Browsing by Author "Tasi, Chi-Tsung"

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Reduction of Defects in AlGaN Grown on Nanoscale-Patterned Sapphire Substrates by Hydride Vapor Phase Epitaxy
(2017-06-01)
Tasi, Chi-Tsung
;
Wang, Wei-Kai
;
Tsai, Tsung-Yen
;
Huang, Shih-Yung
;
Horng, Ray-Hua
In this study, a 3-m-thick AlGaN film with an Al mole fraction of 10% was grown on a nanoscale-patterned sapphire substrate (NPSS) using hydride vapor phase epitaxy (HVPE). The growth mechanism, crystallization, and surface morphology of the epilayers were examined using X-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy at various times in the growth process. The screw threading dislocation (TD) density of AlGaN-on-NPSS can improve to 1-2 x 10(9) cm(-2), which is significantly lower than that of the sample grown on a conventional planar sapphire substrate (7 x 10(9) cm(-2)). TEM analysis indicated that these TDs do not subsequently propagate to the surface of the overgrown AlGaN layer, but bend or change directions in the region above the voids within the side faces of the patterned substrates, possibly because of the internal stress-relaxed morphologies of the AlGaN film. Hence, the laterally overgrown AlGaN films were obtained by HVPE, which can serve as a template for the growth of ultraviolet III-nitride optoelectronic devices.
Structural and Stress Properties of A1GaN Epilayers Grown on A1N-Nanopatterned Sapphire Templates by Hydride Vapor Phase Epitaxy
(2018-09-01)
Tasi, Chi-Tsung
;
Wang, Wei-Kai
;
Ou, Sin-Liang
;
Huang, Shih-Yung
;
Horng, Ray-Hua
In this paper, we report the epitaxial growth and material characteristics of AlGaN (Al mole fraction of 10%) on an AlN/nanopatterned sapphire substrate (NPSS) template by hydride vapor phase epitaxy (HYPE). The crystalline quality, surface morphology, microstructure, and stress state of the AlGaN/AlN/NPSS epilayers were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The results indicate that the crystal quality of the A1GaN film could be improved when grown on the A1N/NPSS template. The screw threading dislocation (TD) density was reduced to 1.4 x 10(9) cm(-2) for the AlGaN epilayer grown on the A1N/NPSS template, which was lower than that of the sample grown on a flat c-plane sapphire substrate (6.3 x 10(9) cm(-2)). As examined by XRD measurements, the biaxial tensile stress of the AlGaN film was significantly reduced from 1,187 MPa (on A1N/NPSS) to 38.41 MPa (on flat c-plane sapphire). In particular, an increase of the Al content in the overgrown AlGaN layer was confirmed by the TEM observation. This could be due to the relaxation of the in-plane stress through the AlGaN and AlN/NPSS template interface.
Surface evolution and effect of V/III ratio modulation on etch-pit-density improvement of thin AlN templates on nano-patterned sapphire substrates by metalorganic chemical vapor deposition
(2018-10-15)
Wang, Tzu-Yu
;
Tasi, Chi-Tsung
;
Lin, Ku-Yen
;
Ou, Sin-Liang
;
Horng, Ray-Hua
A low defect-density AlN template film using V/III ratio modulation and nano-patterned sapphire substrate (NPSS) was achieved by metalorganic chemical vapor deposition. In contrast to conventional high-quality AlN/NPSS obtained with high coalescence thickness ( > 6 mu m) using high growth temperatures ( >= 1250 degrees C), this study reveals the high crystallinity of AlN with the lower thickness of 2.55 mu m grown under a lower temperature of 1130 degrees C. It could effectively increase the heater lifetime and reduce the epi-wafer warpage. The growth of AlN/NPSS dominated by epitaxial lateral overgrowth achieves a dramatic reduction of full width at half maximum values along (10 (1) over bar2) plane from 1640 to 714 arcsec, and lowest dislocation density of approximately 1 x 10(8) cm(-2), as well as a ultra-low etching pit density of 2.3 x 10(5) cm(-2). The crack-free AlN/NPSS with a compressive stress owing to the tensile stress was relaxed by the existence of some key-shaped holes upon the patterned region. Details of the surface evolution, mechanism and dislocation behavior of AlN/NPSS will be discussed and these results demonstrate this low-defect template technique of high potential for AlGaN-based device applications.