Development of Thin Film and Nanorod ZnO-Based
LEDs and Sensors
S. J. Pearton(1), W. T. Lim(1), J. S. Wright(1), R. Khanna(1), L. Voss(1), L.
Stafford(1), L. C. Tien(1), H. S. Kim(1), D.P. Norton(1), J.-J. Chen(2), H.T.
Wang(2), B.S. Kang(2), F. Ren(2), J. Jun (3), J. Lin(3), A.Osinsky(4) and
A.Dabiran(4)
(1) MSE, (2) Chem. Engin., (3) ECE, University of Florida, Gainesville, FL
32611
(4) SVT Associates, Eden Prairie, MN 55344
Supported in part by NSF DMR 0400416
(Verne Hess) and DOE DE-FC26-04NT42271
(Ryan Egidi)
Introduction
Bandgap (eV)
µe (cm2/V-sec)
µh (cm2/V-sec)
me
mh
Exciton binding
energy (meV)
GaN
3.4
220
10
0.27mo
0.8mo
28
ZnO
3.3
200
5-50
0.24mo
0.59mo
60
• Direct, wide bandgap
• Bulk ZnO (n-type) commercially
available
• Grown on inexpensive (glass)
substrates at low temperatures
• High exciton binding energy
• Heterojunction by substitution in
Zn-site
– Cd ~ 3.0 eV
– Mg ~ 4.0 eV
Potential Applications
UV/Blue optoelectronics
Transparent transistors
Nanoscale detectors
Spintronic devices
• Ease of synthesis of nanowires
• Obstacle: good quality,
reproducible p-type
Zn(Mg,Cd)O alloys
The ternary system CdO-ZnO-MgO covers a large bandgap range
B
C
D
E
F
G
< Single quantum wells >
8
Bandgap Energy (eV)
M gO
7
A lN
6
5
M gS
4
G aN
3
2
ZnSe
CdS
G aP
6 H -S iC
CdO
In N
1
0
2 .5
M gSe
ZnS
ZnO
ZnTe
A lA s
G aA s
CdSe
In P
Si
In A s
Ge
3 .0
3 .5
4 .0
4 .5
5 .0
5 .5
L a ttic e C o n sta n t (Å )
6 .0
6 .5
Zn0.95Cd0.05O/ZnO Heterojunction Band
Offsets by XPS (samples grown by SVT-Andrei
ZnO substrate
Zn 2p3
Intensity (a.u.)
Osinsky)
VBM
EZnO
V
=1.33 eV
4
3
2
Samples grown by rf plasma assisted MBE
1
0
ZnO
EZnO
Zn2p3 - EV
Conduction band offset 0.30 eV
ZnCdO
EV
Valence band offset 0.17 eV
=1.18 eV
4
3
2
1
0
Binding Energy (eV)
ZnCdO
EZnCdO
Zn2p3 - EV
1024
1020
1016 10
Binding Energy (eV)
2.9 eV bandgap for ZnCdO
XPS performed at UF, Charles Evans and
Associates
0.1 µm ZnCdO/0.1 µm ZnO
VBM
Zn 2p3
Intensity (a.u.)
Intensity (a.u.)
Binding Energy (eV)
0
Energy Band Diagram of
Zn0.95Cd0.05O/ZnO Heterojunction
ZnCdO
ECZnCdO
EgZnCdO=2.90 eV
EVZnCdO
(EV – EZn 2p3)ZnCdO
=1020.85 eV
EZn 2p3ZnCdO
ZnO
EC=0.30eV
ECZnO
EgZnO =3.37 eV
EVZnO
EV=0.17eV
(EV – EZn 2p3)ZnO
=1020.83 eV
EZn 2p3ZnO
ΔEv = (EZn-2p-EV)thick ZnCdO-( EZn-2p-EV)ZnO- (EZn-2p-EZn-2p)ZnCdO/ZnO
ZnCdO is an attractive option as the narrow bandgap active region in
ZnO-based heterojunction LEDs (ZnMgO band offset almost all in
c (Ohm-cm2)
Ohmic Contacts to ZnCdO
10
-2
10
-3
10
-4
Ti/Au
Ti/Pt/Al/Au
200
300
400
500
600
Annealing Temperature (oC)
The minimum contact resistivity
Ti/Au  2.3x10-4Ωcm2 at
450oC anneal
Ti/Al/Pt/Au1.6x10-4Ωcm2
at 500oC anneal
Severe degradation after 600oC
anneal
Optical Microscopy Images of
Metal on ZnCdO
