Mechanical Spectroscopy Study on the Light Soaking Effect on Hydrogenated Amorphous Silicon

For hydrogenated amorphous silicon (a-Si:H) films deposited at temperatures between 423 K and 623 K (a-Si:H_423K and so on), the light-induced changes in the internal friction between 80 K and 400 K were studied. The internal friction is associated with H₂ motion in microvoid networks, and shows the mild temperature dependence between about 80 K and 300 K (Q^-1_80-300K) and the almost linear increase above 300 K (Q^-1_>300K). Both Q^-1_80-300K and Q^-1_>300K decrease with increasing the deposition temperature, and show the mild temperature dependence in a-Si:H_623K. The white light soaking with 100 mW/cm² (WLS₁₀₀ and so on) below 300 K caused a change in Q^-1_80-300K and no changes in Q^-1_>300K, respectively, and the light-induced changes in Q^-1_80-300K recovered after annealing at 423 K. The wide distribution of activation energies for H₂ motions between microvoids indicate that most of neighboring microvoids are connected through windows, i.e., the microvoid networks are existing in a-Si:H, and the spatially loose or solid structures are responsible for the low or high activation energies for the H₂ motion between microvoids, respectively. Furthermore, the light-induced hydrogen evolution (LIHE) was observed for WLS₂₀₀ to WLS₄₀₀ in a vacuum between 400 and 500 K, resulting in the disappearance of the internal friction due to the H₂ motion in the microvoid network.