Instability of the free surface of undopeda-Si:H studied by photoluminescence and electron-spin-resonance measurements

Changes of photoluminescence efficiency and of electron-spin-resonance (ESR) signal with time after deposition have been observed for a very thin layer of undoped hydrogenated amorphous silicon (a-Si:H). It has been confirmed experimentally that the origin of these time dependences resides at the free surface of the a-Si:H layer. The relationship between the observed time dependences of these quantities differs depending on observation time. In the time range between 2 and 30 d after deposition especially, the photoluminescence efficiency and ESR signal both increase with time. This is unexpected because the inverse relation between those quantities has been known to hold in thick undoped a-Si:H films. From considerations of time evolution of the photoluminescence efficiency and ESR spin density at each observation-time region, it is inferred that gradual changes of the electronic states of dangling bonds with time are taking place at the free surface of an undoped a-Si:H film.