The influence of mask area ratio on GaN regrowth by epitaxial lateral overgrowth

The optical performance of blue-light emitting GaN is greatly influenced by the density and distribution of threading dislocations (TDs), which can be reduced and controlled by varying the ratio of mask-to-window width during epitaxial lateral overgrowth (ELOG). Here we study the evolution of microstructure and optical property along with the spatial distribution of defects in ELOG GaN films by varying the fill factor (FF), defined as mask-to-window width ratio, which was varied from one to five while the window width was maintained at 5 μm. We employed a two-step ELOG method for this study by way of metal organic chemical vapor deposition, where SiO2 was used as the mask and grown on a low-temperature grown GaN layer, which acted as the buffer and seed layer on a sapphire substrate. The patterns were comprised of circular dot arrays arranged along the 〈1 1¯ 0 0〉 directions of the underlying GaN and developed by the conventional photolithography method. X-ray rocking curves show that all the resulting films are of excellent epitaxial quality, of which the sample with FF=5 exhibits the best quality from the narrowest peak broadening. Cathodoluminescence spectra show the near bandgap UV emission as well as a yellow emission, which could have arisen from TDs. Transmission electron microscopy results show that the TDs emerging through the windows can be effectively impeded by the mask and bent toward the faceted planes of GaN pyramid islands.