Heteroepitaxy of Group-III-Nitrides for the Application in Laser Diodes

Abstract

The scope of this thesis is the epitaxy of group-III nitride heterostructures on (0001) sapphire substrates for the fabrication of semiconductor laser diodes. MOVPE as well as MBE were used to deposit the structures. The growth of the epilayers was studied with regard to the threading dislocation density, the strain state and the thermodynamic stability. In general, a low threading dislocation density is obtained for growth processes showing an elongated island growth mode, as those grow free of dislocations. According experiments are discussed on the background of a thermodynamic description of the decomposition reaction. The threading dislocation density was estimated by x-ray diffraction, using a model based on the concept of mosaic crystals. It describes the microstructure of the layer as many dislocation-free, but tilted and twisted crystals. It was shown by plan-view TEM images, that the edge- and screw-type threading dislocation densities can be determined within reasonable limits of accuracy. The strain state of the studied GaN layers is found to correlate with the island diameter at coalescence. The thermal compressive strain at room temperature is partly compensated by an intrinsic, tensile strain, which forms during growth and depends on the average island size. It might be a consequence of the gap closure process during island coalescence. The photoluminescence of InGaN quantum wells was studied for different well thicknesses and Si doping levels of the GaN barriers. The structures were simulated numerically, taking piezoelectric fields into account. The results point towards a light emission mechanism governed by well thickness fluctuations superimposed by the piezoelectric field. The increased electron density induced by the Si doping is proposed to block the localized states of the threading dislocations, acting as nonradiative centers. The optimized structures allowed to realize the first GaN laser grown in Bremen.