As annealing temperature increases, metals start to form intermetallic
compounds.
Ti/Au to ZnCdO
Ti/Al/Pt/Au to ZnCdO
Reference
450oC
10 µm
350oC
Reference
350oC
600oC
450oC
600oC
Smoother morphology after
Reacted appearance after 350oC
annealing even at 600oC
Ti/Au more thermally stable than Ti/Al/Pt/Au
More information: AES Depth profile
AES Depth Profile of Ti/Au to
ZnCdO
8000
o
Annealing at 500 C
Au
100
Count/sec
-8000
C
8000
Ti
Ti
Zn
o
Annealing at 450 C
Au
0
Au
-8000
8000
Au Au Au Au
Au
O
Ti
Ti O
C
Zn
Au
As received
Au
0
Au
-8000
0
Au Au Au Au
O
Au
C
500
1000
1500
Kinetic energy (eV)
Au Au
Atomic concentration (%)
Au
50
C
Zn and Ti outdiffusion to the
surface by 450oC
Zn
Ga
Ti
N
Cd
0
100
o
Annealing at 450 C
Au
50
Zn
O
N
Cd
0
100
Au
As received
Ti
Zn
O
50
Ga
C
0
N
Cd
0
500
C
N
O
Ti
Zn
Ga
Cd
Au
Ga
Ti
C
Au Au
2000
O
Au
0
o
Annealing at 500 C
1000
1500
2000
2500
3000
o
Sputter Depth (A)
The formation of the TiOx interfacial
region is evident after annealing
 improved contact resistance
AES Depth Profile of Ti/Al/Pt/Au to
ZnCdO
8000
Au
O
Al
o
Annealing at 450 C
0
-8000
8000 Au
0
-8000 Au
0
Au Au Au Au
Au
C
Au
Au
o
Annealing at 500 C
Annealing at 500 C
Au
Au
C
O
Al
Au Au Au Au
As received
O
C
50
50
2000
Al outdiffusion to the surface by
450oC in the metallization
scheme
N
o
Annealing at 450 C
Pt
Au
Ga
Zn
O
Al
Ti
0
100
Au
Pt
Al
As received
Ti O
50
C
0
Ga
Zn
N
Cd
1000
C
N
O
Al
Ti
Zn
G
C
Pt
Au
N
C
Au Au Au Au
Au
1000
1500
Kinetic energy (eV)
Ga
Ti
C
Au
Zn
O
Al
0
100
0
500
Au
Pt
Atomic concentration (%)
Count/sec
8000
0
-8000
100
o
2000
3000
4000
5000
6000
o
Sputter Depth (A)
Outdiffusion of Pt, Al, and Ti at higher
anneal temperatures and oxidation of
the Ti
TI/Au Ohmic Contact to Al-doped nZnO
160
19
-3
N ~ 1.32x10 cm
18
-3
N ~ 9.09x10 cm
As deposited
150
140
130
120
As deposited
110
100
90
100
200
300
400
Annealing Temperature (oC)
500
19
Rc (Ohm-cm2)
Rs (Ohm/square)
170
-3
N ~ 1.32x10 cm
18
-3
N ~ 9.09x10 cm
1E-6
1E-7
As deposited
As deposited
100
200
300
400
Annealing Temperature (oC)
500
The as-deposited contacts are ohmic with excellent specific contact resistivity of
2.4x10-7 Ω cm2
Subsequent annealing produces a minimum value of 6x10-8 Ω cm2 after
processing at 300oC
Carrier tunneling and additional annealing further reduces the specific contact
resistance
Tunneling of Ti/Au Contact to Aldoped n-ZnO
800 Å
200 Å
1 μm ZnO:Al
2
Specific contact resistance (Ohm-cm )
Au
Ti
Temperature range: 25~225oC
Independence of temperature
tunneling is the dominant
current transport mechanism
The relation between the
specific resistivity and doping
concentration:
-6
10
-7
8x10
6x10
-7
4x10
-7
2x10
-7
Annealed at 150oC
0.0020
0.0025
0.0030
1/T (1/K)
R SCR  exp[
0.0035
2
 S me
*

