1. 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

2. Acknowledgements
The authors greatly acknowledge the support for this research as sponsored by AFOSR grant under grant number F , by the Army Research Office under grant no. DAAD and the NSF (DMR , Dr. L. Hess).

3. 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

4. Potential Applications UV/Blue optoelectronics Transparent transistors
Background
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
Ferromagnetism at practical Tc when doped with transition metals
Obstacle: good quality, reproducible p-type
Obstacle: development of thermally stable contacts
GaN ZnO
Bandgap (eV)
µe (cm2/V-sec)
µh (cm2/V-sec)
me mo 0.24mo
mh mo 0.59mo
Exciton binding 28 60
energy (meV)
Potential Applications
UV/Blue optoelectronics
Transparent transistors
Nanoscale detectors
Spintronic devices

5. Problem
Creation of thermally stable, reliable low resistance Ohmic and Schottky contacts
Typical Schottky contacts yield ΦB = 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
No trend in barrier height to metal work function has limited understanding of barrier height enhancement. Does the MIS structure improve barrier height and ideality or no?

6. 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 UHV (10-10 Torr)
3 min UV ozone treatment
µm range
Backside Ohmic contacts Ti/Au (200A / 2000A)
Ar plasma-assisted rf sputtering
Contacts 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
N type comes from impurities in matl. H, O vac.

7. Initial Results Typical Morphology Room Temp 77K Deposition
Smooth pattern
Some distortion on
Low temp deposition
Pt, Pd

8. Initial Results – I-V Characteristics
Make note of Ohmic behavior in Ti and Ni contacts; Schottky behavior in Pt and Pd contacts. Schottky Mott Model – Pt and Pd have higher work functions than electron affinity for ZnO.
Schottky-Mott model correlation

9. I-V Response – Ohmic Contacts
Increase in resistance for low-temp deposited contacts. Resistance decreases w/ contact area – expected.

10. I-V Response – Schottky Contacts
Resistance higher again for low-temp contacts.

11. Barrier Height & Ideality – Pt + Pd
Deposition n
Φb (eV) Pt
Room Temp ~2
0.37
Low Temp
1.6
0.5 Pd
1.9
0.44
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

12. Pd Annealing – I-V Response
Ideality ↓ w/ each anneal
n~1-2 after 300
↑ Barrier height
MIS structure
Au contacts = no ΔΦb w/ temp
Au contacts don’t oxidize easily – MIS structure unlikely to develop – results with little change to barrier height over annealing temperature

13. Pd Annealing – Leakage Current
↓ over whole anneal for RT
No Δ for LT
MIS structure
Effect of surface area ↓

14. AES – As-deposited Depth Profile
Surface scans had three components: Ni, Oxygen from a native oxide, and atmospheric carbon. Sharp interfaces are observed. No major difference btw RT and LT.
Room Temperature Low Temp

15. AES – Post-anneal Depth Profile
No difference noticed by AES, even with the differences noticed in electrical characteristics. If MIS structure is present, AES cannot detect it.
Room Temperature Low Temp

16. 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