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The maximum output power of GaN-based high-electron mobility transistors islimited by high channel temperature induced by localized self-heating whichdegrades device performance and reliability. With generated heat fluxes withinthese devices reaching magnitude close to ten times of that at the sun surface,chemical vapor deposition (CVD) diamond is an attractive candidate to aid inthe extraction of this heat in order to keep the operating temperatures ofthese high power electronics as low as possible. Due to the observedinhomogeneous structure, CVD diamond membranes exhibit a 3D anisotropic thermalconductivity which may result in significantly different cooling performancefrom expected in a given application. In this work, time domainthermoreflectance (TDTR) is used to measure the thermal properties of an11.8-{\mu}m CVD diamond membrane from its nucleation side. Starting with a spotsize diameter larger than the thickness of the membrane, measurements are madeat various modulation frequencies from 1.2 MHz to 11.6 MHz to tune the heatpenetration depth, and subsequently the part of diamond sampled by TDTR. Wedivide the membrane into ten sublayers and assume isotropic thermalconductivity in each sublayer. From this, we observe a 2D gradient of thedepth-dependent thermal conductivity for this membrane. By measuring the sameregion with a smaller spot size at multiple frequencies, the in-plane andcross-plane thermal conductivity are extracted respectively. Through this useof multiple spot sizes and modulation frequencies, the 3D anisotropic thermalconductivity of CVD diamond membrane is experimentally obtained by fitting theexperimental data to a thermal model. This work provides insight toward animproved understanding of heat conduction inhomogeneity in CVD polycrystallinediamond membrane that is important for applications of thermal management ofhigh power electronics.

Probing Growth-Induced Anisotropic Thermal Transport in CVD Diamond Membranes by Multi-frequency and Multi-spot-size Time-Domain Thermoreflectance