Mechanisms of metastability in hydrogenated amorphous silicon

We survey theoretical approaches to understanding the diverse metastable behavior in hydrogenated amorphous silicon. We discuss a recently developed network-rebonding model involving bonding rearrangements of silicon and hydrogen atoms. Using tight-binding molecular dynamics we find non-radiative recombination can break weak silicon bonds with low activation energies, producing dangling bond-floating bond pairs. The transient floating bonds annihilate generating local hydrogen motion and leaving behind isolated dangling bonds. Charged defects are also observed. Major experimental features of metastability including electron-spin resonance, t(1/3) kinetics, dangling-bond H anti-correlation, and hysteretic annealing are explained. In the second part we focus on large metastable structural changes observed in a-Si:H. We find H atoms have a local metastability involving the flipping of the H to the backside of the Si-H bond that results in a local increase of strain and increase of dipole moments. This naturally explains the larger infrared absorption found after light soaking, and may be related to other large structural changes in the network. Directions for future research are surveyed. (C) 2002 Published by Elsevier Science B.V.