Threading dislocations with edge components in GaN epilayers grown on Al 2 O 3 substrates
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Tomoya Ogawa Department of Physics, Gakushuin University, Mejiro, Tokyo 171-8588, Japan (Received 16 January 2001; accepted 15 June 2001)
Two types of threading dislocations with edge components were investigated by a high-resolution transmission electron microscope in undoped GaN epilayers grown on Al2O3 substrates. One is a fully filled core with regular contraction and stretch of bright dots, and the other is incompletely filled with one bright dot less and irregular contraction and stretch of bright dots. The bright dots were distorted and degenerated into bright line segments at cores in areas with smaller local dislocation intervals. The calculated results suggested that the distorted bright regions are attributable to the glide and/or climb caused by nearby dislocation interactions.
I. INTRODUCTION
GaN and its related compound semiconductors have the advantages of wide direct band gaps, high external luminescence quantum efficiencies, high breakdown fields, and excellent chemical stability. Breakthroughs in their epitaxial process1–3 have led to wide applications in fabricating high-brightness blue light emitting diodes, blue laser diodes, ultraviolet (UV) detectors, and hightemperature and high-power transistors. The most widely employed substrate for their epitaxy is sapphire (Al2O3). However, the approximate 14% lattice mismatch and 80% thermal-expansion difference between the substrate and the epilayers usually bring about a high density of growth defects in the epilayers. To improve the device and material quality, the physical origins of the growth defects have attracted extensive studies. Threading dislocations were suggested to be formed by boundaries due to misorientation and surface steps.4–6 Nanopipes and precipitates were believed to result from V-shaped indentations during coalescence.7–9 Although most threading dislocations were complicated in the epilayers, the theoretical and experimental investigations concentrate on the core structure of the pure edge dislocations.10,11 Recently, the dislocations were observed to propagate laterally in lateral epitaxial overgrowth GaN.12 However, the information on core images of the curving dislocations is still little, and the curving mechanism of the dislocations has not been clarified.
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J. Mater. Res., Vol. 16, No. 9, Sep 2001 Downloaded: 13 Mar 2015
In this work, the threading dislocations with edge components were investigated by a high-resolution transmission electron microscope (HREM) in undoped GaN epilayers grown on Al2O3 substrates. The core images were examined in detail. The distortions of bright diffraction regions were analyzed by calculating the interaction forces between two parallel edge dislocations lying along the [0001] direction. The physical origins of the distortions of bright diffraction regions are discussed. II. EXPERIMENTAL
Undoped GaN epilayers under study were grown on (0001) sapphire substrates by metalorganic vapo
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