Ellipsometric studies on zinc sulfide thin films grown by atomic layer epitaxy

Refractive index and thickness measurements of zinc sulfide thin films were carried out using spectroscopic ellipsometry. The films studied were grown by atomic layer epitaxy (ALE). The thicknesses of the films were between 26 and 658 nm. In the analysis of the ellipsometric data a two-layer film model was used. The modeling was performed for all the films in the wavelength range 400-600 nm and for some films in the range 350-700 nm. The top layer thickness increased from about 5 to 20-30 nm as a function of the total film thickness. At λ=500 nm, the real part n of the refractive index in the top layer was 1.30-2.00 and in the lower layer 2.14-2.41, also depending on the total film thickness. The reference bulk value was 2.42. The imaginary part of the refractive index of the layers could be modeled to be 0.01 in the range 400-600 nm. At 350 nm k was of the order of 0.05. From the refractive index results the average volume fraction of zinc sulfide in the films was determined using effective medium approximation (EMA). The volume fraction increased from about 0.6 to 0.95-0.97 in the thickness range 26-400 nm. After 400 nm the fraction was slightly reduced. The EMA results indicate that the first stages of crystallization of zinc sulfide on the amorphous glass substrate are highly nonideal and that the surface roughness increases with the total film thickness. Except for the boundary regions, the ALE-grown zinc sulfide thin films exhibit very good crystalline properties also in terms of an optical analysis.