1. Kelly Ip PhD Defense ~ July 1, 2005 ~ University of Florida ~ Materials Science and Enginering Process Development for ZnO-based Devices

2. ~ University of Florida ~ Materials Science and Engineering ~ Outline  Introduction  Inductively-coupled plasma (ICP) etching  Hydrogen in ZnO  Contact metallization  Ohmic contacts  Schottky contacts  p-n junction diode  Conclusions

3. ~ University of Florida ~ Materials Science and Engineering ~ Introduction  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  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) 3.43.3 µ e (cm 2 /V-sec) 220200 µ h (cm 2 /V-sec) 105-50 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

4. ~ University of Florida ~ Materials Science and Engineering ~ ICP Etching  Wet etching  HCl, HNO 3, NH 4 Cl, and HF  Generally isotropic with limited resolution and selectivity  High-density plasma etching  Anisotropic with high resolution  Favored by modern manufacturing environment  Bulk, wurtzite (0001) ZnO from Eagle-Picher  Gas chemistry:  Cl 2 /Ar (10/5 sccm) & CH 4 /H 2 /Ar (3/8/5 sccm)  Constant ICP source power at 500W and process pressure at 1 mTorr  Varied rf chuck power: 50 – 300W

5. ~ University of Florida ~ Materials Science and Engineering ~ ICP Etching - Etch Rates CH 4 /H 2 /Ar ~3000 Å/min Cl 2 /Ar ~1200 Å/min

6. ~ University of Florida ~ Materials Science and Engineering ~ ICP Etching - Etch Mechanism Ion-Assisted Etch Mechanism ER  E 0.5 -E TH 0.5 Vapor pressure of etch products: (CH 4 ) 2 Zn 301 mTorr at 20°C ZnCl 2 1 mTorr at 428 °C E TH ~ 96 eV for CH 4 /H 2 /Ar

7. ~ University of Florida ~ Materials Science and Engineering ~ ICP Etching - Photoluminescence Optical degradation even at the lowest rf power

8. ~ University of Florida ~ Materials Science and Engineering ~ ICP - AFM Control 50 W rf 100 W rf 200 W rf300 W rf Zn and O etch products removed at same rate

9. ~ University of Florida ~ Materials Science and Engineering ~ ICP Etching - AES and SEM CH 4 /H 2 /Ar 200W rf O Zn O Control

10. ~ University of Florida ~ Materials Science and Engineering ~ ICP Etching - Summary  Dry etching is possible with practical etch rates using CH 4 /H 2 /Ar  Surface is smooth and stoichiometric  Anisotropic sidewalls  Optical quality is sensitive to ion energy and flux

11. ~ University of Florida ~ Materials Science and Engineering ~ Hydrogen in ZnO  Hydrogen  Predicted role as shallow donor  Introduced from growth ambient  Present in optimal plasma etch chemistry  Understand diffusion behavior and thermal stability  Bulk, wurtzite (0001) ZnO, undoped (n~10 17 cm -3 ) from Eagle-Picher  Hydrogen incorporation  Ion Implantation of 2 H or 1 H (100keV, 10 15 - 10 16 cm -2 )  2 H plasma exposure in PECVD at 100-300°C, 30 mins  Post-annealing: 500 - 700°C

12. ~ University of Florida ~ Materials Science and Engineering ~ Hydrogen in ZnO - Implanted - SIMS Removal of 2 H below SIMS limit at 700°C Thermally less stable than GaN (>900ºC)

13. ~ University of Florida ~ Materials Science and Engineering ~ Hydrogen in ZnO - Implanted - RBS/C 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

14. ~ University of Florida ~ Materials Science and Engineering ~ Hydrogen in ZnO - Implanted - PL Severe optical degradation even after 700ºC anneal Point defect recombination centers dominate

15. ~ University of Florida ~ Materials Science and Engineering ~ Hydrogen in ZnO - Plasma - SIMS Large diffusion depth 2 H diffuses as an interstitial, with little trapping by the lattice elements or by defects or impurities

16. ~ University of Florida ~ Materials Science and Engineering ~ Hydrogen in ZnO - Plasma/annealed - SIMS 2 H completely evolve out of the crystal at 500°C

