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<ul><li> 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)</li></ul><p> 2. Special Recognition </p> <ul><li>XRD: Wen-Yu Shiao </li></ul><ul><li>TEM: Yung-Sheng Chen </li></ul><p> 3. Acknowledgements </p> <ul><li>Advisor: C. C. Yang </li></ul><ul><li>Nanoimprint lithography: Epistar Corporation </li></ul><ul><li>MBE: Dr. Kent Averett </li></ul><ul><li>Raman measurement: Hsu-Cheng Hsu </li></ul><ul><li>CL measurement: Wei-Chao Chen </li></ul><ul><li>PECVD: Cheng-Hung Lin and Kun-Ching Shen </li></ul><p> 4. Outline </p> <ul><li>Introduction </li></ul><ul><li><ul><li>Limiting Factors for the Nitride-based LED Development </li></ul></li></ul><ul><li><ul><li>Methods of Reducing Threading Dislocation Density </li></ul></li></ul><ul><li><ul><li>Motivations of the Research </li></ul></li></ul><ul><li><ul><li>Overview of Nitride Nanocolumn (NC) or Nanowire Growth </li></ul></li></ul><ul><li>Part 1: MOCVD Overgrowth on MBE-grown GaN NCs </li></ul><ul><li>Part 2: MOCVD Overgrowth on MOCVD-grown GaN NCs </li></ul><ul><li>Conclusions</li></ul><p> 5. </p> <ul><li>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 ) </li></ul><ul><li>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). </li></ul><p>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 </p> <ul><li>Mechanisms for enhancing LED efficiency using nitride NCs </li></ul><ul><li>Lower dislocation density or higher crystal quality </li></ul><ul><li>Lateral strain relaxation for increasing indium content </li></ul><ul><li>Scattering for enhancing light extraction </li></ul><ul><li>High-quality GaN template with coalescence overgrowth </li></ul><p>Threading dislocation Coalescence overgrowth of NC Substrate Overgrown thin film 10. Overview of Nitride Nanocolumn or Nanowire Growth with MBE </p> <ul><li>Self-organized growth </li></ul><ul><li>--- vertical to the template and high column density; but random distribution and non-uniform size distribution </li></ul><ul><li>--- E. Calleja et al.,Mater. Sci. Eng. B82 , 2 (2001). </li></ul><ul><li>K. Kusakabe et al.,Jpn. J. Appl. Phys. 40 , L192 (2001). </li></ul><ul><li>L. W. Tu et al.,Appl. Phys. Lett. 82 , 1601 (2003). </li></ul><ul><li>J. E. Van Nostrand et al.,J. Cryst. Growth 287 , 500 (2006). </li></ul><ul><li>R. Calarco et al.,Nano Lett. 7 , 2248 (2007). </li></ul><p> 11. </p> <ul><li>Coalescence overgrowth</li></ul><ul><li>InGaN/GaN LEDs </li></ul><p>Overview of Nitride Nanocolumn or Nanowire Growth with MBE </p> <ul><li><ul><li>Jpn. J. Appl. Phys., Part 240 , L192 (2001) </li></ul></li></ul><ul><li><ul><li>J. Vac. Sci. Technol. B25, 964 (2007) </li></ul></li></ul><p> 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. </p> <ul><li>Self-organized growth of GaN nanorods </li></ul><ul><li><ul><li>Appl. Phys. Lett.81 , 2193 (2002) </li></ul></li></ul><ul><li>Self-organized growth of InGaN nanorods </li></ul><ul><li><ul><li>Phys. Stat. Sol. (b) 241, 2802 (2004) </li></ul></li></ul><ul><li>Blue emission from InGaN/GaN QW nanorod arrays </li></ul><ul><li><ul><li>Appl. Phys. Lett.87 , 093115 (2005) </li></ul></li></ul><ul><li>Fabrication of free-standing GaN </li></ul><ul><li><ul><li>Phys. Stat. Sol. (c) 4, 2268 (2007) </li></ul></li></ul><p>Overview of Nitride Nanocolumn or Nanowire Growth with HVPE InGaN GaN 14. Overview of Nitride Nanocolumn or Nanowire Growth with VLS method </p> <ul><li>--- Most of them have random orientations and not vertical to the template </li></ul><ul><li>---Impurity incorporation into NC or nanowire due to the use of a catalyst may degrade device performance. </li></ul><p>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 </p> <ul><li>Top-down method </li></ul><ul><li>--- E lectron-beam lithography has been used. </li></ul><ul><li>--- The dry etching procedure normally generates defect states on the column surfaces.