Vertical power devices enabled by bulk GaN substrates

Abstract

The realization of next-generation vertical GaN devices relies heavily on advances in both epitaxial growth of GaN drift layers on commercially available GaN substrates and selective area n-type and p-type doping in a planar process. Although homoepitaxial GaN is expected to provide more control over growth characteristics (e.g. crystallinity and doping), its reliability and reproducibility suffer at the hand of the native GaN substrates available to date. The variations in commercial GaN substrates span defect density, surface roughness, wafer bow, and photoluminescence properties. Thus, elucidating the role of GaN substrate properties on the growth and characteristics of resulting homoepitaxial GaN films has emerged as a new challenge towards next-generation GaN power devices. . In many other semiconductor materials such as Si and SiC, ion implantation is a routine step in most processing sequences for selective area doping and greatly facilitates manufacturing by avoiding the complicated etch/regrowth process. The ability to implant and activate dopants, particularly p-type dopants, in GaN still remains a challenge though. The NRL-developed symmetric multicycle rapid thermal annealing (SMTRA) technique has been shown to activate up to ~10% of the implanted Mg dopant atoms using a combination of a temporary thermally stable capping layer, annealing in a nitrogen overpressure, and performing a well-optimized annealing temperature profile including multiple spike anneals. This paper will present an assessment of substrate-dependent effects on the quality of homoepitaxial GaN films, evaluate ion implantation processing for selective area doping, address basic vertical devices to identify process module development toward practical MOSFET devices.