Jia Grace Lu

Dept. of Chemical Engineering and Materials Science &

Dept. of Electrical Engineering and Computer Science

University of California, Irvine

Metal Oxide Metal Oxide NanowiresNanowires: Synthesis, : Synthesis, Characterization and Device Applications Characterization and Device Applications

(courtesy ZL Wang)

Challenges and ProspectChallenges and Prospect

# of Tr

ansisto

rs/Chip

4Kb64Kb

1Mb

16Mb

256Mb

10

100

1000

10000

1970 1980 1990 2000 2010 2020Year

1.E+04

1.E+06

1.E+08

1.E+10

1.E+12Minimum Dimension (nm)

““ThereThere’’s Plenty of Room at the Bottoms Plenty of Room at the Bottom””-- Richard Feynman, 1959Richard Feynman, 1959

Vision: Vision: To synthesize To synthesize nanoscalenanoscale building blocks with building blocks with precisely controlled size and composition, and precisely controlled size and composition, and assemble them into larger structures with unique assemble them into larger structures with unique properties and functions.properties and functions.

““ThereThere’’s Plenty of Room at the Bottoms Plenty of Room at the Bottom””

Characterization Characterization (Electron (Electron Microscopy, SPM, Microscopy, SPM, LowLow--T, HighT, High--B)B)

Nano Materials Nano Materials ((nanotubenanotube, , nanowiresnanowires, , ultra thin filmsultra thin films) & & Device Fabrication Device Fabrication

Nano Device Nano Device Application (FET,Application (FET,Sensor, Sensor, SET SET ……))

Source DranNanowire

SiO2

Ni/Au Ni/Au

Gate

Science and Engineering in the Science and Engineering in the nanonano--WorldWorld

Semiconductor NanomaterialsQuasiQuasi--oneone--dimensional semiconductor nanostructures: dimensional semiconductor nanostructures:

Q1D physical properties Q1D physical properties High aspect ratioHigh aspect ratioConductivity control (Conductivity control (pp--type/ type/ nn--type)type)BandgapBandgap engineeringengineering

Electronic and photonic device applicationsElectronic and photonic device applicationsField effect transistors Schottky-barrier rectifiersLED & LasersSensorsSensorsSingle electron transistors Memory cells Logic gates

NanoNano MaterialsMaterials

15o

Ga2O3 Fe2O3B

Ag-TCNQ ZnOGaN

MotivationMotivation

Device applications:Device applications:Transparent electronicsTransparent electronicsUV light emitter and UV light emitter and detectordetectorChemical sensorsChemical sensorsElectroElectro--mechanical mechanical devicesdevices

Lower power consumptionLower power consumptionHigher sensitivityHigher sensitivityBetter gate modulationBetter gate modulationFaster response timeFaster response timeTunable band gapTunable band gap

NanowiresZnO

conducting channel

λD

Synthesis and Characterization Synthesis and Characterization of of NanowiresNanowires

CVD synthesisCVD synthesisElectrical transportElectrical transportScanning surface potential microscopyScanning surface potential microscopyChemicalChemical sensing propertysensing propertyOptical propertyOptical property

Single Crystal Single Crystal NanowiresNanowires : Vapor: Vapor--LiquidLiquid--Solid MechanismSolid Mechanism

O2Catalyst

Zn O2

Catalyst: Au Alloy Supersaturation Crystal growth

Single Crystal Single Crystal NanowiresNanowires

• VLS growth mechanism

• SEM image of nanowires, with diameter ranging from ~20 to 100 nm

• High resolution TEM shows ZnO nanowires are single crystal. The distance between neighboring lattice planes is 0.52 nm, andgrowth direction is along [001] -- c axis

Chang et al., Chem of Materials (2004)

Ar+O2

Zincpowder

Quartzvial

Open endof vial Nano-

Crystals collecting substrate

20 30 40 50 60 70 80 90

0

1000

2000

3000

(200

)(2

01)

(203

)

(202

)

(112

)

(103

)(110

)

(102

)

(100

)(0

02)

ZnO Nanostructures JCPDS 89-0510

2θ

Inte

nsity

(a.u

.)

(101

)

C axis

HRTEM done in Japan

Out

b1

b2

b3

b4

b5

Control of Nanostructure MorphologyControl of Nanostructure Morphology

Furnace

QuartzTube

B A ZnC

O2 Gas Flow

Quartz Vial

Si wafer

- Si wafer

- Resist: PMMA

- E-beam lithography

- Catalyst deposition

Control of PositionControl of Position

- Si wafer

- Resist: PMMA

- Catalyst deposition

- Ebeam lithography

Si wafer- Liftoff:

remove PMMA

- CVD growth

Control of PositionControl of Position

Home-made anodic alumina membrane (AAM), 5 micron thick.

