R. Prieto-Alcón

Optical properties of thermally evaporated amorphous As40S60−xSex films

Abstract Compositional dependencies of the optical properties of as-deposited amorphous As40S60−xSex films (x=0, 20, 30, 40 and 60 at.%), prepared by thermal evaporation, have been studied. The refractive index, n, and absorption coefficient, α, have been determined from the upper and lower envelopes of the transmission spectra, measured at normal incidence, in the spectral range from 400 to 2200 nm. An improved optical characterisation method for non-uniform films based on Swanepoel’s ideas, which takes into account the weak absorption in the substrate, has been successfully employed, and it has allowed us to determine both the average thickness, d , and the refractive index, n, of the films, with accuracies better than 1%. It has been found that the refractive index of the As40S60−xSex samples increases with increasing x, over the entire spectral range, which is related to the increased polarizability, αp, of the larger Se atoms, in comparison with S atoms. The values of the As effective coordination number, Nc, have been estimated from an analysis of the dispersion of the refractive index, and an increase in Nc with increasing Se content, from around 3.0 to 3.4, has been inferred.

Optical properties of thin-film ternary Ge10As15Se75 chalcogenide glasses

Abstract The optical properties of ternary chalcogenide amorphous thin films of chemical composition Ge 10 As 15 Se 75 , deposited by vacuum thermal evaporation, have been determined and analysed. Normal-incidence optical transmission spectra have been measured in the range from 400 to 2500 nm. From these transmission spectra, the optical constants and average thickness of this particular amorphous ternary material were accurately calculated using an optical-characterisation method based on creating the upper and lower envelope curves of the spectrum, which also allows to obtain a parameter indicating the degree of film-thickness uniformity. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model. The optical-absorption edge is described using the non-direct transition model proposed by Tauc and the optical band gap is calculated from the absorption coefficient values by Taucs extrapolation procedure. A comparison between these optical properties and those corresponding to the As 30 Se 70 and Ge 25 Se 75 binary thin films (previously reported) is also presented.

Optical properties and structure of amorphous (As0.33S0.67)100−xTex and GexSb40−xS60 chalcogenide semiconducting alloy films deposited by vacuum thermal evaporation

Amorphous films with compositions (As0.33S0.67)100−xTex (x = 0, 1, 5 and 10 at.%) and GexSb40−xS60 (x = 10, 20 and 30 at.%) have been prepared by thermal evaporation. The compositional dependences of their optical properties, with increasing Te and Ge content, respectively, are explained in terms of the modifications occurring in the film structure: Te joins the glassy network through the substitution of S atoms in the AsS3 pyramidal units, to form new AsS2Te mixed pyramidal units, on the one hand, and, on the other hand, there is a presence of GeS4 and S3Ge–GeS3 structural units, when the Ge content is increased. The refractive-index dispersion has been analysed on the basis of the Wemple–DiDomenico single-oscillator approach. It has been found that the refractive index increases in the As–S–Te system, with increasing Te content, while it decreases instead, in the Ge–Sb–S system, with increasing Ge content. The behaviour of the Tauc gap, when the composition of the material is varied, shows, as expected, just the opposite trends.

On the irreversible photo-bleaching phenomenon in obliquely-evaporated GeS2 glass films

Abstract The photo-induced shift in the optical-absorption edge and changes in the optical constants ( n , k ), and their dispersion in the vicinity of the absorption edge, have been studied in obliquely-deposited thin films of the GeS 2 chalcogenide glass, prepared by vacuum thermal evaporation onto glass and silicon wafer substrates. Light exposure of the GeS 2 films in air was found to be accompanied by an irreversible bleaching. The appearance of GeOGe vibrational bands in the IR transmission spectra clearly indicates the extrinsic origin of photo-bleaching: it is most probably a result of the photo-oxidation of the layers. On the other hand, the irradiation of the GeS 2 films (deposited at an angle of incidence of ≈ 70°) showed a decrease of thickness (and physical densification) of ≈ 8%. Furthermore, the dispersion of the refractive index of the as-deposited and exposed films can be described using the Wemple-DiDomenico (W-D) single-oscillator formula. Exposure-induced changes of the parameters of the W-D formula, E 0 and E d , follow the increase of the Tauc gap and the decrease of the film thickness, respectively.

Thermal relaxation of the structural and optical properties of amorphous As40S60−xSex films

This paper deals with the structural and optical properties of virgin (i.e., as-evaporated) and annealed (i.e., thermally relaxed) amorphous As 40 S 60-x Se x films (x = 0, 20, 30, 40 and 60 at.%), which were prepared by vacuum evaporation. Structural properties have been inferred from the X-ray diffraction patterns and the Raman spectra of the thin-film samples. The decrease in the intensity of the first sharp diffraction peak after annealing suggests that the thermal relaxation is connected with a decrease in the free volume leading to an increase of the structural compactness. A significant decrease in film thickness has been invariably found after annealing in all cases, which supports this conclusion. Changes found in the Raman spectra of the film samples after annealing indicate a reduction in the concentration of homopolar bonds, which are present in the molecular fragments embedded in the amorphous matrix. Thus, the thermodynamic state of the system is shifted towards equilibrium as a result of thermally induced polymerization (homogenization). These structural changes also resulted in significant changes in both the refractive index and optical band gap, E opt g .