Structural heterogeneity in chalcogenide glass films prepared by thermal evaporation

GeSe₂ and Ge₂₈Sb₁₂Se₆₀ chalcogenide glass thin films have been deposited on single crystal silicon substrates by vacuum thermal evaporation. The surface morphology of these films has been investigated by field emission-scanning electron microscopy and atomic force microscopy, revealing heterogeneities in their microstructure consisting of granular regions ∼15–50 nm in size, which were coarser in the case of the GeSe₂ films. Typical RMS film surface roughness values were ∼0.9–1.3 nm.     

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.     

Spin-coating of Ge23Sb7S70 chalcogenide glass thin films

Thin film Ge₂₃Sb₇S₇₀ chalcogenide glass has emerged as an important material system for photonic applications due to its high non-linear refractive index. However, one of the challenges is developing low-cost methods to deposit films of glassy material while retaining glass stoichiometry and high film quality. In this paper, we demonstrate a spin-coating technique for the deposition of such films. The dissolution mechanisms of Ge₂₃Sb₇S₇₀ in different solvents are studied in order to select the optimal solvent for film deposition. We show that the use of amine-based solvents allow the deposition of stoichiometric films in contrast to alkaline solutions. Films with low surface roughness (RMS roughness <5 nm) and controlled thickness (100–600 nm) can be deposited from solutions. We also show that annealing the films in vacuum decreases the amount of residual solvent, the presence of which is expected to lead to variation in optical properties of the thin films.     

Glassy state and structure of Sn–Sb–Se chalcogenide alloy

The effect of Sn addition on the glass transition and structure of c-Sb₂₀Se₈₀ chalcogenide alloy have been studied by X-ray diffraction and differential scanning calorimetric studies. The increase in the glass forming region and the glass transition temperature with the addition of Sn is discussed by considering the formation of [SnSe₄] tetrahedra, another type of network former, which inhibits the crystallization. The differential scanning calorimetric studies on Sn_xSb₂₀Se_80−x (8 ⩽ x ⩽ 18) glassy samples reveal a single glass transition temperature for all values of x while a single crystallization peak was obtained only for 10 ⩽ x < 12. The X-ray diffraction studies reveal that the glass crystallizes to Sb₂Se₃ and SnSe₂ phases upon annealing. The glass formation and composition dependence of glass transition temperature in the Sn–Sb–Se chalcogenide alloy could be understood by considering the topological phase transitions and a chemically ordered network model.     

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.     

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.     

Surface morphology of spin-coated As–S–Se chalcogenide thin films

Surface morphology and roughness of amorphous spin-coated As–S–Se chalcogenide thin films were determined using atomic force microscopy. Prepared films were coated from butylamine solutions with thicknesses d ∼ 100 nm and then annealed in a vacuum furnace at 45 °C and 90 °C for 1 h for their stabilization. The root mean square surface roughness analysis of surfaces of as-deposited spin-coated As–S–Se films indicated a very smooth film surface (with R_q values 0.42–0.45 ± 0.2 nm depending on composition). The nanoscale images of as-deposited films confirmed that surface of the films is created by domains with dimensions 20–40 nm, which corresponds to diameters of clusters found in solutions. The domain character of film surfaces gradually disappeared with increasing annealing temperature while the solvent was removed from the films. Middle-infrared transmission spectra recorded a decrease of intensities of vibration bands connected to N–H (at 3367 and 3292 cm⁻¹) and C–H (at 2965, 2935 and 2880 cm⁻¹) stretching vibrations. Temperature regions of solvent evaporation T = 60–90 °C and glass transformation temperatures T_g = 135–150 °C of spin-coated As–S–Se thin films were determined using a modulated differential scanning calorimetry.     

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.     

Holographic recording in amorphous chalcogenide thin films

Holographic recording by He–Ne laser (line 632.8 nm) light in amorphous As_0.55Se_0.45 thin films for different film thickness and grating period was studied. A strong dependence of the diffraction efficiency of the gratings on the readout light wavelength (650 nm, 805 nm and 1150 nm) was observed. A decrease in diffraction efficiency for longer wavelengths is explained by a decrease in the photoinduced changes of refractive index. It is shown that high efficiency gratings can be recorded in As_0.55Se_0.45 films with a thickness of ∼1 μm.     

Transient photoconductivity in amorphous Se70Sb20Ag10 thin films

Se₇₀Sb₂₀Ag₁₀ bulk material was obtained using a conventional melt quenching technique. Thin films of a-Se₇₀Sb₂₀Ag₁₀ were prepared by vacuum evaporation technique in a base pressure of 10⁻⁴ mbar onto well-cleaned glass substrates. Dark, light conductivity and transient photoconductivity have been studied. The photoconductivity increases initially and then saturates with time when illuminated with low intensity light (<400 Lux); however, when illuminated with higher intensity light (>600 Lux) the photoconductivity increases initially, attains a maximum and then decreases with time. This behavior of photoconductivity has been studied at various temperatures. The results have been explained on the basis of Dember voltage and the interaction between photoexcited holes and the trapped electrons on the surface.     

