Process Optimization and Development for ZnO Optoelectronics and Photodiodes

1. Process Optimization and Development for ZnO Optoelectronics and Photodiodes Jon Wright Dept. of Materials Science and Engineering, Univ. of Florida, Gainesville, FL Jan 18, 2007

2. Outline • Introduction & Motivation • Background • Contacts (Ohmic + Schottky) • Ion Implantation (Group V) • Project Objectives • Methodology • Preliminary Results • Ir/Au Ohmic Contacts • Surface Treatment Analysis • Conclusions & Timeline

3. Direct, wide bandgap High excitonic binding energy – 60 meV Inexpensive growth Easily etched (acids and alkalis) Radiation stability ZnO – Basic (Electrical) Properties

4. ZnO vs. GaN • Bulk ZnO (n-type) commercially available • Grown on inexpensive substrates at low temperatures • Lower exciton energy for GaN • Heterojunction by substitution in Zn-site • 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 GaNZnO Bandgap (eV)3.43.2 µe (cm2/V-sec)220200 µh (cm2/V-sec)105-50 me0.27mo0.24mo mh0.8mo0.59mo Exciton binding28 60 energy (meV) Potential Applications UV/Blue optoelectronics Transparent transistors Nanoscale detectors Spintronic devices

5. Motivation ZnO-based electronic devices • UV light-emitting diodes • Optoelectronics • Transparent thin-film transistors • Flat panel displays • Solar cells • Piezoelectric transducers • Gas-sensors • Photonic devices • High density data storage

6. Earlier Metallizations Ti/Au, Zn/Au, Al/Pt Re/Ti/Au, Ru, Pt/Ga ρsc 10-3 – 10-7Ω.cm2 c-TLM reduces steps Au ↓ sheet resistance Surface carrier ↑ annealing Adv: oxygen loss Disad: surface degradation Surface cleaning ↓ b Limited info w/ p-ZnO Ohmic contacts to n-ZnO K. Ip et al. AIP (2004).

7. Schottky Contacts to ZnO • Schottky Obstacles • Surface states • Defects @ surface layer • Metal/ZnO intermixing • Typically Au, Ag, Pd, Pt • Φb ~ 0.6-0.84 eV • n > 1 (~1-2+) • Poor thermal stability • High n factor • Tunneling • Interface layer • Surface conductivity • Deep recomb. centers

8. p-type Doping in ZnO • Several deposition methods • Group V: N, P, As, Sb – all on O sites • MBE requires low temp for high dopant conc. • Crystal quality poor below 500°C • Post-deposition annealing results inconsistent • Hole conc. ~ 1015-1017 cm-3 • Limitations in band edge electroluminescence • Deep traps: non-radiative recombination centers • Low density of holes at junction • Diffusion of carriers away from active region

9. p-type Ion Implantation for ZnO • Dopant beam makes vacancies for acceptors • Questions: • Correct ion dosage • Limiting residual damage • Maximizing acceptors • Need for understanding • Damage accumulation • Thermal stability of defects

10. Project Objectives The goals of this project are three fold: • Optimization of Ohmic contacts to ZnO • Ir, Re, WNx, TiNx, ZrNx, and TaNx • Optimization of Schottky contacts to ZnO • Ir, Re, WNx, TiNx, ZrNx, and TaNx • Investigation of electrical properties for implanted Group V dopants in ZnO Aim: Develop processes for ZnO devices • Specifically for UV optoelectronics and LEDs • Realization of p-type ZnO nanowire devices

11. Why Use These Materials? • Nitrides have excellent electrical properties • Highly conductive • High melting temperature • Strong bonds lead to low diffusivity probability • Thermally stable – some Nitrides up to 800°C on GaN • Ir, Re successful novel metallizations for GaN • Superb thermal stability • Group V elements most promising p-type dopants • Difficulty with shallow acceptor levels due to defect states • Group I elements tend to occupy interstitial sites (act as donors)

12. Methodology – Ohmic Contacts Processing • Surface Treatment/Cleaning • Photolithography – c-TLM pattern if possible [J. Chen thesis] • Sputter deposit metallization scheme • Novel metallizations include Au overlayer • Lift-off • Anneal (300°C-1000°C, 1 min, N2 or O2)

13. Methodology – Schottky Contacts Processing • Sample Treatment/Cleaning • Photolithography for Ohmic contact (outer ring) • Sputter deposit Ti/Au (basic Ohmic contact) • Lift-off • RTA anneal 450°C , 30 sec N2 ambient • Schottky photolithography realignment • Sputter deposit metallization scheme • Novel metallizations include Au overlayer • Lift-off • Anneal contacts (300°C-1000°C, 1 min, N2 or O2)

14. Methodology – Contact Measurements • Electrical Characterization • Contact resistance • 4-probe TLM measurement • 2-probe C-TLM measurement • ΔAnnealing temperature • ΔAnnealing time • Variation in measurement temperature (RT – 300°C) • Schottky Diode parameter measurements • Auger Electron Spectroscopy • Scanning Electron Microscopy • Thermal stability measurements

15. Methodology – Ion Implantation • N, P, As dopants @ doses 1013-1014 cm-2 • Implantation temp varied RT – 300°C • Annealed between 600 – 950°C • RTA • PLD chamber, O2 ambient (in-situ) • Hall measurements used to calculate: • Carrier type • Carrier density • Acceptor ionization energy • Use of Oxygen to reduce vacancies • Depth Profiles by AES/SIMS

16. Ion Implantation → ZnO Nanowires • Ability to create pn junction is paramount • Acceptor implantation + characterization • Why Nanowires? • FETs, photodetectors, gas sensors, nano-cantilevers • Allow investigation of carrier transport properties (1-D) • Surface quality, ambient environment critical to character of device • ZnO nanorods (d ~130 nm) grown by MBE • p-type nanowires by injection of acceptors • Contacts on wires using p-type Ohmic metals • Nanowire pn junctions • Masked implantation OR focused ion beam • Determination of EA, ρ – activation kinetics

17. Prelim Research – Ir/Au Ohmic Contacts

18. Ir/Au Contacts – AES Profiles Only slight intermixing btw Au and Ir layers until 800°C(+)

19. Ir/Au Contacts – Thermal Stability Pre-anneal No change to Rsh after 30 days 30 Days

20. Ir/Au Contacts – N2 vs. O2 Anneal Resistance increased w/ O2 anneal – IrO2 layer

21. Ir/Au Contacts – N2 vs. O2 Anneal AES can not detect IrO2 layer, however more interdiffusion of Ir w/ N2 anneal

22. Prelim Research – Surface Treatment

23. Surface Treatment – IV Character All treatments result in Ohmic contacts except for Oxygen plasma.

24. Investigation Timeline

25. Acknowledgements • Advisory Committee • Prof. S.J. Pearton (Chair) • Prof. C.R. Abernathy • Prof. D.P. Norton • Prof. R. Singh • Prof. F. Ren • Contributors • Dr. L. Stafford, Dr. B.P. Gila, L.F. Voss, R. Khanna, H-T. Wang, S. Jang