(
B
ND
)]
Wet Chemical Etching
Process involves either oxidation or reduction of semiconductor
surface followed by removal of the soluble reaction product
High selectivity
Isotropic etch profile
Ability to remove undesirable ions and contaminants from
Ohmic ring
Ohmic
ring
the wafer surface
Photoresist
Film to be etched
Underlying Film
p-ZnO
p-ZnMgO
ZnO
n-ZnMgO
n+-ZnO
substrate
Isotropic etch profile
ZnO LED cross section structure
Etching of ZnCdO (samples grown
at SVT )
100
Etch Rate (nm/min)
90
HCl
H3PO4
RT
Using dilute HCl and H3PO4 mixtures
Controllable etch rates in the range
(<100 nm min-1) for mesa formation
80
70
60
50
40
30
0.0015 0.0020 0.0025 0.0030 0.0035 0.0040
Concentration (M)
Etch rate (nm/min)
0.0031M HCl, Ea=0.37 Kcal/mol
6
0.0029M H3PO4, Ea=0.38 Kcal/mol
5
4
2.8
2.9
3.0
3.1
3.2
-1
1000/T(K )
3.3
3.4
Solution temperature in the range of 2575oC
The etch rate is diffusion-limited
Selective Etching of ZnCdO over
ZnO
Optical microscopy
minimum undercut
Etch rate is independent of orientation
100μm
60
Etch selecivity
HCl
H3PO4
The selectivity with HCl/H2O was over
30
The maximum selectivity with H3PO4
/H2O was ~15
40
20
0
0.0020
0.0025
0.0030
Concentration (M)
0.0035
Etching of ZnMgO
1000
HCl
H3PO4
Etching rate (nm/min)
Etching rate (nm/min)
2
6x10
2
5x10
750
500
250
0
0.005
0.010
0.015
0.020
Concentration (M)
0.025
4x10
2
3x10
2
2x10
2
0.024M HCl
Ea= 3.29 Kcal/mol
0.024M H3PO4
10
2
Ea= 2.07 Kcal/mol
2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5
1000/T (K-1)
Solution temperature in the range of 25-75oC
The etch rate is diffusion-limited
HCl
H3PO4
50
40
Etch selecivity
Etching rate (nm/min)
Selective Etching of ZnMgO over
ZnO
30
20
10
ZnO substrate
0
0.0
0.2
0.4
0.6
0.8
1.0
Concentration (M)
1.2
450
400
350
300
250
200
150
100
50
0
0.010
HCl
H3PO4
0.015
0.020
0.025
Concentration (M)
The selectivity with HCl/H2O was over 250
The maximum selectivity with H3PO4/H2O was ~30
0.030
•
Site-selective growth of ZnO nanorods possible using a
catalysis-driven molecular beam epitaxy method.
RF PLASMA
Zn flux
RHEED SCREEN
OZONE
GENERATOR
O3/O2
Zn
Zn
ION
GAUGE
Ag catalyst
particles
e-GUN
EFFUSION CELL
SUBSTRATE
HEATER
•
Growth of ZnO on Ag-coated Si via MBE.
•
Nominal Ag film thickness: 20 ~ 200 Å.
(Coalesce into islands at growth temp.)
•
Oxygen source: ozone/oxygen mixture
•
Growth Temperature: 300°C ~ 600 °C.
O2/O3 flux
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
-. Nano sensors (UV, chemical, bio.)
Insulator
-. Nanoelectronics
 Structure of Nanodevice
-. Electrode : Al/Pt/Au by sputtering
-. Diameter of ZnO nanowire : 130 nm
-. Channel Length : 3.5 m
