Transcript Thermally Stable Boride Contacts on n

Effect of Cryogenic Temperature
Deposition of Various Metal Contacts
to Bulk, Single-Crystal n-type ZnO
J. Wright1, L. Stafford1, B.P. Gila1, D.P. Norton1,
S.J. Pearton1, Hung-Ta Wang2, F. Ren2
1Department
of Materials Science and Engineering, University of Florida, Gainesville,
FL 32611
2Department of Chemical Engineering, University of Florida, Gainesville, FL 32611
Acknowledgements
The authors greatly acknowledge the support for this
research as sponsored by AFOSR grant under grant
number F49620-03-1-0370, by the Army Research Office
under grant no. DAAD19-01-1-0603 and the NSF (DMR
0400416, Dr. L. Hess).
Introduction
• ZnO has intriguing, useable properties:
– bandgap (3.2 eV)
– exciton binding energy
• (~60 meV – vs. 24 meV for GaN)
• Potential in UV LEDs, transparent FETs
– Efficient excitonic emission processes → high
temp applications
• Prototype ZnO-based LEDs up to 400°C
Background
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
3.2
200
5-50
0.24mo
0.59mo
60
• Bulk ZnO (n-type) commercially
available
• Grown on inexpensive
substrates at low temperatures
• 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
• Ferromagnetism at practical Tc
when doped with transition
metals
• Obstacle: good quality,
reproducible p-type
• Obstacle: development of
thermally stable contacts
Problem
• Creation of thermally stable, reliable low resistance
Ohmic and Schottky contacts
• Typical Schottky contacts yield ΦB = 0.6-0.8 eV
– Au, Ag, Pd
– Trend with ΦB often does not correlate to metal work function
• Typical Ohmic contacts yield ρc = 10-3 – 10-7 Ω cm2
(annealing < 500°C)
– Ti/Au, Zn/Au, Al/Pt, Pt/Ga
– Thermal stability of Ohmic contacts is extremely poor
• Interest in use of cryogenic deposition temperatures
– Mechanisms for barrier height enhancement are unknown
– Ideality factor of Au contacts for InP increased – MIS structure
Experiment
• Undoped, single-crystal ZnO (0001) (Zn face)
– n = 1017 cm-3;
μ = 190 cm2/V s
• Ti, Ni, Pt, Pd (1000 A) – Deposited at RT and 77K
– Evaporator system @ UHV (10-10 Torr)
– 3 min UV ozone treatment
– 200-800 µm range
• Backside Ohmic contacts Ti/Au (200A / 2000A)
– Ar plasma-assisted rf sputtering
– Contacts annealed @ 450°C for 1 minute O2 ambient
• Post-deposition annealing (up to 300°C, 30 min anneals)
• AES
• SEM/Optical Micrograph
• Electrical characteristics by I-V curves
Initial Results
Typical Morphology
• Room Temp
• 77K Deposition
Smooth pattern
Some distortion on
Low temp deposition
Pt, Pd peeling @77K
Initial Results – I-V Characteristics
Current (A/cm
ZnO 400m device
room temp deposition
20
)
40
75
2
2
)
60
100
Ti
Pt
Ni
Pd
Current (A/cm
80
0
-20
-40
-60
-80
-1.0
-0.5
0.0
0.5
Voltage (V)
Schottky-Mott model correlation
1.0
ZnO 400m device
low temp deposition
50
25
Ti
Pt
Ni
Pd
0
-25
-50
-75
-100
-1.0
-0.5
0.0
0.5
Voltage (V)
1.0
I-V Response – Ohmic Contacts
40
10
)
2
20
40
Ni // ZnO as-deposited
RT200m
LT200m
RT400m
LT400m
RT800m
LT800m
Current (A/cm
Current (A/cm
2
)
30
0
-10
-20
-30
-40
-0.2
-0.1
0.0
0.1
Voltage (V)
0.2
20
0
Ti // ZnO as-deposited
RT200m
LT200m
RT400m
LT400m
RT800m
LT800m
-20
-40
-0.6
-0.4
-0.2
0.0
0.2
Voltage (V)
0.4
0.6
I-V Response – Schottky Contacts
2
2
10
10
1
1
10
0
10
Current (A/cm
)
2
10
-1
-1
10
Pt // ZnO as-deposited
-2
10
RT200m
LT200m
RT400m
LT400m
RT800m
LT800m
-3
10
-4
10
-5
10
-1.0
0
10
Current (A/cm
2
)
10
-2
10
RT200m
RT400m
RT800m
LT200m
LT400m
LT800m
-3
10
-4
10
-5
10
-6
10
Pd // ZnO as-deposited
-7
-0.5
0.0
0.5
Voltage (V)
1.0
10
-1.0
-0.5
0.0
0.5
Voltage (V)
1.0
Barrier Height & Ideality – Pt + Pd
Pt
Pd
Deposition
n
Φb (eV)
Room Temp
~2
0.37
Low Temp
1.6
0.5
Room Temp
1.9
0.44
Low Temp
1.7
0.69
• Ideality factors lower for low temp deposition
• Barrier height enhanced by low temp
– More abrupt interface
– Need for C-V measurements, XPS
• Ideality ↓ w/
each anneal
• n~1-2 after 300
• ↑ Barrier height
– MIS structure
• Au contacts =
no ΔΦb w/ temp
Barrier Height (eV)
Pd Annealing – I-V Response
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
Pd // ZnO
RT200m
RT400m
RT800m
50
100
LT200m
LT400m
LT800m
150
200
250
300
Annealing Temperature (C)
Pd Annealing – Leakage Current
• No Δ for LT
– MIS structure
• Effect of
surface area ↓
0.0
-0.5
Leakage Current (mA)
• ↓ over whole
anneal for RT
-1.0
-1.5
-2.0
Pd // ZnO @ 0.5V
-2.5
RT200m
RT400m
RT800m
-3.0
-3.5
-4.0
0
50
LT200m
LT400m
LT800m
100 150 200 250 300
Annealing Temperature (C)
AES – As-deposited Depth Profile
100
100
Ni
80
70
60
Zn
50
O
40
30
20
C
10
00
Ni
90
Atomic Concentration (%)
Atomic Concentration (%)
90
80
70
60
50
O
40
30
20
10
500
1000
1500
Zn
00
C
500
1000
Sputter Depth (Å)
Sputter Depth (Å)
Room Temperature
Low Temp
1500
AES – Post-anneal Depth Profile
100
90
Pd
80
70
Zn
60
50
40
O
30
20
0
90
Pd
80
70
60
Zn
50
40
O
30
20
10
10
0
Atomic Concentration (%)
Atomic Concentration (%)
100
100 200 300 400 500 600 700 800 900 1000
Sputter Depth (Å)
Room Temperature
0
0
100 200 300 400 500 600 700 800 900 1000
Sputter Depth (Å)
Low Temp
Final Conclusions
• Deposition of Ti, Ni, Pt and Pd contacts on ZnO by low-temperature
evaporation has been investigated.
• Room-temperature deposition has been investigated for
comparison.
• Ni and Ti samples show Ohmic behavior
– Resistance increased with low-temperature deposition
• Pt and Pd samples show Schottky behavior
– Room Temp Φb ~ 0.4 eV
– Low Temp Φb > 0.5 eV
• Subsequent annealing of Pd increased Φb by ~0.6 eV
– Leakage current (@ -0.5V) decreased for RT contacts
• Mechanisms for behavior differences - interfacial oxide layer