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.     

On the optical properties of bismuth oxide thin films prepared by pulsed laser deposition

The optical properties of bismuth oxide films prepared by pulsed laser deposition (PLD), absorption in the photon energy range 2.50-4.30 eV and optical functions (n, k, ?₁, and ?₂) in the domain 3.20-6.50 eV, have been investigated. As-prepared films (d=0.05-1.50 μm) are characterized by a mixture of polycrystalline and amorphous phases. The fundamental absorption edge is described by direct optical band-to-band transitions with energies 2.90 and 3.83 eV. The dispersion of the optical functions provided values of 4.40-6.25 eV for electron energies of respective direct transitions. In the spectral range 400-1000 nm, bismuth oxide films show a normal dispersion, which can be interpreted in the frame of a single oscillator model.     

Optical spectra of Zn1-xBexTe mixed crystals determined by IR-VIS-UV ellipsometry and photoluminescence measurements

Spectroscopic ellipsometry in the photon energy range from 0.04 eV to 6.50 eV is used for investigation of the optical response of Zn_1-xBe_xTe crystals grown by a high-pressure Bridgman method in the composition range x ≤ 0.12. Infrared spectra display absorption bands centred between 411 cm⁻¹ and 420 cm⁻¹ associated with BeTe-type optical phonon modes. The positions of the transverse-optical and longitudinal-optical phonon modes have been found by modelling the line shape of the complex dielectric functions, ε˜ and Im(−ε˜⁻¹), using a classical damped Lorentzian oscillator approach. Ellipsometric measurements in the VIS-UV range allow determination of the fundamental energy-gap (E₀) and the higher threshold energies (E₁, E₁ + Δ₁, E₂) originating from the band edge and spin-orbit splitting critical points. We have found that the Be content x = 0.12 causes an increase of the fundamental energy gap about 0.15 eV at room temperature when compared to the E₀ = 2.23 eV of ZnTe crystal at the same temperature. Photoluminescence spectra were measured in the temperature range from 30 K to room temperature. Luminescence at temperature T > 200 K is very weak. The peak positions of the exciton emission lines agree well with the E₀ band-gaps derived from ellipsometric data if corrected for their temperature dependence.     

Properties of electron-beam-evaporated tin oxide films

Transparent conducting thin films of tin oxide were prepared by electron beam evaporation of sintered pellets of SnO₂ under controlled conditions. Variations in such parameters as the substrate temperature T_s and the post-deposition annealing temperature T_A and time t_A were studied. Structural, electrical and optical properties were measured to characterize the films. The film structure changed gradually from amorphous to crystalline (SnO phase) as T_S was varied from 150 to 350 °C. A sharp decrease in the room temperature resistivity together with the growth of a crystalline phase occured in the as-deposited films at T_S ≈ 250°C. On annealing in air (T_A = 550°C, t_A = 2 h) a radical structural transformation from amorphous to crystalline occurred with a sharp fall in resistivity for T_S ⩽ 225°C. For T_S = 350°C the lowest resistivity achieved for the undoped annealed films was 6.6×10^-3 Ω cm, the average visible transmittance was about 90% and the structure was characteristic of pure SnO₂.     

Structural analysis and luminescent study of thin film zinc germanate doped with manganese

Thin films of zinc germanate doped with manganese (Zn₂GeO₄:Mn) were fabricated by radio frequency magnetron sputtering, and their structural characteristics and luminescent properties were studied. The Zn₂GeO₄:Mn films exhibited a pronounced absorption edge at around 271 nm and a high optical transparency in the visible wavelength region with a peak transmittance of 0.927 at 691 nm. While the as-deposited Zn₂GeO₄:Mn films had an amorphous structure, the annealed films possessed a rhombohedral polycrystalline structure with a random crystallographic orientation of grains. The broad-band photoluminescence (PL) emission was observed from the annealed Zn₂GeO₄:Mn films. The PL emission spectrum showed a peak maximum at around 537 nm in the green range, which was accounted for by the intrashell transition of 3d⁵ orbital electrons from the ⁴T₁ lowest excitation state to the ⁶A₁ ground state in the divalent manganese ions. Two discrete peaks were observed in the PL excitation spectrum at 256 and 296 nm, which are considered to be associated with the band-to-band absorption of the host and the sub-band absorption from defect states, respectively. The green cathodoluminescence (CL) emission was obtained from the annealed Zn₂GeO₄:Mn films with a peak centered at around 534 nm, analogous to the PL emission spectrum.     

