Oral Defense

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  • 1. Growth of GaN Nanocolumns and Their Coalescence Overgrowth Using Metalorganic Chemical vapor Deposition and the Characterization Study (Tsung-Yi Tang) (Dr. C. C. Yang)

2. Special Recognition

  • XRD: Wen-Yu Shiao
  • TEM: Yung-Sheng Chen

3. Acknowledgements

  • Advisor: C. C. Yang
  • Nanoimprint lithography: Epistar Corporation
  • MBE: Dr. Kent Averett
  • Raman measurement: Hsu-Cheng Hsu
  • CL measurement: Wei-Chao Chen
  • PECVD: Cheng-Hung Lin and Kun-Ching Shen

4. Outline

  • Introduction
    • Limiting Factors for the Nitride-based LED Development
    • Methods of Reducing Threading Dislocation Density
    • Motivations of the Research
    • Overview of Nitride Nanocolumn (NC) or Nanowire Growth
  • Part 1: MOCVD Overgrowth on MBE-grown GaN NCs
  • Part 2: MOCVD Overgrowth on MOCVD-grown GaN NCs
  • Conclusions

5.

  • Because of the 36 % lattice mismatch between GaN and sapphire substrate, the density of threading dislocation in GaN is too high (10 9 10 10cm -2 )
  • Because of the 11 % lattice mismatch between GaN and InN, high-indium incorporation is difficult andthe internal quantum efficiency is low when the indium content is high (green-red range).

Limiting Factors for the Nitride-based LED Development Threading dislocations 6. Methods of Reducing Threading Dislocation Density Epitaxial Lateral Overgrowth APL71 , 2639 (1997) Facet-controlled ELOG JCG 221, 316 (2000) Pendeoepitaxy APL75 , 196 (1999) 7. Methods of Reducing Threading Dislocation Density Insertion of inter-mediate layer Patterned sapphire Insertion of LT AlN and SiN JAP99 , 123518 (2006) Multiple insertions of SiN JAP101 , 093502 (2007) Cantilever epitaxy APL77 , 3233 (2000) 8. Motivations of the Research Reduction of residual strain and threading dislocation density Journal of Crystal Growth287 , 500 (2006) Dislocation-free NCs Nano letters6 , 1808 (2006) Strain-free NCs Jpn. J. Appl. Phys. Vol. 40 (2001) 9. Motivations of the Research

  • Mechanisms for enhancing LED efficiency using nitride NCs
  • Lower dislocation density or higher crystal quality
  • Lateral strain relaxation for increasing indium content
  • Scattering for enhancing light extraction
  • High-quality GaN template with coalescence overgrowth

Threading dislocation Coalescence overgrowth of NC Substrate Overgrown thin film 10. Overview of Nitride Nanocolumn or Nanowire Growth with MBE

  • Self-organized growth
  • --- vertical to the template and high column density; but random distribution and non-uniform size distribution
  • --- E. Calleja et al.,Mater. Sci. Eng. B82 , 2 (2001).
  • K. Kusakabe et al.,Jpn. J. Appl. Phys. 40 , L192 (2001).
  • L. W. Tu et al.,Appl. Phys. Lett. 82 , 1601 (2003).
  • J. E. Van Nostrand et al.,J. Cryst. Growth 287 , 500 (2006).
  • R. Calarco et al.,Nano Lett. 7 , 2248 (2007).

11.

  • Coalescence overgrowth
  • InGaN/GaN LEDs

Overview of Nitride Nanocolumn or Nanowire Growth with MBE

    • Jpn. J. Appl. Phys., Part 240 , L192 (2001)
    • J. Vac. Sci. Technol. B25, 964 (2007)

12. Overview of Nitride Nanocolumn or Nanowire Growth with MBE Patterned growth with focused-ion-beam or electron-beam lithography---K. Kishino et al.,J. Cryst. Growth 311 , 2063 (2009). ---S. Ishizawa et al.,applied physics express 1 , 015006 (2008) Selective-Area Growth of GaN nanocolumns on Si(111) substrates using nitrided Al nanopatterns by rf-plasma-assisted molecular-beam epitaxy Ti-mask selective-area growth (SAG) by rf-plasma-assisted molecular beam epitaxy demonstrating extremely uniform GaN nanocolumn arrays 13.

