Two-level systems and growth-induced metastability in hydrogenated amorphous silicon

TL;DR

This study investigates the low-temperature specific heat of hydrogenated amorphous silicon, revealing the roles of metastable hydrogen and two-level systems, and how annealing affects these properties.

Contribution

It provides new insights into the relationship between hydrogen content, growth conditions, and low-temperature thermal properties in amorphous silicon.

Findings

Excess specific heat is linked to metastable hydrogen and tunneling two-level systems.

Annealing reduces heat capacity and eliminates the Schottky anomaly.

Growth temperature and hydrogen content non-monotonically affect low-temperature properties.

Abstract

Specific heat measurements from 2 to 300 K of hydrogenated amorphous silicon prepared by hot-wire chemical vapor deposition show a large excess specific heat at low temperature, significantly larger than the Debye specific heat calculated from the sound velocity. The as-prepared films have a Schottky anomaly that is associated with metastable hydrogen in the amorphous network, as well as large linear and excess cubic term commonly associated with tunneling two-level systems in amorphous solids. Annealing at 200 {\deg}C, a temperature that enables hydrogen mobility but not evaporation, irreversibly reduces the heat capacity, eliminating the Schottky anomaly and leaving a reduced linear heat capacity. A non-monotonic dependence on growth temperature and H content is observed in all effects, except for sound velocity, which suggests that the tunneling two-level systems and the Schottky anomaly are associated with atomic hydrogen and require low density regions to form, while sound velocity is associated with the silicon network and improves with increasing growth temperature.