Selective area growth of GaN nano islands by metal organic chemical vapor deposition: Experiments and computer simulations

We report the properties of GaN islands selectively grown by MOCVD on (0001) GaN/sapphire templates and on bare (0001) sapphire substrates. The approach allows us to grow GaN crystals with control over size and density through optical (micro-scale) and scanning electron microscope (SEM) e-beam (nano-scale) lithography. We have obtained complete pyramidal and prismatic hexagonal GaN islands. The growth characteristics are found to be very different from what we obtain for epitaxial layers, an effect which is attributed to loading and surface diffusion of source materials on the hard mask. From the dependence of growth rate of the pyramids with pitch we evaluate Ga surface diffusion length of ∼ 10 urn over SiO₂ mask. We present a growth model based on diffusion and compare the results of modeling with our data. Using micro-Raman spectroscopy the stress in pyramidal and prismatic islands was evaluated to be 50 ± 17 MPa and 84 ± 20 MPa, respectively. This implies that the islands are nearly relaxed compared to similar thickness of uniform layers. Room temperature SEM cathodoluminescence (CL) shows a narrow linewidth consistent with high quality of GaN layers. The GaN islands, of varying shapes, were overgrown with InGaN multiple quantum wells, with optical properties measured using SEM-CL.