No 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.