Two-Level Systems in Evaporated Amorphous Silicon

TL;DR

This study investigates two-level systems in evaporated amorphous silicon, revealing their dependence on atomic density and structural heterogeneity, and showing their proportional relationship consistent with existing models.

Contribution

It demonstrates that TLS densities in a-Si are proportional and depend on atomic density, emphasizing the role of structural heterogeneity rather than intrinsic amorphous properties.

Findings

TLS densities vary over three orders of magnitude.

TLS are proportional and depend on atomic density.

Heterogeneous structure is necessary for TLS formation.

Abstract

In $e$-beam evaporated amorphous silicon ($a$-Si), the densities of two-level systems (TLS), $n_{0}$ and $\overline{P}$, determined from specific heat $C$ and internal friction $Q^{-1}$ measurements, respectively, have been shown to vary by over three orders of magnitude. Here we show that $n_{0}$ and $\overline{P}$ are proportional to each other with a constant of proportionality that is consistent with the measurement time dependence proposed by Black and Halperin and does not require the introduction of additional anomalous TLS. However, $n_{0}$ and $\overline{P}$ depend strongly on the atomic density of the film ($n_{\rm Si}$) which depends on both film thickness and growth temperature suggesting that the $a$-Si structure is heterogeneous with nanovoids or other lower density regions forming in a dense amorphous network. A review of literature data shows that this atomic density dependence is not unique to $a$-Si. These findings suggest that TLS are not intrinsic to an amorphous network but require a heterogeneous structure to form.