P.J.S. Ewen

Photo-induced structural and physico-chemical changes in amorphous chalcogenide semiconductors

Abstract The various photo-induced phenomena that occur in chalcogenide glasses are classified and described, with particular emphasis on the photo-dissolution effect. The detailed mechanisms responsible for many of these processes are still unknown, although in the case of photo-darkening in annealed a-As2S3 films, Raman experiments indicate that a light-induced change in the bond distribution from that for a chemically-ordered network towards one characteristic of a random network may be the principal cause. New results on the photo-dissolution of silver into As-S films are presented which indicate that the actinic radiation initiating the effect is absorbed in the photo-doped layer, close to the interface with the undoped region of the chalcogenide film. The basic mechanism responsible for the effect, however, is still not known.

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.

Photoinduced changes in the linear and non-linear optical properties of chalcogenide glasses

Abstract The linear refractive index, n, and the position of the absorption edge for thin chalcogenide/chalcohalide films from the systems As–S, As–S–X (X=Bi, Tl or I), and Ge–S–Me (Me=Bi, Tl, As) have been measured before and after exposure to above-bandgap illumination. For compositions containing either As or Ge, the absorption edge was shifted to longer wavelengths (i.e. photodarkening occurred) for As-containing films, and to shorter wavelengths (i.e. photobleaching occurred) for Ge-containing films. As–S–Ge films exhibit either photodarkening or photobleaching, depending on Ge content. The largest values of these edge shifts ranged from 20 up to 60 nm, in the case of Ge31S63Tl6. The conditions for synthesizing the glasses studied are reported. A formula for predicting the non-linear refractive index, n2, for chalcogenide/chalcohalide glasses from the dispersion of n, and which enables n2 to be related to structural parameters, has been developed and compared with existing formulae for other types of glass and crystals. Using the various formulae and the measured values of n, n2 for these materials has been predicted. The results indicate that glasses with compositions near As42S58 or As2S3Tl0.13 may, after UV exposure, exhibit significantly larger values of n2 than as-deposited As40S60.

Near-infrared optical nonlinearities in amorphous chalcogenides

Abstract The measurement of the nonresonant third-order nonlinearity of three chalcogenide glasses is reported. Using the Z-scan technique, both the refractive and the absorptive parts of the nonlinearity are resolved, including their sign. It is shown that Ag-doping of As 2 S 3 produces a change of sign and a dramatic enhancement of the nonlinear refractive index.

Index of refraction of Ag-doped As33S67 films: Measurement and analysis of dispersion

Abstract We report data on the dispersion of the index of refraction of silver-doped As33S67 thin films in the visible and near-infrared regions. The measured data is analysed on the basis of a single-effective-oscillator model proposed by Wemple and DiDomenico. The silver content dependence of the oscillator energy Eo seems to vary in proportion to optical gap Eg opt of the sample, whereas the dispersion energy or oscillator strength Ed increases with increasing silver content. This latter fact indicates that the effective coordination of the cation increases as silver is incorporated into the chalcogenide host matrix.

Interference grating fabrication in spin-coated As2S3 films

Abstract The optical properties and solubility of spin-coated As2S3 films significantly change after silver is introduced by photodiffusion. The different types of holographic grating that can be fabricated in spin-coated Ag/As2S3 double film structures by photodiffusion and subsequent selective etching are described in this paper. The silver diffusion occurs under the influence of visible monochromatic light and the grating pattern is produced by a holographic arrangement.

Kinetics of the thermally and photoinduced solid state reaction of Ag with As33S67 films

The kinetics of the solid state chemical reaction of silver with amorphous As33S67 films in a conventional sandwich structure was measured by optically monitoring the change in thickness of the undoped chalcogenide. For the thermally stimulated process, the kinetics data indicate a two-stage process, with the second stage having a sublinear time dependence. The effect of broadband illumination is to reduce the activation energy by approximately one order of magnitude, but the photoinduced process still has two identifiable stages. In the case of the photostimulated process, rate coefficients and activation energies have been measured and the composition and structure of the reaction products have been determined by Rutherford backscattering spectroscopy (RBS). The RBS spectra obtained at different stages of the photoinduced reaction process show a step-like form for the Ag concentration profile during the course of the reaction and indicated a homogeneous distribution of Ag in the final reaction products,...

Photodoped chalcogenides as potential infrared holographic media

The extension of holographic techniques from the visible to the infrared is important. Potentially, holographic diffractive elements have a large range of uses in this wave band. Examples include mirrors, lenses, filters, and beam combiners. All these elements would have similar advantages to those enjoyed by their visible band diffractive analogs. The metal photodissolution effect in chalcogenides shows promise as one of the few techniques for producing low-loss holographic materials for use at any given wavelength from 0.6 to beyond 16 microm. To date, the work has concentrated on the photodissolution of silver into arsenic sulfide glasses. Both bulk and surface relief gratings can be fabricated simply by holographic or mask exposure. In principle, kinoforms (e.g., blazed zone plates) and Fresnel lenses can also be made. The results of material studies show that phase gratings with high modulation and low absorption can be produced. A coupled-wave analysis is used to calculate the likely grating performance, and some initial grating characterization results are presented. The limitations of the medium are discussed and possible solutions are considered.

Photo-induced dissolution effect in Ag/As33S67 multilayer structures and its potential application

Abstract A new technique for preparing metal-doped layers using photo-induced metal dissolution in chalcogenide glasses is presented. This technique is based on multilayer structures formed by alternating metal (Ag) and chalcogenide (As 33 S 67 ) layers prepared by vacuum evaporation. Such multilayer structures have a greater sensitivity to illumination and a larger photo-dissolution rate in comparison to the conventional double layer structure Ag/As 33 S 67 . The multilayer structure was used to fabricate a phase grating for operation in the IR region.

Focused ion beam micromachining of three-dimensional structures and three-dimensional reconstruction to assess their shape

Focused ion beam (FIB) systems are ideal tools for micromachining three-dimensional structures. To mill a shape correctly numerous factors, such as the shape of the ion beam and the re-deposition of the sputtered materials have to be accounted for during the ion milling. In order to alter the ion milling process to account for these and other factors, the shape of the milled structure and how it differs to the intended shape has to be determined. For a non-symmetrical structure with high depth-to-width aspect ratios a cross-section through its centre, prepared using the FIB system, will not be representative and atomic force microscopy cannot be used because of the geometry of the atomic force microscopes tip. Here, the use of three-dimensional reconstruction from a sequential set of FIB-prepared two-dimensional cross-sections milled through a structure to determine its shape is outlined.