METASTABLE HYDROGEN-ATOM TRAPPING IN HYDROGENATED AMORPHOUS-SILICON FILMS - A MICROSCOPIC MODEL FOR METASTABLE DEFECT CREATION

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) ti me dependence (characteristic time tau(SE) and dispersion parameter be ta). In this study, a clearly different (one-parameter) analytic funct ion is obtained from a microscopic model based on the formation and tr apping of metastable hydrogen (MSH) atoms. In this microscopic model d educed 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) co nfigurations and trapped at either broken bonds or SI-H bonds, corresp onding respectively to light-induced annealing and light-induced creat ion of defects. Competition between trapping sites results in a satura tion of the defect density N(t) at a steady-state value N-SS An implic it 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, in dicating that the use of a statistical distribution of trap energies i s not necessary. A comparison of the empirical SE parameters with the microscopic 'MSH model' shows that these parameters are strongly relat ed to the steady-state value N-SS.