Raman spectra of As4S4 polymorphs: Structural implications for amorphous As2S3 films

Raman-scattering spectra of α-As₄S₄ and β-As₄S₄ have been determined at 300 and 10 K. Although similar in overall aspect, the spectral signatures of the two polymorphs are clearly distinct. We have made a careful comparison of these first-order crystal line spectra to the sharp features reported in the Raman spectra of freshly-deposited films of amorphous As₂S₃. Prior proposals for the presence of As₄S₄ molecules in the unannealed films are supported by these comparisons, but recent contentions that actual microcrystals of β-As₄S₄ are present in the as-deposited material are clearly contradicted by the absence of any of the lattice-phonon lines which are prominent in the crystals at frequencies below 70 cm^-1.     

Photoelastic properties of amorphous As2S3

The piezobirefringence of amorphous As₂S₃ was measured in the wavelength range 0.69 to 1.15 μ, and the pressure dependence of the index of refraction was determined at 0.633 and 1.15μ. The dispersion of the photoelastic constants of As₂S₃ is discussed using a one-oscillator model, and compared with the results of a similar analysis performed on GaAs.     

Acousto-induced structural change in amorphous As2S3

The acoustic flux injection method was applied to amorphous As₂S₃. The velocity and the attenuation constant for shear wave at ～ 100MHz were measured through the observation of the Brillouin scattering of the light beam from a He-Ne laser. The optical transmission around absorption edge was found to change after the injection of acoustic fluxes. It was also found that the sound velocity decreased with increasing injection time of acoustic fluxes and the amount of the change of the sound velocity reached ～ 20% after 10⁵ injections. The changes in the optical transmission and the sound velocity tended to be erased by annealing below the glass transition temperature. These phenomena are considered to be due to structural changes caused by the injection of strong acoustic fluxes.     

Resonance Raman scattering from amorphous As2S3

Resonance Raman scattering from amorphous As₂S₃, obtained by temperature tuning the band edge through the He-Ne 6328 Å laser line, is reported.     

Band to band radiative recombination in amorphous As2S3

Radiative recombination from band states is observed in amorphous As₂S₃. The spectral dependence of the radiation shows that recombination occurs before thermalization takes place.     

Thermally and photo-induced changes in the valence states of vapour-deposited As2S3 films: A comparative photoemission study of As2S3 and As4S4

During thermal annealing or light irradiation, the changes in the valence states of vapour-deposited As₂S₃ and As₄S₄ films were observed by UPS. The experimental results reveal that an as-deposited As₂S₃ film contains considerable numbers of As₄S₄ molecular units, which polymerize or cross link to form a As₂S₃ glassy network on annealing or irradiation.     

Electronic structure of As4S5 as a local structural model for amorphous As2S3 film

We have studied the electronic structure of As₄S₅ molecule as a model of the local pyramidal structure in amorphous As₂S₃ film. Emphasis has been put on the analysis of the behaviour of a trapped electron in this model concerning the consequent structural relaxation. It has been found that As₄S₅ species has affinity for an excess electron, and that the attachment of an excess electron promotes the cleavages of specific As-S bonds.     

Photo-lightening effect in crystalline As2S2 films

A photo-induced shift of the optical transmission edge to a shorter wave-length in crystalline As₂S₂ films has been studied at room temperature and liq. N₂ temperature. This effect has been investigated by X-ray diffraction and scanning electron micrograph observations.     

Photo-synthesis of As3S2 crystal from AsAs2S2 mixture

Existence of As₃S₂ compound which is stable in a certain temperature range higher than room temperature has been confirmed in AsAs₂S₂ system. Photo-synthesis effect of As₃S₂ crystal from As and As₂S₂ mixture, namely light induced conversion from a crystalline state to another crystalline state, has also been investigated.     

Photoemission study of the density of valence states of amorphous As2S3 and Sb2S3

The densities of valence states of amorphous As₂S₃ and Sb₂S₃ have been investigated by means of X-ray photoemission and ultraviolet photoemission spectroscopy. The spectra are interpreted on the basis of existing band structure calculations.     

Evidence for reversible photostructural change in local order of amorphous As2S3 film

As₂S₃ films were exposed to band-gap illumination both at 290 and 77 K alternately, and after each exposure optical and X-ray measurements were carried out. It has been clarified that the reversible photodarkening should be attributed to the reversible change in local order of the network.     

