Transcript Kelly Ip Defense - Stephen J. Pearton

Process Development for
ZnO-based Devices
Kelly Ip
PhD Defense ~ July 1, 2005
~ University of Florida ~
Materials Science and Enginering
Outline
Introduction
 Inductively-coupled plasma (ICP) etching
 Hydrogen in ZnO
 Contact metallization



Ohmic contacts
Schottky contacts
p-n junction diode
 Conclusions

~ University of Florida ~ Materials Science and Engineering ~
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 substrates at
low temperatures
High exciton binding energy
Heterojunction by substitution in
Zn-site


Potential Applications
UV/Blue optoelectronics
Transparent transistors
Nanoscale detectors
Spintronic devices



Cd ~ 3.0 eV
Mg ~ 4.0 eV
Nanostructures demonstrated
Ferromagnetism at practical Tc
when doped with transition metals
Obstacle: good quality,
reproducible p-type
~ University of Florida ~ Materials Science and Engineering ~
ICP Etching

Wet etching



High-density plasma etching





Anisotropic with high resolution
Favored by modern manufacturing environment
Bulk, wurtzite (0001) ZnO from Eagle-Picher
Gas chemistry:


HCl, HNO3, NH4Cl, and HF
Generally isotropic with limited resolution and selectivity
Cl2/Ar (10/5 sccm) & CH4/H2/Ar (3/8/5 sccm)
Constant ICP source power at 500W and process pressure
at 1 mTorr
Varied rf chuck power: 50 – 300W
~ University of Florida ~ Materials Science and Engineering ~
ICP Etching - Etch Rates
5000
300
3000
250
2000
200
150
1000
DC Bias (-V)
4000
Etch Rate (Å/min)
350
Cl2/Ar dc bias
CH4/H2/Ar dc bias
Cl2/Ar etch rate
100
CH4/H2/Ar etch rate
0
50
50
100
150
200
250
300
RF Power (W)
CH4/H2/Ar ~3000 Å/min
Cl2/Ar ~1200 Å/min
~ University of Florida ~ Materials Science and Engineering ~
ICP Etching - Etch Mechanism
Ion-Assisted Etch Mechanism
ER  E0.5-ETH0.5
Cl2/Ar Chemistry
CH4/H2/Ar Chemisty
Etch Rate (Å/min)
3000
2500
ETH ~ 96 eV for
CH4/H2/Ar
Vapor pressure
of etch products:
2000
1500
(CH4)2Zn
1000
301 mTorr at 20°C
500
ZnCl2
0
10
12
14
16
18
20
1 mTorr at 428 °C
Square Root (25+Vb)
~ University of Florida ~ Materials Science and Engineering ~
ICP Etching - Photoluminescence
Log PL Intensity (Arb.)
3.0
PL Intensity (Arb.)
2.5
Control
2.0
1.5
50W rf
100W rf
300W rf
200W rf
1.0
0.5
0.0
2.0
2.5
3.0
Energy (eV)
3.5
Control
50W rf
100W rf
300W rf
200W rf
1
0.1
0.01
2.0
2.5
3.0
Energy (eV)
Optical degradation even at the lowest rf power
~ University of Florida ~ Materials Science and Engineering ~
3.5
ICP - AFM
Control
ZnO
CH4/H2/Ar
500W ICP Power
50 W rf
100 W rf
RMS Roughness (nm)
8
6
4
Control
2
0
200 W rf
300 W rf
50
100
150
200
250
300
rf Power (W)
Zn and O etch
products removed at
same rate
~ University of Florida ~ Materials Science and Engineering ~
ICP Etching - AES and SEM
Min: -9203
Max: 6112
dN(E)
Control
O
Zn
50
250
450
650
850
1050 1250 1450
Kinetic Energy (eV)
1650
1850
2050
Min: -9742
Max: 5973
dN(E)
O
Zn
50
250
450
650
CH4/H2/Ar
200W rf
850
1050 1250 1450
Kinetic Energy (eV)
1650
1850
2050
~ University of Florida ~ Materials Science and Engineering ~
ICP Etching - Summary
Dry etching is possible with practical etch
rates using CH4/H2/Ar
 Surface is smooth and stoichiometric
 Anisotropic sidewalls
 Optical quality is sensitive to ion energy
and flux

