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

Nanowire Synthesis

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

Source Drain

VgVds

GateTi/Au

SiO2

P++Si

Nanowire (Channel)

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.

Surface Potential Results

Surface Topology Surface Potential