Al2O3 substrate

Al2O3 substrate

Al2O3 substrate

AAOtemplate

BottomelectrodeAdhesion

layer

Short Snnanorod

ZnOnanowires

• Vertical aligned ZnO nanowires array has promising potentials for nanoelectronics/optoelectronics.

• Anodic alumina membrane is an ideal template to integrate vertical aligned nanowires.

Schematic of nanowire synthesis process

400 nm

Control of Alignment Control of Alignment

Vg

Source Drain

Back Gate

SiO2

P++Si

Vds

NW

Furnace

QuartzTube

B A ZnC

O2 Gas Flow

Quartz Vial

Control of Electric PropertiesControl of Electric Properties

-2 -1 0 1 2-3

-2

-1

0

1

2

3

I ds (µ

A)

Vds (V)

Chip B Chip C

Higher Conductivity

-20 -10 0 10 20

0

50

100

150

200

Vg,t(C)I ds

(nA

)

Vg (V)

Chip B Chip C

-20 -10 0 10 20

0

50

100

150

200

I ds (

nA)

Vg (V)

Chip B Chip C

g,tV (B)

SiO2/p++ Si

Define alignment marks using ebeam lithography and metal deposition

Disperse NWs on substrateand locate the coordinates

1st resist layer: MMA/MAA2nd resist layer: PMMA

e-e-

Ebeam writing

Developing

Metal depositionLiftoff

Electrode Contact using Ebeam-lithography

Electrode Contact using Photolithography

SiO2/p++ Si

Disperse NWs

Spin coat resist

Cover the resist with a mask

UV

UV Exposure

DevelopingMetal depositionLiftoff

Band Diagram of Electrode ContactBand Diagram of Electrode Contact

Ohmic contactSchottky junction

EF-ZnOEF - Ni

Before contact

Ec

Ev

Ni ZnO

After contact

Work function (Φ) of materials:

ΦTi = 4.3 eV

ΦNi = 5.2 eV

ΦZnO = 4.3 eV

EZnO = 3.4 eV

ΦNi > > ΦZnO > ΦTi

Ti ZnO

φBEF - Ni

Source DrainNanowire

SiO2

p++ Si

Vg

Vds

Gate

NanowireNanowire Field Effect Transistor Field Effect Transistor

e

V

LCn gt⋅−=

)/2ln(/2/ 0 rhLC πεε≈

))/2ln(/2/()/( 0 rhLVdVdI dsg πεεµ =

r: NW radius; L: NW channel length; h: gate oxide thickness;

Vgt: gate threshold voltage; dI/dVg: transconductance.

Capacitance per unit length:

Carrier concentration per unit length:

Mobility:

I-V curves under different gate voltages

(b)

-10 -5 0 5 10 15

0

8

16

24

32

I (nA

)

Vg (V)

Vds=100mV Vds=75mV Vds=50mV Vds=25mV

Vgt=-8.37V

Transconductance at different source-drain biases

Electrical Transport PropertiesElectrical Transport Properties

• Three-terminal electrical measurements show n-type behavior.

• Carrier concentration ne ~ 107 cm-1; and mobility µe~ 80 cm2/V•s

-4 -2 0 2 4

-1600

-1200

-800

-400

0

400

Vds (V)

I (nA

)

Vg=6 V Vg=4 V Vg=2 V Vg=0 V Vg=-2 V Vg=-4 V Vg=-6 V

Z. Fan et al., APL (2004)

High Performance High Performance ZnOZnO NW FETNW FET

-10 -5 0 5 10

-200

-100

0

100

200

Ids

(nA

)

Vds (mV)

Vg=-20V Vg=-16V Vg=-12V Vg= -8V Vg= -4V Vg= 0V

-30 -20 -10 0 10 20 30

0

100

200

300

Ids

(nA

)

Vg (V)

2mV 4mV 6mV 8mV10mV

ZnO nanowires show significant enhancement in mobility after surface passivation.