17. ~ University of Florida ~ Materials Science and Engineering ~ Hydrogen in ZnO - Plasma - CV Effects 2 H plasma treatment Passivate the compensating acceptor impurities Induces a donor state and increases the free electron concentration Suggest H from growth process n-type conductivity probably arises from multiple impurity sources

18. ~ University of Florida ~ Materials Science and Engineering ~ Hydrogen in ZnO Implanted 2 H is slightly more thermally stable: trapping at residual damage in the ZnO by the nuclear stopping process Implanted Plasma exposure

19. ~ 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

20. ~ University of Florida ~ Materials Science and Engineering ~ Ohmic Contacts  Require low specific contact resistance  Surface treatments  As-received  Organic solvents (trichloroethylene, methanol, acetone, 3 mins each)  H plasma  Ti/Al/Pt/Au metal scheme on n-type ZnO  Bulk  PLD films  Au/Ni/Au and Au on p-type ZnMgO

21. ~ University of Florida ~ Materials Science and Engineering ~ Ohmic Contacts - Ti/Al/Pt/Au on Bulk Bulk n-ZnO Metals Cross-sectional view of circular TLM R1R1 RORO ρ c lowest at 250 °C anneal ρ c ~ 6  10 -4  cm 2 Severe contact degradation after 600 °C anneal  C = R S L T 2 Marlow and Das, Solid-State Electron. 25 91 (1982)

22. ~ University of Florida ~ Materials Science and Engineering ~ Ohmic Contacts - Ti/Al/Pt/Au on Bulk - AES

23. ~ University of Florida ~ Materials Science and Engineering ~ Ohmic Contacts - Ti/Al/Pt/Au on Bulk - SEM

24. ~ University of Florida ~ Materials Science and Engineering ~ Ohmic Contacts - Growth: n-type ZnO:P Films  N-type phosphorus-doped ZnO film on (0001) Al 2 O 3 grown by PLD  Post-growth annealing  Increase anneal temperature  Decrease carrier concentration and Hall mobility  Increase resistivity  Reduction of shallow state density  P dopants activation as acceptors in O site Post-growth Anneal T (°C) Carrier conc (#/cm 3 ) Resistivity (  cm) Hall mobility (cm 2 /Vs) 30 1.5  10 20 0.00218.5 425 6  10 19 0.0137.8 450 2.4  10 18 1.31.9 500 3.2  10 17 12.81.5 600 7.5  10 15 4631.8 Heo et al APL 83 1128 (2003) Post-growth Anneal T (°C) Carrier conc (#/cm 3 ) Resistivity (  cm) Hall mobility (cm 2 /Vs) 30 1.5  10 20 0.00218.5 425 6  10 19 0.0137.8 450 2.4  10 18 1.31.9 500 3.2  10 17 12.81.5 600 7.5  10 15 4631.8

25. ~ University of Florida ~ Materials Science and Engineering ~ Ohmic Contacts - Ti/Al/Pt/Au ZnO:P Films Nonalloyed: n = 1.5  10 20 cm -3  c = 8.7  10 -7  -cm 2 Annealed: Measured at RT: n = 6.0  10 19 cm -3  c = 3.9  10 -7  -cm 2 Measured at 200 °C n = 2.4  10 18 cm -3  c = 2.2  10 -8  -cm 2 Ti/Al/Pt/Au (200/800/400/800)Å on PLD ZnO:P films

26. ~ University of Florida ~ Materials Science and Engineering ~ Ohmic Contacts - p-type ZnMgO Films Ohmic behavior after annealing  500 °C Ti/Au more thermally stable than Ni/Au contacts Severe degradation of Ni/Au after 600 °C anneal S. Kim et al APL 84 1904 (2004)

27. ~ University of Florida ~ Materials Science and Engineering ~ Ohmic Contacts - p-type ZnMgO Films Specific contact resistance after 600 °C anneal Au: 2.5  10 -5  cm 2 Au/Ni/Au: 7.6  10 -6  cm 2

28. ~ University of Florida ~ Materials Science and Engineering ~ Ohmic Contacts - Au/ZnMgO

29. ~ University of Florida ~ Materials Science and Engineering ~ Ohmic Contacts - Au/Ni/Au/ZnMgO

