. For film
thickness dependency, carrier density is decreased and mobility is increased
with thicker thickness.
Chapter 4 Electrical characteristics of -FeSi2
55
Reference
[4.1] K. Takakura, T. Suemasu, Y. Ikura, F. Hasegawa, “Control of the
control type of nondoped high mobility b-FeSi2 films grown from Si/Fe
multilayers by change of Si/Fe ratios, Jpn J. Appl. Phys, 309, 233-236 (2000)
[4.2] J. Tani, H. Kido, “First-principle study of native point defects in
-FeSi2”, Journal of Alloys and Compounds, 352 153-157 (2003)
[4.3] S.B. Zhang, J.E. Northrup, “Chemical potential dependence of defect
formation energies in GaAs: Application to Ga self-diffusion, Phys. Rev. Lett,
67, 2339 (1991)
[4.4] D.B. Laks, V. Meregalli, “Theory of FeSi2 direct gap semiconductor on
Si(100)”, Phys. Rev. B, 45,10965 (1992)
[4.5] R. Ohki, Y. Hoshi, “Properties of ITO thin films sputter-deposited by
using various kinds of rare gases”, Technical report of IEICE, CPM 99-91
(1999)
[4.6] J.A. Thornton, D. W. Hoffman, “ Internal stresses in amorophous silicon
films deposited by cylindrical magnetron sputtering using Ne, Ar, Kr, Xe, and
Ar+H2”, J. Vac. Sci. Technol., 18, 203 (1981)
[4.7] D.K. Schroder, “SEMICONDUCTOR MATERIAL AND DEVICE
CHARACTERIZATION Third Edition”, 466-471 (2006)
Chapter 5 Demonstration of -FeSi2 Schottky solar cell
56
Chapter 5
Demonstration of -FeSi2
Schottky solar cell
5.1 Introduction
5.2 I-V characteristics of -FeSi2/p+-Si Schottky solar cell
5.3 Equivalent circuit
5.4 Conclusion
Reference
Chapter 5 Demonstration of -FeSi2 Schottky solar cell
57
5.1 Introduction
As described in chapter 1, -FeSi2 has been attracted as solar cell
material. The highest efficiency of -FeSi2 is 3.7 % under AM 1.5, 100
mW/cm2 illumination so far [5.1]. The open circuit voltage was 0.45 V, short
circuit current density is 14.8 mA/cm2, and fill factor is 0.55. However, it is
-FeSi2/Si heterojunction solar cell; fabricated on p-type silicon substrate. An
expanse of depletion layer of -FeSi2 is much smaller than that of Si because
the carrier density of Si and -FeSi2 are ~1015 cm-3 and ~1018 cm-3
respectively. In other words, light response of this sample could be caused by
Si rather than -FeSi2.
We fabricated Schottky solar cell to confirm light response of -FeSi2
itself. In this study, we used p+-Si whose impurity concentration is ~1018 cm-3
instead of metal. An activation energy of Si is ~20 meV. On the other, an
activation energy of -FeSi2 is ~ 200 meV. Therefore, if -FeSi2/p+-Si Schottky
solar cell sample is cooled, the carrier density of -FeSi2 decreases and that
of Si is not changed.
Figure 5.1 Schematic device structure of -FeSi2/p+-Si schottky solar cell
p+-Si
SiO2 SiO2
Al
-FeSi2
Schottky
Chapter 5 Demonstration of -FeSi2 Schottky solar cell
58
5.2 J-V characteristics of -FeSi2/Si Schottky solar cell
For 200×200 mm2 without any surface protection or anti-reflection
coating, Figure 5.2 compares the J-V characteristics of -FeSi2/p+-Si Schottky
solar cell under dark and light conditions at 90 K. Not only the forward
current but also reverse current were increased by light illumination. This
could be because band-to-band tunneling probability of carriers was
increased by light excitation. The open-circuit voltage Voc and short-circuit
current density Jsc were not verified. Therefore, we cannot confirm power
generation of -FeSi2/p+-Si solar cell.
Figure 5.2 J-V characteristics of -FeSi2/p+-Si Schottky solar cell
0 0.2-0.2-0.4-0.6-0.8-1.0
Voltage (V)
0
-40
-30
-20
-10
Cu
rrent density (
mA
/cm
2)
light
dark
800 oC, 5min
annealing in F.G.
