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

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We have measured the subgap otpical 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 approx.1.4 eV below the conduction band. While doping also creates defects approx.1.4 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.