~ University of Florida ~ Materials Science and Engineering ~
Hydrogen in ZnO

Hydrogen






Understand diffusion behavior and thermal stability
Bulk, wurtzite (0001) ZnO, undoped (n~1017cm-3)
from Eagle-Picher
Hydrogen incorporation



Predicted role as shallow donor
Introduced from growth ambient
Present in optimal plasma etch chemistry
Ion Implantation of 2H or 1H (100keV, 1015 - 1016 cm-2)
2H plasma exposure in PECVD at 100-300°C, 30 mins
Post-annealing: 500 - 700°C
~ University of Florida ~ Materials Science and Engineering ~
Hydrogen in ZnO - Implanted - SIMS
10
20
15
-2
H 10 cm ZnO
5 min anneals
3
H Concentration (atoms/cm )
2
10
19
As Implanted
10
18
10
17
500°C
600°C
700°C
10
16
10
15
0.5
1.0
1.5
2.0
Depth (m)
Removal of 2H below SIMS limit at 700°C
Thermally less stable than GaN (>900ºC)
~ University of Florida ~ Materials Science and Engineering ~
Hydrogen in ZnO - Implanted - RBS/C
600
Depth (Å)
10000 7500 5000 2500
0
1
RBS Yield
100 keV H ZnO
400
virgin
16
2
110 cm
200
0
0.4
0.6
0.8
1.0
1.2
Energy (MeV)
1.4
1.6
Minimal affect on BS yield near surface
Small increase in scattering peak (6.5% of the random level
before implantation and 7.8% after implantation)  the
nuclear energy loss profile of 100keV H+ is max
~ University of Florida ~ Materials Science and Engineering ~
Hydrogen in ZnO - Implanted - PL
10
1
Control
500 °C
600°C
700°C
As-implanted
0
10
-1
10
-2
Severe optical degradation
even after 700ºC anneal
Point defect recombination
centers dominate
0.014
0.012
10
-3
2.0
2.5
Energy (eV)
3.0
3.5
PL Intensity (Arb.)
PL Intensity (Arb.)
10
0.010
700°C Anneal
600°C Anneal
500°C Anneal
As-implanted
0.008
0.006
0.004
2.0
2.5
3.0
Energy (eV)
~ University of Florida ~ Materials Science and Engineering ~
3.5
Concentration (atoms/cc)
Hydrogen in ZnO - Plasma - SIMS
10
18
10
17
10
16
10
15
2
200°C
100°C
0
5
10
H plasma treatment
15
20
300°C
25
30
Depth (m)
Large diffusion depth
2H diffuses as an interstitial, with little trapping by the lattice
elements or by defects or impurities
~ University of Florida ~ Materials Science and Engineering ~
Hydrogen in ZnO - Plasma/annealed - SIMS
18
10
2
H plasma 200°C
Post treatment anneal
No anneal
Concentration (atoms/cc)
17
10
400°C, 5min
16
10
500°C, 5min
15
10
0
5
10
15
20
25
Depth (m)
2H
completely evolve out of the crystal at 500°C
~ University of Florida ~ Materials Science and Engineering ~
Hydrogen in ZnO - Plasma - CV
17
4x10
Effects 2H plasma
treatment
2
-3
Dopant Concentration (cm )
H Plasma exposure at 200°C & 600°C Anneal
2
H Plasma exposure at 200°C
As Grown
Passivate the
compensating acceptor
impurities
17
3x10
17
2x10
17
1x10
0.0
0.1
0.2
0.3
Induces a donor state
and increases the free
electron concentration
Depth (µm)
Suggest H from growth process
n-type conductivity probably arises from multiple impurity sources
~ University of Florida ~ Materials Science and Engineering ~
Hydrogen in ZnO
100
% H Remaining
100
2
1
60
0
200
400
600
Anneal Temperature (°C)
40
2
% H Remaining
80
10
5 min Anneal
Implanted2H
Implanted
2
Plamsa
Plasma H
20
0
exposure
0
100
200
300
400
500
600
700
Anneal Temperature (°C)
Implanted 2H is slightly more thermally stable:
trapping at residual damage in the ZnO by the
nuclear stopping process
~ University of Florida ~ Materials Science and Engineering ~
Hydrogen in ZnO - Summary






