Amorphous As2S3 Research Articles

Subwavelength-period gratings in amorphous chalcogenide thin films

Thin films of amorphous chalcogenide semiconductor As2S3 and As?S?Se systems were studied to record refractive-index and surface-relief modulated gratings with a period of 0.15?1??m. The period and profile of the surface-relief modulated gratings were measured by AFM. The polarization properties of refractive-index modulated gratings are shown.

Quantum efficiency of light-induced defect creation in hydrogenated amorphous silicon and amorphous As2Se3

The quantum efficiency (QE) of light-induced metastable defect creation in hydrogenated amorphous silicon (a-Si : H) and amorphous As2Se3 (a-As2Se3) by bandgap and subgap illumination has been deduced from photocurrent measurements. The QE decreases with increasing number of absorbed photons. A higher QE for a-As2Se3 than for a-Si : H has been observed and this is interpreted in terms of the higher structural flexibility of a-As2Se3. We have also found that, for both materials, subgap illumination yields a higher QE than does bandgap illumination.

A nanoscale characterisation of extended defects in glassy-like As2Se3 semiconductors with PAL technique

A meaningful interpretation of positron lifetime characteristics for glassy-like g-As2Se3 is developed taking into account calculations of Jensen et al. (J. Non-Cryst. Solids 170 (1994) 57) for positrons trapped by free-volume extended defects in orthorhombic As2Se3 and void volume distribution for 146-atoms layer-biased model of amorphous As2Se3 presented by Popescu (J. Non-Cryst. Solids 35–36 (1980) 549). The obtained results are compared for samples having different thermal pre-history. Two groups of experimental results with close lifetime characteristics are distinguished for each of the investigated samples. This feature is explained in terms of average positron lifetime by applying two-state positron trapping model for mathematical treatment of the obtained spectra.

Simulations of arsenic selenide glasses

In this paper we make some general comments on the methods used for modellingglasses and use glassy GeSe2 as an example. We then emphasize recent work onAs–Se glasses, including glassy As2Se3 and amorphous AsSe. The topology andelectronic and vibrational properties of the models are discussed and with theiruse we infer the microscopic nature of the valence band x-ray photoemission(XPS) signal of g-As2Se3 and AsSe. We note that the XPS signal may beinterpreted as arising from three distinct subbands with well defined structuralorigins.

Strong Bragg gratings photoinduced by 633-nm illumination in evaporated As2Se3 thin films.

Bragg gratings are used in several photonic devices to reflect, and thus to isolate, specific wavelengths of light. Gratings can be photoinduced in chalcogenide glasses by illumination of bandgap light in an interference pattern. We used holographic interferometry to create Bragg gratings in amorphous As2Se3 thin films with a period of 0.56 microm by illumination with 633-nm light. The quality of the gratings was tested in real time, and refractive-index modulations as high as 0.037 were measured. These gratings were found to be stable over a period of several months if they were kept in the dark.

Natural realgar and amorphous AsS oxidation kinetics

The oxidation rates of natural realgar and amorphous synthetic AsS by dissolved oxygen were evaluated using mixed flow reactors at pH 7.2 to 8.8 and dissolved oxygen contents of 5.9 to 16.5 ppm over a temperature range of 25 to 40°C. The ratios of As/S are stoichiometric for all amorphous AsS oxidation experiments except for two experiments conducted at pH ∼8.8. In these experiments, stoichiometric ratios of As/S were only observed in the early stages of AsS (am) oxidation whereas lower As/S ratios were observed during steady state. For realgar oxidation experiments, the As/S ratio is less than the stoichiometric ratio of realgar, ranging between 0.61 and 0.71. This nonstoichiometric release of As and S to solution indicates that realgar oxidation is more selective for S after the rates of oxidation become constant. All measured oxidation rates at 25°C can be described within experimental uncertainties as follows: Table 1 Rate Expression Activation Energy (kJ/mol) R (Realgar/As) = 10 −9.63(±0.41)[DO] 0.51(±0.08)[H +] −0.28(±0.05) 64.2 ± 9.8 R (Realgar/S) = 10 −9.74(±0.35)[DO] 0.54(±0.05)[H +] −0.31(±0.04) 62.2 ± 9.0 R (AsS(am)) = 10 −13.65(±0.82)[DO] 0.92(±0.08)[H +] −1.09(±0.10) 124 ± 18.8 where R signifies the steady-state oxidation rate (mol m −2s −1), [DO] is dissolved oxygen concentration (M), and [H +] is the proton concentration (M). Arsenic (III) and As(V) are both present in solution, and As(III) is the dominant species in most experiments. Intermediate sulfur species besides sulfate, sulfite, and thiosulfate are the important products during realgar and AsS (am) oxidation. Comparison of realgar and AsS (am) oxidation rates shows that at similar conditions, the rates of AsS (am) are always faster by about a factor ranging from 2 to 38. The oxidation of realgar involves breaking bonds in the realgar crystal, whereas AsS (am) oxidation does not include crystallographic framework destruction due to the amorphous nature of the solid.

