Structural Relaxation and Structural Memory at Amorphous Silicon Dangling Bonds

We examine the energy and time scales of configurational relaxation around the dangling bond defect, D, in hydrogenated Amorphous silicon (a-Si:H). After D captures or emits charge, its bond angle, electron energy eigenvalues and local structural environment all change. This determines the measured electronic energy levels; we use previous theoretical results and experimental data to estimate the density of gap states in the different atomic configurations of D. We also describe D relaxation effects observed in experiments, including the very slow relaxations found in recent transient capacitance measurements. To explain the unusual T-independent kinetics of transient capacitance carrier emission, we propose a model of “structural memory” in a-Si:H. After carrier capture, neighbors of D retain memory of their pre-capture configuration for seconds at 300K. The rate-limiting step to carrier emission is an effectively one-dimensional random walk of these neighbors through their configuration space and back to the pre-capture configuration. The final, activated, step of emission is very rapid. We describe analytic and monte Carlo calculations that support the structural memory Model and propose possible microscopic Mechanisms.