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.     

Photoelectrical properties of (Sb15As30Se55)100−xTex (0 ⩽ x ⩽ 12.5 at.%) thin films

This paper reports photoelectrical properties of (As₃₀Sb₁₅Se₅₅)_100?xTe_x amorphous chalcogenide films (0 ? x ? 12.5 at.%) through measurements of ‘steady state’ and ‘transient’ photocurrents. The composition dependence of the steady state photocurrent at room temperature shows that the photoconductivity increases while the photosensitivity decreases with increasing Te content. A study of photoconductivity of (As₃₀Sb₁₅Se₅₅)_100?xTe_x at different levels of light intensity reveals that, the photoconductivity increases exponentially with increase in light intensity. The Photocurrent (I_ph) when plotted against light intensity (G) follows a power law (I_ph = G^γ) the exponent γ for (As₃₀Sb₁₅Se₅₅)_100?xTe_x films has been found nearly 0.5 suggesting bimolecular recombination. The transient photoconductivity shows that the lifetime of the carrier decreases with increasing the light intensity. This decrease suggests that the photoconductivity mechanism in our samples was controlled by the transition trapping processes. The increase of Te content results in a monotonic decrease in the band gap and the free carrier life time of (As₃₀Sb₁₅Se₅₅)_100?xTe_x thin films. These results were interpreted on the basis of the chemical-bond approach.     

Waveguides based on Te2As3Se5 thick films for spatial interferometry

Planar waveguides were obtained by deposition of Te₂As₃Se₅ films on polished As₂S₃ glass substrates by RF-sputtering or thermal evaporation. The RF-sputtered films were porous and had a column-like structure. By contrast, the thermal evaporated films were homogeneous and dense. They were adhesive, transparent up to 18 μm and characterized by a refractive index of 2.821 ± 0.005 at 10.6 μm. M-lines measurements highlighted the existence of several guided mode lines, proving that the manufactured structures behaved as waveguides. The geometry of the film was modified by physical or reactive ion etching. Some quasi vertical ribs up to 2.11 μm in depth were etched by reactive ion etching under a CF₄/O₂ atmosphere.     

Preparation of mixed arsenic/antimony chalcogenide glasses and some optical and thermo-physical properties

The preparation of mixed glasses of As₂S_3−xSe_x (x = 0–3) and (1 − y) · As₂S₃y · Sb₂S₃ (y = 0–1) has been carried out by an in situ pouring technique. X-ray diffraction (XRD) was used to confirm the glassy nature of the materials and monitor devitrification. Visible-IR transmission, photoluminescence, refractive index and micro-Raman were measured as a function of composition. Microhardness (MH) and thermal expansion coefficient (TEC) were also measured. Raman peaks in As₂S₃ and As₂Se₃ were observed around 338 cm⁻¹ and 230 cm⁻¹, respectively in this first composition series in which S was replaced by Se. When As was replaced by Sb, in the case of second composition series, the As₂S₃ related Raman peak became broader and shifted to lower wave number, reflecting some structural change/devitrification. MH increased (1.31–1.50 GPa) with Se and Sb content while the TEC was found to decrease (2.5–1.4 × 10⁻⁵/K). The progressive increase in the content of either Se or Sb in As₂S₃ is anticipated to modify bond lengths and bond angles. The combined effect of these structural modifications would change the local structure of the glass forming a more rigid glass network thereby increasing the hardness and decreasing TEC.     

Nanostructural and optical features of amorphous AlxSi0.45−xO0.55 (0 ⩽ x ⩽ 0.05) thin films prepared by RF-magnetron sputter techniques

The nanostructural, chemical, and optical features of Al_xSi_0.45−xO_0.55 (0 ⩽ x ⩽ 0.05) thin films were investigated in terms of Al concentration and post-deposition annealing conditions; the films were prepared by co-sputtering a Si main target and Al-chips, and the annealing was carried out at temperatures of 400–1100 °C. The a-Si_0.45O_0.55 films prepared without Al-chips and annealed at 800 °C contain ∼3.5 nm-sized Si nanocrystallites. The photoluminescence (PL) intensity as well as the volume fraction of Si nanocrystallites increased with increasing the concentration of Al to a certain level. In particular, the intensity of the PL spectra of the Al_0.025Si_0.425O_0.550 films which were annealed at 800 °C increased significantly at wavelengths of ∼580 nm. It is highly likely that the observed increase in the PL intensity is caused by the raise in the total volume of the ∼3.5 nm-sized nanocrystallites in the films. The addition of Al as well as the post-deposition annealing allow adjustment and control of the nanostructural and light-emission features of the a-SiO_x films.     