Q1D concentration ~ 9 x 107 cm-1 ; 3D concentration ~1.16 x 1018 cm3

Q1D carrier mobility ~ 1200 cm2/Vs

ThermionicThermionic EmissionEmission

-1.0 -0.5 0.0 0.5 1.0-800

-600

-400

-200

0

200

400

600

I (nA

)

Vds (V)

290 K 333 K 373 K 428 K 473 K

Con

duct

ance

(nS

)

1000/T2.0 2.4 2.8 3.2 3.6

103

210

110

010

(c)

• Electric current was observed to increase monotonically with temperature.

• An effective energy barrier height of 0.3 eV is estimated from the thermionic emission model: I ~ exp(-φb/kT).

Top view of ZnO nanowire array embedded in AAM.

•Chemical vapor deposition is used to grow ZnO nanowires array.

•Electrical transport through individual nanowires is characterized with conductive AFM probes.

Vertical Nanowire Array

I-V characteristic of an individual vertical ZnO nanowire in AAM.

Scanning Surface Potential MicroscopyScanning Surface Potential Microscopy

Source Drain

Back gate

Vac

VdsSiO2

~

Vtip

sampletipdc VVV −=

acdc VVdzdC

F ⋅=

• The uniformity of nanowire and the property of electrode contact was studied using scanning surface potential microscopy by probing local electric potential distribution.

• Conductive AFM tip is biased with a DC signal and an AC signal. AC signal is used to drive tip resonant vibration and DC signal can be controlled by feedback loop to trace the surface potential on sample.

Surface Potential and Tip GatingSurface Potential and Tip Gating

Topography and surface potential image of a ZnO nanowire FET.

Cross section analysis of surface potential image gives the potential drop on device: VAB = 0.26 V; VBC = 0.30 V; VCD = 0.89 V

Source

Drain

Scanning direction

Vds

Vg

-1.0 -0.5 0.0 0.5 1.00

1

2

3

4

5

Ids

(nA

)

Time (s)

Back gating Back gating vsvs tip gatingtip gating

Source DrainNanowire

Back gateVg

Vds

SiO2

Source Drain

Back gate

Vg

Vds

SiO2

-10 -5 0 5 10-1200

-1000

-800

-600

-400

-200

0

200

Vds (V)

I (nA

)

Vbg=-20 VVbg=-10 VVbg=0 VVbg=10 VVbg=20 V

-10 -5 0 5 10-500

-400

-300

-200

-100

0

I (nA

)

Vds (V)

Vtg=-20VVtg=-10VVtg=0VVtg=10VVtg=20V

• Electrical transport results are obtained for both AFM tip gating and Si back gating.

• It is observed that negative tip gating has significant local depletion effect on FET and increases the height of effective barrier for electron conduction. However, positive tip gating does not strongly enhance the conductance of NW.

Back gating Back gating vsvs tip gatingtip gating

• Gate potential affect conductance of NW by band bending.

• Si Back gate potential affects the entire FET, but AFM tip gate potential only affects the local region and creates an energy barrier at negative Vg.

Back gating Tip gating

EFDrai n SourceNW

(d)

φbs EF

φbd

Von

Drai n SourceNWEF

Von

Drai n Source

φbt

NW

φ bs

Ecnegat i ve Vg

posi t i ve VgEF

Chemical SensingChemical Sensing

Adsorbed molecules(H2,O2,NO2,NH3,CO, C2H5OH,…)

Nanowire

Current transport

Electrons

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

∆G/G

0

NW radius (nm)

O2

Source Drain

Gate

Nanowi r e channel

Depletion region

increases with reduced nanowire radius.

S = ∆G/GoSensitivity:

-6 -4 -2 0 2 4 6-6

-4

-2

0

2

4

Vds (V)

I (µ

A)

0ppm 10ppm 20ppm

-10 -5 0 5 10 150.00.51.01.52.02.53.0

I (µA

)

Vg (V)

0 ppm 10 ppm 20 ppm

Vds= 5 V

NO2

NONO22 OxidingOxiding SensingSensing

• Response time is defined as time when conductance drops 1/e from maximum.

• Response time decreases with increasing concentration.

0 100 200 300 400

0

200

400

600

800

1000 Start of NO2

1 ppm

20 ppm10 ppm

5 ppmCur

rent

(nA

)

Time (s)

2 ppm

0 5 10 15 200

10

20

30

40

Res

pons

e tim

e (s

)

NO2 concentration (ppm)

T=A×N -0.6

Gate Tunable SensitivityGate Tunable Sensitivity

-8 -4 0 4 80.0

0.2

0.4

0.6

0.8

1.0

Vg (V)δG

/G0

Sensitivity to5 ppm NO 2

bVa

Sg +

∝

-6 -4 -2 0 2 4 6

-0.4

-0.2

0.0

0.2

0 ppm0.2 ppm0.4 ppm0.6 ppm

Vg = -5 V

I (µA

)

Vds (V)

Chemical Sensing properties of nanowire FET largely depend on gate potential.

