Configuration-coordinate model for photodarkening in amorphous As2S3

Annealing characteristics of photodarkening have been studied experimentally in order to investigate a model represented by a configuration diagram which includes excited and ground states. Barrier heights of the ground states, having double wells, are fairly scattered around 0.5 ～ 1.5 ev from site to site, and the energy difference between the wells is 0.1 eV. The relaxation energy of the excited states is estimated to be 0.05 eV. The fact that band-gap illumination determines the position of optical absorption edges of the specimen irrespective of the previous history, is understandable with this model. Further, it is shown from annealing kinetics that the origins of the photodarkening and mid-gap-absorption are not the same.     

High-field behaviour of amorphous AsSe films

Thin films of composition AsSe_χ where 0.8 ≤ χ ≤ 1.0 obtained by the evaporation of both crystalline and glassy As₂Se₃ have been examined for their high-field behaviour. The films exhibit two essentially linear I–V regions with a smooth nonlinear change over. While the initial linear region may perhaps correspond to a Poole-Frenkel emission, the very high-field linear region is likely to arise from a distortion of the band shapes and lowering of the mobility gap.     

Electrical conductivity and thermoelectric power in amorphous As2Se3Tlx semiconductors

Dc conductivity and thermopower measurements as a function of temperature have been performed on solid As₂Se₃Tl_x glasses with x ranging from 0 to 1.99. Investigations have been extended to low temperatures. By substitution of Tl the conductivity can be increased by more than 5 orders of magnitude without changing the type of transport mechanism. Band-like conduction is observed with holes being the dominant charge carriers. The three-dimensional network of As₂Se₃ is assumed to be modified by the Tl atoms; this leads mainly to a decrease in the activation energy.     

Holographic recording in amorphous As2S3 films at 633 nm

Holographic grating recording with focused (light intensity I = 14–124 W/cm²) and unfocused (I = 0.50–0.78 W/cm²) 633 nm He–Ne laser sub-bandgap light in non-annealed and annealed a-As₂S₃ films has been experimentally studied. The focused light recording is found to be much more efficient (diffraction efficiency up to 14.9%, specific recording energy down to 216 J/(cm² %)) than the unfocused light recording (0.11%, 72 400 J/(cm² %)). Some other properties are also different. The hologram lifetime of more than two years and positive refractive index changes take place in the case of focused recording versus two days and negative refractive index changes for unfocused recording. The absence of measureable photoinduced thickness changes and a slight photobleaching are common properties. The focused light recording is explained by the photostimulated relaxational structural changes (RSC) accompanied by the photoinduced generation and recharging of D-centers. The unfocused light recording is explained by the photorientation of D-centers with some contribution of RSC.     

Medium range order in amorphous AsSe systems

The effects of photon and As on the crystallization of a-Se were investigated and the structure factors of a-As_xSe_1-x (0≤×≤0.36) systems were measured by neutron diffraction. We obtained the information about the medium range order (MRO) in these systems. The length of the MRO in these systems is 10～15A.     

Formation and stability of amorphous molecular systems: As the prototype of functional amorphous organic materials

Structural changes in vapor-deposited amorphous states of simple organic compounds were studied by Raman scattering, X-ray diffraction, and light interference in the sample. Two types of processes, namely direct crystallization and structural relaxation leading to glass transition, were observed depending on the flexibility of the molecule. Gradual relaxations were also found to occur in amorphous states of butyronitrile and 1,2-dichloroethane at temperatures much lower than their T_g or crystallization temperature. The observed structural changes are discussed by referring to the high enthalpy of the vapor-deposited amorphous systems.     

Experimental characterization of amorphous As–Se–Sb alloys

Thin films were thermally evaporated from ingot pieces of the As₃₀Se_70−xSb_x (with 2.5 ⩽ x ⩽ 17.5 at.%) glasses under vacuum of ∼10⁻⁵ Torr. Increasing Sb content was found to affect the thermal and optical properties of these films. Non-direct electronic transition was found to be responsible for the photon absorption inside the investigated films. The chemical bond approach has been applied successfully to interpret the decrease of the glass optical gap with increasing Sb content. Decreasing the thermal stability of the As₃₀Se_70−xSb_x specimen by increasing Sb content is responsible for occurring the amorphous–crystalline process at lower temperatures. Binary As₂Se₃ and Sb₂Se₃ phases are the main components of the stoichiometric As₃₀Se₆₀Sb₁₀ composition.     

Optical constants of As2Se3–Ag4SSe–SnTe amorphous thin films

Amorphous thin films from the As₂Se₃–Ag₄SSe–SnTe system were prepared by thermal vacuum evaporation from the corresponding bulk glasses. Their transmission, reflection, scattering and ellipsometric spectra were measured in the UV, VIS and NIR regions, and the results were mathematically simulated using the Tauc–Lorentz dispersion model in order to obtain the refractive index and extinction coefficient. The compositional dependence of the derived optical properties was found and discussed.     

