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We have applied the dual-beam photocurrent spectroscopy to study the excited defect states in a-Si:H. The pump beam is used to create the excited state while the second beam is used as a probe. It is shown that the anomalous band in dual-beam photocurrent spectra of a-Si:H results from a combination of two processes: photocurrent enhancement due to excitations by the probe light for the D0 states to the conduction band. Using the dual-beam photocurrent spectra, we measured the optical transition energy (0.77 eV) and the electron correlation energy (0.16 eV) for the filled D- defects (i.e., excited D0 defects). The dual-beam photocurrent spectroscopy may also be used for study of deep gap states in other thin film semiconductors.

It is shown that the anomalous band in dual-beam CPM (DBCPM) spectra results from a combination of two processes: photocurrent enhancement due to excitations by the probe light from the valence baud to theD+ defect states, and photocurrent reduction due to excitations by the probe light from the D° defect states to the conduction band. From the DBCPM spectra, the electron correlation energy for the (filled)D - defects is determined to be 0.25 eV.

Under a dc light bias, the ac constant photocurrent method (CPM) yields an anomalous subgap optical absorption over that without the bias. When the intensity of the bias is high, an absorption band appears. However, absorption measured by ac photothermal deflection spectroscopy (PDS) is much less affected by the bias. A simple model qualitatively explains the effect on the PDS spectra and suggests that the anomalous CPM absorption is an apparent one which represents a variation of the recombination lifetime.