90 K
Chapter 5 Demonstration of -FeSi2 Schottky solar cell
59
5.3 Equivalent circuit of solar cell
The J-V characteristic of an illustrated solar cell that behaves as the
ideal diode is described by [5.2]
J = 𝐽0 [𝑒𝑥𝑝 (𝑞𝑉
𝑘𝑇) − 1] − 𝐽𝑝ℎ (5.1)
This behavior can be described by a simple equivalent circuit, in which a
diode and a current source are connected in parallel. The firs term in Eq.
(5.1) describes the dark diode current density and the second term describes
the photo-generated current density. In practice, the FF is influenced by the
shunt resistance, Rp, of a solar cell. The leakage current which is
characterized by the shunt resistance causes the voltage drop. The influence
of this parameter on the J-V characteristic of the solar cell can be studied
using the equivalent circuit presented in Figure 5.3. The J-V characteristic of
the one-diode equivalent circuit with the shunt resistance is described by
J = 𝐽0 [𝑒𝑥𝑝 (𝑞𝑉
𝑘𝑇− 1)] +
𝑉
𝑅𝑝− 𝐽𝑝ℎ (5.2)
Figure 5.3 The equivalent circuit of a solar cell with shunt resistance Rp
V
+
-A
Iph Rp
Chapter 5 Demonstration of -FeSi2 Schottky solar cell
60
In order to testify the light response of -FeSi2 thin film, we subtracted
the value of shunt resistance Rp from the measured current. That is
𝐽𝑖𝑑𝑒𝑎𝑙 = 𝐽𝑡𝑜𝑎𝑙 −𝑉
𝑅𝑝 (5.3)
where Jtotal is measured current.
Figure 5.4 shows subtracted current density characteristic. This result
revealed that -FeSi2 film has the ability of solar energy conversion. The
leakage current which passes the shunt resistance is caused by the current
through local defects in junction. Figure 5.5 shows shunt resistance
dependency on measurement temperature. The shunt resistance showed
reduction with lower measurement temperature. This indicates that it has
semiconductor nature. However, conversion efficiency is much smaller than
theoretical efficiency. In a practical solar cell, the series resistance Rs is
included in addition to shunt resistance. Figure 5.6 shows the equivalent
circuit including the series resistance. The series resistance is introduced by
the resistance of the main current path through which the photo-generated
carriers arrive to the external circuit. The contribution to the series
resistance comes from the bulk resistance of the junction, the contact
resistance between the junction and electrodes, the resistance of the
electrodes. The four defect levels extracted in chapter 4 could work as
efficient trap centers for photo-generation carriers [5.3, 5.4]. This results in
high series resistance and large leakage current. Therefore, it could be the
key to reduce the defect concentration for high conversion efficiency.
Chapter 5 Demonstration of -FeSi2 Schottky solar cell
61
Figure 5.4 J-V characteristic of -FeSi2/p+-Si Schottky solar cell. In this
result, we subtracted the current which passes the shunt resistance.
Figure 5.5 Shunt resistance dependence on temperature
0-10-20-30-40-50
Voltage (mV)-60 10
0
2
4
6
8
-2
-4
-6
-8
-10
Cu
rre
nt d
en
sity (m
A/c
m2)
light
Voc Jsc
V
+
-A
IphRp
ideal solar cell
0 100 200 300
40K
90K
300K
104
105
106
107
Temperature (K)
Shu
nt
resi
stan
ce (
)
Chapter 5 Demonstration of -FeSi2 Schottky solar cell
62
Figure 5.6 The equivalent circuit of a solar cell with shunt resistance Rp and
series resistance Rs
5.4 Conclusion
We measured J-V characteristic of -FeSi2/p+-Si Schottky solar cell at 90
K to confirm light response of -FeSi2 film. However, the open-circuit voltage
and the short-circuit current can be rarely obtained. In order to testify the
solar energy conversion ability of -FeSi2 thin film, we consider the J-V
characteristic of ideal solar cell. The open-circuit voltage and the
short-circuit current is slightly obtained. Therefore, it is revealed that
-FeSi2 has solar energy conversion ability. However, the value is much
smaller than theoretical value. It is essential to increase the shunt and to
reduce the series resistance for higher performance of -FeSi2.
V
+
-A
Iph Rp
Rs
Chapter 5 Demonstration of -FeSi2 Schottky solar cell
63
Reference
[5.1] Z. Liu, S. Wang, N. Otogawa, Y. Suzuki, M. Osamura, Y. Fukuzawa, T.
