DANGLING-BOND LEVELS AND STRUCTURE RELAXATION IN HYDROGENATED AMORPHOUS-SILICON

Tight-binding molecular-dynamics calculations are utilized to study th e spatial extent and time scales of the structure relaxation, followin g a change of the charge state of dangling bonds in hydrogenated amorp hous silicon. Structural relaxation is found to be local, primarily in volving large displacements (>0.1 Angstrom) of the nearest neighbors o f the dangling bond and of a few nearby H atoms. Calculated optical tr ansition levels have the D- level below both D-0 levels and the D+ lev el above the D-0 levels. A smooth energy surface is found for transiti ons between the neutral and charged dangling-bond configurations. Mole cular-dynamics simulations show that electron levels relax in tens of picoseconds following electron capture qr emission by a dangling bond, but large oscillations of the gap levels may be present as a result o f the strong coupling between the charge and local structure. The resu lts do not appear to support either the slow relaxation model of Cohen , Leen, and Rasmussen, or the D structural memory model of Branz and F edders.