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, ChihYang Chang3,G.C. Chi3,Wei-Ming Wang3 and LiChyong 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
FESEM image & CL spectrum
of a single GaN NW with two electrodes
500μm
5μm
Ti/Au Pad
SiNx/Si
Ti/Au Pad
C L Intensity (a. u.)
7000
sin g le G a N N W
th e p e a k p o sitio n : 3 .4 7 e V
6000
5000
4000
2 .8
3 .0
3 .2
3 .4
3 .6
E n e rg y (e V )
3 .8
4 .0
4 .2
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
InN NWs grown by catalytic thermal-CVD
HRTEM image
30
35
40
 (440)
45
50
2  (d e g re e )
XRD spectrum
(103)
(110)
(102)
(413)
(332)
In (110)
(411)
 (400)
(002)
 (222)
(100)
25
5 nm
In N
 In 2 O 3
In te n s ity (a .u .)
0 .3 08 n m
(101)
(a )
55
60
Temperature-dependent I-V curve of a InN NW
0 .0 0 0 6
0 .0 0 0 4
C u rre n t (A )
0 .0 0 0 2
0 .0 0 0 0
-0 .0 0 0 2
In N C 6 3 -9 4 -9
400K
350K
300K
250K
200K
150K
100K
50K
4K
-0 .0 0 0 4
-0 .0 0 0 6
-1 .0
-0 .5
0 .0
V o lta g e (V )
2300
2250
R e sista n ce (  )
2200
D a ta : R vs T _ B
M o d e l: T E M c o e f. o f re s is tivity
C h i^2
R ^2
= 2 0 5 .7 7 1 4 3
= 0 .9 8 1 7
R0
2 1 4 5 .1 1 4 2 9
0 .0 0 0 4 7
300
±0

