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1.1 Foreword The III-V nitrides have long been viewed as promising semiconductor materials for their application in the blue and ultraviolet wavelengths optical devices, as well as high power and high temperature electronic devices. In the absence of a suitable gallium nitride (GaN) substrate, GaN, and related III-V materials are heteroepitaxially grown on sapphire or other substrates. GaN grown on sapphire normally contains a high density of threading dislocations in the range of 1010 cm-2 (Lester, 1995; Qian, 1995a; Hong & Cho, 2009) due to lattice constant and thermal expansion coefficient mismatches between GaN and sapphire. Besides threading dislocations, there are many other structural defects, such as, inversion domain, stacking mismatch boundaries, micropipes/nanopipes or voids, and surface pits. These defects will cause the periodicity of the crystal to be disrupted over distances of several atomic diameters from the defect and affect the optoelectronic properties of the devices. For example, threading dislocations have been found to act as nonradiative centers and scattering centers in electron transport that is detrimental to the performance of light emitting diodes and field-effect transistor (Ng et al., 1998). Dislocations defects cause rapid recombination of holes with electrons without conversion of their available energy into photons, i.e., nonradiative recombination, which causes heating up of the crystal and making optoelectronic devices malfunction (Hong & Cho, 2009; Garni et al., 1996). With the advancement of crystal growth technology, crystal defects in GaN have been reduced tremendously. The threading dislocations density in the GaN films has been reduced from the range of 1010 cm-2 to 105 cm-2. However, effort to further reduce the density of structural defects in GaN is strongly driven by the growth of high crystal quality thin films for fabrication of high performance optoelectronic devices. |
