Materials Issues for Vertical Gallium Nitride Power Devices

Abstract

In this paper we are addressing some of the fundamental materials issues for the development of vertical GaN-based power devices. Major components of such device are the n+ GaN freestanding substrate on which a thick (~50 µm), low defect density and low carrier concentration (<1016 cm-3) n-GaN drift region is grown homoepitaxially. We show that the hydride-vapor-phase-epitaxy (HVPE) is a method capable of producing economically free standing n+ GaN substrates as well as the required thick and low defect and carrier concentration n-GaN drift region. The formation of freestanding GaN substrates by a natural separation mechanism from the sapphire substrate is demonstrated and effectively eliminates the need for post-growth processes such as laser liftoff, chemical etching or mechanical lapping to form freestanding GaN substrates. These results were accounted for by calculating the thermal stresses in the GaN film and substrate as a function of film thickness using Stoney’s equation and assuming that the GaN buffer undergoes decomposition at the growth temperature. The structure of these films was determined by x-ray diffraction and the dislocation density was measured to be as low as 5x106 cm-2. The lowest carrier concentration in these heteroepitaxially grown films was found to be 1017 cm-3. Furthermore, we have identified the origin of this n-type auto-doping and proposed method to reduce the carrier concentration to values 1016 cm-3 or lower.