Ion-beam doping of silver in amorphous As2S3 thin films

Double layers of metallic Ag/amorphous (a-)As2S3 on silica substrates were irradiated with 300 keV He+ ions to examine doping of Ag+ into a-As2S3 by dense electronic excitation. The irradiating ions passed through the double layers and came to rest in the silica substrates. Thus, energy deposition from energetic He+ ions into the double layers was completely almost due to electronic processes (99.4%). Ionized silver found to diffuse into a-As2S3 as a result of electronic excitation by ion-beam irradiation. Rutherford backscattering spectroscopy revealed that the highest concentration of silver doped into a-As2S3 was 35 at. %. When the resulting Ag-doped a-As2S3 was illuminated by band gap light, photo-surface-deposition of metallic silver particles, which was not observed for Ag photo-doped specimens, was observed.

Photoinduced refractive index change in As2Se3 by 633nm illumination

Photodarkening of amorphous As2Se3 thin films was generated by a 633-nm HeNe laser. The refractive index and absorption coefficient of the chalcogenide glass was determined, both before and after exposure, by analyzing the material's transmission spectrum. In order to accurately determine the optical constants, the thin film's non-uniform thickness was accounted for. The increase in the refractive index and the coefficient of absorption was investigated and was found to demonstrate saturation with increased exposure time. Index changes as high as 0.05, or 2%, were obtained in As2Se3, a promising glass for all-optical switching.

Ultrafast Optical Kerr Effect in Amorphous As2Se3 FilmInduced by Ultrashort Laser Pulses

Nonresonant third-order nonlinear optical properties of amorphousAs2Se3 films were investigated experimentally by the method offemtosecond optical heterodyne detection of the optical Kerr effect at 805 nmwith 80 fs ultrafast pulses. The results have shown that the values of realand imaginary components of third-order susceptibility of amorphousAs2Se3 films were 5.6×10-12 esu and -2.0×10-13 esu, respectively. Amorphous As2Se3 films demonstrateda very fast response time within 200 fs. The ultrafast response and largethird-order nonlinearity are attributed to the ultrafast distortion of theelectron cloud of As2Se3 films.

Photodarkening in Amorphous As2S3 Thin Films

The photodarkening effect in amorphous As2S3 films is studied.The optical absorption edge shifts to a lower energy after illumination atthe bandgap light of 514.5 nm wavelength by an argon laser. The shift inwell-annealed films can be recovered by annealing at 180°C for 1 h but inun-annealed films it is irreversible. In addition, it was found that themagnitude of photodarkening ΔE increased with the increase ofillumination light intensity and illumination time. The reversibility ofphotodarkening in amorphous As2S3 films can be applied in opticalmemories.

Short range structure in the amorphous As–S–Br system

The short-range structure in the amorphous As–S–Br system has been investigated through Raman spectroscopy, EXAFS, X-ray diffraction and neutron diffraction measurements using (As 2S 3) 1− x Br x and As 0.4− x S 0.6Br x alloys. Raman bands relating to AsBr 3 molecular units, which have not been observed in the amorphous As–Se–Br system, appear in the present As–S–Br alloys. The intensity of the Raman bands assigned to S 8-ring clusters increases at larger x in amorphous (As 2S 3) 1− x Br x alloys, reflecting that, owing to the preferential formation of the As–Br bonding, extra S atoms in the amorphous AsS 3/2 network matrix associate with each other to form S 8-ring clusters. Interatomic distances of As–S, As–Br and S–S pairs involved in the nearest neighbor shell in the present As–S–Br alloys are estimated to be 2.26, 2.36 and 2.05 Å, respectively, by least squares fitting of observed EXAFS, and X-ray diffraction and neutron diffraction data. Further, total coordination numbers of constituent atoms in the system remain unchanged at any x and are computed to be three for As, two for S and unity for Br, respectively.