  • Self-organized growth of GaN nanorods
    • Appl. Phys. Lett.81 , 2193 (2002)
  • Self-organized growth of InGaN nanorods
    • Phys. Stat. Sol. (b) 241, 2802 (2004)
  • Blue emission from InGaN/GaN QW nanorod arrays
    • Appl. Phys. Lett.87 , 093115 (2005)
  • Fabrication of free-standing GaN
    • Phys. Stat. Sol. (c) 4, 2268 (2007)

Overview of Nitride Nanocolumn or Nanowire Growth with HVPE InGaN GaN 14. Overview of Nitride Nanocolumn or Nanowire Growth with VLS method

  • --- Most of them have random orientations and not vertical to the template
  • ---Impurity incorporation into NC or nanowire due to the use of a catalyst may degrade device performance.

phys. stat. sol. (b)241 , 2775 (2004)(111) MgO Nature materials 3, 524 (2004) J. Am Chem. Soc.123, 2793 (2001) 15. Overview of Nitride Nanocolumn or Nanowire Growth with MOCVD

  • Top-down method
  • --- E lectron-beam lithography has been used.
  • --- The dry etching procedure normally generates defect states on the column surfaces.
  • Patterned growth
  • Appl. Phys. Lett.89 , 233115 (2006).(interferometry lithography)
  • J. Appl. Phys. 100 , 054306 (2006).(AAO lithography)

Nanotechnology17 , 1454 (2006) 16.

  • Pattern growth- pulsed growth mode
    • Nano Lett.6 , 1808 (2006)
  • Our consideration
    • High quality (surface defectstate, impurity incorporation)
    • High density
    • Regular arrangement

Overview of Nitride Nanocolumn or Nanowire Growth with MOCVD 17. Outline

  • Introduction
  • Part 1: MOCVD Overgrowth on MBE-grown GaN NCs
    • Sample Structure and Growth Conditions
    • Characterization: PL, CL, AFM, SEM, XRD, and TEM
    • Summary
  • Part 2: MOCVD Overgrowth on MOCVD-grown GaN NCs
  • Conclusions

18. MBE-grown GaN Nanocolumns Template Si (111) NCs (810 O C) AlN (710 O C) Column diameter: 100nm Column density: 10 9/cm 2 Grown by Dr. Kent Averett 19. Growth Parameters of MOCVD-Overgrown GaN Pressure: 200 torr TMG flow rate: 17 mol/min NH 3flow rate: 1000 sccm V/III ratio: 2600 Growth rate: 0.44 nm/sec Temperature: 800O C, 900O C, 1000O C Thickness: 700 nm, 2.5 m Overgrown GaN NCs AlN Si 20. SEM and CL Images

  • The overgrown layer shows relatively stronger emission when compared with that from the NCs.
  • The dark regions are always located around the boundaries of the domains. In other words, the optical property near the center of a domain is much better than that near its boundary.

Growth temperature: 1000 O C, thickness: 2.5 m 21. Comparison between the Overgrown Sample and a GaN Thin Film (PL measurement) a high-quality GaN thin film Substrate: sapphire FWHM of the peak of (0002) XRD curve: 190 arcsec FWHM of the peak of (10-12) XRD curve: 296 arcsec Thickness of GaN layer: 2-3 m The comparison shows that the overgrown sample has better optical quality than the GaN thin film. Wavelength (nm) Growth temperature: 1000 O C, thickness: 2.5 m 22. AFM and PL Measurements

  • An AFM image of 7 m x 7 m in dimensions demonstrating part of a hexagon.
  • The difference in height between the maximum and the minimum, indicated with the two marks, is about 14nm, and the surface roughness is about 5.7nm.

Growth temperature: 1000 O C, thickness: 2.5 m 20nm -20nm 0 0 2 4 6 m 23. Two-beam X-ray Diffraction (Conventional Measurement) MOCVD overgrowth samples: A: 800o C 700 nm thick (1274 arcsec) B: 900o C 700 nm (1435) C1: 1000o C 700 nm (2653) C2: 1000o C -- > 2.5 m (6245) Comparison samples: GaN1: good GaN film 2 m (201) GaN2: poor GaN film 2 m (1012) XRD results: courtesy of Wen-Yu Shiao (0002) plane 24. 1 m 1 m Sample A Sample B Sample C1 Sample C2 Cross-section SEM Images 25. Three-beam Depth-dependent X-ray Diffraction Results Three-beam X-ray diffraction geometry Depth-dependent X-ray diffraction results c-axis C1 C2 26. Summary

  • Higher growth temperature leads to better crystal quality. For the growth temperature of 1000 O C, the overgrown GaN layer even shows better optical properties than conventional GaN grown on sapphire.
  • The overgrown GaN layer shows stronger CL emission than nano-columns, which may be attributed to the relative high growth temperature in MOCVD growth process.
  • Hexagonal structures are observed on the surface. It is believed that the surface morphology can be improved by using regular and uniform NCs.

27. Outline

  • Introduction
  • Part 1: MOCVD Overgrowth on MBE-grown GaN NCs
  • Part 2: MOCVD Overgrowth on MOCVD-grown GaN NCs
    • Overgrown Undoped GaN
    • Overgrown QWs and LED
    • Summary
  • Conclusions

28. MOCVD Patterned Growth of GaN Nanocolumns 80nm SiO 2 Holes fabricated with nano-imprint lithography (courtesy of Epistar) Ga