$Ab$ $initio$ hydrogen dynamics and the morphology of voids in amorphous silicon

TL;DR

This study uses ab initio simulations to investigate hydrogen atom behavior and void morphology in amorphous silicon, revealing hydrogen diffusion, surface microstructure, and the formation of H2 molecules within nanovoids at different temperatures.

Contribution

It provides new insights into hydrogen dynamics, surface chemistry, and void shape in amorphous silicon using density-functional theory simulations.

Findings

Hydrogen forms monohydride and dihydride bonds on void surfaces.

A significant fraction of hydrogen exists as H2 molecules inside voids.

Void shapes are characterized using convex-hull and Gaussian broadening methods.

Abstract

This paper presents an $ab$ $initio$ study of hydrogen dynamics inside nanometer-size voids in $a$-Si within the framework of the density-functional theory for a varying hydrogen load of 10 to 30 H atoms/void at the low and high temperature of 400 K and 700 K, respectively. Using the local density approximation and its generalized-gradient counterpart, the dynamics of hydrogen atoms inside the voids are examined with an emphasis on the diffusion of H atoms/molecules, and the resulting nanostructural changes of the void surfaces. The results from simulations suggest that the microstructure of the hydrogen distribution on the void surfaces and the morphology of the voids are characterized by the presence of a significant number of monohydride Si-H bonds, along with a few dihydride Si-H$_2$ configurations. The study also reveals that a considerable number of (about 10--45 at.%) total H atoms inside voids can appear as H$_2$ molecules for a hydrogen load of 10--30 H atoms/void. The approximate shape of the voids is addressed from a knowledge of the positions of the void-surface atoms using the convex-hull approximation and the Gaussian broadening of the pseudo-atomic surfaces of Si and H atoms.