Gate adjusts the position of the Fermi level of the channel, affect the surface processes.

à Gate voltage tunes the detection range.

Z. Fan and J. G. Lu, APL ( 2005).Z. Fan, et al, APL (2004).

-6 -4 -2 0 2 4 6

-8

-4

0

4

8

Vds (V)I (

µA)

0 ppm10 ppm20 ppm30 ppmVg = 30 V

-6 -4 -2 0 2 4 6

-1.2

-0.8

-0.4

0.0

0.4

Vg= 0 V

0ppm1ppm2 ppm5 ppm10 ppm20 ppm

I (µA

)

Vds (V)

NO2

Electrically Refreshable SensingElectrically Refreshable Sensing

0 200 400 600 800 1000 1200

200

400

600

800

1000

1200

1400

T = 300 K

I (n

A)

Time (s)

10 ppmNO2

T = 453 K

Vg= 0 V

Vg= -60 V

-200 0 200 400 600 800 1000 1200 1400 1600-150

0

150

300

450

600 10 ppmNO2

I (nA

)

Time (s)

T = 300 K

At room temperature, chemisorbed molecules are quite difficult to be desorbed.

Thermal desorption and ultra-violet illumination are conventional ways to refresh nanowire.

For oxidizing gas, a strong negative gate field greatly facilitates desorption and is capable to quickly refresh nanowire sensor.

Z. Fan and J. G. Lu, APL (2005).

DistinguishabilityDistinguishability

0 20 40 60 80 1000

20

40

60

80

100

2.5 ppm NO2,k=0.12

I (nA

)

Time (s)

5 ppm NO2,k=0.10

10 ppm NO2,k=0.16

10 ppmNO2

recoveryregion

Vg =-20 V Vg =0 V

800 850 900 950 1000 1050

-40

0

40

80

120

160

I (nA

)

Time (s)

0 20 40 60 800

20

40

60

80

2% NH3,k=0.23

1% NH3, k=0.25

0.5% NH3 , k=0.27

I (nA

)

Time (s)

Temporal response of NO2 and NH3 to a negative gate voltage pulse demonstrates a potential distinguishable sensing.

Gate Refresh Voltage

• Different activation energy for NO2 and NH3adsorption.

• Distinguishable gate refresh voltage.

VgSiO2

p ++ Si

S D

Vds

Reducing Sensing PropertiesReducing Sensing Properties

0 1000 2000 3000 4000 5000-20

0

20

40

60

80

100

0.5%CO on

0.5%CO on

0.5%CO on

0.5%CO off

I (nA

)

Time (s)

T= 500K

20% O2 in

0.5%CO on

0.5%CO off

0.5%CO off

0 400 800 1200 160035

40

45

50

55

1% NH3off1% NH3off0.5% NH3off

1% NH3 on1% NH3on

I (nA

)

Time (s)

0.5% NH3 on

T=500K

Exposure nanowire to CO at 500K in O2 ambient increase nanowire conductance by reaction:

−− +→+ eCOOCO 2

Exposure nanowire to NH3 at 500K in Ar ambient increase nanowire conductance also, but by electron direct transferring from NH3 to nanowire.

Ef

EcEf

500 K300 KEv

Ec

Ev

µNH3 µNH3

Optical Property of Optical Property of ZnOZnO

Photoluminescence and photoconductivity spectrum of single ZnO nanowire

Z. Fan, et al., APL (2004).

200 400 600 800 1000

0

40

80

120

160

Cur

rent

(nA

)

Input optical wavelength (nm)

Vds= 0.2 V

200 400 600 800

100

200

300

400

500

600

700

Wavelength (nm)

PL

inte

nsity

(a. u

.)

0 50 100 150 200

4

8

12

16

20

24

28

0 20 40 60 80

0

20

40

60

80

Turn on laser

Cur

rent

(nA

)

(min)

Turn off laser

Cur

rent

(nA

)

Time (s)

Vacuum

Air

Due to the adsorption of oxygen:Due to the adsorption of oxygen:OO22 + + ee ?? OO22--

Td ~ 8 s in airTd > 1 hr in vacuum(Td is defined as the time for current drop from Imaxto Imax/e)

PhotoconductivityPhotoconductivity

-2 -1 0 1 2 3-20

0

20

40

60

80

100

120

140

160

-4 0 4 8 12

0

10

20

30

Cur

rent

(nA

)

Gate Voltage (V)

Cur

rent

(nA

)

Drain voltage (V)

Without 633nm illumination

With 633 nmillumination

Without 633nm illumination

With 633nm illumination

• Light illumination results in an increase in nanowire carrier concentration and conductance, and a slight decrease in mobility.

