Czasopismo | INŻYNIERIA MATERIAŁOWA | Rocznik 2013 - zeszyt 4

Recognition and characterization of defects in GaN epilayers

nr katalogowy: 79713

Defects in epitaxial films may degrade device performance. A variety of defects can arise during the growth, processing and operation of devices. It is important to identify their nature. The most potent origin of defects is the lattice mismatch between epilayer and substrate that arises due to differences in symmetry, structure and orientation. The lattice mismatch must be accommodated by misfit dislocations, for epitaxial film with thickness greater than the critical value. Sapphires are the most commonly used substrates, with about 16% lattice mismatch, and SiC, with a mismatch of about 3.5% and with closer thermal expansion coefficients to those of GaN [1]. Moreover, the high thermal conductivity and the availability of conducting as well as semi-insulating substrates makes SiC attractive for the realization of both transistors and optoelectronic devices. In the case of GaN epitaxy, the use of off-oriented instead of exact-oriented SiC substrates results in an improved surface smoothness and in reduced dislocation density under optimized step flow growth conditions. GaN epilayers have generated significant interest for usage in optoelectronic device applications since the material’s large direct band gap can be employed for the production of blue-green and ultraviolet wavelengths [2]. It is challenging that devices comprising such epilayers exhibit satisfactory performance despite a multiplicity of microstructural defects such as threading and misfit dislocations, nanopipes, inversion domain boundaries, stacking faults on basal and prismatic crystallographic planes and grain boundaries. TEM investigations were performed on epitaxial layers GaN/SiC and GaN/sapphire grown by Metal Organic Chemical Vapour Deposition (MOCVD) technique [3, 4]. Studies were carried out on samples prepared in both plan-view and crosssectional orientation using JEM 3010 Jeol microscope operating at 300 kV. DISLOCATIONS Threading dislocat[...]

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