Atomistic Origin of Urbach Tails in Amorphous Silicon

TL;DR

This study uses ab initio calculations to reveal that Urbach tails in amorphous silicon originate from filamentary structural features like short and long bonds, linking electronic tail states to network disorder and defects.

Contribution

It identifies the structural filamentary features responsible for Urbach tails in amorphous silicon using first-principles calculations, connecting electronic states to network topology.

Findings

Urbach tails are associated with filamentary structures in a-Si.

Structural relaxation and defects induce filament formation.

Tail states are localized on structural filaments.

Abstract

Exponential band edges have been observed in a variety of materials, both crystalline and amorphous. In this paper, we infer the structural origins of these tails in amorphous and defective crystalline Si by direct calculation with current {\it ab initio} methods. We find that exponential tails appear in relaxed models of diamond with suitable point defects. In amorphous silicon (a-Si), we find that structural filaments of short bonds and long bonds exist in the network, and that the tail states near the extreme edges of both band tails are are also filamentary, with much localization on the structural filaments. We connect the existence of both filament systems to structural relaxation in the presence of defects and or topological disorder.