Thermal stability and diffusion behavior of hydrogen
in ZnO
T  700 °C completely evolved the implanted H from
ZnO
Residual implant-induced defects severely degrade
optical properties and minimal affect crystal structure
Plasma: incorporation depths of about 30 m for 0.5
hr at 300°C
T  500 °C to remove H introduced by plasma
exposure
Thermal stability of the hydrogen retention :


direct implantation > plasma exposure
Trapping at residual implant damage
~ University of Florida ~ Materials Science and Engineering ~
Ohmic Contacts
Require low specific contact resistance
 Surface treatments





Ti/Al/Pt/Au metal scheme on n-type ZnO



As-received
Organic solvents (trichloroethylene, methanol,
acetone, 3 mins each)
H plasma
Bulk
PLD films
Au/Ni/Au and Au on p-type ZnMgO
~ University of Florida ~ Materials Science and Engineering ~
1.0x10
-2
8.0x10
-3
6.0x10
-3
4.0x10
-3
2.0x10
-3
2
Specific Contact Resistivity (·cm )
Ohmic Contacts - Ti/Al/Pt/Au on Bulk
Cross-sectional view of circular TLM
Metals
RO
As Received
H2 Plasma
Organic Solvents
250
300
350
400
450
Anneal Temperature (°C)
R1
Bulk n-ZnO
RT 
RS
2
  R1  LT I O ( RO / LT ) LT K O ( R1 / LT ) 
 

ln

  RO  RO I1 ( RO / LT ) R1 K1 ( R1 / LT ) 
C = RS LT2
Marlow and Das, Solid-State Electron. 25 91 (1982)
ρc lowest at 250 °C anneal
ρc ~ 610-4 cm2
Severe contact degradation
after 600 °C anneal
~ University of Florida ~ Materials Science and Engineering ~
Ohmic Contacts - Ti/Al/Pt/Au on Bulk - AES
As Received 450°C
As Received 250°C
100
100
Al
Pt
Ti
80
60
Zn
40
O
20
Atomic Concentration (%)
Atomic Concentration (%)
Au
80
Ti
Pt
Au
60
O
Al
40
Zn
20
C
0
C
0
0
1000
2000
3000
4000
0
1000
Depth (Å)
As Received 350°C
4000
100
Ti
80
Atomic Concentration (%)
Atomic Concentration (%)
3000
As Received 600°C
100
Au
Pt
60
O
Zn
Al
40
20
80
60
Pt
40
Al
O
Zn
Au
20
Ti
C
0
2000
Depth (Å)
0
1000
2000
Depth (Å)
3000
4000
0
C
0
1000
2000
3000
4000
Depth (Å)
~ University of Florida ~ Materials Science and Engineering ~
Ohmic Contacts - Ti/Al/Pt/Au on Bulk - SEM
~ University of Florida ~ Materials Science and Engineering ~
Ohmic Contacts - Growth: n-type ZnO:P Films

Post-growth
Post-growth
Anneal
T
Anneal
T
(°C)
(°C)
30 30
425425
450450
500500
600
600
Increase anneal temperature


Decrease carrier
concentration and Hall
mobility
Increase resistivity
Carrier conc
Carrier 3conc
(#/cm 3)
(#/cm )
 20
20
1.5
1.5 10
10
19
19
6 610
10
18
2.410
1018
2.4
17
3.210
1017
3.2
15
7.5  10
7.5  1015
Hall
Hallmobility
Resistivity
mobility2
Resistivity
(  cm)
2(cm /Vs)
( cm)
(cm /Vs)
0.002
0.002
18.5 18.5
0.013
0.013
7.8 7.8
1.3
1.9 1.9
1.3
12.8
1.5 1.5
12.8
463
1.8
463
1.8
20
600 °C

Reduction of shallow state
density
P dopants activation as
acceptors in O site
10
2
10
1
10
0
10
-1
10
-2
10
-3
500 °C
15
450 °C
10
Carrier Mobility
Resistivity
425 °C
5
0
10
Heo et al APL 83 1128 (2003)
3
16
10
17
10
18
10
19
10
20
-3
Carrier Concentration (cm )
~ University of Florida ~ Materials Science and Engineering ~
Resistivity (-cm)

