Low dislocation density AlGaN epilayers by epitaxial overgrowth of patterned templates

Abstract

We present an epitaxial overgrowth process for reducing threading dislocations in AlxGa1−xN over the entire compositional range. This process avoids the use of UV-absorbing GaN layers and results in a spatially uniform defect reduction which eliminates the need for precise alignment of devices to low-defect areas of the wafer. Using the described overgrowth process, we demonstrate Al0.3Ga0.7N and Al0.6Ga0.4N epilayers with a dislocation density of 2×108cm−2 and 5×108cm−2 respectively, rendering them suitable as templates for deep-UV bottom-emitting LEDs and laser diodes. The process involves patterning of submicron-wide-stripes, less than 1μm in height, into an AlGaN/AlN/sapphire template and subsequent regrowth of a 5–10μm thick AlGaN epilayer. The sub-micron width of the mesa allows for bending of threading dislocations that would continue to thread vertically through wider mesas. Utilizing 1.3mm-thick sapphire substrates (3× thicker than commonly used), epilayer cracking from regrowth is eliminated and wafer bow over a 2-in. diameter substrate is reduced to less than 15μm. We observed a 7× increase in photoluminescence intensity from GaN–AlGaN multi-quantum well structures emitting at 340nm and a 15× increase in electroluminescence from laser diode heterostructures when grown on patterned Al0.3Ga0.7N templates.