Electrodeposited and selenized CIGS thin films for solar cells

Cu(In,Ga)Se₂ polycrystalline thin films were deposited adopting the potentiostatic electrochemical method on Mo/soda lime glass substrate. All the as-deposited Cu(In,Ga)Se₂ thin films were annealed in a selenium atmosphere at 550 °C for 1 h to improve the film crystalline properties. The selenized CIGS thin films were characterized by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM).The results indicate Cu(In,Ga)Se₂ thin films have single chalcopyrite structure and the grain size varies from 0.8 to 2.5 μm.     

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.     

Fine embossing of chalcogenide glasses – a new fabrication route for photonic integrated circuits

For the first time embossing of ribs, from 1 to 10 μm wide and ∼10 mm long, has been carried out in chalcogenide glass layers sputtered onto semiconductor wafer substrates, with potential to act as monomode waveguides; these features have been similarly embossed in the surface of bulk chalcogenide glasses. The embossing shows very good replication of the GaAs mould patterning to 1 μm definition, with evidence also for sub-micron replication. For the embossing, thin coatings of the chalcogenide glasses were sputtered onto wafer substrates as follows: (i) a 6 μm layer of Ge₁₇As₁₈Se₆₅ (at.%) onto porous Si-on-Si wafer substrates and (ii) a 4 μm layer of Ge₁₅As₁₅Se₁₇Te₅₃ onto uncoated GaAs substrates. The Ge₁₇As₁₈Se₆₅ sputtered glass layer on porous Si-on-Si was demonstrated to slab waveguide at 1.55 μm wavelength; it was designed to achieve monomode waveguiding at 1.55 μm after embossing, for the 5 μm wide rib. The series of ribs, 1–10 μm wide, were successfully embossed in the Ge₁₇As₁₈Se₆₅ glass sputtered layer on porous Si-on-Si, but cracking of the glass layer occurred during the embossing process. Successful embossing of ribs without the glass layer cracking was achieved for the Ge₁₅As₁₅Se₁₇Te₅₃ sputtered glass layer on uncoated GaAs. Due to its relative simplicity, it is likely that hot embossing of this type of glass-based matrix offers an extremely promising route for producing high-resolution, guided-wave optical components and circuitry at low-cost, high-volume, and for a wide wavelength range.     

Electron beam induced surface modification of amorphous Sb2Se3 thin film

The change in the surface morphology of amorphous Sb₂Se₃ thin films during the electron beam irradiation has been studied mainly by atomic force microscopy (AFM). Electron beam at accelerating voltages 30 kV is focused onto the surface of the specimens of 100-μm thickness, and then the surface morphology of each specimen has been observed by AFM in air. The modification of the film surface includes lateral and vertical resizing which is typically in the micrometer and sub-micrometer range. Protrusions above the surface as high as 90 nm are observed at 180 pA electron beam current, whilst trenches as deep as 97 nm are observed at 800 pA electron beam current (total thickness of thin film is 100 nm). The dependence of patterns characteristics on irradiation parameters such as exposure time and beam current has also been studied. Physical mechanisms for trench and mound formation are proposed.     

Dependence of photo-oxidation on Ag(Cu)-content in Ag(Cu)–As2Se3 films

We report the dependence of photo-oxidation (i.e., the formation of As₂O₃ microcrystals) on Ag (or Cu) content in Ag-doped As₂Se₃ and Cu-doped As₂Se₃ films. These chalcogenide films were prepared by thermal evaporation and photodoping, and their film surface was illuminated in air with an Ar laser beam of a wavelength of 0.5145 μm. From viewpoint of applications, we paid attention to the probability and the beginning optical intensity of the As₂O₃ microcrystals formation, and the photodarkening effect as a function of Ag (Cu) content. It has been confirmed that the addition of metals Cu or Ag into As₂Se₃ films is very useful in for suppressing or weakening such a harmful oxidation reaction. It has also been found that there is a distinct difference in these properties between Ag–As₂Se₃ and Cu–As₂Se₃ films, which is attributed to the difference in the coordinate number between Ag and Cu atoms.     

