Electrical and optical properties of In2Se3 thin films

In₂Se₃ thin films were grown with good stoichiometry at a substrate temperature around 460 K in the α phase and were shown to remain in the β phase above 480 K. The Hall coefficient and the d.c. conductivity of polycrystalline In₂Se₃ films grown on Pyrex and mica substrates were studied in the temperature range 77–530 K. After thermal treatment above 473 K of the α phase thin film, the electrical resistivity decreased and the sample remained in an irreversible phase. This is explained in terms of structural changes at high temperatures. The mobility behaviour of the β phase annealed thin films is illustrated. We use the Petritz barrier model to explain the activation energy of the mobility as due to the grain boundaries of the polycrystallites.The optical properties (refractive index and absorption coefficient) are also reported. The direct band gaps of In₂Se₃ thin films are 1.43 eV and 1.55 eV for the α phase and β phase respectively. These values are obtained from transmission measurements and are confirmed through photoconductivity measurements.     

4.6 Some optical properties of thin Cd3As2 films

The transmission of vacuum deposited Cd₃As₂ films has been measured in the spectral range 0.6–15 ωm. The absorption coefficient α has been calculated from the data for crystalline films obtained at substrate temperature T_s = 440 K and also for amorphous films obtained on substrates at T_s = 300?393 K. The position of the low-energy absorption edge in the spectral range 0.1–0.3 eV is discussed and the value Δ_g^opt = 0.06 eV for indirect transion Γ → Γ₁₅ is obtained for crystalline films. For amorphous films, a shift of the minimum absorption edge to $\text{Δ}\text{E}{}_{\mathrm{g}}{}^{\mathrm{opt}}\text{? 0.4?0.5}\text{eV}$ was observed. For higher energies two direct transitions have been established: the first at E₁ = 0.9 eV in the photon energy range 0.9–1.2 eV.and the second at E₂ = 1.2?1.3 eV in the photon energy range 1.3?1.6 eV. The last value is in good agreement with the value calculated by Lin Chung and with reflectivity data; it corresponds to the transition Γ₁₅ → X₁ in the hypothetical band model of Lin Chung. Differences in the reflective index values for crystalline and amorphous Cd₃ As₂ are also discussed.     

Photo-induced changes in optical properties of As2S3 and As2Se3 films deposited at normal and oblique incidence

Photostructural transformations in amorphous chalcogenide films have been a subject of intensive research so far. In this paper we discuss the changes in the optical properties of typical As-based chalcogenide glasses (As₂S₃ and As₂Se₃) on exposure to ultraviolet (UV) light. An attempt has been made to systematically investigate the optical parameters like extinction coefficient, refractive index and optical bandgap of the films by measuring the same for as-grown and UV-exposed amorphous films of As₂S₃ and As₂Se₃ prepared by vacuum evaporation technique.     

Photothermal measurements on optical thin films

An overview of the application of the photothermal technique for optical as well as thermophysical characterizations of thin films is given. The peculiarities of this technique are discussed in some detail, and selected important results are pointed out. Emphasis is placed on the influence of both residual absorption and randomly distributed inhomogeneities in thin films on their laser-damage resistance.     

Structural and optical properties of CdGa2Se4 thin films

CdGa₂Se₄ thin films were prepared by vapour deposition onto either room temperature or preheated quartz and glass substrates (T _S) or they were deposited at room temperature and then annealed at about (T _A) 623 K. The films thus prepared were crystalline with a thiogallate tetragonal structure. The optical constants (the refractive index n and the absorption index, k) were determined for CdGa₂Se₄ films deposited onto quartz substrates held at either room temperature or at T _S = 573 K. These constants were also determined for preannealed films (T _A = 623 K). Plots of (hv)² = f(hv) and (hv)^1/2 = g(hv) were linear, indicating the existence of both direct and indirect optical transitions. It was found that the values of E _g ^d and E _g ^ind for as-deposited CdGa₂Se₄ films were 2.46 and 1.91 eV, respectively. The corresponding values for the annealed films and the films deposited at T _S = 573 K were 2.56 and 2.06 eV, respectively.     

