Effects of ion bombardment on the structural and optical properties in hydrogenated silicon thin films

Abstract

Structural and optical properties of two series of hydrogenated silicon films prepared by reactive radiofrequency magnetron sputtering technique at low substrate temperature, and with applied rf-powers of 150W (series A) and 300W (series B) were investigated. The Fourier transform infrared spectroscopic analysis showed changes in the nature of SiH bonding absorption spectra for both wagging and stretching vibration modes between the two series films. By varying the electrode gap spacing, D, (D=5, 6 and 7cm) and keeping all the parameters of the plasma constant, a gradual change in the hydrogen-bonding configurations was also observed. The effects of this change, on the structural and the optical properties of the material, were studied by means of Raman spectroscopy and spectroscopic ellipsometry (1.5–5eV) measurements, complemented with standard optical transmission. The results of this investigation clearly show that the films of the series A present a completely amorphous structure whatever are the D value, with a constant band gap ET of 1.68eV, typical of hydrogenated amorphous silicon (a-Si:H) films; the decrease in D leads only to an increase in the film compactness. However, the films of the series B are well crystallized, and exhibit a mixture of small and large Si crystallite sizes. Both the volume fractions of these crystallites, Fc, increase with decreasing in D. The optical gap shows decreasing trend, with constant hydrogen content, as Fc increases. The values of ET lie between those of a-Si:H and c-Si materials, and consequently they have been attributed to the increase in Fc values. It is suggested that ion bombardment plays a crucial role in increasing the compactness of the films and one or more specific properties.