Direct measurement of gap-state absorption in hydrogenated amorphous silicon by photothermal deflection spectroscopy

Abstract

We have measured the subgap optical absorption of undoped, singly doped, and compensated hydrogenated amorphous silicon down to 0.6 eV using the sensitive technique of photothermal deflection spectroscopy. We show that this absorption is due to silicon dangling-bond defects located \ensuremath{\sim}1.3 eV below the conduction band. While doping also creates defects \ensuremath{\sim}1.3 eV below the conduction band, compensation removes them. The results suggest that for the undoped material the density-of-states maximum found in field-effect measurements is due to silicon dangling bonds.