10
As-grown
2

N-type phosphorus-doped
ZnO film on (0001) Al2O3
grown by PLD
Post-growth annealing
Carrier Mobility (cm /V-s)

Ohmic Contacts - Ti/Al/Pt/Au ZnO:P Films
Ti/Al/Pt/Au (200/800/400/800)Å on
PLD ZnO:P films
Nonalloyed:
n = 1.5  1020 cm-3
-3
c = 8.7  10-7 -cm2
2
Specific Contact Resistance (-cm )
10
-4
10
Annealed:
-5
10
As-deposited
200°C anneal,
measured at 30 °C
200°C anneal,
measured at 200°C
-6
10
-7
10
15
10
16
10
17
10
18
10
19
20
10
10
21
10
Measured at RT:
n = 6.0  1019 cm-3
c = 3.9  10-7 -cm2
Measured at 200 °C
n = 2.4  1018 cm-3
c = 2.2  10-8 -cm2
-3
Carrier Concentration (cm )
~ University of Florida ~ Materials Science and Engineering ~
Ohmic Contacts - p-type ZnMgO Films
5.0
0.50
2.5
Current (mA)
0.25
Current (mA)
Ni/Au (200/800 Å)
No anneal
o
250 C
o
400 C
o
500 C
0.00
-0.25
-0.50
-0.50
-0.25
0.00
0.25
0.50
Bias (V)
-2.5
-5.0
-0.50
-0.25
0.00
0.25
0.50
• Ohmic behavior after annealing  500 °C
16000
Resistance ()
Ti/Al (200/800 Å)
Bias (V)
20000
12000
• Ti/Au more thermally stable than Ni/Au
contacts
Ti/Au Ohmic contact
Ni/Au Ohmic contact
8000
• Severe degradation of Ni/Au after 600
°C anneal
4000
0
0.0
No anneal
o
250 C
o
400 C
o
500 C
o
600 C
0
100
200
300
400
500
o
Annealing Temperature ( C)
600
S. Kim et al APL 84 1904 (2004)
~ University of Florida ~ Materials Science and Engineering ~
Ohmic Contacts - p-type ZnMgO Films
Au/Zn0.9Mg0.1O:P0.02
Au/Ni/Au/Zn0.9Mg0.1O:P0.02
0.04
0.04
600 °C Annealed
600 °C Annealed
0.02
Current (A)
Current (A)
0.02
As-deposited
0.00
0.00
-0.02
-0.02
-0.04
-5.0
-0.04
-5.0
-2.5
0.0
2.5
5.0
As-deposited
-2.5
0.0
2.5
Bias (V)
Bias (V)
Specific contact resistance after 600 °C anneal
Au: 2.5  10-5 cm2
Au/Ni/Au: 7.6  10-6 cm2
~ University of Florida ~ Materials Science and Engineering ~
5.0
Ohmic Contacts - Au/ZnMgO
Au/Zn0.9Mg0.1O:P0.02 As-deposited
Atomic Concentration (%)
100
Au
80
60
O
40
Zn
20
0
Mg
C
0
500
1000 1500 2000 2500 3000 3500
Sputter Depth (Å)
Au/Zn0.9Mg0.1O:P0.02 600ºC Annealed
Atomic Concentration (%)
100
Au
80
60
O
40
Zn
20
0
C
0
Mg
500
1000 1500 2000 2500 3000 3500
Sputter Depth (Å)
~ University of Florida ~ Materials Science and Engineering ~
Ohmic Contacts - Au/Ni/Au/ZnMgO
Au/Ni/Au/Zn0.9Mg0.1O:P0.02 As-deposited
Atomic Concentration (%)
100
Au
80
Ni
60
O
40
Zn
C
20
0
Mg
0
500
1000
1500
2000
2500
Sputter Depth (Å)
Au/Ni/Au/Zn0.9Mg0.1O:P0.02 600ºC Annealed
Atomic Concentration (%)
100
80
Au
60
O
40
Ni
Zn
20
Mg
C
0
0
500
1000
1500
2000
2500
Sputter Depth (Å)
~ University of Florida ~ Materials Science and Engineering ~
Ohmic Contacts - Summary
Ti/Al/Pt/Au to n-type ZnO (bulk, thin film)
 No significant improvement from H2
plasma treatment or organic solvent
cleaning
 AES revealed Ti-O interfacial reactions and
intermixing between Al and Pt layers
T250°C
 Au/Ni/Au to p-type ZnMgO: lower C than
Au alone