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.     

Optical and electrical properties of as-prepared and annealed (Se80Te20)100 − xAgx (0 ≤ x ≤ 4) ultra-thin films

Optical and electrical properties of the (Se₈₀Te₂₀)_100 ? xAg_x (0 ≤ x ≤ 4) ultra-thin films have been studied. The ultra-thin films were prepared by thermal evaporation of the bulk samples. Thin films were annealed below glass transition temperature (328 K) and in between glass transition temperature and crystallization temperature (343 K). Thin films annealed at 343 K showed crystallization peaks for Se–Te–Ag phases in the XRD spectra. The transmission and reflection of as-prepared and annealed ultra-thin films were obtained in the 300–1100 nm spectral region. The optical band gap has been calculated from the transmission and reflection data. The refractive index has been calculated by the measured reflection data. It has been found that the optical band gap increases, but the refractive index, extinction coefficient, real and imaginary dielectric constant decrease with increase in Ag content. The optical band gap and refractive index show the variation in their values with increase in the annealing temperature. The extinction coefficient increases with increasing annealing temperature. The surface morphology of ultra-thin films has been determined using a scanning electron microscope (SEM). The measured dc conductivity, under a vacuum of 10^? 5 mbar, showed thermally activated conduction with single activation energy in the measured temperature range (288–358 K) and it followed Meyer–Neldel rule. The dc activation energy decreases with increase in Ag content in pristine and annealed films. The results have been analyzed on the bases of thermal annealing effects in the chalcogenide thin films.     

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.     

Single oscillator energy and dispersion energy of uniform thin chalcogenide films from Cu–As–S–Se system

This paper presents some of the results obtained by using the modified envelope method, which takes substrate absorption into account. Samples investigated in this paper are the series of amorphous thin chalcogenide uniform films from system Cu_x[As₂(S_0.5Se_0.5)₃]_100−x. Thin films were deposited under vacuum on glass substrates by thermal evaporation technique, from previously synthesized bulk samples. The dispersion of the refractive index is discussed in terms of the single oscillator model proposed by Wemple and DiDomenico. By using this model, i.e. by plotting (n² − 1)⁻¹ against (ℏω)² and fitting a straight line, oscillator parameters, E₀ – the single oscillator energy and E_d – the dispersion energy, were directly determined.     

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).     

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.     

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.     

The effect of Tb doped PbTiO3 inducing layer on texture and tunable property of sol–gel derived Pb0.4Sr0.6TiO3 thin films grown on ITO/glass substrate

Highly (1 0 0)-oriented Pb_0.4Sr_0.6TiO₃ (PST40) thin films have been prepared on the Tb doped PbTiO₃ (PTT) thin film coated ITO/glass substrate by sol–gel technique. The PTT inducing layers are (1 0 0)-oriented and can help to control the orientation of PST40 thin films. Crystallization of the PST40 thin film with the PTT inducing layer is more perfect than that without PTT layer due to less distortion in the thin film. The dielectric tunability of the PST40 thin film with PTT layer therefore reaches 65%, which is 85% higher than that without PTT layer. The dielectric loss of the PST thin film is only 0.05. These results indicate that (1 0 0)-oriented Tb doped PbTiO₃ can be used as an inducing layer for highly (1 0 0)-oriented tunable materials on ITO/glass substrate.     

Structural change of laser-irradiated Ge2Sb2Te5 films studied by electrical property measurement

Sheet resistance of laser-irradiated Ge₂Sb₂Te₅ thin films prepared by magnetron sputtering was measured by the four-point probe method. With increasing laser power the sheet resistance undergoes an abrupt drop from 10⁷ to 10³ Ω/□ at about 580 mW. The abrupt drop in resistance is due to the structural change from amorphous to crystalline state as revealed by X-ray diffraction (XRD) study of the samples around the abrupt change point. Crystallized dots were also formed in the amorphous Ge₂Sb₂Te₅ films by focused short pulse laser-irradiated, the resistivities at the crystallized dots and the non-crystallized area are 3.375 × 10^?3 and 2.725 Ω m, sheet resistance is 3.37 × 10⁴ and 2.725 × 10⁷ Ω/□ respectively, deduced from the I–V curves that is obtained by conductive atomic force microscope (C-AFM).     

Te-rich Ge–As–Se–Te bulk glasses and films for future IR-integrated optics

Ge₁₅As₁₅Se_70−xTe_x materials with x = 56, 60 and 63, of potential use in IR-integrated optics, were prepared by the classical melt-quenching method. A macroscopic phase separation was observed with a crystalline phase on the top and an amorphous one at the bottom. The glasses from the bottom were transparent from 1.9 to 16 μm without any purification of the elemental precursors Ge, As, Te and Se. The higher the Te/Se content in the glasses the lower their glass transition temperature and thermal stability. Films 7–12 μm thick of the above stated compositions were deposited by thermal evaporation. The higher the tellurium content, the larger the optical band gap shift of the films in the infra-red and the higher the refractive index.