Virtual substrates for wide bandgap Al_yX_1-yN growth

Lattice-matched substrates are critical for growth of high quality, compositionally-targeted ternary Al_yX_1-yN compounds with properties suitable for a wide variety of next-generation opto- and power electronic applications. [DOI:10.1149/2.0111702jss] We have identified the (111) plane of transition metal carbides and nitrides as lattice matched “virtual” substrate layers that have additional benefits of electrical conductivity and appropriate coefficients of thermal expansion for nitride layer at both growth and operating temperatures. [arXiv:2208.11769 2022]



In this work, (111)-oriented TaC and ZrN are grown by RF sputtering and optimized as substrate layers for Al_yGa_1-yN and Al_yGd_1-yN, respectively. TaC is demonstrated as a full proof of concept. (111)-stabilized TaC layers with a rock-salt crystal structure are annealed to improve surface crystal quality and surface morphology and then used as a template for growth of an Al_0.7Ga_0.3N layer by molecular beam epitaxy. X-ray diffraction (XRD) demonstrates epitaxial registry of the grown layer to the substrate. High resolution transmission electron microscopy (TEM) is used to investigate interface behavior, showing regions of abrupt interface transitions and nitrogen polar termination of the Al_0.7Ga_0.3N. We also grew (111)-oriented ZrN on Ti-Zr-N graded buffer layers as a lattice-matched virtual substrate for Al_yGd_1-yN. Structure, morphology, and strain are analyzed using XRD and atomic force microscopy. Structure and quality of initial Al_0.8Gd_0.2N thin films are investigated for both in-situ and ex-situ growths.