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.     

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.     

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.     

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.     

X-ray diffraction measurement of liquid As2Se3 by using third-generation synchrotron radiation

X-ray diffraction (XD) measurements of liquid As₂Se₃ were carried out in the temperature range up to 1600 °C where the temperature is well beyond the semiconductor to metal (SC–M) transition temperature around 1000 °C. The measurements were done by using third-generation synchrotron radiation at SPring-8 and the obtained structure factors have been much improved compared to previous in house XD measurements with regard to the momentum transfer range and the data statistics. The deduced pair distribution functions show that with increasing temperature, the position of the first peak does not change within the errorbar and the coordination number gradually decreases up to 1600 °C irrespective of the SC–M transition. These results coincide with those of the first-principle molecular dynamics simulation.     

K X-ray spectra of amorphous and crystalline silicon

K X-ray emission and absorption spectra of crystalline and amorphous silicon have been analysed and compared to band structure calculations. The experimental results confirm the influence of rings and bound-angle fluctuations on different regions of the valence and conduction band of amorphous silicon.     

Temperature dependence of electrical conductivity and hall effect of Ga2Se3 single crystal

Electrical conductivity (σ) and Hall coefficient (R_H) of single crystal grown from the melt have been investigated over the temperature range from 398 K to 673 K. Our investigation showed that our samples are p-type conducting. The dependence of Hall mobility an charge carrier concentration on temperature were presented graphically. The forbidden energy gap was calculated and found to be 1.79 eV. The ionization energy of impurity level equals 0.32 eV approximately. At 398 K the mobility equals to 8670 cm² V⁻¹ s⁻¹ and could described by the law μ = aTⁿ (n = 1.6) in the low temperature range. In the high temperature range, adopting the law μ = bT^–m (as m = 1.67), the mobility decreases. This result indicates that in the low temperature range the dominant effect is scattering by ionized impurity atoms, whereas in the high temperature range the major role is played by electron scattering on lattice vibrations (phonons). At 398 K the concentration of free carriers showed a value of about 1.98 × 10⁷ cm⁻³.     

Infrared laser ablation of glassy As2Se3

In this study, we report the first measurements of wavelength and pulse length dependence of ablation thresholds in glassy As₂Se₃. Ablation yields, obtained from time-of-flight spectra were compared to laser intensity curves to obtain laser ablation thresholds. The wavelength dependence of the ablation thresholds were obtained by tuning the Vanderbilt–Keck free electron laser in the range of 3.2–5 μm. Pulse dependence of the ablation thresholds were obtained with a 5 μs macropulse in the wavelength range 3.2–5 μm and with macropulses ranging from 200 to 1000 ns at the wavelength of 3.5 μm. The wavelength dependence of the ablation threshold did not indicate that impurity absorption was a major factor in the ablation process in glassy As₂Se₃. The ablation threshold decreased with wavelengths corresponding to higher photon energies. In addition, a sharp increase in ablation threshold was observed with decrease in the macropulse length. Optical images of ablation craters at intensities above the threshold reveal significant melting on the sample surface. We will discuss the wavelength and pulse length dependence of the laser ablation threshold in glassy As₂Se₃ in terms of a multi-photon process and a cumulative heating process.     

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.     

Morphological features of synthetic crystals of As2Se3

Scanning electron microscope (SEM) studies have been carried out on vapour grown arsenic triselenide platelet crystals and whiskers grown under argon and normal air ambients. The crystal morphology and microstructure were found similar in both types of crystals. While investigating the microstructure the layered structure of these crystals could be discerned under SEM. Layers with average thickness of 2–2.5 μm could be distinctly observed for the first time in As₂Se₃ crystals. This might be an important observation.     

One- and two-phonon vibrational spectra of amorphous As- and Sb-chalcogenide systems

One- and two-phonon infrared and Raman spectra of As- and Sb-chalcogenide systems are reported. Both one- and two-phonon Raman and infrared vibrational spectra exhibit two modes type behaviours characteristics to the vibration modes of AsSe₃, AsS₃, and SbSe₃ pyramids. In the infrared spectra, the larger peak shifts are observed for one phonon frequencies than for two phonon frequencies with the change of the composition x, and its suggests rather big local field effects. The two-phonon coupling parameters of the combination modes are estimated.     

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.     

The vibrational spectra of B2O3-GeO2 glasses

Infrared and Raman spectra were measured for a series of glass compositions in the binary B₂O₃-GeO₂ system at room and elevated temperatures. No coordination changes were detected from spectral analysis for either boron or germanium with concentration or temperature changes. Interpretation of the vibrational spectra indicated that both boroxol rings and BO₃-triangular units were present as the basic units in glasses with higher B₂O₃ concentrations. Only BO₃-triangular units were detected as the basic units in glasses with lower B₂O₃ concentrations. Raman spectra of glasses measured at higher temperatures (>T_g) indicated that their boroxol ring concentrations decreased as a function of temperature. The enthalpy for ring rupture in 35GeO₂–65B₂O₃ glass was 7.7 kcal/mol. Also, spectral analysis indicated that the boroxol ring concentrations of these glasses at room temperature were dependent upon their previous heat treatments.     

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.     

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.