Bond exchange, network restructuring and recrystallization in vacuum-ultraviolet-irradiated a-Si:H films

The structural changes resulting from vacuum ultraviolet (VUV) (hν>100 eV) irradiation of hydrogenated amorphous Si films on Si(1 0 0) were investigated by spectroscopic ellipsometry (SE). Linear regression analysis of the pseudo-dielectric function using the Bruggeman effective medium approximation and a two-layer model comprising amorphous (a-Si), crystalline (c-Si) and void components was used to determine the volume of each component in the layers. Irradiation at 200°C increased the a-Si volume and decreased the void volume, indicating that the Si–H bond cleavage resulting from electronic excitation is followed by the formation of Si–Si bonds contributing to the amorphous network. Irradiation at 400°C, however, decreased both the a-Si and void volumes and increased the c-Si volume, indicating the occurrence of thermally assisted network restructuring from the amorphous phase to the crystalline phase.