Materials Issues for Vertical Gallium Nitride Power Devices

摘要

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 (＜10~(16) 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 effectively eliminates the need for post-growth processes such as laser liftoff, chemical etching or mechanical lapping to form freestanding GaN substrates. A number of GaN thick films were grown onto sapphire substrates by the Hydride Vapor Phase Epitaxy (HVPE) method with thickness varying from 150μm to 3.8mm using either a low-temperature GaN or an AlN buffer as the nucleation step. We have found that samples grown on a low temperature GaN buffer naturally delaminate from the sapphire substrate post-growth over the entire thickness range studied. However, the GaN films grown on AlN buffers did not delaminate. 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 5×10~6 cm~(-2). The lowest carrier concentration in these heteroepitaxially grown films was found to be 10~(17) cm~(-3). Furthermore, we have identified the origin of this n-type auto-doping and proposed method to reduce the carrier concentration to values 10~(16) cm~(-3) or lower.