Transducer-Less Thermoreflectance Technique for Measuring Thermal Properties of the Buried Buffer Layer and Interface in GaN-based HEMTs

Abstract

Measuring the thermal properties of the buried GaN buffer layer and interface in GaN high-electron mobility transistor (HEMT) structures is of crucial importance. This remains challenging with the traditional pump–probe thermoreflectance techniques due to their limited thermal penetration depths and the difficulty in extracting the large number of unknown parameters in the multilayer HEMT structure. This work applies a transducer-less transient thermoreflectance technique (TL-TTR) for the characterization. We experimentally and numerically investigate the dynamic thermal transport process of the TL-TTR measurement, benchmarking against that of the traditional metal transducer transient thermoreflectance (MT-TTR) measurement. The significantly different heat absorption and dissipation processes in the two measurements lead to the distinctive measurement sensitivities. Notably, the sensitivity of TL-TTR signal to all unknown thermal properties follows a distinctive trend over the measurement time region, demonstrating the technique’s capability of measuring the buried buffer thermal conductivity and the thermal boundary conductance (TBC) across the buffer/substrate interface. This is illustrated by measuring three different GaN-on-SiC wafers with highly Fe-doped buffer layers. The uncertainties of buffer thermal conductivity and TBC, determined by TL-TTR, are as low as ±6 and ±13%, respectively. In contrast, MT-TTR measures the buffer thermal conductivity and TBC with the uncertainties as large as ±20 and ±30%. The TL-TTR technique enables a non-invasive platform to characterize the thermal properties of the advanced GaN-based materials with complex structures and achieve a more in-depth understanding the phonon transport mechanisms.