Abstract
Surface states of liquid-encapsulated Czochralski and molecular beam epitaxy grown n-type GaAs samples coated with Na2S and (NH4)2S films are studied with deep level transient spectroscopy measurements employing metal-insulator-semiconductor structures. Two deep levels at 0.58 and 1.15 eV below the conduction-band edge are found in the as-grown samples. The Ec−0.58 eV deep level is found to be greatly reduced after sulfide treatment, while the Ec−1.15 eV deep level remains with a very high density. The surface Fermi level is shifted deeper into the energy band gap toward the Ec−1.15 eV deep level after surface treatment. This new, deeper pinning position is believed to be the cause of the greatly enhanced photoluminescence intensity recently reported. This new pinning position is deeper than some of the recently reported sodium sulfide/GaAs results but consistent with the high Schottky barrier height, 1.15 eV measured for the gallium-rich GaAs surface.
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