Ootsuka, Y. Nakayama, H. Tanoue, Y. Makita, “A thin-film solar cell of
high-quality -FeSi2/Si heterojunction prepared by sputtering”, Solar Energy
Materials & Solar Cells, 90, 276-282 (2006)
[5.2] S.M. Sze, Kwok K. Ng, “Physics of Semiconductor Devices Third
Edition”, 719-736, (2007)
[5.3] K. Wunstel, P. Wagner, “IRON-RELATED DEEP LEVELS IN
SILICON”, Solid State Communications, 40, 797-799 (1981)
[5.4] T. Suemasu, T. Fujii, K. Takakura, F. Hasegawa, “Dependence of
photoluminescence from -FeSi2 and induced deep levels in Si on the size of
-FeSi2 balls embedded in Si crystals”, Thin Solid Films, 381, 209-213 (2001)
Chapter 6 Conclusion
64
Chapter 6
Conclusion
Chapter 6 Conclusion
65
In this thesis, we studied semiconductor silicides for high efficiency thin
film photovoltaic devices. In this chapter, the studies are summarized below.
In chapter 1, maximum conversion efficiency of BaSi2/-FeSi2 tandem
silicide solar cell is calculated. It ideally achieves 40 %. However, carrier
density of semiconductor silicides is too high to be used for photovoltaic
devices.
In chapter 3, multi-sputtering process to form -FeSi2 is investigated in
order to control composition ratio. Absorption edge of 0.8 is obtained in the
film formed by our Fe/Si stacked process same as one formed by FeSi2 target
sputtering. And -FeSi2 crystalline phase can be detected. Therefore, -FeSi2
with bandgap of 0.8 eV can be formed by our multi-sputtering process.
In chapter 4, electrical characteristics of -FeSi2 are investigated. It is
confirmed that resistivity of -FeSi2 depends on Fe/Si composition ratio. In
our case, Si-rich condition with Si /Fe ratio of 2.25 has shown the largest
resistivity of ~0.6 cm. From temperature dependency, two types of defect
levels, one related to composition of -FeSi2 and the other related to
crystalline defects have been detected. Carrier density and mobility of
-FeSi2 are 7.66×1018 cm-2 and 23.6 cm2/Vs respectively.
In chapter 5, light response of -FeSi2 has been characterized by
fabricating Schottky-type -FeSi2 solar cell. However, the open-circuit
voltage and short-circuit current cannot be obtained. And so, we subtract the
value of shunt resistance from the measured current. In the result, the
open-circuit voltage and the short-circuit current is slightly obtained.
Equivalent circuit modeling has revealed the presence of shunt resistance
which suggests semiconductor nature within the film. In the future, more
fine composition ratio control, improvement of crystalline quality and
increase of shunt resistance is required for -FeSi2 thin film photovoltaic
device.
Acknowledgements
66
Acknowledgements
First of all, I would like to express my gratitude to my supervisor Prof.
Hiroshi Iwai for his continuous encouragement and advices for my study. He
also gave me many chances to attend conferences. The experiences are
precious for my present and future life.
I also thank Prof. Yoshinori Kataoka and Associate Prof. Kuniyuki
Kakushima for many kindness supports.
I deeply thank Prof. Takeo Hattori, Prof. Kenji Natori, Prof. Kazuo
Tsusui, Prof. Prof. Hitoshi Wakabayashi, Prof. Nobuyuki Sugii, Prof. Akira
Nishiyama, and Associate Prof. Parhat Ahmet for useful advice and great
help whenever I met difficult problem.
I would like to thank Mr. Katsuaki Aoki and Mr. Akito Sasaki at Toshiba
Materials Co., LTD. for giving me chance of cooperative research.
Also, I thank research colleagues of Iwai Lab for their friendship, active
discussions and many of encouraging words. Especially, I have good
friendships with Hiroshi Oomine, Takuya Seki, Atsushi Takemasa, Shuhei
Hosoda, Ryo Yoshihara and Song Jinhan.
I would like to appreciate the support of secretaries, Ms. Nishizawa and
Ms. Matsumoto.
Finally, I appreciate to my parents Tetsuji and Yoshimi, my brother Yuji
and my sister Shiori. I can keep researching thanks to my family
cooperation.
Taichi Inamura
February, 2014