Device-Level Impact of Highly Anisotropic Thermal Conductivity of AlN/GaN Digital Alloys

AlN/GaN digital alloys (i.e., short period superlattices) have the advantage of being an ultra-wide bandgap due to the quantum confinement effect, preservation of high mobility (no strong alloy scattering), and higher thermal conductivity than AlGaN random alloys. Herein, we investigate the influence of AlN/GaN thickness combinations on digital alloy (DA) thermal conductivity. Using time-domain thermoreflectance and steady-state thermoreflectance, we measure the anisotropic thermal conductivity of AlN/GaN DA’s, with thickness combinations: (2 nm/1 nm), (4 nm/1 nm), (6 nm/1 nm), (11 nm/1 nm). DA's with thick AlN layers exhibit highly anisotropic thermal conductivity (i.e., 4x difference between cross-plane and in-plane). Using finite-element modeling, we thermally simulate AlGaN and AlN/GaN DA devices. Improved lateral heat spreading in DA devices results in lower hot-spot temperatures compared to AlGaN devices. As the first thermal measurements of AlN/GaN DA’s, this study provides guidance for future device design.