• Power law dependence of photocurrent to the input optical power is due to the finite number of impurity states.

(b)75

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0

15

30

45

60

Cur

rent

(nA

)

Laser power (mW)

Vds= 2 V

I=A×P0.43

PhotoluminescencePhotoluminescence

Blue-shift as diameter decreases

Diameter dependence

340 360 380 400 420-2000

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

22000

24000

26000

392nm

383nm

Inte

nsity

/ a.

u.

Wavelength/ nm

Dia(100-200nm) Dia(20-100nm) Dia(6-20nm)

381nm

Temperature dependence

LT PL spectroscopy indicate the nanowiresare of high crystal quality.

(data taken at Göttingen University)

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6

300 K

200 K

100 K

12 K

Inte

nsity

/ a.

u.energy / eV

Polarization Dependent Polarization Dependent PhotodetectionPhotodetectionLight

NW

E||

E?

• Nanowire conductance is minimized when the incident field is polarized perpendicular to the long axis.

• Such anisotropy has potential application for polarization dependent photodetector and optical gated switch.

Z. Fan et al., APL (2004)

30 60 90 120 150 180 210

600

800

1000

1200

1400

1600

365 nm UV

Vds

= 2 V

Vds

= 5 V

Polarization Angle (degree)

Cur

rent

(n

A)

E

E

broadband light

Nanowire diameter

Summary on Summary on ZnOZnO NanowireNanowire

Single crystal ZnO nanowires are synthesized and configured as field effect transistors. Electrical transport studies show ZnO nanowires are n-type semiconductor.

Scanning surface potential microscopy demonstrates that electrical property of the nanowire is uniform.

Chemical sensing results indicate the future application of ZnO FET as a self-contained, gate tuned, and gate refreshable chemical sensor.

Optical studies show its potential applications in nanoscale optoelectronics.

Gallium OxideGallium Oxide

nn Ceramic metal oxide Ceramic metal oxide nn n n --type semiconducting at high type semiconducting at high

temperature temperature nn wide bandgap ~ 4.9eVwide bandgap ~ 4.9eVnn Carrier concentration ~ 10Carrier concentration ~ 101313 cmcm--33 at at

10001000°°CC(M. Fleischer and (M. Fleischer and H.MeixnerH.Meixner, , J. J. ApplAppl. Phys.,. Phys., 74, 300, 1993)74, 300, 1993)

nn BaseBase--centered monoclinic structurecentered monoclinic structurenn a =12.23 a =12.23 ÅÅ, b =3.04 , b =3.04 ÅÅ, c =5.8 , c =5.8 ÅÅ

nn Potential Applications:Potential Applications:nn High temperature gas sensorHigh temperature gas sensornn Optoelectronics deviceOptoelectronics device

Electron MicroscopyElectron Microscopy

nn Dimension:Dimension: length~10length~10--3030µµm, m, diameter~20diameter~20--200nm200nm

nn VLS (VaporVLS (Vapor--LiquidLiquid--Solid) catalytic growth Solid) catalytic growth mechanismmechanism

nn BaseBase--Centered Monoclinic crystalline structureCentered Monoclinic crystalline structurenn Several growth direction can be observed:Several growth direction can be observed:

A.A. [111] spacing ~0.25nm[111] spacing ~0.25nmB.B. [001] spacing ~0.55nm[001] spacing ~0.55nmOthers [200], [Others [200], [--130] 130] ……

Au

A

15o

B

P. Chang et al., APL (2006)

XRD & PL CharacterizationsXRD & PL Characterizations

30 40 50 60 70 80

0

1000

(510

)(-11

1)

(403

,512

,-71

2)

(600

,-31

2,11

2)

(-31

1,-4

02,2

02)(0

02,-

202)

(111

)

(-40

1,40

0)

Ga2O3 nanowires

JCPDS 43-1012

Inte

nsi

ty (a

.u.)

2θ (degree)

• CuKα (λ=1.54 Å) source• As grown nanowire sample matches the β-Ga2O3diffraction pattern• Single phase• The strongest peak is (111)

300 350 400 450 500 550

0

30000

60000

Inte

nsi

ty (a

.u.)

