Metastable hydrogen atom trapping in hydrogenated amorphous silicon films: A microscopic model for metastable defect creation

Abstract

In hydrogenated amorphous silicon (a-Si: H) films, the increase in the metastable defect density under high-intensity illumination is usually described by an empirical two-parameter stretched-exponential (SE) time dependence (characteristic time τ_SE and dispersion parameter β). In this study, a clearly different (one-parameter) analytic function is obtained from a microscopic model based on the formation and trapping of metastable hydrogen (MSH) atoms.

In this microscopic model deduced from experimental observations, assuming that MSH atoms are the only mobile species, only three elemental chemical reactions are thus significant; MSH are produced from doubly hydrogenated (Si-H H-Si) configurations and trapped at either broken bonds or Si-H bonds, corresponding respectively to light-induced annealing and light-induced creation of defects. Competition between trapping sites results in a saturation of the defect density N(t) at a steady-state value N _SS. An implicit analytic function is obtained for the continuous-wave illumination time dependence of the metastable defect density; a one-parameter fit of this analytical function to experimental data is generally good, indicating that the use of a statistical distribution of trap energies is not necessary. A comparison of the empirical SE parameters with the microscopic ‘MSH model’ shows that these parameters are strongly related to the steady-state value N _SS.