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)
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)
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)
Overview of Nitride Nanocolumn or Nanowire Growth with MBE
Jpn. J. Appl. Phys., Part 2 40 , L192 (2001)
J. Vac. Sci. Technol. B 25, 964 (2007)
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
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
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
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
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
1 m 1 m Sample A Sample B Sample C1 Sample C2 Cross-section SEM Images
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
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.
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.
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.
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.
The FWHMs of the control, NCs, and overgrowth samples are 220, 303, and 256 arcsec, respectively.
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.
Hole diameter: 250 nm
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
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
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
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
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
Edge effect due to different Thermal Expansion Coefficient
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 2 mask due to their different thermal expansion coefficients.
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.
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.
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
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 %
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.
Smaller NC and spacing size lead to higher overgrowth quality, including a lower TD density and a larger lateral domain size.
We presented the emission enhancement results of the blue and green-emitting InGaN/GaN QW and LED structures based on NC growth and coalescence overgrowth. Significant enhancements (up to 80 % output intensity increase in the blue LED) were observed. For LED application, the TD density reduction in an overgrown GaN template could more effectively enhance the emission efficiency of a blue LED, when compared with a green LED.
The coalescence overgrowth of MBE-grown NC is demonstrated. The characterization results show superior properties of overgrowth sample compared to the control sample although hexagonal structures can be observed on the surface.
The coalescence overgrowth of MOCVD-grown NC is demonstrated. The overgrowth process improve the properties of GaN crystal. Smaller NC and spacing size lead to higher overgrowth quality.
Significant enhancement of output intensity of the overgrown blue LED is observed.