Investigation of Confined and Interface Phonons in Acoustically Mismatched Heterostructures using Brillouin – Mandelstam Light Scattering Spectroscopy*

We report on the development of the advanced Brillouin – Mandelstam spectrometer tool and its use for the investigation of the confined and interface phonons in the acoustically mismatched heterostructures [1]. We studied the acoustically soft PMMA layers deposited on the acoustically hard single-crystal diamond substrate. The acoustic impedance is defined as a product of the mass density and sound velocity. Diamond and PMMA have a large acoustic impedance mismatch, allowing for the observation of confined and interface phonons. We observed evenly spaced peaks in the spectral range between the acoustic phonon peaks of diamond and the acoustic phonon peaks of PMMA layers suggesting the appearance of many confined polarization branches. We discuss the dependence of the observed phonon spectra on the thickness of the PMMA layer and the quality of the interface. The obtained results have implications for thermal transport across dissimilar interfaces. Diamond has been investigated for applications in electronics as a thermal management substrate and a wide-band-gap semiconductor channel. The knowledge of the phonon spectra of the diamond-based heterointerfaces is important for both types of applications.