A.P. Firth

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.

Reversible photodarkening and structural changes in As2S3 thin films

Abstract The photodarkening effect in evaporated As2S3 films has been studied. Optical transmissivity was measured and its dependence on intensity, wavelength, temperature and previous thermal history was found. A structural investigation was also made using Raman spectroscopy. A feature at 231 cm−1 in the Raman spectra, which is associated with homopolar As–As bonds, is enhanced after photodarkening. A model based on the redistribution of chemical bonds under illumination is used to explain the results.

Photodissolution of silver in arsenic sulphide films — an exafs study

Abstract Silver K-edge EXAFS measurements have shown that the photodissolved silver site in arsenic sulphide films is very similar to that in the bulk ternary glasses of the general composition Ag x (As 2 S 3 )( 1−x ). Neither the films nor the glasses show a marked resemblance to any of the simple crystalline sulphides. In particular silver is 3-fold coordinated to sulphur without any pronounced AgAg or AgAs correlations. In those cases where a surfeit of silver has been added the EXAFS of the photodoped films exhibit a second neighbour peak which is interpreted as an incomplete dissolution of silver.

Optically induced crystal-to-amorphous-state transition in As2S3

The optically induced transition of crystalline As 2 S 3 (orpiment) to the amorphous state was studied using Raman spectroscopy. The illumination of As 2 S 3 crystals by Kr laser light (530.9 and 568.2 nm wavelength) rapidly reduces the intensities of the narrow Raman lines of c-As 2 S 3 , while increasing the intensities of broad Raman bands (340 cm -1 ), 231 cm -1 ) typical of the amorphous state. The origin of the effect is non-thermal ; the light probably decreases the kinetic barrier for the disordered crystal-to-amorphous-state transition.

Optical monitoring of photodissolution kinetics in amorphous As-S films

Abstract The kinetics of the photodissolution of silver into amorphous As-S films of various compositions in the range As40S60 to As20SB0 has been investigated by monitoring the changes that occur in their reflectivity during the photodissolution process. It was found that as the S content was increased above 60at.% the rate of photodissolution increased to a maximum at around As33S67 and then decreased, the maximum rate being approximately double that for As20S80. This is attributed to the fact that only compositions within a few at.% of As33S67 yield a homogeneous material when photodoped; compositions outside this range yield a photodoped product with a phase-separated structure which may impede the passage of electrons through the photodoped layer. The effect of using silver-copper alloys as the metal source instead of pure silver has also been investigated and was found to increase the photodissolution rate by up to 12%. The photodissolution rate and the form of the time dependence of the process wer...

Inorganic Resists Based On Photo-Doped As-S Films

In recent years there has been considerable interest in inorganic resist systems based on the photo-doping of amorphous chalcogenide films, the majority of the research being devoted to Ge-Se films. This paper presents a detailed investigation of inorganic resists based on the photo-doping of Ag into As-S films. It is shown that high resolution patterns can be produced in such resists using holography or optical lithography and that they are compatible with wet-chemical or plasma etching. Structural studies using Raman spectro-scopy indicate that for best resolution the composition of the As-S film should be close to AS33S67 since on photo-doping it will yield a single-phase homogeneous material. A possible mechanism for the photo-doping process is described based on a tarnishing-type photo-chemical reaction. It is shown that the actinic radiation initiating the photo-dissolution effect is absorbed primarily in the photo-doped layer, close to the interface with the undoped As-S region.

An optical reflectivity study of Ag photo-dissolution into AsS films

Abstract The optical reflectivity of As30S70 films has been monitored during the photo-dissolution of Ag. Comparison with a computer simulation suggests that the doped and undoped regions remain distinct throughout the process, the doped region consisting of a homogeneous AsSAg phase. It is argued that photo-dissolution may occur as a result of the photo-deposition of Ag at the interface between the doped and undoped regions.

Compositional dependence of photodissolution kinetics in amorphous AsS films

Abstract The kinetics of the photodissolution of Ag into amorphous AsS films of various compositions in the range As40S60 to As20S80 has been investigated by monitoring the changes that occur in their reflectivity during the photodissolution process. It was found that as the S content was increased abobe 60 at.% the rate of photodissolution increased to a maximum around As33S67 and then decreased, the maximum rate being approximately double that for As20S80. This is attributed to the fact that only compositions within a few atomic percent As33S67 yield a homogeneous material when photodoped.

Explanation of the laser-induced oscillatory phenomenon in amorphous semiconductor films

Abstract A new model is presented which explains the recently observed oscillatory phenomena in self-supporting a-GeSe2 films. The model is based on a combination of thermal and photoinduced effects. It can also be used for predicting the experimental conditions for observing light-induced bistability and oscillatory phenomena in other amorphous semiconductor films.

Chalcogenide gratings produced by the metal photodissolution effect

A theoretical analysis of transmissive gratings of the type likely to be produced by the metal photodissolution effect in chalcogenide glasses has been carried out and used to predict diffraction efficiencies for a typical chalcogenide system, namely As30S70 glass photodoped with Ag. This was done to assess the potential for using these devices as diffractive elements for the 8-14 mu m IR region. For bulk diffractive devices with a rectangular modulation profile, predicted efficiencies in the multiwave regime were over 90% and in the volume regime up to 100%, irrespective of profile. Surface relief gratings with a rectangular profile yielded a maximum efficiency of at least 73%, The refractive index of Ag-doped and undoped As30S70 glass as a function of wavelength was measured for use in the grating analysis and was found to be approximately constant above 1 mu m, the value depending on the amount of Ag photodoped in the glass. For As30S70 containing approximately 27 at.% Ag the refractive index at 2 mu m is 2.8 compared with 2.2 for undoped As30S70, and this large difference is expected to be retained beyond 10 mu m.