Wide Bandgap Semiconductor Nanowires for Sensing
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Transcript of Wide Bandgap Semiconductor Nanowires for Sensing
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