Refractive-index change caused by electrons in amorphous AsS and AsSe thin films doped with different metals by photodiffusion

The refractive-index change caused by electrons was measured in amorphous AsS and AsSe thin films. Films were coated with different metals. Diffraction gratings were written by electron-beam lithography. The interactions of electrons in films with and without the photodiffusion of overcoated metal were compared. Incoming electrons caused metal atom and ion diffusion in both investigated cases. The metal diffusion was dependent on the metal and it was found to influence the refractive index. In some cases lateral diffusion of the metal was noticed. The conditions for applications were verified.

Kinetic rates of amorphous As2S3 oxidation at 25 to 40°C and initial pH of 7.3 to 9.4

The experimental studies of As2S3 (am) oxidative dissolution were investigated by use of particle sizes of 8 to 44 μm in a mixed flow reactor at 25 to 40°C and initial pH 7.3 to 9.4 to determine the effect of dissolved oxygen content, temperature, sulfate, and pH at 0.01 M ionic strength. Most experiments ran for ∼30 h; however, some longer (52 h) and shorter (6–10 h) experiments were also conducted. In most experiments, the As and S release rates had reached an apparent steady state after 20 h. For most experiments, oxidation became stoichiometric after a steady state condition was achieved. The arsenic and sulfur release rates after 20 h in most experiments are in the order of 2.70 × 10−10 to 4.90 × 10−9 mol m−2 s−1 in the presence of oxygen.The rate laws of As2S3 (am) oxidation were only determined for experiments that ran for ∼30 h at 25 to 40°C and steady state pH 6.9 to 7.9 and are given as follows: Rate law:Activation energy:DE-01R(As)=10−16.77[DO]0.42[H+]−1.2616.8±5.0 kJ mol−1R(S)=10−16.99[DO]0.35[H+]−1.2516.3±5.4 kJ mol−1, where R is the rate of As2S3 (am) destruction (mol m−2s−1), [DO] is the concentration of dissolved oxygen (M), and [H+] is the concentration of protons (M). The low activation energy of As2S3 (am) oxidative dissolution indicates diffusion control as the dominant mechanism. Moreover, most solid surfaces after the experiments appear smooth and undisturbed.In all experiments, As(III) is present in proportions ranging from ∼49 to 92% of total As at steady-state conditions. Sulfur oxyanions occurred as sulfate (SO42−), sulfite (SO32−), and polythionates (SnO62−) where polythionates are found to be the dominant sulfur species. The acid generated from As2S3 (am) at 25°C and pH 7 to 9 can be expressed as follows: DE-0.2[H+]prod=7.89[H+]initial+(2.37×10−7), where [H+]prod is the proton release from the oxidation of As2S3 (am) (mol/g), [H+]initial is the initial amount of proton, and 2.37 × 10−7 is an empirical factor.

Experimental study and configurational-coordinate model for photo-induced effects in amorphous As2Se3 films

The optical absorption of As2Se3 thin films is studied in the UV–VIS spectral range and the value of the optical energy gap is determined. The effect of photo-irradiation on the optical absorption and transmission of thin film samples is also investigated. The optical energy gap was found to decrease with photo-irradiation time. The results of photo-irradiation are discussed in correlation with the structural aspects of As2Se3. A model is proposed to account for the structural changes, resulting from photo-irradiation, causing the decrease of the energy gap. The effect of γ-radiation on the optical absorption of As2Se3 thin films was studied also and no detectable effect on the value of the optical energy gap was observed.

Mechanism of photoinduced changes in the structure and optical properties of amorphous As2S3.