</li></ul><ul><li>Patterned growth </li></ul><ul><li>Appl. Phys. Lett.89 , 233115 (2006).(interferometry lithography) </li></ul><ul><li>J. Appl. Phys. 100 , 054306 (2006).(AAO lithography) </li></ul><p>Nanotechnology17 , 1454 (2006) 16. </p> <ul><li>Pattern growth- pulsed growth mode </li></ul><ul><li><ul><li>Nano Lett.6 , 1808 (2006) </li></ul></li></ul><ul><li>Our consideration </li></ul><ul><li><ul><li>High quality (surface defectstate, impurity incorporation) </li></ul></li></ul><ul><li><ul><li>High density </li></ul></li></ul><ul><li><ul><li>Regular arrangement </li></ul></li></ul><p>Overview of Nitride Nanocolumn or Nanowire Growth with MOCVD 17. Outline </p> <ul><li>Introduction </li></ul><ul><li>Part 1: MOCVD Overgrowth on MBE-grown GaN NCs </li></ul><ul><li><ul><li>Sample Structure and Growth Conditions </li></ul></li></ul><ul><li><ul><li>Characterization: PL, CL, AFM, SEM, XRD, and TEM </li></ul></li></ul><ul><li><ul><li>Summary </li></ul></li></ul><ul><li>Part 2: MOCVD Overgrowth on MOCVD-grown GaN NCs </li></ul><ul><li>Conclusions</li></ul><p> 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 </p> <ul><li>The overgrown layer shows relatively stronger emission when compared with that from the NCs. </li></ul><ul><li>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.</li></ul><p>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 </p> <ul><li>An AFM image of 7 m x 7 m in dimensions demonstrating part of a hexagon.</li></ul><ul><li>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. </li></ul><p>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 -- &gt; 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 </p> <ul><li>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.</li></ul><ul><li>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.</li></ul><ul><li>Hexagonal structures are observed on the surface. It is believed that the surface morphology can be improved by using regular and uniform NCs.</li></ul><p> 27. Outline </p> <ul><li>Introduction </li></ul><ul><li>Part 1: MOCVD Overgrowth on MBE-grown GaN NCs </li></ul><ul><li>Part 2: MOCVD Overgrowth on MOCVD-grown GaN NCs </li></ul><ul><li><ul><li>Overgrown Undoped GaN </li></ul></li></ul><ul><li><ul><li>Overgrown QWs and LED </li></ul></li></ul><ul><li><ul><li>Summary </li></ul></li></ul><ul><li>Conclusions</li></ul><p> 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 29. Growth Conditions </p> <ul><li>NC growth </li></ul><ul><li><ul><li>Continuous growth (5sec) </li></ul></li></ul><ul><li><ul><li><ul><li>Temperature: 1050 O C </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Pressure: 100 torr </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>V/III ratio: 1100 </li></ul></li></ul></li></ul><ul><li><ul><li>Pulsed growth mode </li></ul></li></ul><ul><li><ul><li><ul><li>Temperature: 1050 O C </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Pressure: 100 torr </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Duration and flow rate of TMG: 20 sec and 12.5 mol/min </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Duration and flow rate of ammonia: 30 sec and 500 sccm </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Growth rate: 2 m/ hr </li></ul></li></ul></li></ul><p> 30. Growth Conditions </p> <ul><li>Coalescence overgrowth </li></ul><ul><li><ul><li><ul><li>Temperature: 1050 O C </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Pressure: 200 torr </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>V/III ratio: 3900 </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Growth rate: 1.3 m/ hr </li></ul></li></ul></li></ul><ul><li><ul><li><ul><li>Thickness: 2 m </li></ul></li></ul></li></ul><p>Four patterns are fabricated by nanoimprint lithography. The hole diameters of the four patterns are 250, 300, 450, and 600 nm, corresponding to center-to-center spacing sizes of 500, 600, 900, and 1200 nm, respectively. 