Characterization of p-In2Se3 thin films

Indium selenide thin films were deposited onto glass substrates kept at 150 °C by thermal evaporation of -In₂Se₃. Some of the films were annealed at 150 °C and 200 °C and they all were found to exhibit p-type conductivity without intentional doping. Scanning electron microscopy (SEM) established that the films have an atomic content of In₅₁Se₄₉. X-ray diffraction (XRD) indicated that the as-grown films were amorphous in nature and became polycrystalline -In₂Se₃ films after annealing. The analysis of conductivity temperature-dependence measurements in the range 320–100 K revealed that thermal excitation and thermionic emission of the carriers are the predominant conduction mechanisms above 200 K in the amorphous and polycrystalline samples, respectively. The carrier transport below 200 K is due to variable range hopping in all the samples. Hall measurements revealed that the mobility of the polycrystalline films is limited by the scattering of the charged carriers through the grain boundaries above 200 K. © 2001 Kluwer Academic Publishers     

Electrical breakdown in amorphous As2S3 films

We investigated the electrical breakdown under d.c. electric fields of amorphous As₂S₃ films about 100–1000 Å thick sandwiched between two aluminium electrodes. It was found that the breakdown field decreased with increases in either the temperature or the thickness of the sample. Attempts were made to fit the observed electrical breakdown voltages with several previously proposed theories of thermal and electrical breakdown. It was found that a satisfactory fit could only be obtained by a modification of Klein's theory provided that the current was observed to depend on the voltage in an exponential manner in accordance with the Poole mechanism in which the carriers are released from high density localized states within the forbidden energy region. According to this modification, the breakdown field should depend inversely on the temperature and this agrees well with the experiment. The formation of localized conducting channels within the sample because of breakdown was verified by taking a scanning electron micrograph of the junction area of the broken film.     

Heat treatment effect on the structural and optical properties of Ga2Se3-Ga2S3 thin films

The structural properties of the 50 mol% Ga₂Se₃-50 mol% Ga₂S₃ system in thin-film form were studied using an X-ray diffraction technique. As-deposited films had an amorphous nature, whereas films heat treated for 2 h at 400 °C in a vacuum of 10^–2 Pa had a single-phase (cubic) polycrystalline nature with lattice constanta=0.532 nm. The optical properties of Ga₂Se₃-Ga₂S₃ thin films as-deposited and as-heat treated were also studied. It was found that heat treatment strongly affects the optical constants as well as the energy gap, which can be attributed to compositional as well as structural changes.     

The optical absorption edge in amorphous thin films of MoO3-In2O3

A study of the effects of changes in composition, film thickness, substrate deposition temperature and annealing on the optical properties of MoO₃-In₂O₃ is presented. The results are found to be compatible with the reduction in the value of optical energy gap of these materials as the molar fraction of In₂O₃ in the MoO₃ thin film increases. This decrease of optical gap may be attributed to the incorporation of In(III) ions in an MoO₃ lattice. The decrease in optical band gap with increasing thickness may be interpreted in terms of the incorporation of oxygen vacancies which are also believed to be the source of conduction electrons in the MoO₃-In₂O₃ complex. The decrease of band gap with increasing substrate temperature may be attributed to the enhanced ordering of the samples and the decrease of band gap with annealing may be attributed to a reduction in the concentration of lattice imperfections.     

Optical properties of Sb2Se3 thin films

The optical constants (the refractive index n and the absorption index k) of Sb₂Se₃ thin films deposited at room temperature on quartz have been calculated in the wavelength range (5000–2000 nm) using a transmission spectrum. Both n and k were found to be practically independent on either time, up to 6 months, or the film thickness in the range of 102–760 nm. Beyond the absorption edge, the absorption is due to allowed indirect and direct transitions with energy gaps of 1.225 and 1.91 eV, respectively. The value of the optical gap depends on the annealing temperature. X-ray analysis showed that the prepared films at room temperature had amorphous structure while the films annealed at 200°C for 1 h were verified to be crystalline.     