ZnO Nanorod MOS FET
Oxide
Gate
Gate oxide
((Ce,Tb)MgAl11O19)
Source
(Al/Pt/Au)
Gate(Al/Pt/Au)
Nanowire
Drain
(Al/Pt/Au)
Source
Drain
Insulator (SiO2)
Nanowire
Si
8x10
-8
V G=0 V
V G = -0 .5 V
•
Apply the stable oxide((Ce,
Tb)MgAl11O19 ) for each device
Can be used as passive layer in
gas, humidity, chemical sensor
6x10
V G = -1 V
-8
V G = -1 .5 V
V G = -2 V
V G = -2 .5 V
I D S (A )
•
4x10
2x10
-8
-8
0
0
2
4
6
V D S (V )
8
10
pH sensor with gateless nanorod FET
Nanowire
1 .6 x1 0
-7
non UV
U V (3 6 5 n m )
Microchannel
1 .2 x1 0
Insulator
Insulator (SiO2)
I D S (A )
8 .0 x1 0
-7
-8
2
4 .0 x1 0
Si
pH
3
4
5
-8
6
7
8
9
10
11
12
0 .0
0
100
200
300
400
500
600
T im e (s e c )
300
C o n d u c ta n c e (n S )
electrode
(Al/Pt/Au)
non UV
U V (3 6 5 n m )
250
200
8.5 nS/ pH in the dark
20 nS/ pH under UV(365nm)
150
100
50
0
2
3
4
5
6
7
8
9
10
11
12
pH
Appl. Phys. Lett., 86, 112105 (2005)
ZnO Nano-Rods for Hydogen Sensing
D
S
Al/Pt/Au
ZnO M-NRs
Al2O3 Substrate
a)
b)
Schematic of Multiple ZnO Nano-Rods
Close-Up of Packaged ZnO NanoRod Sensor
• ZnO currently used for
detection of humidity, UV
light and gas detection
• Easy to synthesize on a
plethora of substrates
• Bio-safe characteristics
• Large chemically sensitive
surface to volume ratio
• If coated with Pt or Pd, can
increase device’s sensitivity
to hydrogen
• High compatibility to
microelectronic devices
Single nanorod hydrogen gas sensor
Pt-ZnO Nanorod
Electrode (Al/Pt/Au)
Δ R/R (Sensitivity)
0.01
ZnO nanorod with Pd
0.00
N2 O2
-0.01
Air
Air
Air
Air
-0.02
-0.03
10ppm
H2
-0.04
100ppm
H2
0
30
60
250ppm
H2
90
Time(min)
500ppm
H2
120
150
Insulator
Al/Pt/Au
Current status of ZnO LED research
1. Nitrogen doping [ Tsukazaki et al. Nat. Mater. 4, 42 (2005) ]
• Growth method
: L-MBE (repeated-temperature-modulation epitaxy)
• Structure
: p-ZnO:N / i-ZnO / n-ZnO:Ga LED on a ScAlMgO4 substrate
(a) Structure
(b) Current-voltage
(c) Electroluminescence
Current status of ZnO LED research
2. Phosphorus doping [ Lim et al. Adv. Mater. 18, 2720 (2006) ]
• Growth method
: Sputtering system
• Structure
: p-ZnO:P / n-ZnO:Ga LED on a sapphire substrate
: Mg0.1Zn0.9O energy barrier layer
(a) Current-voltage
(b) Electroluminescence
Current status of ZnO LED research
3. Arsenic doping [ Ryu et al., Appl. Phys. Lett. 88, 241108 (2006) ]
• Growth method
: Hybrid beam deposition (HBD)
• Structure
: p-ZnO:As / active layer / ZnO substrate
: BeZnO/ZnO active layer (seven quantum wells)
(a) Structure
(b) Current-voltage
(c) Electroluminescence
Device Fabrication
Cermet: (0001) undoped, I grade
n0=1017 cm-3; μe=190 cm2/V·s
Proc. of SPIE, Vol.5941, 59410D-1(2005)
Implantation

