Improved defect-pool model for charged defects in amorphous silicon

We have developed an improved defect-pool model for the calculation of the density of dangling-bond states in amorphous silicon. The results of this improved defect-pool model are contrasted with earlier work, from which we have eliminated some errors and approximations. We show that the calculated energy-dependent density of states depends on the specific microscopic reaction involving hydrogen, in contrast to previous conclusions. We calculate the bulk density of states, using the best input parameters drawn from experiment, and conclude that the best agreement with experimental results is found for a rather wide defect pool and for a microscopic model where two Si-H bonds break for every weak bond converted into two dangling bonds. The calculations predict that there are approximately four times as many charged defects as neutral defects in good-quality intrinsic amorphous silicon. We argue that this picture of the density of states is consistent with a wide range of experimental results. We show how this important conclusion depends on the principal parameters of the defect-pool model and investigate how the density of states would change with different parameters.