~ University of Florida ~ Materials Science and Engineering ~
Schottky Contacts
Previous Works
 Metals: Au, Ag, Pd
 Schottky barriers heights ~ 0.6-0.8 eV
 Barrier heights not following the difference in the
work function value  interface defect states
determine contact characteristics
 Au is unstable even at 60°C
This Work
 Investigate the effect of UV surface cleaning
 Metal schemes:


PLD n-type film: Pt
Bulk: Pt, W, W2B, W2B5, CrB2
~ University of Florida ~ Materials Science and Engineering ~
Schottky Contacts - Pt/Au on Bulk
30 min ozone
0.020
0.5
0.015
Current (A)
Current (mA)
No ozone
1.0
0.0
-6
0.010
-0.05
0.00
0.05
0.000
0.0
0.10
0.1



B = 0.696 eV
 = 1.49
Js = 6.17  10-6 A-cm-2
0.3
0.4
0.5
-2
0
0.0
-0.5
Current (mA)

No ozone treatment: Linear I-V
Ozone treatment:
0.2
Bias (V)
30 min ozone
Bias (V)

-2
Js = 6.17  10 A-cm
0.005
-0.5
-1.0
-0.10
B = 0.696 eV
 = 1.49
-1.0
-1.5
-2.0
-2.5
-3.0
-10
-8
-6
-4
Bias (V)
~ University of Florida ~ Materials Science and Engineering ~
Schottky Contacts - UV Ozone - AFM
No Ozone Treatment
30 min Ozone Treatment
~ University of Florida ~ Materials Science and Engineering ~
Schottky Contacts - UV Ozone - XPS
30 min Ozone
60000
150000
40000
100000
20000
50000
0
O
O
-20000
C
Zn
N(E)
N(E)
No Ozone
0
-50000
Zn
Zn
Zn
-40000
-60000 Zn
1000
C
O
-100000
800
600
400
200
0
Zn
1000
Energy (eV)
C 1s peak
800
600
400
200
Energy (eV)
No ozone (at. %) 30 min (at.%)
No Ar+ sputter
29.5
5.8
1 min Ar+ sputter
5.3
1.1
2 min Ar+ sputter
2.6
0.1
Desorption of
surface C
contaminants
~ University of Florida ~ Materials Science and Engineering ~
0
Schottky Contacts - W/Pt/Au on Bulk
0.3
0.5
As-sputtered
30 min ozone
no ozone
Current (mA)
Current (mA)
1.0
0.0
-0.5
0.2
700 °C, 1 min anneal
30 min ozone
No ozone
0.1
0.0
-1.0
-0.1
-0.010 -0.005 0.000 0.005 0.010
-0.4
-0.2
Bias (V)
Sputter-induced damages
 Non-rectifying for 250
°C and 500 °C anneal
 Rectifying after 700 °C
anneal
0.0
0.2
0.4
Bias (V)
No ozone
30 min ozone
B (eV)
0.45
0.49

4.5
3.2
Js (A-cm-2)
8.4310-2
2.1110-2
~ University of Florida ~ Materials Science and Engineering ~
Schottky Contacts - W/Pt/Au - AES
30 min ozone, as-sputtered
30 min ozone, 700 °C anneal
25000
Zn
25000
20000 C
O
W
15000
Au
10000
Pt
Zn
20000
O
C
15000
W
10000
Au
Pt
5000
5000
0
Intensity (arb. unit)
Intensity (arb. unit)
30000
Zn
0
200
400
600
Time (s)
800
1000
0
0
200
400
600
800
1000
Time (s)
• Post-deposition annealing  500 °C: no detectable
intermixing
• 700 C anneal: Zn diffused out to the Au-Pt interface,
independent of whether the samples had been exposed to
ozone
~ University of Florida ~ Materials Science and Engineering ~
Schottky Contacts - W2B5 vs. W2B
W2B5/Pt/Au as deposited
W2B/Pt/Au as deposited
W2B5/Pt/Au 600ºC annealed
W2B/Pt/Au 600ºC annealed
~ University of Florida ~ Materials Science and Engineering ~
Schottky Contacts - Summary






