E. Márquez

Calculation of the thickness and optical constants of amorphous arsenic sulphide films from their transmission spectra

The interference modulated transmission spectra T( lambda ) at normal incidence for amorphous arsenic sulphide semiconducting films deposited by thermal evaporation were obtained in the spectral region from 300 nm up to 2000 nm. The straightforward analysis proposed by Swanepoel (1983), which is based on the use of the extremes of the interference fringes, has been applied in order to derive the real and imaginary parts of the complex index of refraction and also the film thickness. Thickness measurements made by a surface profiling stylus have also been carried out to cross check the results obtained by the method employing only T( lambda ). In addition, the optical band gap Egopt has been determined from the absorption coefficient values using Taucs procedure, i.e. from the relationship alpha (h nu )=K(h nu -Egopt)2/h nu , where K is a constant. Finally, it is emphasised that accurate results were achieved not only with the above mentioned glass composition As2S3, but also in the case of the non-stoichiometric composition As30S70.

Optical characterization of wedge-shaped thin films of amorphous arsenic trisulphide based only on their shrunk transmission spectra

Abstract Inhomogeneities in thin films have a large influence on the optical transmission spectrum. If not corrected for, this may lead to too large calculated values of the absorption coefficient or the apparent presence of an absorption band tail as well as serious errors in the values of the refractive index and film thickness. The effect of thickness variation on the optical transmission spectrum is taken into consideration in the present case of amorphous arsenic trisulphide thin films prepared by thermal evaporation. The analytical expressions proposed by Swanepoel, enabling the derivation of the optical constants of a wedge-shaped thin film from its shrunk transmission spectrum only, have successfully been applied. Finally, the refractive-index dispersion is discussed in terms of the single-oscillator Wemple and DiDomenico model.

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.

Method for determining the optical constants of thin dielectric films with variable thickness using only their shrunk reflection spectra

Thickness inhomogeneities in thin films have a large influence on their optical transmission and reflection spectra. If not taken into account, this may lead to rather large calculated values for the absorption coefficient or the erroneous presence of an absorption-band tail, as well as to significant errors in the calculated values of the refractive index and the film thickness. The effect of thickness variation on the optical reflection spectrum of a thin dielectric film covering a thick non-absorbing substrate, is analysed in detail in this paper, and analytical expressions are presented for such a reflection spectrum and its upper and lower envelopes. A method is suggested for determining the refractive index n(λ) and the extinction coefficient k(λ), as well as the average thickness and the thickness variation, of a thin dielectric film with variable thickness, by using only the two envelopes of the corresponding shrunk reflection spectrum. This method is used for the geometrical and optical characterization of thermally-evaporated amorphous chalcogenide films, deposited on glass substrates.

Derivation of the optical constants of thermally-evaporated uniform films of binary chalcogenide glasses using only their reflection spectra

Abstract Optical reflection spectra, at normal incidence, of binary chalcogenide glass thin films of chemical compositions As25S75 and Ge33Se67, deposited by thermal evaporation, were obtained in the 400 nm to 2200 nm spectral region. The optical constants of these particular amorphous materials were accurately determined using an optical characterization method proposed by Minkov, based on the maximum and minimum envelopes of the reflection spectrum, which allows to obtain both the real and the imaginary parts of the complex refractive index, and the film thickness. The dispersion of n is discussed in terms of the single-oscillator Wemple–DiDomenico model. The optical gap has been determined from the absorption coefficient values by Taucs procedure.

Structural characteristics and some concepts related to switching properties in As0.45Se0.10Te0.45 glassy alloy

A structural model of the bulk Aso.4sSeo.roTeo.4s amorphous alloy has been built by the random Monte Carlo technique. The model is in good agreement with the atomic radial distribution function obtained by X-ray diffraction. The mean coordination number obtained was 2.3, thus allowing the sample to be characterized as a bistable material. Chalcogenide glasses are studied mainly because they present a switching phenomenon and a memory effect, and therefore can be used in the making of a great number of electronic devices [l]. The main interest in the analysis of the glass system As-Se-Te [2,3], amongst other reasons, is the dependence of the electrical conductivity on sample composition, as well as presenting memory effect together with the switching process. For these reasons, a structural analysis as well as an electrical study of the glassy alloy As0.45Se0.10Te0.45 has been undertaken, given that the establishment of the structural units, the distribution of chemical bonds between atoms, the radii of the coordination spheres and the mean values of the bonds, besides being of intrinsic interest, allows the process of electrical conductivity to be explained, as it is closely linked to the type and amount of chemical bonds existing in the material. The ‘4s0.45Se0.10Te0.45 composition was prepared so that the influence of the amount of Te could be observed, previous studies having been carried out with a smaller atomic fraction of Te [2,3]. Electrical resistance measurements of the composition with a greater amount of Te indicate that it reduces electrical resistance, due to the dislocation of covalent bonds as a result of the metallization of the chemical bond. Analogous behaviour has been reported in the literature [41. The number of bonds between each of the elements,

Refractive-index dispersion and the optical-absorption edge of wedge-shaped thin films of metal - chalcogenide glasses

The optical constants and average thickness of metal - chalcogenide glass films of chemical composition , with x = 5, 10 and 15 at.%, are very accurately determined by a novel method, based only on the transmission spectra at normal incidence, measured over the 400 - 2200 nm spectral range. This useful optical method takes into consideration the non-uniform thickness of the thermally evaporated films (if not corrected, this may lead to too-large calculated values of the absorption coefficient, as well as serious errors in the values of the refractive index and film thickness). The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. The optical-absorption edges are described using the non-direct transition model proposed by Tauc and the optical energy gaps are calculated by Taucs extrapolation. It has been found that the value of the refractive index increases clearly with copper content, whereas the optical band gap decreases from 1.80 to 1.64 eV.

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.

Electrical conductivity and phenomenology of switching in the glassy alloy Ge0.09As0.20Te0.71

Abstract The essential characteristics of the switching phenomenon, excluding memory effects, have been analyzed in the bulk chalcogenide semiconductor glass Ge 0.09 As 0.20 Te 0.71 . A device has been designed and built to activate the samples correctly, and with which the contact pressure of the electrodes can be regulated. One of the results of this research is the constancy of the electrical power with which switching takes place. The functional dependence between the physical variables, delay time and applied voltage, is typical of bulk glasses. We have found a dependence of electrical conductivity on the applied field, which shows a deviation from the linear behaviour of the I - V characteristics in the blocking state. Also, the temporal dependence of the intensity has been studied, for voltages lower and higher than V th , which provides information about the evolution of the temperature of the material during electrical stimulation. The experimental results support an electrothermal model for the switching effect in the glass studied, and with the configuration of the electrodes used. The electronic contribution to the switching process is demonstrated by the dependence of electrical conductivity on the applied field.