Vanadium oxide films were deposited at different sputtering times on sapphire substrates by radio frequency reactive magnetron sputtering. The effects of thickness on the surface structural, morphology and optical-switching properties of the films were studied comprehensively. X-ray diffraction results show that all the prepared films are polycrystalline vanadium pentoxide films (V 2 O 5 ) on (001) preferred orientation. Both root-mean-square roughness and average surface grain size decrease with the increase of thickness. With the thickness reduction, the closing time decreases first and then increases at the turning point of 350 nm, and the maximum and the minimum are 2.7 ms at 600 nm and 1.7 ms at 350 nm, respectively. The recovery time decreases rapidly with the reduction of the film, which varies from 40 ms at 600 nm to 27 ms at 150 nm. More concerning, the dynamic range of transmittance varies greatly from thickness, and the transmittance ratio before and after the phase transition covers from 3.75 at 150 nm to the maximum of 7.3 at 600 nm under a continuous probe light of 1064 nm.

The predominance of Si-H bonding and the origin of {111} platelets in hydrogenated Si remain important unsolvedproblems in the study of H in Si.Recent theoretical and experimental results indicate that H predominately enters the Si network in pairs. A promising diatomic H configuration consists of a bond centered H closely associated with an antibonding centered H. In this work, we show that adjacent diatomic H pairs have a binding energy of 0.2 eV/2H. The binding originates from relaxation of strained Si-Si backbonds. Further clustering of the H pairs eliminates all strained bonds, forming a hydrogenated platelet oriented along the {111} plane. The binding energy of 3.95 eV/2H for the platelet is 0.15 eV lower than that for interstitial H2 molecules in c-Si. Lattice expansion makes the platelets energetically more competitivewith the lowest energy Si-H bonding confi gration at hydrogenated Si (111) surfaces. These higher level complexes explainthe formation of platelets, Raman spectra, and absence of gap states in hydrogenated c- Si as well as the clustered phaseseen in NMR and of H evolution and diffusion in hydrogenated amorphous Si.