Dual-beam photocurrent spectroscopy in undoped A-Si:H: A method for study of excited deep gap states in thin film semiconductors

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 from the valence band to the D⁺ states, and photocurrent reduction due to excitations by the probe light from 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.