Experimental evidence of impurity-induced selective short-mean-free-path phonon scatterings in <i>β</i>-Ga2O3

Abstract

Understanding the phonon–impurity interactions is essential to modulate and evaluate the lattice thermal conductivity (LTC) of materials for heat dissipation and thermoelectric conversion. The phonon–impurity scattering theorem has been established for several decades and argues a primary effect on phonons with high frequency or short mean free path (MFP). However, all measurements of crystals with impurity so far have indicated a controversial full-MFP-range suppression of phonon contribution to LTC. Here, by employing an improved frequency-domain thermoreflectance method with ultrahigh heating frequency reaching 200 MHz, we measured the LTC of a promising ultrawide-bandgap semiconductor β-Ga2O3 and its phonon MFP spectra down to about 50 nm and reported an impurity-induced selective reduction of spectral contribution of phonons with MFPs shorter than about 500 nm. This work uses an improved thermoreflectance method with ultrahigh heating frequency as an effective approach to probe the phonon physics, provides fundamental experimental evidence for selective interactions between impurity and high-frequency phonons, and will guide the material design for desired thermal transport performance.