Polycrystalline Mo as the substrate for heteroepitaxy of GaN using sputtered AlN buffer layer

Abstract

GaN and related III-nitrides have attracted significant attention due to their excellent performance and extensive applications. However, the substrates for epitaxial growth of III-nitride films are limited to a few options, such as SiC, Si, and sapphire, which suffer from significant shortcomings including high cost, lattice mismatch, and thermal expansion coefficient mismatch. In this study, AlN film with c-axis orientation was deposited on a 2-inch polycrystalline Mo substrate using reactive magnetron sputtering, leveraging the advantages of Mo. Additionally, the influence of a two-dimensional graphene (Gr) insertion layer on the epitaxy of III-nitrides on Mo was investigated. The introduction of Gr slightly reduced the grain size of the AlN by about 10 nm. However, the Gr induced some in-plane tensile strain in the AlN film, which compensated the compressive strain in the subsequently grown GaN, resulting in a more undistorted GaN lattice with a c-axis strain of only 0.01 %. Continuous GaN films were successfully epitaxially grown on the sputtered AlN buffer layers, which are with c-axis preferred orientation and ultraviolet emission at ∼3.36 eV. The grain size of GaN increased by about 5 nm and the full width at half maximum of the photoluminescence spectra also decreased by about 2.5 nm after the insertion of Gr. Our investigation indicates that Mo or Gr/Mo substrates are promising candidates for the heteroepitaxial growth of GaN films using sputtered AlN buffer layers. This work also provides a valuable strategy for low-cost and high-quality heteroepitaxy of other III-nitrides.