Variation in the Nanostructural Features of the nc-Si:H Thin Films with Substrate Temperature

Abstract

We investigated the nanostructural, chemical and optical properties of nc-Si:H films according to deposition conditions. Plasma enhanced chemical vapor deposition(PECVD) techniques were used to produce nc-Si:H thin films. The hydrogen dilution ratio in the precursors, [SiH4/H2], was fixed at 0.03; the substrate temperature was varied from room temperature to 600 oC. By raising the substrates temperature up to 400 oC, the nanocrystalite size was increased from ~2 to ~7 nm and the Si crystal volume fraction was varied from ~9 to ~45% to reach their maximum values. In high-resolution transmission electron microscopy(HRTEM) images, Si nanocrystallites were observed and the crystallite size appeared to correspond to the crystal size values obtained by X-ray diffraction(XRD) and Raman Spectroscopy. The intensity of highresolution electron energy loss spectroscopy(EELS) peaks at ~99.9 eV(Si L2, 3 edge) was sensitively varied depending on the formation of Si nanocrystallites in the films. With increasing substrate temperatures, from room temperature to 600 oC, the optical band gap of the nc-Si:H films was decreased from 2.4 to 1.9 eV, and the relative fraction of Si-H bonds in the films was increased from 19.9 to 32.9%. The variation in the nanostructural as well as chemical features of the films with substrate temperature appears to be well related to the results of the differential scanning calorimeter measurements, in which heatabsorption started at a substrate temperature of 180 oC and the maximum peak was observed at ~370 oC.