Department of Mechanical Engineering
Kenneth E. Goodson
Cho, J., Francis, D., Altman, D.H., Asheghi, M., and Goodson, K.E., "Phonon Conduction in GaN-Diamond Composite Substrates," Journal of Applied Physics, accepted and in press.
The integration of strongly contrasting materials can enable performance benefits for semiconductor devices. One example is composite substrates of gallium nitride (GaN) and diamond, which promise dramatically improved conduction cooling of high-power GaN transistors. Here, we examine phonon conduction in GaN-diamond composite substrates fabricated using a GaN epilayer transfer process through transmission electron microscopy, measurements using time-domain thermoreflectance, and semiclassical transport theory for phonons interacting with interfaces and defects. Thermoreflectance amplitude and ratio signals are analyzed at multiple modulation frequencies to simultaneously extract the thermal conductivity of GaN layers and the thermal boundary resistance across GaN-diamond interfaces at room temperature. Uncertainties in the measurement of these two properties are estimated considering by means of thermal analysis, which yields regions of coupled values for the two properties on a contour plot that are consistent with the measurement. A presence of near-interfacial defects in the GaN and diamond, as well as the volume resistance of an intermediate, disordered SiN layer between the GaN and diamond dominates the measured GaN-diamond thermal boundary resistances as low as 17 m2 K GW–1. The GaN thermal conductivity data are consistent with the semiclassical phonon thermal conductivity integral model that accounts for the size effect as well as phonon scattering on point defects at concentrations near 3 × 1018 cm–3.