1. Growth of GaN Nanocolumns and Their Coalescence Overgrowth Using Metalorganic Chemical vapor Deposition and the Characterization Study 以有機金屬氣相沉積法從事氮化鎵奈米柱生長和接合再生長以及其特性研究 研究生： 唐宗毅 (Tsung-Yi Tang) 指導教授： 楊志忠博士 (Dr. C. C. Yang)

6. Methods of Reducing Threading Dislocation Density Epitaxial Lateral Overgrowth APL 71 , 2639 (1997) Facet-controlled ELOG JCG 221, 316 (2000) Pendeoepitaxy APL 75 , 196 (1999)

7. Methods of Reducing Threading Dislocation Density Insertion of inter-mediate layer Patterned sapphire Insertion of LT AlN and SiN JAP 99 , 123518 (2006) Multiple insertions of SiN JAP 101 , 093502 (2007) Cantilever epitaxy APL 77 , 3233 (2000)

8. Motivations of the Research Reduction of residual strain and threading dislocation density Journal of Crystal Growth 287 , 500 (2006) Dislocation-free NCs Nano letters 6 , 1808 (2006) Strain-free NCs Jpn. J. Appl. Phys. Vol. 40 (2001)

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

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 3 flow rate: 1000 sccm V/III ratio: 2600 Growth rate: 0.44 nm/sec Temperature: 800 O C, 900 O C, 1000 O C Thickness: 700 nm, 2.5  m Overgrown GaN NCs AlN Si

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

23. Two-beam X-ray Diffraction (Conventional Measurement) MOCVD overgrowth samples: A: 800 o C – 700 nm thick (1274 arcsec) B: 900 o C – 700 nm (1435) C1: 1000 o C – 700 nm (2653) C2: 1000 o 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

28. MOCVD Patterned Growth of GaN Nanocolumns 80nm SiO 2 Holes fabricated with nano-imprint lithography (courtesy of Epistar) GaN NCs maintain their geometry after they emerge from the growth mask if the growth conditions are changed into a pulsed MOCVD growth mode before the NCs emerge from the growth mask. Sapphire 2  m undoped GaN Sapphire 2  m undoped GaN time NH 3 TMGa Flow rate

31. Regularly Arranged GaN Nanocolumns - 1 Template hole diameter: 250 nm Hexagonal column size: ~300 nm Center-to-center spacing: 500 nm Clean NC bottom 1  m (a) (b) 100 nm SiO 2 SiO 2 250 nm (hole) 300 nm (column)

32. Hole diameter: 450 nm Column diameter: 500nm Separation: 900nm Hole diameter: 600 nm Column diameter: 800nm Separation: 1200nm Regularly Arranged GaN Nanocolumns - 2

34. CL and AFM Results 1  m Overgrown layer Nanocolumns Template Pit density: 2X10 7 cm -2 Roughness: 0.411nm AFM image 5  m*5  m Control sample Pit density: 3X10 8 cm -2 Roughness: 0.843nm Hole diameter: 250 nm Overgrown layer Template Nanocolumns 1  m

37. Comparisons of Internal Quantum efficiency Sample E: GaN template Samples A, B, C, D: NCs with hole sizes at 250, 300, 450, and 600 nm Samples AO, BO, CO,DO: Overgrowth samples with hole sizes at 250, 300, 450, and 600 nm NCs Overgrown layers 10X 7X

38. Comparisons of Dislocation Density Based on a depth-dependent X-ray diffraction measurement technique Edge and screw dislocation densities at the level of 10 7 cm -2 are achieved. The lateral domain size has been significantly increased. >10X >3X >3X

39. Comparison of Dislocation Density, Internal Quantum Efficiency, and Surface Roughness 0.81 6.63 x 10 8 1.09 x 10 8 1.1 0.834 E (GaN template) 1.71 1.32 x 10 8 9.81 x 10 7 1.6/3.9 0.665 DO/D (600 nm) 1.73 9.24 x 10 7 8.11 x 10 7 3.1/4.2 0.473 CO/C (450 nm) 2.01 6.21 x 10 7 5.09 x 10 7 4.1/7.1 0.425 BO/B (300 nm) 2.24 5.04 x 10 7 3.09 x 10 7 6.7/9.9 0.411 AO/A (250 nm) Lateral domain size (  m) Edge dislocation density (cm -2 ) Screw dislocation density (cm -2 ) IQE (%) Surface roughness (nm) Overgrowth sample

40. Cross-sectional TEM Images of Nanocolumns Threading dislocation is terminated at the bottom of a hole when the hole size is small. 250-nm hole size 450-nm hole size Two threading dislocations merge into one. Courtesy of Yung-Sheng Chen c-axis SiO 2 NC 100 nm template SiO 2 NC c-axis 200 nm template

41. Cross-sectional TEM Images of Overgrowth Samples New dislocations are formed on the masks when they are narrow. Such dislocations may disappear along overgrowth. 250-nm hole size 600-nm hole size Similar to ELOG 1 6 5 4 3 2 7 500 nm c-axis SiO 2 NC layer template overgrowth c-axis SiO 2 1  m NC layer overgrowth template 2 3 5 9 6 7 8 1 4 10

44. Sample Structures Sapphire Overgrown thin film 5 QWs 1  m 4  m nGaN/1  m uGaN Well:3nm Barrier: 15nm Undoped GaN Sapphire Overgrown thin film 5 QWs 120nm pGaN Undoped GaN Quantum well (QW) structure LED structure Conventional GaN template 2  m 5  m uGaN 80 nm SiO 2 mask Growth temperatures for blue and green emission are 715 O C and 675 O C, respectively. The growth temperature of barrier is 850 O C.

45. Blue LED Structure on Coalescence Overgrown GaN Template IQE A quick test shows ~80 % increase in output intensity. Scattering of the residual NC pattern may also help in enhancing light extraction. Wavelength: 460 nm 49.2% 20.1% ~80 % L-I curves

46. Green QW and LED on Coalescence Overgrown GaN Template 9.1 % increase IQE of green QW structure 14.1 % 10.4 % IQE of the green LED The reduction of dislocation density does not seem to significantly help in enhancing the efficiency of a green LED. The low miscibility between GaN and InN is the major cause for the low efficiency of a green LED. Wavelength: 520 nm 21.2 % 12.4 %