Wavelength (nm)

100K 150K 200K 250K 300K

414nm

• Blue-Green broad emission is contributed by Gallium-oxygen vacancy pair (acceptor-donor) formed trapped excitons

(T. Harwing, et al., J. Solid State Chem., 24, 255, 1978)(Vasil’tasiv et al., Ukr. Fiz. Zh., 33, 1320, 1988)

νh++′→+ •oGaox

ox

Gao V )V ,V (V )V ,V (

nn Intrinsic:Intrinsic:It shows very low conductance (insulating, high It shows very low conductance (insulating, high resistance~10resistance~101616ΩΩ) at room temperature ) at room temperature

nn nn--type doping:type doping: SbSb5+5+, W, W6+6+, , etcetcnn pp--type doping:type doping:

Zn atoms can be acceptorsZn atoms can be acceptorsnn Ionic crystal radii size:Ionic crystal radii size:

ZnZn2+ 2+ : 0.074nm, Ga: 0.074nm, Ga3+ 3+ : 0.062nm : 0.062nm (R. D. Shannon, (R. D. Shannon, ActaActa CrystallographicaCrystallographica, A32, 751, (1976), A32, 751, (1976)

ooGa O2V 'Zn22ZnO ++=••

nn Setup (diffusion doping):Setup (diffusion doping):Pure Zn powder in Pure Zn powder in ArAr inert ambient, inert ambient, 450? for 15 minutes

Extrinsic DopingExtrinsic Doping

Transport Properties (ITransport Properties (I--V)V)

-20 -10 0 10 20

-10

0

10

20

30

Cu

rren

t (p

A)

Bias Voltage Vds (Volt)

Vg= 30V Vg= 0V Vg=-30V

• Improved transport properties - up to 3 order increase in magnitude

• p-type semiconducting behavior - from gate voltage dependence

Vds

VgSiO 2

p ++ Si

)/2ln(2(th)V 0g

rhep

πεε×=

ds0V2)/2ln(

VI

πεεµ

rhLdd

gh ×=

Carrier concentration

Carrier mobility

(R.Martel et al., Appl. Phys. Lett. , 73, 2447, 1998)

Transport Properties (ITransport Properties (I--Vg)Vg)

-80 -60 -40 -20 0

0

2

4

6

8

Cu

rren

t (n

A)

Gate Voltage Vg (Volt)

dI/dVg= -0.988x10-9 A/V

Vg(th)= -74.7 V

ex. h = 200nm, r = 60nm, L = 2.12µm, ε = 3.9 (permittivity of SiO2)

p = 5.3×108 cm-1 (4.6×1018 cm-3) µh = 3.5×10-2 cm2/Vs

Vg

Source Drain

Back Gate

Vds

NW

Transconductance dI/dVg = -0.988×10-9 A/V

Threshold VoltageVg(th) = -74.7 V

-40 -20 0 20 40

100

200

300

400

Vds = 2.0 V

I (pA

)

Vg (V)

•(b)

As-grown Fe2O3 nanobelts are configured as field-effect transistors showing n-type behavior.

For the p-type Zn doping, the nonlinear I-V arises from the Schottky contact formation due to nanobelt Fermi level shifts as a result of doping.

-6 -4 -2 0 2 4 6

-900

-600

-300

0

300

600

900

I (pA

)

Vds

Vg = 10 VVg = 0 VVg = -10 V

EFEc

Ni Fe2O3

φφ

NiFe2O3

Vacuum

p-type Zn doping

EF EVNiFe2O3

φNi

VacuumφFe2O3

Ec

-6 -4 -2 0 2 4 6 8

-80

-40

0

40

80

-120 -80 -40 0 4045505560657075

I (nA

)

Vg (V)

Vds=5VI (nA

)

Vds (V)

Vg=-100 VVg=-50 VVg=0 VVg=100 V

Z. Fan et al., APL (2005)

Extrinsic Doping Extrinsic Doping -- FeFe22OO33 NanobeltsNanobelts

““Small wonders, endless frontiersSmall wonders, endless frontiers””“…nanotechnology will be central to the next epoch of the information age, and will be as revolutionary as science and technology at the micron scale have been since theearly ’70s.” –– John Armstrong, 1991John Armstrong, 1991

Vo

Vi1 VcVi2

Vcc

AAO template

Bottomelectrodes

Top electrodes

Nanowires

SiO2p ++ Si

S D

Vg Vds