Optoelectronic and Structural Properties of Undoped Microcrystalline Silicon Thin Films: Dependence on Substrate Temperature in Very High Frequency Plasma Enhanced Chemical Vapor Deposition Technique

Abstract

The effects of substrate temperature on optoelectronic and structural properties of undoped microcrystalline silicon thin films have been investigated. The undoped silicon films have been deposited by the very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique using a SiH4 and H2 gas mixture at 105 MHz plasma excitation frequency and moderately low power density of 70 mW/cm2. The effect of the systematic variation of substrate temperature (from 180°C to 370°C) on film properties has been studied, while keeping the other parameters constant. The deposition rate is considerably high (2.6 Ås-1) at 180°C and remains almost constant over the whole temperature range. Dark conductivity for all the films lies around ∼10-6 Scm-1. Low subband gap absorption has been observed by photothermal deflection spectroscopy for these microcrystalline films. Increase of substrate temperature improves the microcrystallinity of the film, which is confirmed from structural studies. Crystalline grain size also increases with increase of substrate temperature and a maximum of 460 Å has been achieved at 370°C. A satisfactory correlation is observed among the results of different structural studies: Raman spectroscopy, infrared spectroscopy, X-ray diffraction, transmission electron microscopy and atomic force microscopy.