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Comparative Raman and HRTEM Study of Nanostructured GaN Nucleation Layers and Device Layers on Sapphire (0001)

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Raman spectroscopy in conjunction with high-resolution transmission electron microscopy (HRTEM) has been used to study structural characteristics and strain distribution of the nanostructured GaN nucleation layer (NL) and the GaN device layer on (0001) sapphire substrates used for light-emitting diodes and lasers. Raman peaks corresponding to the cubic and the hexagonal phase of GaN are observed in the Raman spectrum from 15 nm and 45 nm NLs. A comparison of the peak intensities for the cubic and hexagonal phases of GaN in the NLs suggests that the cubic phase is dominant in the 15 nm NL and the hexagonal phase in the 45 nm NL. An increase in the density of stacking faults in the metastable cubic GaN (c-GaN) phase with increasing growth time lowers the system energy as well as locally converts c-GaN phase into hexagonal GaN (h-GaN). It also explains the observation of the more intense peaks of h-GaN in the 45 nm NL compared to c-GaN peaks. For the sample wherein an h-GaN device layer was grown at higher temperatures on the NL, narrow Raman peaks corresponding to only h-GaN were observed, confirming the high-quality of the films. The peak shift of the E2H(LO) mode of h-GaN in the NLs and the h-GaN film suggests the presence of a tensile stress in the NL which is attributed to defects such as stacking faults and twins, and a compressive stress in high-temperature grown h-GaN film which is attributed to the thermal-expansion mismatch between the film and the substrate. The peak shifts of the substrate also reveal that during the low temperature growth of the NL the substrate is under a compressive stress which is attributed to defects in the NL and during the high temperature growth of the device layer, there is a tensile strain in the substrate as expected from differences in coefficients of thermal expansion of the film and the substrate during the cooling cycle.
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Document Type: Research Article

Publication date: 2008-11-01

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  • Journal for Nanoscience and Nanotechnology (JNN) is an international and multidisciplinary peer-reviewed journal with a wide-ranging coverage, consolidating research activities in all areas of nanoscience and nanotechnology into a single and unique reference source. JNN is the first cross-disciplinary journal to publish original full research articles, rapid communications of important new scientific and technological findings, timely state-of-the-art reviews with author's photo and short biography, and current research news encompassing the fundamental and applied research in all disciplines of science, engineering and medicine.
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