Stability as Regards Film Thickness, Homogeneity and Optical Properties of Thin a-Si:H Films

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Abstract
The instability, or other words, the spontaneous changes in the properties of the material (ageing) that occur in thin a-Si:H films in the atmosphere in the long term have been determined as a function of the preparation parameters–pressure, power, gas flow rate and substrate temperature. The repeated monitoring of the material properties of the original films required the use of a non-destructive technique. The chosen technique encompassed the obtaining of transmission spectra of the film-substrate configuration and from this the film thickness, the homogeneity parameter Δd and the optical properties, n(λ), α(λ) and E 0 were obtained. Of the four preparation parameters that were varied, substrate temperature played a dominant role as regards the material properties and stability of these films. Experimental results of a number of films that were deposited at temperatures equal to, or lower than room temperature consistantly underwent the following changes in the long term: increases in film thickness and E 0, and decreases in the values of the parameters Δd, n(λ) and α(λ). The effect of ageing, as seen from the results, was not only restricted to the surface, but appeared throughout the whole thickness of the film.