We propose a mechanism of photostructural changes in amorphous As2S3 ( a-As2S3) on the basis of ab initio molecular orbital calculations on clusters of atoms modeling the local structure of the amorphous system. We have found that trigonal AsS3 pyramidal units can be transformed into a fivefold coordinated As site having four As-S bonds and one As-As bond via a photoionization process. This photoinduced coordination defect center exhibits a lower photoabsorption energy as compared with the usual pyramidal structure, explaining the observed photodarkening effect of a-As2S3.

On the linearity of holographic recording in amorphous As2S3 films

Optical recording in amorphous arsenic trisulfide was investigated for amplitude to phase conversion to be exploited in applications such as holographic recording and the fabrication of diffractive optical elements. The main result of this study is the demonstration that it is possible to obtain a linear relation between the spatial variation of the recording intensity and the recorded phase modulation. The extent of the linear recording region depends strongly on the recording wavelength. At short wavelength (458 nm in these experiments), where the rate of the photoinduced structural changes is strong, the recording is practically linear from zero exposure up to about 25% of the maximum value of the refractive index change. With higher exposure, saturation effects start to play an important role and degrade the linearity of the response. Recording with longer wavelength (488 and 514.5 nm) is less effective (absorption becomes weaker) and then relaxation processes in the material become significant. If the competition between photoinduced processes and relaxation processes is strong, the linear relationship between exposure and refractive index change can no longer be linear. Nevertheless, even for recording with the longer wavelengths, a region of exposures can be found within which variations of intensity can be converted into variations of phase with a linear relationship.

Temperature dependence of the optical gap in thin amorphous films of As2S3, As2Se3 and other basic non-crystalline chalcogenides

Optical transmittivity in the UV–VIS region of thermally evaporated amorphous As2S3, and As2Se3 thin films was measured in the temperature region 15K<T≤300K. The temperature dependence of the optical gap (Eg(T)) was found to be in agreement with Fan's one-phonon approximation or with Cody's one-oscillator model. The parameters necessary to describe the Eg(T) dependences for non-crystalline Se, As2S3, As2Se3, GeS2 and GeSe2, according to a simple linear relation, Varshni's relation, and Fan's or Cody's model are summarized.

In-Situ Ellipsometric Observations of Thickness Change in the Layers of Ag/a-As2S3 Film System with Progression of Photodoping

The phenomenon of Ag-photodoping in an amorphous As2S3 film was examined by an ellipsometric method. The observed ellipsometric parameters Ψ and Δ were fitted using multilayer models. It was observed that the thickness of each layer of the Ag, Ag-doped As2S3 and As2S3 system showed a timed-dependency. From these behaviors, the main features of photodoping in a double-layer, Ag/As2S3 were confirmed. The composition of Ag-doped As2S3 layer was estimated to be Ag2.6As2S3 from the amounts of consumed Ag and As2S3.

Holographic manifestations of D centres in amorphous As2S3 films

The concept of D centres introduced by N. F. Mott, E. A. Davis and R. A. Street for chalcogenide glasses is used to explain three holographic effects in amorphous As2S3 films experimentally observed by authors at room temperature. These three effects include subbandgap light holographic recording (previously assumed to be impossible), strong stimulation of bandgap light holographic recording by subbandgap readout light, and diffraction efficiency oscillations at low recording intensities in the presence of subbandgap readout light. The first and the second effects are found to be due to the subbandgap light induced reorientation and generation of D− and D+ centres whereas the third effect is explained by the subbandgap light stimulated D centre diffusion. The phenomenon of photoinduced anisotropy involved in the first two effects is analysed, too.

Analysis of asymmetric Z-scan measurement for large optical nonlinearities in an amorphous As_2S_3 thin film

We have extended the conventional Z-scan theory by employing an aberration-free approximation of a Gaussian beam through a nonlinear medium and derived a simple analytical formula for Z-scan transmittance, including the effects of both nonlinear absorption and nonlinear refraction, which could be applicable to the sample with large nonlinear phase shifts. We verified the extended Z-scan theory in an amorphous chalcogenide As2S3 thin film by measuring the Z-scan transmittance with both open and closed apertures. The nonlinear refractive index γ=7.6×10-5 cm2/W and the nonlinear absorption coefficient β=1.6 cm/W of As2S3 were measured at subbandgap 633-nm illumination.