Optical properties and structure of amorphous films Agx(As0.33S0.67−ySey)100−x

As₃₃S_67−ySe_y, where y = 0, 16.75, 33.5, 50.25 and 67, amorphous thin films were prepared by a vacuum thermal evaporation technique. The films with known silver concentrations and good optical quality were prepared by thermal vacuum evaporation of a silver film on the top of As₃₃S_100−ySe_y films with sequential step-by-step optically- and thermally-induced diffusion and dissolution (OIDD) of silver. The range of silver content was x = 0–25 at.%. The kinetics of OIDD of silver were measured optically by monitoring the change of thickness of the undoped part of the chalcogenide during broadband illumination. Compositions of the reaction products have been determined by scanning electron microscope with energy-dispersive X-ray microanalyser EDS. Optical properties $(T\text{,}n\text{,}{E}_{\text{g}}^{\text{opt}})$ of thin films were measured and/or calculated by the Swanepoel method [R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214]. The refractive index increase with increasing silver and selenium concentration has been shown. The difference of the refractive index (Δn) between undoped and silver doped films was ∼0.4 and between As₃₃S₆₇ and As₃₃Se₆₇ was films ∼0.42. Non-linear indices of refraction were estimated according to Tichy’s formula [H. Ticha, L. Tichy, J. Optoel, Adv. Mat. 4 (2002) 381]. The values of non-linear refractive index grew with increasing silver and selenium content. The difference of optical bandgap, $\mathrm{\Delta }{E}_{\text{g}}^{\text{opt}}$, between undoped As₃₃S₆₇ and fully doped films with Ag and Se was ∼1 eV. Raman spectroscopy showed a decrease in S–S or Se–Se bonds with increasing silver content.     

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.     

Incipient amorphous-to-crystalline transition in HfO2 as a function of thickness scaling and anneal temperature

A series of 1.4, 1.8, and 4.0 nm thick HfO₂ films deposited on Si(1 0 0) substrates have been measured by extended X-ray absorption fine-structure prior to anneal processing, following a standard post deposition anneal of 700 °C for 60 s in NH₃ ambient, and following an additional rapid thermal anneal cycle of 1000 °C for 10 s in N₂ ambient. Analysis of the second coordination shell gives clear evidence of increased ordering with increasing film thickness at each temperature. Similarly, increased ordering with increasing anneal temperature is evident for each film thickness. Although X-ray diffraction and high resolution transmission electron microscopy indicated the 1.4 nm HfO₂ samples to be amorphous, EXAFS has distinguished nanocrystalline from amorphous states for these films.     

Optical properties of Ge–Se–Te wedge-shaped films by using only transmission spectra

Amorphous Ge₁₀Se_90?xTe_x (with x = 0, 5, 10 and 15 at.%) thin films were prepared by thermal evaporation method. The optical transmission spectra of these films were measured in the wavelength range of 500–2500 nm in order to drive the refractive index and the absorption coefficient of these films. Applying the analytical expressions proposed by Swanepoel, enabling the calculations of optical constants of thin films with non-uniform thickness with high accuracy. Furthermore, the dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. It was found that, the mechanism of the optical absorption follows the rule of the allowed non-direct transition. The optical band gab, E_g, and the oscillator energy, E_o, decrease while the dispersion energy, E_d, increases by increasing Te content. The relationship between the obtained results and the chemical compositions of the Ge₁₀Se_90?xTe_x thin films were discussed in terms of the chemical bond approach, the excess of Se–Se homopolar bonds and the cohesive energy (CE).     

Improvement of the electrical properties of Se3Te1 thin films by In additions

The electronic and photoconductivity properties of semiconducting chalcogenide glasses have been largely stimulated by attractive micro-electronic device applications. The present paper aims to study the effect of In additions on the steady state and transient photoconductivity of amorphous In_x(Se₃Te₁)_100 ? x (0 ≤ x ≤ 10 at.%) chalcogenide films. It was found that, the Indium additions lead to the decrease of both the activation energies (ΔE_dc in the dark and ΔE_ph for the photoelectrical conduction) and the optical band gap E_g that improved the electrical properties of these films. The photoconductivity increases while photosensitivity changes from 8.73 to 7.18 with the increase of In content. The exponential dependence of photocurrent on the light intensity suggests that, the recombination mechanism in these films is due to bimolecular recombination. The transient photoconductivity measurements stated that, the carrier lifetime decreased by the increase of the light intensity and In content. The obtained results were discussed in terms of the width of localized states (Mott and Davis model) and the chemical-bond approach.     

