DPG conference in Dresden 2011

Fabrication and Characterization of Well-Aligned Zinc Oxide Nanowire Arrays and their realizations in Schottky-Device Applications

Kin Mun Wong1, Liaoyong Wen1, Yaoguo Fang2, Fabian Grote1, Hui Sun1, and Yong Lei2

1Institute of Materials Physics and Center for Nanotechnology, University of Muenster, Wilhelm-Klemm-Str. 10, 48149 Muenster, Germany

2Institute of Nanochemistry and Nanobiology, Shanghai University, Shanghai 201800, China

Outline

• Fabrication of well aligned ZnO NWs arrays using UTAM as a substrate

• Device application of the ZnO NWs arrays

as Schottky Diodes

Fabrication of well aligned vertical ZnO NWs on ultrathin alumina membranes

Well vertical alignment of theZnO NWs due to the well orderedunderlying UTAM pore structure

ZnO NWs on the UTAM are grownby CVD at 550 oC under 400 mbarat a constant nitrogen flow of 150 cm3/min

0.5 μｍ

UTAM

Au nanodots

(a)

１ μ ｍ

(b)

1 μm

(c)

(a) (b)

(c)(d)

(a) Thermal Evaporation of Au on the UTAM surface

(b) ZnO + C → Zn + CO (by graphite), negative charges (e.g. C2O42-) attract Zn atoms to the UTAM for the nucleation process to begin

(c) Mergence of the Zn atoms into (0001)-oriented quasihexagonal ZnO nuclei upon precipitation and oxidation

(d) Due to the intense (0001)-oriented growth direction, vertical ZnO NWs are formed after prolonged growth from the nuclei

Growth process of the vertical well ordered ZnO nanowires arrays

Process of fabricating horizontal arraysof ZnO NWs Schottky diodes

The ZnO NWs as constructed by our method are densely packed (with approximately 4x105 NWs in an area of 1 cm-2) and are demonstrated to be functional as Schottky diodes.

The overall process of fabricating horizontal arrays of ZnO NWs Schottky diodes

(a) (b)

(c) (d)

Length of the ZnO NWs reach181 m in 40 minutes

Rapid growth of the NWs due to the plentiful supply of zinc vapor and oxygen near the substrate

Temperature : 950 oC

Growth of the ZnO NWs by the CVD process

(a) (b)

(c) (d)

Dry contact printing process for the realization of the well aligned arrays of ZnO Schottky diodes

NWs were effectively sheared from the donor substrate

NWs are aligned along the [0001] direction (or c-axis), retaintheir orginal alignment on thedonor substrate

Ag electrodes on the ZnO NWsformed by thermal evaporationSeparation between the electrodes

20 m

Directional sliding of the donorsubstrate with respect to thereceiver substrate

90% of the NWs are highlyAligned in the direction of theSliding motion

(a)

I-V Characterization of the Zinc Oxide Nanowire Arrays

Movement of the positive andnegative probes along the neighbouring pairs of electrodesfor the I-V measurements

(b)

Rectifying behavior in the negative voltage region with some differences in the forward biased current for all the 9 electrode pairs

(b)

Decrease of the foward biased current when the number of sequential electrodes increases due to the increased number of Schottky barriers

Increase in the turn on voltage

I-V characteristics of horizontally aligned ZnO NWs arrays of Schottky diodes

(a)

Zinc and oxygen alternating layers along the c-axis results in a spontaneous polarization, P which leads to a potential gradient along the length of the individual NW

P, which is oriented along the NW direction (towards the source electrode) leads to a build in potential decreasing along the NW c-axis direction

I-V characteristics of nine different single ZnO NW Schottky diode

SEM image of a single ZnO NW device on the Si substrate

Length of the ZnO NW ~ 100 m

25 μm

(a)

-3 -2 -1 0 1 2 3

0

4

8

12

16

20

1# electrode

2# electrode

3# electrode

4# electrode

5# electrode

6# electrode

7# electrode

8# electrode

9# electrode

I (nA

)

Vds (voltage)

(b)

8 10 12 14 16 18 200,4

0,5

0,6

0,7

0,8

0,9

1,0

Single ZnO NW Schottky diodes

Ideality Factor

Bar

rier

Hei

gh

t, d

(eV

)

The ideality factor and the barrier height of the single ZnO NW Schottky diode is obtained fromthe respective I-V characteristics

The high ideality factor is due to the interface states of the thin interfacial layer between the Ag electrode and the ZnO NW as an interfacial layer with surface states forms easily on the surface of the ZnO NW

Weak linear dependency of the barrier height on the ideality factor indicates lateral inhomogeneity of the ZnO NW Schottky contacts due to the difference in the inhomogeneity in the Schottky contact corresponding to the different diode-to-diode interface

Conclusion

•Fabrication of well-aligned arrays of vertical ZnO nanowires using an UTAM as a substrate

•Fabrication of a large area of repeatable, densely packed and horizontally well-aligned arrays of ZnO NWs Schottky diodes along their c-axis using a simple and viable two-step method

•Our technique for the realization of Schottky device applications opens a possibility for the mass scale production of the ZnO NW based devices for flexible electronics.

Surface Nano-Structuring Group

Group Leader: Prof. Dr. Yong Lei

Group Members: Stefan Bartels Fabian Grote Christian Heckel Peter Heß Stefan Ostendorp Dr. Hui Sun Nina Winkler Dr. Kin Mun Wong Feng Xu Dr. Shikuan Yang Dr. Huaping Zhao

Acknowledgements

Funding

Thank you for your attention