RESUMEN
Understanding the growth mechanisms of III-nitride nanowires is of great importance to realise their full potential. We present a systematic study of silane-assisted GaN nanowire growth on c-sapphire substrates by investigating the surface evolution of the sapphire substrates during the high temperature annealing, nitridation and nucleation steps, and the growth of GaN nanowires. The nucleation step - which transforms the AlN layer formed during the nitridation step to AlGaN - is critical for subsequent silane-assisted GaN nanowire growth. Both Ga-polar and N-polar GaN nanowires were grown with N-polar nanowires growing much faster than the Ga-polar nanowires. On the top surface of the N-polar GaN nanowires protuberance structures were found, which relates to the presence of Ga-polar domains within the nanowires. Detailed morphology studies revealed ring-like features concentric with the protuberance structures, indicating energetically favourable nucleation sites at inversion domain boundaries. Cathodoluminescence studies showed quenching of emission intensity at the protuberance structures, but the impact is limited to the protuberance structure area only and does not extend to the surrounding areas. Hence it should minimally affect the performance of devices whose functions are based on radial heterostructures, suggesting that radial heterostructures remain a promising device structure.
RESUMEN
Time-resolved cathodoluminescence offers new possibilities for the study of semiconductor nanostructures - including defects. The versatile combination of time, spatial, and spectral resolution of the technique can provide new insights into the physics of carrier recombination at the nanoscale. Here, we used power-dependent cathodoluminescence and temperature-dependent time-resolved cathodoluminescence to study the carrier dynamics at trench defects in InGaN quantum wells - a defect commonly found in III-nitride structures. The measurements show that the emission properties of trench defects closely relate to the depth of the related basal plane stacking fault within the quantum well stack. The study of the variation of carrier decay time with detection energy across the emission spectrum provides strong evidence supporting the hypothesis that strain relaxation of the quantum wells enclosed within the trench promotes efficient radiative recombination even in the presence of an increased indium content. This result shines light on previously reported peculiar emission properties of the defect, and illustrates the use of cathodoluminescence as a powerful adaptable tool for the study of defects in semiconductors.