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: 500nmSeparation: 900nm Hole diameter: 600 nm Column diameter: 800nmSeparation: 1200nm Regularly Arranged GaN Nanocolumns - 2 33. Coalescence Overgrowth Results Hole diameter for column growth: 250 nm </p> <ul><li>The overgrown surface is quite smooth. </li></ul><ul><li>The overgrown layer (I), the NC layer (II), and the GaN template layer (III) can be identified. </li></ul><ul><li>The original column walls are depicted by the vertical dashed lines.</li></ul><p>(a) 1 m (b) 1 m I II III (c) SiO 2 SiO 2 SiO 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 35. PL Measurements 10 K RT </p> <ul><li>For control sample, spectra at 10 K include one major peak around 355.5 nm due to donor-bound exciton (DBE) recombination and two minor peaks at 354.8 and 356 nm, corresponding to free exciton A (FE(A)) and acceptor-bound exciton (ABE), respectively. </li></ul><ul><li>By using the proportionality factor of K = 21.2 meV/GPa, we find that at room temperature a stress of 0.66 GPa is built during the overgrowth process.</li></ul><p>Hole diameter: 250 nm 36. Raman and XRD Measurements </p> <ul><li>NC is almost strain-free. Based on the stress-shift coefficient of 4.2 cm-1 GPa-1, one can estimate the stress built in the overgrown layer to give 0.67 GPa. </li></ul><ul><li>The FWHMs of the control, NCs, and overgrowth samples are 220, 303, and 256 arcsec, respectively.</li></ul><ul><li>The rocking curve of the NCs sample can be decomposed into two components, including the broader one from the NCs and the narrower one from the template beneath. different columns still have significant variations in crystal orientation even though they stem from the same GaN template.</li></ul><p>Hole diameter: 250 nm 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 NCsOvergrown 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 7cm -2are achieved. The lateral domain size has been significantly increased. &gt;10X &gt;3X &gt;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 42. Edge effect due to different Thermal Expansion Coefficient </p> <ul><li>The termination or bending of a TD around the hole region is caused by the strain, compressive or tensile, induced in GaN by the SiO 2mask due to their different thermal expansion coefficients. </li></ul><ul><li>TDs can easily penetrate into the overgrown layer through the large windows, leading to the poor-quality window regions and high-quality mask regions in the lateral dimension.</li></ul><p>mask Thin solid films514 , 344 (2006) 43. Outline </p> <ul><li>Introduction </li></ul><ul><li>Part 1: MOCVD Overgrowth on MBE-grown GaN NCs </li></ul><ul><li>Part 2: MOCVD Overgrowth on MOCVD-grown GaN NCs </li></ul><ul><li><ul><li>Overgrown Undoped GaN </li></ul></li></ul><ul><li><ul><li>Overgrown QWs and LED </li></ul></li></ul><ul><li><ul><li>Summary </li></ul></li></ul><ul><li>Conclusions</li></ul><p> 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 2mask Growth temperatures for blue and green emission are 715O C and 675O C, respectively. The growth temperature of barrier is 850O 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 % 47. Summary </p> <ul><li>The coalescence overgrowth has been successfully implemented on the NCs grown by patterned growth and pulsed growth mode. The measurement results of depth-dependent XRD, PL, and AFM show the superior properties of the overgrown thin film than those of a standard GaN thin film directly grown on sapphire substrate. </li></ul><ul><li>Smaller NC and spacing size lead to higher overgrowth quality, including a lower TD density and a larger lateral domain size.</li></ul><ul><li>We presented the emission enhancement results of the blue a...</li></ul>