Gap-State Profiles of a-Si: H Deduced from Below-Gap Optical Absorption

We determined the gap-state profiles N ( E ) of undoped and P-doped a-Si: H from their below-gap optical absorption spectra through theoretical simulation under the assumption of constant matrix elements (i.e., Davis-Mott approximation). It is demonstrated that the conduction band tail has a sharper edge than the valence band, and the incorporation of P atom creates a broad band in N ( E ) around 0.6 eV above the mobility edge of the valence band. The origin of the broad band is speculated to be 3-fold coordinated P-atom or its relevant defect. A validity of the Davis-Mott approximation is confirmed experimentally.

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