dose 1: 10keV, 2×1013 cm-2
dose 2: 30keV, 5×1013 cm-2
dose 3: 65keV, 9×1013 cm-2
dose 4: 140keV, 2.4×1014 cm-2
Au (80nm)
Ni (20nm)
N+ implanted ZnO (300nm)
ZnO
substrate
Ti (20nm)
Au (200nm)
Thermal activation
(RTA, furnace; T=600~1000°C)
Backside metal: Ti/Au(20/200nm)
Front-side metal: Ni/Au(20/80nm)

Diode I-V Characteristics
0.04
1 x1 0
+
0
800C O 2 RTA
0.02
0.01
600C, O2, 2 mins.
800C, O2, 2 mins.
950C, O2, 2 mins.
1 x1 0
C u rre n t(A )
Current(A)
0.03 N implanted ZnO
0.00
-0.01
-0.02
-15
1 x1 0
1 x1 0
-0.03
-0.04
1 x1 0
-10
-5
0
5
Voltage(V)
10
15
1 x1 0
-------- lin e a r fit, slo p e = 1 .4
-2
-4
-6
-8
-1 0
-1 0
-8
-6
-4
-2
0
2
4
V o lta g e (V )
Leakage current~10-4A @ -6V
Ideality factor~11
Device fabrication
Light emission from ZnO pn homojunction device
Electroluminescence at 120K
12000
T= 298 K
400000
U n-im planted Z nO
Im planted Z nO
350000
300000
250000
200000
150000
100000
50000
0
350
400
450
500
550
w a ve le n g th (n m )
600
E L in te n s ity (a rb . u n it)
P L in te n s ity (a rb . u n it)
450000
10000
8000
6000
I= 3 0 m A
T = 120 K
T = 298 K
4000
2000
0
350
400
450
500
550
w a ve le n g th (n m )
600
Vertical ZnO NWs/PEDOT LED
Nanowire Array
The cross section schematic of ZnO nanowires LED
8
1 .0 0 x 1 0
-7
1.4
L -I C u rv e
6
P L at R T
1.0
V o ltag e (V )
P L Intensity ( a. u. )
1.2
0.8
0.6
0.4
7 .5 0 x 1 0
-8
4
5 .0 0 x 1 0
-8
2
0.2
0.0
2 .5 0 x 1 0
0
350
400
450
500
550
600
W avelen gth ( n m )
650
700
0 .0 0 0
0 .0 0 5
0 .0 1 0
0 .0 1 5
C u rre n t (A )
0 .0 2 0
0 .0 2 5
-8
L ig h t in ten sity (m W )
I-V c u rv e
(3 7 8 )
Summary
Valence and conduction band offsets of the Zn0.95Cd0.05O/ZnO
material system are 0.17 eV and 0.30 eV, respectively. In the
ZnMgO, the band offset is mainly in the valence band
Ohmic contacts fairly simple on n-and p-ZnO, but Schottky contacts
are difficult (low barrier height, leaky).
The etch selectivity of ZnCdO/ ZnO with HCl/H2O >30
Some rudimentary LEDs demonstrated by groups worldwide-need to
show robust bandedge EL on cheap, large area substrates if there is
any chance of finding a niche relative to the nitrides
Functional nanowires with excellent structural and optical qualitymany types of sensors demonstrated-Electrical transport properties of
single ZnO nanowires, Pt/ZnO nanowire Schottky Diode, depletion-mode ZnO
nanowire field-effect transistor, UV, pH, & gas sensor
Lots of room to study transport/functionality in radial and longitudinal
wires
Conclusions


Site-selective growth of ZnO nanowires using catalyst, Ag, by molecular Beam
Epitaxy
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

Nano-devices using ZnO nanowires

Electrical transport properties of single ZnO nanowire

Pt/ZnO nanowire Schottky Diode

Depletion-mode ZnO nanowire field-effect transistor

UV, pH, & gas sensor