Ozone treatment removes surface C contamination
Pt contacts: ozone treatment produces transition
from ohmic to rectifying behavior
W contacts: require annealing T  700°C to repair
sputter-induced damages
AES revealed intermixing of metal layers and outdiffusion of Zn to Au-Pt interface
Low barrier heights for boride contacts
W2B showed good thermal stability  high
temperature ohmic contacts
~ University of Florida ~ Materials Science and Engineering ~
p-n Junction Diode - Growth and Structure
Circular ohmic contact
(50 to 375 m diameter)
Pulsed
laser deposition (PLD)
(0001)
bulk ZnO substrate
Zn0.9Mg0.01O:P0.02
Zn0.9Mg0.1O: P0.02 PLD film (~1.4 m)
Buffer n-ZnO PLD film (~0.8 m)
KrF
Bulk ZnO (0.5 mm, n ~ 1017 cm-3)
Full backside ohmic contact
target
excimer laser ablation source
Laser
repetition rate: 1 Hz
Laser
pulse energy density: 3 J-cm-2
400 °C, O2 overpressure
of 20 mTorr
Ohmic contacts:
Growth:
Undoped buffer layer
necessary for good
rectifying behavior

p-ZnMgO: Pt/Au (200/800Å)

n-ZnO: Ti/Al/Pt/Au 200/400/200/800Å)

Annealed at 200 °C, 1 min, N2 ambient
~ University of Florida ~ Materials Science and Engineering ~
p-n Junction Diode - IV Characteristics
0.04
Measured at room temp:
Pt/Au 50 m Diode
Current (A)
0.02
0.00
-0.02
-0.04
-10.0
-7.5
-5.0
-2.5
0.0
2.5
VRB
–9.0 V
Js
4.610-9 A·cm-2
Vf
4.0 V
RON
14.5 m ·cm-2
5.0
Bias (V)
~ University of Florida ~ Materials Science and Engineering ~
p-n Junction Diode - Reverse Breakdown
0.00
-0.02
Pt/Au 210 m diode
Measurement Temperatures
30 °C
50 °C
100 °C
150 °C
200 °C
Reverse Breakdown Voltage (V)
Current (A)
-0.01
7
-0.03
-0.04
-8
-6
-2
-4
Bias (V)
0
6
Pt/Au
5
4
3
VRB  VRB0[1   (T  T0 )]
2
1
0
50
100
150
200
Measurement Temperature (°C)
Temperature coefficient:
Slightly negative ~ .1 to .2 V/K
Presence of defects
Non-optimized growth and processing
~ University of Florida ~ Materials Science and Engineering ~
250
p-n Junction Diode - Summary




Demonstrated ZnO-based p-n junctions
ZnMgO/ZnO heterostructure system
n-type ZnO buffer on the ZnO substrate is critical
in achieving acceptable rectification in the
junctions
Important step in realizing minority carrier devices
in the ZnO system
~ University of Florida ~ Materials Science and Engineering ~
Conclusions

ICP etching



H in ZnO




Straightforward n-type
Rapidly improving for p-type
Schottky contacts to ZnO



Much less thermally stable than GaN
Completely evolve out by 700°C anneals
Ohmic contacts to ZnO


Methane-based chemistry
Practical etch rate but optical degradation
Low B for both n-type and p-type
Surface states dominate transport mechanism
p-n junction diode using ZnMgO/ZnO demonstrated
~ University of Florida ~ Materials Science and Engineering ~
Acknowledgements

Committee members:






Prof. Stephen Pearton, Chair
Prof. Cammy Abernathy
Prof. David Norton
Prof. Rajiv Singh
Prof. Fan Ren, External
Contributors:
Y.-W. Heo
Y. Li
B. Luo
B.P. Gila
E.S. Lambers
K.H. Baik
A.H. Onstine M.E. Overberg J.R. LaRoche
~ University of Florida ~ Materials Science and Engineering ~