Metastable states in undoped and dopeda-Si:H studied by photomodulation spectroscopy

Changes in the gap states in undoped, phosphorus-, and boron-doped a-Si:H produced by light soaking (LS), thermal quenching (Q), and annealing (A) are studied by photomodulation spectroscopy. First a method for analyzing the photomodulation spectra is explained and applied to annealed samples. Then the changes in the spectra produced by LS and Q are analyzed. It is shown that LS increases the density of dangling bonds ${\mathit{D}}_{\mathit{i}}$ in undoped a-Si:H. In a-Si:H:P, LS increases the density of dangling bonds ${\mathit{D}}_{\mathrm{P}}$ relative to the density of ${\mathrm{P}}_4$ states. The LS a-Si:H:P samples also exhibit, in addition to ${\mathit{D}}_{\mathrm{P}}$ defects, defects with similar properties as ${\mathit{D}}_{\mathit{i}}$ defects in undoped a-Si:H. This is a strong support for the presence of more than one kind of defect in a-Si:H:P. In a deposited boron-doped a-Si:H a region of photoinduced transmission was observed that disappears with A, LS, or Q. This change is irreversible, and the spectra of A, LS, or Q samples differ very little. The results are compared with the recent work on metastable states based on different methods.