Noise measurements in electron-beam-evaporated amorphous silicon thin films

The low frequency noise of vacuum-evaporated amorphous silicon films, both intrinsic and prepared under a partial pressure of hydrogen, was measured over a range of temperatures. The current noise spectral density shows a predominance of 1/?^α noise at room temperature, where α lies between 0.8 and 1.04, and the conduction proceeds primarily by the hopping of carriers near the Fermi energy level. As the temperature is raised towards the region of extended states conduction the noise spectrum changes to more of a generation-recombination type with α having values from 1.66 to 2.04 at the highest temperatures. These results imply that no extremely large, very narrow band of gap states was formed under our fabrication conditions and they therefore agree with earlier work on the mixed conduction process and with field effect data.     

The optical properties of CuGa0.5In0.5Se2 thin films

CuGa_0.5In_0.5Se₂ thin films with thickness in the range 50 to 280 nm were deposited by thermal evaporation of prereacted material on glass substrates. The films were found to be polycrystalline with single phase having chalcopyrite structure as that of bulk material. The optical constants of these films were determined by transmittance and reflectance measurements at normal incidence for light in the wavelength range 400 to 1200 nm. Three characteristic energy gaps of 1.30, 1.55 and 2.46 eV were obtained from an analysis of the optical absorption spectrum. The optical constants of the films appear to be independent of the substrate temperature.     

Structural and optical properties of CuIn0.35Al0.65Se2 thin films

CuIn_0.35Al_0.65Se₂ thin films were deposited onto glass substrates by flash evaporation method. The films deposited at substrate temperatures 613–633 K were nearly stoichiometric, polycrystalline and single phase showing chalcopyrite structure with lattice parameters a = 0.569 nm and c = 1.124 nm. The optical absorption studies revealed a threefold optical structure near the fundamental edge with E _g1 = 1.75 eV, E _g2 = 1.81 eV and E _g3 = 1.99 eV. The crystal-field (Δ_CF) and spin–orbit (Δ_SO) splitting parameters were evaluated.     

Electrical transport in amorphous Se1−xSbx thin films

Amorphous Se_1–x Sb _x (x=0.1, 0.15 and 0.2) films of different thicknesses were prepared by the electron gun evaporation technique. Their structure was studied using reflection or transmission electron diffraction patterns. The d.c. electrical conductivity, thermoelectric power and magnetoresistance measurements, were made in the temperature range 80–300 K. These measurements confirm intrinsic conduction in the entire range of investigation, and were found to be independent of the film thickness. The effect of antimony impurity on electrical transport properties of selenium is understood in terms of the reduction of the Se₈ ring population.     

Electrical and optical properties of (Sb2Se3)2 (Sb2Te3)1 thin films

A study of the synthesized (Sb₂Se₃)₂ (Sb₂Te₃)₁ glassy system has been carried out, X-ray diffraction (XRD) patterns and differential thermal analysis (DTA) of the system studied were used to obtain an insight into the structural information. An investigation of the electrical and optical properties of (Sb₂Se₃)₂ (Sb₂Te₃)₁ thin films prepared by thermal evaporation having different thicknesses (89.2, 214, 223 nm) and annealing temperatures ranging from 300 to 473 K has been carried out. The effect of the thickness and heat treatment on the activation energy E for d.c. conductivity and the density of localized states at the Fermi level N(E_F) were carried out. The electrical conductivity measurements depend on the thickness and annealing temperature, and exhibit two types of conduction mechanisms. Optical absorption measurements have been made on as-deposited and annealed films for the investigated system. The optical transition was found to be indirect. The optical energy gap (E_opt) decreases with increasing thickness and annealing temperatures (below T_g). The corresponding band is approximately twice the conduction activation energy. This effect is interpreted in terms of the density of states model proposed by Mott and Davis. © 2002 Kluwer Academic Publisher     

Photo-induced optical changes in amorphous AsS films

Photo-induced optical changes were investigated in thin film structures of As_xS_100?x with x between 10 and 43. For As₄₀S₆₀ the magnitude of the change in optical path length at 6328 Å for the reversible photodarkening process is about one-third of that for the irreversible one. The relaxation time of the former is approximately twice that of the latter.In specimens with excess chalcogen, the influence of annealing becomes weaker with increase in sulphur content. The change in optical path length for the virgin films has a minimum at a composition of about As₂₅S₇₅. These characteristics suggest that sulphur plays an active role in the photo-induced transformation.