30. ~ 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 H 2 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

31. ~ 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, W 2 B, W 2 B 5, CrB 2

32. ~ University of Florida ~ Materials Science and Engineering ~ Schottky Contacts - Pt/Au on Bulk  No ozone treatment: Linear I-V  Ozone treatment:   B = 0.696 eV   = 1.49  J s = 6.17  10 -6 A-cm -2

33. ~ University of Florida ~ Materials Science and Engineering ~ Schottky Contacts - UV Ozone - AFM No Ozone Treatment 30 min Ozone Treatment

34. ~ University of Florida ~ Materials Science and Engineering ~ Schottky Contacts - UV Ozone - XPS C 1s peakNo ozone (at. %)30 min (at.%) No Ar + sputter29.55.8 1 min Ar + sputter5.31.1 2 min Ar + sputter2.60.1 Desorption of surface C contaminants

35. ~ University of Florida ~ Materials Science and Engineering ~ Schottky Contacts - W/Pt/Au on Bulk Sputter-induced damages  Non-rectifying for 250 °C and 500 °C anneal  Rectifying after 700 °C anneal No ozone30 min ozone  B (eV) 0.450.49  4.53.2 J s (A-cm -2 ) 8.43  10 -2 2.11  10 -2

36. ~ University of Florida ~ Materials Science and Engineering ~ Schottky Contacts - W/Pt/Au - AES 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

37. ~ University of Florida ~ Materials Science and Engineering ~ Schottky Contacts - W 2 B 5 vs. W 2 B W 2 B 5 /Pt/Au as deposited W 2 B 5 /Pt/Au 600ºC annealed W 2 B/Pt/Au as deposited W 2 B/Pt/Au 600ºC annealed

38. ~ 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 out- diffusion of Zn to Au-Pt interface  Low barrier heights for boride contacts  W 2 B showed good thermal stability  high temperature ohmic contacts

39. ~ University of Florida ~ Materials Science and Engineering ~ p-n Junction Diode - Growth and Structure Full backside ohmic contact Bulk ZnO (0.5 mm, n ~ 10 17 cm -3 ) Buffer n-ZnO PLD film (~0.8  m) Zn 0.9 Mg 0.1 O: P 0.02 PLD film (~1.4  m) Circular ohmic contact (50 to 375  m diameter)  Pulsed laser deposition (PLD)  (0001) bulk ZnO substrate  Zn 0.9 Mg 0.01 O:P 0.02 target  KrF excimer laser ablation source  Laser repetition rate: 1 Hz  Laser pulse energy density: 3 J-cm -2  Growth: 400 °C, O 2 overpressure of 20 mTorr  Ohmic contacts:  p-ZnMgO: Pt/Au (200/800Å)  n-ZnO: Ti/Al/Pt/Au 200/400/200/800Å)  Annealed at 200 °C, 1 min, N 2 ambient Undoped buffer layer necessary for good rectifying behavior

40. ~ University of Florida ~ Materials Science and Engineering ~ p-n Junction Diode - IV Characteristics Measured at room temp: V RB –9.0 V J s 4.6  10 -9 A·cm -2 V f 4.0 V R ON 14.5 m  ·cm -2

41. ~ University of Florida ~ Materials Science and Engineering ~ p-n Junction Diode - Reverse Breakdown Temperature coefficient: Slightly negative ~.1 to.2 V/K Presence of defects Non-optimized growth and processing

42. ~ University of Florida ~ Materials Science and Engineering ~ 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

43. ~ University of Florida ~ Materials Science and Engineering ~ Conclusions  ICP etching  Methane-based chemistry  Practical etch rate but optical degradation  H in ZnO  Much less thermally stable than GaN  Completely evolve out by 700°C anneals  Ohmic contacts to ZnO  Straightforward n-type  Rapidly improving for p-type  Schottky contacts to ZnO  Low  B for both n-type and p-type  Surface states dominate transport mechanism  p-n junction diode using ZnMgO/ZnO demonstrated

44. ~ 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. HeoY. LiB. Luo B.P. GilaE.S. LambersK.H. Baik A.H. OnstineM.E. OverbergJ.R. LaRoche