Optical Excitation of MOS Interface States.

MOS interface states have been studied in n-type samples by infrared optical excitation. Infrared light in the wavelength range 5 to 15 micrometers was used to excite only shallow states near the conduction band edge. Thermal background radiation was excluded in the irradiation by using cold interference filters for the spectral selection. For a sample with low density of states approx. 10 to the 10th powersq cm ev, a consistent density was observed in the optical measurement with quasistatic measurements and CC-DLTS. The optical absorption cross-section sigma assumes low values which are in favor for a model which interprets interface states by tunneling into a discrete level in SiO2. Another sample with a high density of states shows a complicated behavior with a field dependence and inconsistency of the density of states observed by different techniques which cannot be completely interpreted. The absorption cross-section, however, also assumes low values below 10 to the -18th power sq cm.