Photodarkening in glassy As2S3 under pressure

Photodarkening phenomena have been studied in glassy As₂S₃ films under pressure up to 3 kbar. A remarkable shift of the absorption edge to lower energy by the band gap illumination is observed under pressure. The shift is quenched even after releasing pressure. X-ray diffraction studies have also been performed at various pressures. The first sharp diffraction peak disappears at 50 kbar. The origin of the observed enhancement of the photodarkening may be associated with the increase in the interlayer correlation by application of pressure.     

Kinetics of photo-induced edge shift in optical transmission of amorphous As2S3 film

Photo-induced edge-shift affect in As₂S₃ film has been studies for different intensities of incident light. This process depends only on the total energy E of light. A simple experimental relationship between shift of edge Δλ and E

$\text{1 ?}\text{Δλ(E)}\text{Δλ(∞)}\text{=}\text{exp}\text{(-K·E)}$

has been obtained, which coincides with a theretical prediction deduced from a model based on a first-order reaction.     

Reversible photo-induced volume changes in evaporated As2S3 and As4Se5Ge1 films

Reversible photo-induced volume changes have been observed accompanying reversible optical absorption edge shift produced by successive cycles of band gap illumination and annealing for well-annealed evaporated As₂S₃ and As₄Se₅Ge₁ films. The As₂S₃ film shows an increase in its volume by illumination, while As₄Se₅Ge₁ film shows a decrease. Qualitative discussion has been given in connection with pressure-induced optical constant change.     

Conductivity of the chalcogenide semi-insulator system: As2S3Sb2S3

The conductivities of mixed As₂S₃Sb₂S₃ thin films are discussed. Temperature independent a.c. data are interpreted in terms of a hopping mechanism where correlation effects are important. Excepting As₂S₃, which exhibits extrinsic behaviour, the dielectric constant, a.c. conductivity and d.c. conductivity pre-exponential factor, all scale approximately linearly with composition.     

Photoluminescence decay affected by variable range hopping in band tail in amorphous As2S3

Photoluminescence decay was observed with various excitation energies and at various temperatures in amorphous As₂S₃. The decay at high temperature or with the high excitation energy had the $\text{t}{}^{\mathrm{<mtext>-3</mtext><mtext>2</mtext>}}$ long time behavior. This decay is concluded to arise from the combination of the tunneling recombination and the variable range hopping of carriers in the band tail.     

Reversible photostructural change in melt-quenched As2S3 glass

Reversible phtodarkening and photostructural change accomplished by repeated cycles of band-gap illumination and annealing have been observed in melt-quenched As₂S₃ glass for the first time. The magnitudes of photo-induced shift of optical absorption edge as well as the fraction of photo-expansion observed in melt-quenched As₂S₃ glass coincide quantitatively with those observed in well-annealed evaporated As₂S₃ glass. The experimental data involving results of crystalline As₂S₃ demonstrate that the reversible photostructural change is “unique” to amorphous state and is not affected by the difference in disordered structure originated from preparation technique.     

Hole mobility in evaporated layers of As2S3.CdI2

The hole drift mobility in As₂S₃.CdI₂ has been determined using Spear's method. The room-temperature value is 10^?4$\text{cm}{}^2\text{Vs}$, the activation energy 0.2 eV. The hole yield appears to be controlled by geminate recombination.     

Studies on electrodeposited As2S3 thin films by double exposure holographic interferometry technique

Arsenic trisulphide (As₂S₃) thin films have been deposited onto stainless steel and fluorine doped tin oxide (FTO) coated glass substrates by electrodeposition technique using arsenic trioxide (As₂O₃) and sodium thiosulphate (Na₂S₂O₃) as precursors and ethylene diamine tetracetic acid (EDTA) as a complexing agent. Double exposure holographic interferometry (DEHI) technique was used to determine the thickness and stress of As₂S₃ thin films. It was observed that the thickness of the thin film increases whereas film stress to the substrate decreases with an increase in the deposition time. X-ray diffraction and water contact angle measurements showed polycrystalline and hydrophilic surface respectively. The bandgap energy increases from 1.82 to 2.45 eV with decrease in the film thickness from 2.2148 to 0.9492 μm.     

High field absorption current and space charge measurements on thin films of As2S3

The absorption current in evaporated and blown films of As₂S₃ decays approximately as 1/t. The decay is conclusively shown not to be due to barrier polarization. Substantial space charge accumulation occurs subsequently without further current decay. Rectification in evaporated films at high fields is attributed to non-glassy structure.