The optical constants n and k of thin polycrystalline films of CdSe, ZnSe, CdTe and ZnTe have been measured over the wavelength range 2000-300 nm. From analysis of the absorption data, the three cubic materials ZnSe, CdTe and ZnTe were found to have direct band-gaps with Eg=2.62, 1.50 and 2.23 eV respectively and spin-orbit splittings of the valence band of 0.48, 0.88 and 0.90 eV respectively. CdTe and ZnTe also have indirect band-gaps of 1.82 and 2.56 eV respectively. CdSe has a fundamental direct gap of 1.65 eV but the analysis of the absorption curve at higher energies was limited by the energy bands becoming non-parabolic.
The very complicated formulae which relate the components of the complex refractive index of a thin film to the measurable optical reflection and transmission coefficients have been put into a much simpler form. This greatly simplifies the programming for a computer solution of the equations, and also leads to some useful approximate formulae.
The derivation of the components n and k of the complex refractive index of a thin film from measurements, at normal incidence, of reflectance R and transmittance T is complicated by the existence of multiple solutions of the relevant equations. It is shown that, although use of Tomlin's expressions for (1 ± R)/T rather than Heavens's explicit formulae for R and T separately simplifies the problem, it is still necessary to consider very carefully the multiple solutions. A procedure is given for determining the correct solutions for n and k, and also for accurately fixing the film thickness, provided that the film is a homogeneous single layer on a substrate.
Some experimental results have not yielded to this treatment and in these cases it has been found necessary to assume the presence of a surface layer (often, but not necessarily, an oxide layer) and to make use of the equations in the preceding paper which relate to a double layer on a substrate. It is shown how this may be done in such a way as to obtain the correct dispersion curve for the material, together with its thickness, and that of the surface layer.
The optical constants n and k have been determined over the wavelength range from 2000-250 nm by measurements of reflectance and transmittance at normal incidence and treating these data by the method of Denton et al. (1972). The effects of surface roughness have been taken into account. Analysis of the dependence of absorption on photon energy have shown that the experimental results may be explained by the occurrence of direct transitions from 2.42 eV to 2.82 eV, in the case of CdS, followed by combined direct and indirect transitions beyond 2.82 eV assuming the energy band to be parabolic, or equally well by assuming only direct transitions between nonparabolic bands. The results for ZnS films are similar and may be treated in the same way. It is concluded that these materials both show absorption by direct transitions just beyond the absorption edge, and that at higher energies the form of the absorption curve is probably due to the combined effects of indirect transitions together with direct transitions between nonparabolic bands.
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