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

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

ZnO – Basic (Electrical) Properties Direct, wide bandgap High excitonic binding energy – 60 meV Inexpensive growth Easily etched –(acids and alkalis) Radiation stability PropertyValue Lattice parameters at 300 K (nm) a 0 : c 0 : Density (g cm -3 )5.606 Stable phase at 300 KWurtzite Melting point (ºC)1975 Thermal conductivity0.6, Linear thermal expansion coefficient a 0 : 6.5  c 0 : 3.0  Static dielectric constant8.656 Refractive index2.008, Energy bandgap (eV)Direct, 3.37 Intrinsic carrier concentration (cm -3 ) <10 6 max n-type doping: n ~ max p-type doping: p ~ Exciton binding energy (meV)60 Electron effective mass0.24 Electron Hall mobility, n-type at 300 K (cm 2 V - 1 s -1 ) 200 Hole effective mass0.59 Hole Hall mobility, p-type at 300 K (cm 2 V -1 s -1 )5 - 50

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 T c when doped with transition metals Obstacle: good quality, reproducible p-type GaNZnO Bandgap (eV) µ e (cm 2 /V-sec) µ h (cm 2 /V-sec) m e 0.27m o 0.24m o m h 0.8m o 0.59m o Exciton binding2860 energy (meV) Potential Applications UV/Blue optoelectronics Transparent transistors Nanoscale detectors Spintronic devices

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

Ohmic contacts to n-ZnO Earlier Metallizations –Ti/Au, Zn/Au, Al/Pt Re/Ti/Au, Ru, Pt/Ga –ρ sc – Ω.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 K. Ip et al. AIP (2004).

Schottky Contacts to ZnO Schottky Obstacles –Surface states surface layer –Metal/ZnO intermixing Typically Au, Ag, Pd, Pt –Φ b ~ eV –n > 1 (~1-2+) –Poor thermal stability High n factor –Tunneling –Interface layer –Surface conductivity –Deep recomb. centers ElementWork Function (eV)Ideal Barrier Height (eV) B Cr Pt Ti W Zr

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. ~ 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

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

Project Objectives The goals of this project are three fold: 1.Optimization of Ohmic contacts to ZnO –Ir, Re, WN x, TiN x, ZrN x, and TaN x 2.Optimization of Schottky contacts to ZnO –Ir, Re, WN x, TiN x, ZrN x, and TaN x 3.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

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)

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, N 2 or O 2 )

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 N 2 ambient Schottky photolithography realignment Sputter deposit metallization scheme –Novel metallizations include Au overlayer Lift-off Anneal contacts (300°C-1000°C, 1 min, N 2 or O 2 )

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

Methodology – Ion Implantation N, P, As doses cm -2 Implantation temp varied RT – 300°C Annealed between 600 – 950°C –RTA –PLD chamber, O 2 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

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 E A, ρ – activation kinetics

Prelim Research – Ir/Au Ohmic Contacts

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

Ir/Au Contacts – Thermal Stability 30 Days Pre-anneal No change to R sh after 30 days

Ir/Au Contacts – N 2 vs. O 2 Anneal Resistance increased w/ O 2 anneal – IrO 2 layer

Ir/Au Contacts – N 2 vs. O 2 Anneal AES can not detect IrO 2 layer, however more interdiffusion of Ir w/ N 2 anneal

Prelim Research – Surface Treatment

Surface Treatment – IV Character All treatments result in Ohmic contacts except for Oxygen plasma. Surface TreatmentRshρsc (Ohm/□)(Ohm cm 2 ) Argon Ozone Oxygen BCl Anneal H3PO As-Dep (LTLM)

Investigation Timeline Plan FallSpringSummerFallSpringSummerFallSpringSummerFallSpring Literature Review Fabrication (Ohmic & Schottky) Measurements of Contact Resistance Measurements of Contact Intermixing & Second Phases Thermal Stability Measurements of ZnO Contacts Long-term Aging Studies Ion Implantation (Doping) Thermal Stability Measurements for Annealed ZnO Activation Study for Different Acceptor Dopants Diffusivity Measurements Oral qualifier & defense Dissertation & defense

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