1. Properties of Suspended ZnO Nanowire Field-Effect Transistor
Scott Cromar
Mentor: Jia Grace Lu
University of California, Irvine
31 August 2006

2. ZnO Nanowires (NWs) Advantages Objectives
Wide & direct bandgap semiconductor (Eg=3.35 eV)
Field effect transistors (FETs)
Gas & chemical sensing
Optoelectronics & Micro-lasers
NW Synthesis
Suspended ZnO NW FET fabrication
Electrical contact improvement techniques
Sensing experiment results
Next step
Fan, Z.; Lu, J.G. "Zinc Oxide Nanostructures: Synthesis and Properties." Journal of Nanoscience and Nanotechnology 5 (2005):

3. Vapor Trapping Chemical Vapor Deposition (CVD)
Nanowire Synthesis
Vapor Trapping Chemical Vapor Deposition (CVD) O2 O2
Au catalyst
ZnO nanowire
Catalyst Zn
Au Catalyst
700 °C
Absorbs Zn
Vapor
Au-Zn Alloy
Supersaturation
Nucleation
1D Crystal Structure
ZnO Nanowire
Chang, P.-C.; Fan, Z.; Wang, D.; Tseng, W.-T.; Chiou, W. -A.; Hong, J.; Lu, J. G. “ZnO nanowires synthesized by vapor trapping CVD method.” Chem. Mater. 16 (2004):

4. Nanowire Synthesis Diameter = 30-100 nm O2 Gas Flow Furnace Zn Vapor
Zn Powder
Si Chip
Quartz Vial
Quartz Tube

5. Nanowire Synthesis

6. ZnO NW FET Fabrication Sonicate NW chip in isopropanol
Drop solution on pattern, Ti/Au contact
Search for device w/ optical microscope

7. ZnO NW FET Fabrication Nanowire (Channel) Source Drain Ti/Au SiO2 VdsVg
Vds
Gate
Ti/Au
SiO2
P++Si

8. ZnO NW FET Fabrication

9. Improve Contact Annealing Metal deposition w/ Focused Ion Beam
°C, 30 min.
Metal deposition w/ Focused Ion Beam

10. Gas Sensing NWs have high surface-volume ratio
Suspended NW have more surface area than nonsuspended
Gas molecules on metal-oxide surface alter the electronic properties by chemisorption
Depletion Region
Source
Drain
Gate
NW Channel
Fan, Z.; Lu, J. G. “Chemical sensing with ZnO nanowire field-effect transistor.” IEEE Transactions on Nanotechnology 5 (2006):

11. Gas Sensing Results Sensitivity: Nonsuspended: 90% Suspended: 10%
I-Vds curve for various concentrations of NO2
Conductance of NW exposed to 1000 ppm CO2
Ggas = Transconductance on gas exposure
G0 = Trasconductance in inert environment

12. Next Step
Further investigate the intrinsic properties of suspended ZnO NWs
More gas sensing
Gating characteristics
Mechanical & electrical properties w/ SPM
Identify device applications

13. Acknowledgements Professor Jia Grace Lu CJ Chien Joseph Fan
IM-SURE staff
Carl Zeiss Inc. (SEM use)
Funding through NSF

14. Surface Potential Results
Scanning Probe Microscopy (SPM)
Scanning Surface Potential Microscopy (SSPM)
F = electrostatic force
dC/dz = derivative of the tip-sample capacitance
Vac = magnitude signal applied tip
Vtip – Vsample = potential difference between tip and sample
Fan, Z.; Lu, J. G. “Electrical properties of ZnO nanowire field effect transistors characterized with scanning probes.” Applied Physics Letters 86 (2005):

15. Surface Potential Results
Surface Topology
Surface Potential