Structure, electronic properties, and interactions of defects in epitaxial GaN layers

Gallium nitride (GaN) is a III-V semiconductor which is widely used in optoelectronic devices, especially in lasers and light emitting diodes (LEDs). Due to its wide band gap of approximately 3.4 eV at room temperature [1], GaN - like related ternary group III-nitride semiconductors - is a material of choice for the emission of light within the blue to near ultraviolet range [2]. Due to its application in ultraviolet LEDs, GaN is deployed in microelectronic devices, but also nds use in the medical eld both for diagnostic and therapeutic purposes as well as for the purication of air and water [3]. At present, the ecacy of devices based on GaN is, however, limited by the quality of the bulk crystal material. Since there are hardly any large bulk substrates available [4], almost all GaN lms are grown on substrates which exhibit a large lattice mismatch as well as a large thermal mismatch using heteroepitaxy or are deposited onto pseudo substrates which themselves were nucleated on mismatched substrates [3]. The mismatch induces high concentrations of defects, of which dislocations negatively aect the properties of the device in particular. Dislocations in transistors, for example, can cause a large reverse bias leakage [5] and threading dislocations have been associated with luminescence at various wavelengths in GaN epitaxial layers [6]. Although group III-nitrides and especially GaN are already being used commercially in optoelectronic devices, the knowledge of the general properties of defects within these materials and of the interactions between the defects in particular is still limited: The electronic properties of dislocations, for example, are debated controversially, with dierent groups reporting either only charged or only uncharged dislocations or a mixture of both [7{14] depending on the type of dislocation as well as on the material investigated. In addition, various ideas on the origin of charges spanning from reconstructions of the dislocation cores, impurities, point defects, to strain [14{21] are being discussed.