Investigations on hydrogenated amorphous silicon films grown at high rate in a UHV plasma CVD system

Abstract

In this paper we analyze the results of an extensive characterisation study involving scanning electron microscopy, spectroscopic ellipsometry, Laser Raman spectroscopy, optical band gap, dark and photoconductivity measurements as a function of temperature and light intensity, current-voltage characteristics and capacitance versus frequency measurements on Schottky diodes of a-Si:H films, prepared in a UHV plasma CVD system at varying deposition rates upto ≈ 10.0 Å/s. We find that the values of imaginary part of the dielectric constant ( ϵ 2max), dark and photoconductivity, activation energy, conductivity pre-exponential factor, exponent Λ of photoconductivity decrease, whereas the values of diode quality factor n and density of defect states increase and the values of the width of TO-peak in Raman spectra, photosensitivity and optical band gap do not change significantly with the increasing rate of deposition. The less severe deterioration of opto-electronic properties observed in these films, even when grown at high rates in our plasma CVD reactors, is attributed to the effective control of secondary plasma reactions in the interelectrode space when kept very low (10–12 mm). Implications of obtaining high growth rate for i-layer, in a p-i-n solar cell structure, is discussed with the available results of modelling analysis.