properties of suspended zno nanowire field-effect transistor scott cromar mentor: jia grace lu...
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Properties of Suspended ZnO Nanowire Field-Effect Transistor
Scott Cromar
Mentor: Jia Grace LuUniversity of California, Irvine
31 August 2006
ZnO Nanowires (NWs)
• 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
Advantages
Objectives
Fan, Z.; Lu, J.G. "Zinc Oxide Nanostructures: Synthesis and Properties." Journal of Nanoscience and Nanotechnology 5 (2005): 1561-1573.
Nanowire Synthesis
Vapor Trapping Chemical Vapor Deposition (CVD)
Catalyst ZnO2 O2
•Au Catalyst•700 °C
•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): 5133-5137
•Absorbs Zn Vapor•Au-Zn Alloy Au
catalyst
ZnO nanowire
Nanowire Synthesis
O2 Gas Flow
Furnace
Quartz Tube Si Chip Quartz Vial
Zn PowderZn Vapor
Diameter = 30-100
nm
ZnO NW FET Fabrication
1. Sonicate NW chip in isopropanol2. Drop solution on pattern, Ti/Au contact3. Search for device w/ optical microscope
ZnO NW FET Fabrication
-4 -2 0 2 4
-1600
-1200
-800
-400
0
400
Vds
(V)
I (n
A)
Vg=6 V Vg=4 V Vg=2 V Vg=0 V Vg=-2 V Vg=-4 V Vg=-6 V
Improve Contact
• Annealing– 300 - 700 °C, 30
min.
• Metal deposition w/ Focused Ion Beam
-6 -4 -2 0 2 4 6-60
-40
-20
0
20
40
60
80
100
-6 -4 -2 0 2 4 6
0.0
0.2
0.4
0.6
0.8
1.0
I (n
A)
Vds (V)
Before Annealing
I (nA
)
Vds
(V)
After Annealing at 300 C Before Annealing at 300 C
-6 -4 -2 0 2 4 6-7-6-5-4-3-2-1012
-6 -4 -2 0 2 4 6
-30
-20
-10
0
I (nA
)
Vds (V)
Before FIB
I (u
A)
Vds
(V)
Before FIB After FIB
Source Drain
Gate
NW Channel
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 chemisorptionDepletion Region
Fan, Z.; Lu, J. G. “Chemical sensing with ZnO nanowire field-effect transistor.” IEEE Transactions on Nanotechnology 5 (2006): 393-396.
Gas Sensing Results
-6 -4 -2 0 2 4 6
-30-20-10
01020304050
I (n
A)
Vds
(V)
0 ppm, NO2 20 ppm, NO2 100 ppm, NO2 500 ppm, NO2
0 250 500 750 1000
0
20
40
60
80
100
120
I (n
A)
Time (s)
Vds = 2 V 1000 ppm NO2
Start of NO2
0
0
0 G
GG
G
G gas
• Sensitivity:
– Nonsuspended: 90%– Suspended: 10%
Ggas = Transconductance on gas exposure
G0 = Trasconductance in inert environment
I-Vds curve for various concentrations of NO2
Conductance of NW exposed to 1000 ppm CO2
Next Step
• Further investigate the intrinsic properties of suspended ZnO NWs– More gas sensing– Gating characteristics– Mechanical & electrical
properties w/ SPM
• Identify device applications
Acknowledgements
• Professor Jia Grace Lu• CJ Chien• Joseph Fan
• IM-SURE staff• Carl Zeiss Inc. (SEM use)• Funding through NSF
Surface Potential Results
• Scanning Probe Microscopy (SPM)
• Scanning Surface Potential Microscopy (SSPM)
sampletipac VVVdz
dCF
F = electrostatic forcedC/dz = derivative of the tip-sample capacitanceVac = 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): 032111.