Differential anomalous X-ray scattering studies of amorphous Cd59As41 and Cd26As74

The local structure of amorphous cadmium arsenide semiconducting films has been studied by differential anomalous X-ray scattering. Intensity measurements were carried out on two samples, containing 41 and 74 at.% As, in the vicinity of the absorption K edges of both constituents using synchrotron radiation. The computational procedure, similar to that proposed by Warren for an amorphous sample with more than one kind of atom, was applied to obtain the structural parameters from the experimental data. It has been found that atoms in the amorphous CdAs films remain almost tetrahedrally coordinated and that the investigated films are chemically ordered. The structural changes going from cadmium- to arsenic-rich composition have been revealed. The differential anomalous X-ray scattering technique proved to be effective, providing the evidence for the CdCd and AsAs near neighbour correlations in Cd₅₉As₄₁ and Cd₂₆As₇₄, respectively. The simulations of the differential radial distribution functions have shown that for the amorphous film containing 41 at.% As the distorted tetrahedral structure, intermediate between the CdAs and Si III type structures, is adequate to account for the experimental data. At 74 at.% As, the atomic arrangement can be described satisfactorily by the structural model based on the tetragonal CdAs₂ structure. The structural parameters obtained from the present study and those previously derived using the extended X-ray absorption fine structure and conventional large angle X-ray scattering techniques are compared.     

One- and two-phonon Raman spectra of amorphous AsSe systems

The polarization dependence of the one- and two-phonon Raman scatterings of the amorphous semiconductor As_xSe_1?x systems for 0 ? x ? 0.5 was measured. The analysis of the Raman data by the superposition of the vibrations of AsSe₃ pyramids and SeSe bonds suggests the existence of the intermediate range interaction in the As_xSe_1?x systems.Two phonon Raman spectra for 0.3 ? x are explained by the combination plus overtone of the first order Raman scatterings. For x ? 0.2, however, the overtone processes become predominant.The dependence of the Raman spectra on the As concentration suggests that the chemical orderings are predominant.     

Photo and thermal induced effects on (As2S3)0.85Sb0.15 amorphous thin films

Exposure with above band gap light and thermal annealing at a temperature near to glass transition temperature, of thermally evaporated amorphous (As₂S₃)_0.85Sb_0.15 thin films were found to be accompanied by structural effects, which in turn, lead to changes in the optical properties. The optical properties of thin films induced by illumination and annealing were studied by Fourier Transform Infrared spectrometry, X-ray Photoelectron Spectroscopy and Raman Spectroscopy. Photodarkening or photobleaching was observed in the film depending upon the conditions of the light exposure or annealing. These changes of the optical properties are assigned to the change of homopolar bond densities. The photodarkening in the as-prepared film was seen at low temperature (4.2 K).     

A model for photostructural changes in the amorphous AsS system

The photodarkening effect was studied in thin film, bulk glasses and powdered glasses of composition As₂₉S₇₁, As₄₀S₆₀ and As_42.5S_57.5. The amount of photodarkening produced in a sample depended strongly on the samples state and composition as well as temperature, incident light intensity and wavelength. Using Raman spectroscopy, vibrational bands at 231 and 491 cm^?1 (assigned to AsAs and SS bond vibrations) were found to be enhanced in photodarkened samples. We present a model in which photodarkening is due to the formation of As_n clusters (n ? 2) which are loosely coupled to the amorphous network.     

Molecular beam epitaxy of crystalline and amorphous GaN layers with high As content

We have studied the low-temperature growth of gallium nitride arsenide (GaN)As layers on sapphire substrates by plasma-assisted molecular beam epitaxy. We have succeeded in achieving GaN_1−xAs_x alloys over a large composition range by growing the films much below the normal GaN growth temperatures with increasing the As₂ flux as well as Ga:N flux ratio. We found that alloys with high As content x>0.1 are amorphous and those with x<0.1 are crystalline. Optical absorption measurements reveal a continuous gradual decrease of band gap from ∼3.4 to ∼1.35 eV with increasing As content. The energy gap reaches its minimum of ∼1.35 eV at x∼0.6–0.7. The structural, optical and electrical properties of these crystalline/amorphous GaNAs layers were investigated. For x<0.3, the composition dependence of the band gap of the GaN_1−xAs_x alloys follows the prediction of the band anticrossing model developed for dilute alloys. This suggests that the amorphous GaN_1−xAs_x alloys have short-range ordering that resembles random crystalline GaN_1−xAs_x alloys.     

Structural relaxation and optical properties in amorphous Ge33As12Se55 films

Amorphous Ge₃₃As₁₂S₅₅ films prepared by ultra fast pulsed laser deposition (PLD) have been vacuum annealed over a range of different temperatures. Raman scattering measurements indicated that the features corresponding to Ge–Se and As–Se clusters increase in intensity with increasing annealing temperature (T_a) up to their respective glass transition temperature, and then decrease with further increasing T_a up to 300 °C. Optical property measurements showed that the refractive index deceases but the optical band gap increases with increasing T_a, and both of them could be fitted by the exponential function. The corresponding characteristic time extracted was found not to obey Arrhenius behavior, which is consistent with the existence of a broken network cut by cross-linking bonds and different clusters in films.     

Structure of pulsed-laser deposited arsenic-rich As–S amorphous thin films,and effect of light and temperature

Thin amorphous films from As–S system (∼As₅₀S₅₀) were prepared by the pulsed laser deposition technique. Light- and thermally-induced changes of structure of studied films have been investigated using Raman scattering spectroscopy results and interpreted in terms of chemical reactions and/or phase transitions between individual structural units. The irradiation of as-deposited thin films causes light-induced reactions, in which As₄S₃, α- and β-As₄S₄, and pararealgar/χ-As₄S₄ molecules are formed. Thermal-annealing of exposed thin films leads to the formation of β-As₄S₄ molecules.