Transmissionselektronenmikroskopische Analysen zur Defektreduktion mittels SiNx Zwischenschichten in AlGaN Heterostrukturen
Day of Ph.D. defence: 19.04.2012
AlGaN with high Al concentration has attracted great attention because of its potential application to optoelectronic devices in the UV range due to its large direct band gap of up to 6eV. In order to avoid UV light absorption by any GaN buffer layer, the ternary layer AlGaN should be grown directly on sapphire, as adequate nitride based substrates are not yet readily available. By using a two-step growth process high quality, single crystalline AlGaN quasi-substrates can be achieved by metal organic vapor phase epitaxy (MOVPE) directly on sapphire. However especially a-type threading dislocations (TDs) are responsible for still high dislocation densities at the surface, which can act as non-radiative recombination centres and thus lower the efficiency and the lifetime of the optoelectronic device. A promising possibility to decrease the dislocation density of an AlGaN surface is the in-situ growth of SiNx interlayers. While the dislocation densities have already been reduced successfully in pure GaN by SiNx nano-masks by more than an order of magnitude, the defect reducing effect of such masks was very poor in AlGaN with high Al concentrations so far. By optimizing the growth parameters systematically for the deposition of the SiNx nano-mask, high defect reducing efficiencies of the interlayer could be achieved even in AlGaN with high Al concentrations. The purpose of the present work is to study the defect reducing mechanisms of SiNx interlayers in AlGaN with high Al concentrations in detail by TEM. As TEM allows detecting volume information of a material system with highest resolutions, it becomes possible to analyse layer and defect structures and to establish connections to the epitaxial growth behaviour. The combination of different TEM techniques in this work enabled the development of a complete growth model for the growth of AlGaN on SiNx, delivering deeper insight into the defect reducing mechanisms of in-situ deposited SiNx interlayers in AlGaN.