Comparative studies of GaN, n-GaN and n+-GaN contact layers on GaN/c-Al2O3 virtual substrates synthesized by PA MBE

Abstract

An approach to create ohmic contacts to GaN layers without using the high-temperature annealing procedure is described in the paper. The results of comparative studies of structural, morphological, optical, and electrical properties of undoped GaN and silicon-doped n-GaN and n+-GaN doped contact layers are presented.The contact layer technology can be successfully realized using a plasma-assisted molecular beam epitaxy (PA MBE) setup of industrial type, where due to characteristic design features, the growth of nitride AIIIN compounds occurs using small streams of activated nitrogen, which significantly limits the possibility of varying the technological parameters.For the first time it is demonstrated that using the proposed technology for the creation of post-grown GaN, n-GaN and n+-GaN contact layers to GaN/c-Al2O3 virtual substrates in Ga-enriched conditions of epitaxial PA MBE growth at relatively low temperatures Ts = 700oC an effective filtration of dislocations threading from the buffer GaN layer of the virtual substrate formed by MOCVD can be realized.Calculations of in-plane εxx and out-of-plane εzz deformations, as well as residual biaxial stresses based on X-ray diffraction and Raman microspectroscopy data indicate at high structural quality of the formed contact layer regardless of the silicon doping level. At the same time PA MBE contact layers GaN, n-GaN and n+-GaN demonstrate close optical properties (refractive index and photoluminescence) in a wide range (400–850 nm), as well as minimal influence of defects associated with the formation of silicon clusters at high doping levels.The contact resistance reduced to the pad width determined using the transmission line method for the structure with n+-GaN contact layer was of ∼0.11 Ohm*mm and ∼0.5 Ohm*mm for n-GaN. Note that electrophysical characteristics of n-GaN and n+-GaN contact layers do not degrade under high-dose exposure of high-energy X-ray irradiation.The technique developed in our work to create ohmic contacts to GaN layers, as well as the obtained experimental results, have important implications for understanding the physics of AIIIN nanoheterostructures and will contribute to their potential applications in their fabrication.