Wide Bandgap Semiconductor Nanowires for Sensing

• S.J. Pearton1, B.S. Kang1, B.P.Gila1, D.P. Norton1, L.C.Tien1, H.T.Wang2, F. Ren2, Chih-Yang Chang3,G.C. Chi3,Wei-Ming Wang3 and Li-Chyong Chen4

• 1Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611-6400, U.S.A

• 2Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, U.S.A.

• 3Department of Physics, National Central University, Jhong-Li 320, Taiwan

• 4Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan

GaN Applications

Blue/violet/white/UV LED Blue/green/UV lasers

High power microwave transistors

Robust sensors

GaN NWs grown by catalytic chemical vapor deposition

500μm

5μm

Ti/Au PadTi/Au Pad SiNx/Si

FESEM image & CL spectrum of a single GaN NW with two electrodes

Gate voltage-dependent I-Vsd curves of a single GaN NW

The carrier mobility is estimated at 30 cm2/V·s.The carrier concentration is estimated to be 2×1017 cm-3

25 30 35 40 45 50 55 60

(a)

In (

110)

(411

)

(413

)

(332

)

(

440)

InN In

2O

3

(

400)

(

222)

(103

)

(110

)

(102

)

(101

)

(002

)

(100

)

Inte

nsi

ty (

a.u

.)

2 (degree)

5 nm

0.308 nm

5 nm5 nm

0.308 nm

InN NWs grown by catalytic thermal-CVD

HRTEM image XRD spectrum

Temperature-dependent I-V curve of a InN NW

fAppl. Phys. Lett. 64, p1508-1510 (1994)gSolid-state Electronics, 39, p1289-1294 (1996)hJ. Vac. Sci. Technol. B, 14, p3520-3522 (1996)iThis work

Resistivity comparison between thin film and nanowire (n-type GaN and InN)

　 thin film nanowire

　 resistivity (Ω cm)

contact resistivity (Ω cm2)

resistivity (Ω cm)

contact resistance (Ω)

n-GaN 4.4×10-2 a 3~7×10-6 a,b 56 ~ 1.24×10-4 c,d,e 　 X

InN2.1~

3.1×10-3

f,g,h

1.8×10-7 f 4×10-4 i 2i

aSolid State Electron 41, p165-168 (1997)bAppl. Phys. Lett. 70, p57-59 (1997)cAppl. Phys. Lett. 85, p1636-1638 (2004)dNano Lett. 2, p101-104 (2002)eNano Lett. 3, p1063-1066 (2003)

10101010

Single Crystal Nanowire

• TEM image of an individual ZnO Nanowire.

• An estimated diameter of the wire is 20 nm.

• A small particle embedded at the tip of the wire is Ag or Ag-Zn alloy.

• HR-TEM image and selected area diffraction (SAD) of the nanowire indicates that it is a single crystal ZnO.

00020002

Heterostructured nanowires

Type I Type II

Core (Zn,Mg)O(Hexa.)

Sheath(Zn,Mg)O(Hexa.)

Zn1-xMgxO(x <0.02)(Hexa.)

(Mg,Zn)O(cubic)

Radial heterostructure Axial heterostructure

ZnO(Zn1-XMgX)O

ZnO(Zn1-XMgX)O

Growth condition

-. Zn : 3 × 10-6 mbar

-. Mg : 4 × 10-7 mbar

-. O3/O2 : 5 × 10-4 mbar,

-. Tg= 400C

Growth condition

-. Zn : 3 × 10-6 mbar

-. Mg : 2 × 10-7 mbar

-. O3/O2 : 5 × 10-4 mbar,

-. Tg= 400C

Type I - Radial heterostructured nanowire

Core (Zn,Mg)O(Hexa.)

Sheath(Zn,Mg)O(Hexa.)

-. Nanowire is crystalline with the wurtzite crystal

structure maintained throughout the cross-

section.

-. The higher contrast for the center core region

clearly indicates a higher cation atomic mass.

-. Core : zinc-rich Zn1-xMgxO

-. Sheath : Mg-rich Zn1-yMgyO

10 nm

a b

0002

1120

b

Type II - Radial heterostructured (Zn,Mg)O/(Mg,Zn)O nanowire

Zn1-xMgxO(x <0.02)(Hexa.)

(Mg,Zn)O(cubic)

[0001]

(1ī1

)(200)

(11ī)

0 10 20 30 40 50 600

20

40

60

80

100

120

140

No

rma

lize

d C

ou

nts

Position across Nanowire (nm)

Mg Zn

Compositional line scan across the nanowire (STEM)

-. Core : Zn1-xMgxO Hexagonal Wurtzite structure -. Sheath (Shell): Mg1-xZnxO Cubic Rock salt structure

ZnMg

(Mg,Zn)O nanowire (cubic rock salt structure)

200

020

B=[001]

2.04 Å

Position across nanowire(nm)

Inte

nsit

y(ar

b.)

Growth condition

-. Zn : 3 × 10-6 mbar

-. O3/O2 : 5 × 10-4 mbar,

-. Mg : 8 × 10-7 mbar

-. Tg = 400C

ZnO

hexagonal

wurtzite st.

(Mg,Zn)O

cubic

rock salt st.(Zn1-xMgx)O/(Zn1-xMgx)O hexa. / hexa.wurtzite / wurtzite

Radial heterostructured (Zn,Mg)O

Zn = 3 × 10-6

O3/O2 = 5 × 10-4

Mg = none

Zn = 3 × 10-6

O3/O2 = 5 × 10-4

Mg = 8 × 10-7

Zn = 3 × 10-6

O3/O2 = 5 × 10-4

Mg = 2 × 10-7

Zn = 3 × 10-6

O3/O2 = 5 × 10-4

Mg = 4 × 10-7

[unit: mbar]Tg= 400C

core / sheath(Zn1-xMgx)O / (Mg,Zn)O

hexa. / cubicwurtzite / rock salt st.

core / sheath

Nanowires vs Zn, Mg pressures

I II

Fabrication of ZnO nanowire device

Insulator

Electrode (Al/Pt/Au) Al/Pt/Au

ZnO Nanowire

-. Fundamental understanding of transport

-. Nanoelectronics

-. Nano sensors (UV, chemical, bio.)

