Excess Modes and Enhanced Scattering in Rare-Earth Doped Amorphous Silicon Thin Films

TL;DR

This study investigates how rare-earth doping in amorphous silicon thin films introduces excess vibrational modes, significantly reducing thermal conductivity and increasing specific heat, with implications for thermal management materials.

Contribution

It provides new insights into the vibrational spectrum modifications caused by rare-earth dopants and links microstructural features to thermal transport properties.

Findings

Doping reduces thermal conductivity across a wide temperature range.

Doping increases specific heat, indicating excess vibrational states.

Microstructural analysis shows columnar features influencing scattering.

Abstract

We report specific heat and thermal conductivity of gadolinium- and yttrium-doped amorphous silicon thin films measured using silicon-nitride membrane-based microcalorimeters. Addition of gadolinium or yttrium to the amorphous silicon network reduces the thermal conductivity over a wide temperature range while significantly increasing the specific heat. This result indicates that a large number of non-propagating states are added to the vibrational spectrum that are most likely caused either by localized vibration of the dopant atom in a Si cage, or softening of the material forming the cage structures. High-resolution cross-sectional electron micrographs reveal columnar features in Gd-doped material which do not appear in pure amorphous silicon. Scattering from both the nanoscaled columns and the filled-cage structures play a role in the reduced thermal conductivity in the rare-earth doped amorphous semiconductor. The overall result is an amorphous solid with a large bump in $C/T^{3}$ and no plateau in thermal conductivity.