2150
T0
± 6 .1 0 2 0 3
± 0 .0 0 0 0 3
2100
2050
2000
In N C 6 3 -9 4 -9
R = R 0 [1 +  (T -T 0 )]
1950
50
100
150
200
250
300
T e m p e ra tu re (K )
350
400
450
0 .5
1 .0
Resistivity comparison between thin film and nanowire
(n-type GaN and InN)
thin film
resistivity
(Ω cm)
nanowire
contact resistivity resistivity (Ω contact resistance
(Ω cm2)
cm)
(Ω)
n-GaN
4.4×10-2 a
3~7×10-6 a,b
56 ~
1.24×10-4 c,d,e
InN
2.1~
3.1×10-3
1.8×10-7 f
4×10-4 i
X
2i
f,g,h
a
Solid State Electron 41, p165-168 (1997)
b
Appl. Phys. Lett. 70, p57-59 (1997)
c
Appl. Phys. Lett. 85, p1636-1638 (2004)
d
Nano Lett. 2, p101-104 (2002)
e
Nano Lett. 3, p1063-1066 (2003)
f
Appl. Phys. Lett. 64, p1508-1510 (1994)
Solid-state Electronics, 39, p1289-1294 (1996)
h
J. Vac. Sci. Technol. B, 14, p3520-3522 (1996)
i
This work
g
Single Crystal Nanowire
0002
1010
• 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.
Heterostructured nanowires
Radial heterostructure
Type I
Sheath
(Zn,Mg)O
(Hexa.)
Axial heterostructure
Type II
Core
(Zn,Mg)O
(Hexa.)
Growth condition
-. Zn : 3 × 10-6 mbar
-. Mg : 2 × 10-7 mbar
-. O3/O2 : 5 × 10-4 mbar,
-. Tg= 400C
(Mg,Zn)O
(cubic)
Zn1-xMgxO
(x <0.02)
(Hexa.)
Growth condition
-. Zn : 3 × 10-6 mbar
-. Mg : 4 × 10-7 mbar
-. O3/O2 : 5 × 10-4 mbar,
-. Tg= 400C
ZnO
(Zn1-XMgX)O
ZnO
(Zn1-XMgX)O
Type I - Radial heterostructured nanowire
Sheath
(Zn,Mg)O
(Hexa.)
Core
(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
Type II - Radial heterostructured (Zn,Mg)O/(Mg,Zn)O nanowire
a
(Mg,Zn)O
(cubic)
bb
Zn1-xMgxO
(x <0.02)
(Hexa.)
10 nm
Compositional line scan
across the nanowire (STEM)
(11ī)
-. Sheath (Shell): Mg1-xZnxO
Cubic
Rock salt structure
Zn
140
Mg
Zn
Mg
120
N o rm a liz e d C o u n ts
-. Core : Zn1-xMgxO
Hexagonal
Wurtzite structure
100
80
60
40
20
0
0
10
20
30
40
50
P o s itio n a c ro s s N a n o w ire (n m )
60
(Mg,Zn)O nanowire (cubic rock salt structure)
020
200
2.04 Å
B=[001]
M gO
Intensity(arb.)
In te n s ity (a rb .)
80
Growth condition
Mg
O
Zn
Mg
-. Zn : 3 × 10-6 mbar
60
-. O3/O2 : 5 × 10-4 mbar,
40
O
-. Mg : 8 × 10-7 mbar
20
-. Tg = 400C
Zn
0
0
50
100
150
P o sitio n a cro ss th e n a n o w ire (n m )
Position
across nanowire(nm)
200
Nanowires vs Zn, Mg pressures
ZnO
Radial heterostructured (Zn,Mg)O
I
hexagonal
wurtzite st.
core
/ sheath
(Zn1-xMgx)O/(Zn1-xMgx)O
hexa. / hexa.
wurtzite / wurtzite
(Mg,Zn)O
II
core
/ sheath
(Zn1-xMgx)O / (Mg,Zn)O
cubic
rock salt st.
hexa. / cubic
wurtzite / rock salt st.
 Zn = 3 × 10-6
 Zn = 3 × 10-6
 Zn = 3 × 10-6
 Zn = 3 × 10-6
 O3/O2 = 5 × 10-4
 O3/O2 = 5 × 10-4
 O3/O2 = 5 × 10-4
 O3/O2 = 5 × 10-4
 Mg = none
 Mg = 2 × 10-7
 Mg = 4 × 10-7
 Mg = 8 × 10-7
Tg= 400C [unit: mbar]
Fabrication of ZnO nanowire device
ZnO Nanowire
Electrode (Al/Pt/Au)
Al/Pt/Au
 Motivation
-. Fundamental understanding of transport
-. Nanoelectronics
Insulator
-. Nano sensors (UV, chemical, bio.)
 Structure of Nanodevice
-. Electrode : Al/Pt/Au by sputtering
-. Diameter of ZnO nanowire : 130 nm
-. Channel Length : 3.7 m
Prototype device fabrication sequence
Find Nanowires
Relative To Alignment
Marks
Deposit
SiO2
Spin PMMA
Resist
Design and
Deposit Alignment
Marks
Ethanol and Nanowire
Suspension
E-beam Write
Aligned Pattern
And Develop
Deposit Metal
And Lift Off
Evaporation &
Nanowires Deposition
UV Response of single ZnO nanowire
U V = 3 6 6 n m re sp o n se a t 0 .2 5 V
UV 366nm at VD 0.25V
UV 366nm
D a rk
UV366nm
800
C u rre n t (n A )
20
Dark
0
-2 0
C u rre n t (n A )
40
on
600
400
200
-4 0
off
0
-0 .3
-0 .2
-0 .1
0 .0
0 .1
V o lta g e (V )
0 .2
0 .3
0
100
200
T im e (se c)
300
400
Pt/ZnO nanowire Schottky Diode
Pt/Au (schottky contact)
0 .9
D a rk
UV366nm
Al/Pt/Au
Al/Pt/Au
C u rre n t (n A )
0 .6
0 .3
0 .0
-0 .3
-0 .6
-0 .9
-0 .4
-0 .2
0 .0
0 .2
0 .4
B ia s (V )
1x10
0 .0 0
1
Forward Bias
Reverse Bias
1x10
C u rre n t (n A )
C u rre n t (n A )
-0 .0 5
-0 .1 0
1x10
1x10
-0 .1 5
1x10
-0 .2 0
-1 0
1x10
-8
-6
-4
B ia s (V )
-2
0
0
-1
-2
I=Io(eqV/nkT-1)
Ideality factor = 1.1
-3
-4
0 .0 0
0 .0 5
0 .1 0
B ia s (V )
0 .1 5
0 .2 0
Depletion-mode ZnO nanowire field-effect transistor
Gate oxide
((Ce,Tb)MgAl11O19)
Source
(Al/Pt/Au)
Gate(Al/Pt/Au)
8x10
Nanowire
-8
V G=0 V
Drain
(Al/Pt/Au)
V G = -0 .5 V
6x10
V G = -1 V
-8
V G = -1 .5 V
V G = -2 V
I D S (A )
V G = -2 .5 V
Insulator (SiO2)
4x10
2x10
Si
-8
-8
0
0
2
4
6
8
10
V D S (V )
Gate
6x10
-8
ID S
Oxide
4x10
-8
0.2
I D S (A )
Drain
2x10
Nanowire
0.1
-8
0.0
0
-3
-2
-1
V G (V )
0
g m (m S /m m )
Source
0.3
gm
pH Sensing with Single ZnO Nanowire
electrode
(Al/Pt/Au)
Nanowire
1 .6 x1 0
-7
non UV
U V (3 6 5 n m )
1 .2 x1 0
-7
I D S (A )
Microchannel
Insulator (SiO2)
8 .0 x1 0
-8
2
4 .0 x1 0
3
4
-8
5
6
7
8
9
10
11
12
0 .0
Si
0
100
200
300
400
500
600
T im e (s e c )
C o n d u c ta n c e (n S )
300
non UV
U V (3 6 5 n m )
250
200
150
100
50
0
2
3
4
5
6
7
pH
8
9
10
11
12
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
960
Resistance(ohm)
ZnO nanorod without Pd
950
N2 O
2
10ppm
H2
Air
100ppm
H2
Air
250ppm
H2
Air
500ppm
H2
Air
670
660
650
ZnO nanorod with Pd
640
0
30
60
90
Time(min)
120
150
Wireless Hydrogen Sensor
System Prototype – powered by
Remote Sensor
Central Station
battery
Low-noise
Op Amp
Microcontroller
RX
TX
916
MHz
Microcontroller
16x1 LCD
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)
Conclusions


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

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