Motivation

-. Electrode : Al/Pt/Au by sputtering

-. Diameter of ZnO nanowire : 130 nm

-. Channel Length : 3.7 m

Structure of Nanodevice

Prototype device fabrication sequence

Design　andDeposit　AlignmentMarks

Deposit　SiO2

Evaporation　&　Nanowires　Deposition

Find　NanowiresRelative　To　AlignmentMarks

　　　　　Spin　PMMA　　　　　　　Resist

E-beam　WriteAligned　PatternAnd　Develop

Deposit　MetalAnd　Lift　Off

Ethanol　and　NanowireSuspension

UV Response of single ZnO nanowire

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

-40

-20

0

20

40

Cur

rent

(nA

)

Voltage (V)

Dark UV366nm

Dark

UV 366nm

on

off

UV 366nm at VD 0.25V

-0.4 -0.2 0.0 0.2 0.4

-0.9

-0.6

-0.3

0.0

0.3

0.6

0.9 Dark UV366nm

Cur

rent

(nA

)

Bias (V)

0.00 0.05 0.10 0.15 0.201x10-4

1x10-3

1x10-2

1x10-1

1x100

1x101

Cur

rent

(nA

)

Bias (V)

I=Io(eqV/nkT-1)Ideality factor = 1.1

Forward Bias

Al/Pt/Au Al/Pt/Au

Pt/Au (schottky contact)

-10 -8 -6 -4 -2 0-0.20

-0.15

-0.10

-0.05

0.00

Cur

rent

(nA

)

Bias (V)

Reverse Bias

Pt/ZnO nanowire Schottky Diode

0 2 4 6 8 10

0

2x10-8

4x10-8

6x10-8

8x10-8

I DS(A

)

VDS

(V)

VG=0 V

VG=-0.5 V

VG=-1 V

VG=-1.5 V

VG=-2 V

VG=-2.5 V

Depletion-mode ZnO nanowire field-effect transistor

Source Drain

GateOxide

Nanowire

Si

Insulator (SiO2)

Source(Al/Pt/Au)

Drain(Al/Pt/Au)

Gate(Al/Pt/Au)

Nanowire

Gate oxide((Ce,Tb)MgAl11O19)

-3 -2 -1 0

0

2x10-8

4x10-8

6x10-8

VG(V)

I DS(A

)

0.0

0.1

0.2

0.3 IDS

gm

gm (m

S/mm

)

0 100 200 300 400 500 6000.0

4.0x10-8

8.0x10-8

1.2x10-7

1.6x10-7

1211109876

543

2

I DS(A

)

Time(sec)

non UV UV(365nm)

2 3 4 5 6 7 8 9 10 11 12

0

50

100

150

200

250

300

Co

ndu

ctan

ce(n

S)

pH

non UV UV(365nm)

electrode(Al/Pt/Au) Nanowire

Si

Insulator (SiO2)

Microchannel

pH Sensing with Single ZnO Nanowire

Hydrogen Detection

• Hydrogen has been used as fuels in many NASA’s space exploration missions.

• President Bush’s Hydrogen Fuel Initiative in 2003.

• Why hydrogen sensing?

– Safety!

– Production, Storage, Transport

• Hydrogen concentration in air reaches a dangerous level at 4%. ppm-level detection is needed.

Simple Fabrication Process

• Direct deposition of metal contacts on the silicon substrate with nanorods.

• No need to go through sonication and E-beam lithography to fabricate the sensors.

• The sensor has better sensitivity (more nanorods combined).

Al/Pt/Au

Al/Pt/Au

Hydrogen-Selective Sensing at Room Temperature with ZnO Nanorods

Hydrogen-selective gas sensing at 25C with Pd/ZnO nanorods

0 30 60 90 120 150640

650

660

670

950

960

AirAirAirAir 500ppm H

2

250ppm H

2

100ppm H

2

10ppm H

2

O2

N2

ZnO nanorod with Pd

ZnO nanorod without Pd

Resistance(ohm)

Time(min)

Wireless Hydrogen Sensor System Prototype – powered by

battery

916MHz

TX RXMicro-

controllerLow-noiseOp Amp

Micro-controller

16x1 LCD

Remote Sensor Central Station

Self-Powered Wireless Sensor

• Use energy from ambient– Solar, vibration, ambient RF radiation

• Use energy supplied locally– Hydrogen flow, micro fuel cell, acoustic,

thermal gradient

• Use energy supplied remotely– Wireless power supply (wireless power

transmission)

High quality,single-crystal growth of wide bandgap semiconductor nanowires

Bimodal growth of cored ZnO/(Zn,Mg)O heterostructured nanowires.

Type I -. Core : Zn1-xMgxO (x < 0.02) , Hexagonal wurtzite structure

-. Sheath : Zn1-xMgxO (x >> 0.02), Hexagonal wurtzite structure

Type II -. Core : Zn1-xMgxO (x < 0.02), Hexagonal wurtzite structure

-. Sheath : (Mg,Zn)O, Cubic rock salt structure

(Mg,Zn)O nanowires having cubic rock salt structure

Conclusions

Functional Nano-devices

Pt/ZnO nanowire Schottky Diode

Depletion-mode GaN and ZnO nanowire field-effect transistor

UV, pH, & gas sensors from GaN,InN and ZnO nanowires