METASTABLE STATES IN UNDOPED AND DOPED A-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 D(i) in undoped a-Si:H. In a-Si:H:P, LS increases the density of dangling bonds D(P) relative to the density of P4 states. The LS a-Si:H:P samples also exhibit, in addition to D(P) defects, defects with similar properties as D(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.