Abstract
We investigate the influence of different types of template micro-patterning on defect reduction and optical properties of non-polar GaN using detailed luminescence studies. Non-polar (or a-plane) GaN exhibits a range of different extended defects compared with its more commonly used c-plane counterpart. In order to reduce the number of defects and investigate their impact on luminescence uniformity, non-polar GaN was overgrown on four different GaN microstructures. The micro-patterned structures consist of a regular microrod array; a microrod array where the −c-side of the microrods has been etched to suppress defect generation; etched periodic stripes and finally a subsequent combination of etched stripes and etched microrods (double overgrowth). Overall the presence of extended defects, namely threading dislocations and stacking faults (SFs) is greatly reduced for the two samples containing stripes compared with the two microrod samples. This is evidenced by more uniform emission and reduction in dark regions of non-radiative recombination in room temperature cathodoluminescence imaging as well as a reduction of the SF emission line in low temperature photoluminescence. The observed energy shifts of the GaN near band edge emission are related to anisotropic strain relaxation occurring during the overgrowth on these microstructures. A combination of stripes and microrods is a promising approach for defect reduction and emission uniformity in non-polar GaN for applications in light-emitting devices as well as power electronics.
Highlights
The CL images of the integrated intensity of the GaN near band edge (NBE) emission together with the secondary electron (SE) image of the area imaged by CL are shown in figure 2
The samples using microrods exhibit large dark regions of reduced GaN NBE intensity associated with nonradiative recombination at threading dislocations (TDs) and stacking faults (SFs) as identified by room temperature (RT) CL imaging
The lower energy shoulders on the RT NBE emission, which are related to SFs, are strongly reduced for the samples employing stripes
Summary
In order to address the different growth rates and defect formation in non-/semi-polar materials, overgrowth is performed on micro-patterned GaN templates. The geometry of the patterning has a major impact This approach has been successfully demonstrated to achieve high quality semi-polar (11 ̄22) GaN on m-plane sapphire, where an initial GaN layer was patterned into a regular array of microrods for subsequent overgrowth [9, 10]. We probe the luminescence properties of non-polar (11 ̄20) (or a-plane) GaN overgrown on different GaN microstructures using CL imaging and photoluminescence (PL) spectroscopy in order to investigate the influence of micro-patterning on the presence of defects and spatial uniformity in emission intensity. A series of four different types of micro-pattern are considered: a regular microrod array, etched microrods (‘mushroom configuration’), periodic stripes and a combination of stripes and microrods (‘double overgrowth’)
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