Study of As―Se―Te glasses by neutron-, X-ray diffraction and optical spectroscopic methods

The atomic structures of amorphous As₄₀Se_(60?x)Te_x (x = 10 and 15) and As₄₀Se₆₀ glasses have been investigated by neutron and high energy X-ray diffraction methods. The two datasets were modeled simultaneously by reverse Monte Carlo (RMC) simulation technique. The RMC simulations revealed a glassy network built-up from As(Se, Te)₃ pyramids in which Te atoms substitute Se atoms. The As―Se correlation function shows a strong and sharp first peak at 2.4 Å and two broad and much less intense peaks at 3.7 and 5.6 Å, related to 1st, 2nd and 3rd neighbor distances of the As―Se bonds, respectively. They are an evidence for existence of short and medium ordering in the studied glasses. The similarity of Θ_Te―As―Te and Θ_Se―As―Se bond distributions suggests that Te atoms have a similar role in the structure formation as Se atoms. The FTIR spectra analysis revealed impurity bonds of Se―H, As―O, Se―O, and Te―O in the glasses which contributed to enhanced absorption in visible spectral range. From the ellipsometric data analysis the optical constants and the energetic parameters of the studied glasses were established. The compositional variation of these parameters is explained in terms of chemical bonds formation and change in the density of charged defects.     

Growth and characterization of nanostructured CdS/Polyaniline/CuInSe2 thin films for solar cell applications

In the present paper, we report about synthesis of nanostructured organic–inorganic heterojunction of CdS/Polyaniline/CuInSe₂ thin films by cost effective chemical route at room temperature, for solar cell application. As such obtained thin films are characterized for structural, compositional, morphological, optical and electrical properties by X-ray diffraction (XRD) pattern, energy dispersive X-ray (EDAX) analysis, scanning electron microscopy (SEM), optical absorbance spectra and I–V response respectively. The XRD reveals the polycrystalline nature of the thin films having tetragonal crystal structure and a crystallite size of 19 nm. The presence of observed and expected elements in the EDAX spectra confirms the elemental compositions in CdS/Polyaniline/CuInSe₂ thin films. From SEM images it can be inferred that the surface morphology of the Polyaniline thin films exists like clothing fibers, while CdS/CuInSe₂ shows granular shape particles distributed over the substrate and the SEM of CdS/Polyaniline/CuInSe₂ represents mixing and attachment of circular particles to fiber like structure. The optical absorbance spectra have shown red shift in absorbance strength and energy band gap value of CdS/CuInSe₂ from ~ 1.36 eV to ~ 1.62 eV upon formation of CdS/Polyaniline/CuInSe₂ thin film. The I–V response of CdS/CuInSe₂ and CdS/Polyaniline/CuInSe₂ measured under dark and illumination to 100 mW/cm² light, exhibited the solar characteristics from these graphs and the conversion efficiency calculated is observed to be 0.26 and 0.55% for CdS/CuInSe₂ and CdS/Polyaniline/CuInSe₂ thin films respectively.     

Structural study of undoped and (Mn, In)-doped SnO2 thin films grown by RF sputtering

Undoped and 5%(Mn, In)-doped SnO₂ thin films were deposited on Si(1 0 0) and Al₂O₃ (R-cut) by RF magnetron sputtering at different deposition power, sputtering gas mixture and substrate temperature. X-ray reflectivity was used to determine the films thickness (10–130 nm) and roughness (～1 nm). The combination of X-ray diffraction and Mössbauer techniques evidenced the presence of Sn⁴⁺ in an amorphous environment, for as-grown films obtained at low power and temperature, and the formation of crystalline SnO₂ for annealed films. As the deposition power, substrate temperature or O₂ proportion are increased, SnO₂ nanocrystals are formed. Epitaxial SnO₂ films are obtained on Al₂O₃ at 550 °C. The amorphous films are quite uniform but a more columnar growth is detected for increasing deposition power. No secondary phases or segregation of dopants were detected.     

Formation and structure of crystalline inclusions in As2S3–SbSI and As2Se3–SbSI systems glass matrices

The main aim of the study presented in this paper is the investigation of the structure of (As₂S₃)_100?x(SbSI)_x and (As₂Se₃)_100?x(SbSI)_x (0 ≤ × ≤ 40) glasses by Raman spectroscopy and X-ray methods, also the nature of the crystalline inclusions which arise up in their matrix at heat treatment. We have found that in conditions of continuous heating in the interval “glassforming temperature–crystallization temperature” a crystallization with predominant mechanism of stable phase SbSI separation is taking place. The formation mechanism of crystalline inclusions of antimony sulphoiodide in glass matrix is discussed in the light of our results. It was established that all investigated glasses have a nano-heterogeneous structure.