Optical dispersion of homogeneously mixed ZnSMgF2 films

Mixed films of ZnSMgF₂ were prepared by co-evaporation from a single source. The dispersion of the optical constants was obtained from the reflectance and transmittance data of the films for various compositions in the spectral range 2000–7000 Å. The values of the optical constants of the mixed films at any wavelength lie between the values of the optical constants of pure ZnS and MgF₂ films. The refractive index of the mixed films closely obeys the Lorentz-Lorenz relation at wavelengths at which the absorption is small. The fundamental absorption edge of the mixed films shifts from that of MgF₂ to that of ZnS continuously but non-linearly on increasing the concentration of ZnS in MgF₂. Thus it is possible to use these films as variable frequency cut-off filters. The fundamental absorption edge of the mixed films follows the (hv-E_g)² relation, indicating an indirect optical transition, in sharp contrast to the direct optical transition observed in bulk ZnS.     

Electrical and optical properties of reactive sputtered TiO x thin films for uncooled IR detector applications

Nanostructured titanium oxide (nano-TiO _x ) thin films for uncooled IR detectors were fabricated by dc reactive magnetron sputtering and post-deposition annealed in oxygen atmosphere. The crystalline structure and surface morphology were characterized by glancing incidence X-ray diffraction (GIXRD) and field emission scanning microscopy. The results of GIXRD measurements indicate that TiO _x thin film deposited at room temperature is amorphous. A mixture of anatase and rutile nanocrystalline structure phase were present in oxygen annealed TiO _x thin film. A weak absorption peak around 438 cm^?1 corresponding to Ti–O stretching vibration is observed by Fourier transform infrared spectroscopy with annealed TiO _x thin film. The X-ray photoelectron spectra reveals Ti³⁺ and Ti⁴⁺ ions are coexisting in TiO _x films. The optical spectra of the films indicate that the optical absorption edge of the nano-TiO _x film exhibits a red shift compared to the as-deposited film. Furthermore, compared to bulk TiO _x , a blue shift was observed in both of the deposited and annealed films due to quantum size effect. The dependence of resistivity on temperature reveals both the absolute value of temperature coefficient of resistivity (TCR) and activation energy of TiO _x thin film increase significantly after annealing in oxygen.     

Annealing dependence of optical properties of Ga20S75Sb5 and Ga20S40Sb40 thin films

Amorphous Ga₂₀S₇₅Sb₅ and Ga₂₀S₄₀Sb₄₀ thin films were prepared onto glass substrates by using thermal evaporation method. The effect of annealing (under vacuum) at different temperatures on the optical parameters was investigated in the temperature range 373-593 K. The optical absorption coefficient (α) for the as-deposited and annealed films were calculated from the reflectance and transmittance measurements in the range 190-900 nm. X-Ray diffraction indicates that the as-deposited films and those annealed up to the glass transition temperature (T_g) exhibit amorphous state. On annealing above the glass transition temperature these films show a polycrystalline structure. Analysis of the optical absorption data indicates that the optical band gap E_g^opt of these films obeys Tauc's relation for the allowed non-direct transition. It was found that the optical band gap E_g^opt increases with annealing temperature up to T_g, whereas above T_g there is a remarkable decrease. The obtained results were interpreted on the basis of amorphous- crystalline transformation.     

Dispersive optical constants of thermally deposited AgIn5S8 thin films

AgIn₅S₈ thin films were obtained by the thermal evaporation of AgIn₅S₈ crystals onto ultrasonically cleaned glass substrates. The films are found to exhibit polycrystalline cubic structure. The calculated lattice parameter of the unit cell (a) is 10.78 Å. The transmittance data of the as grown films which was recorded at 300 K in the incidence wavelength (λ) range of 320-1000 nm are used to calculate the refractive, n(λ). The transmittance and reflectance data are also used to calculate the absorption coefficient of the as grown AgIn₅S₈ thin films. The fundamental absorption edge is found to be corresponding to a direct allowed transitions energy band gap. This band-to-band transition energy is found to be 1.78 eV and it is consistent with that reported for AgIn₅S₈ single crystals.     

Temperature dependent optical properties of UV emitting γ-CuCl thin films

In this work, ultra violet emitting properties of RF sputtered CuCl thin films were studied as a function of temperature in the region of 15-300 K. Deposition parameters were optimised to obtain good optical quality films exhibiting an intense and sharp Z₃ free exciton emission (at 383 nm) with an FWHM of ∼ 74 meV at room temperature. At lower temperatures, there are two other emissions observed at 375 and 390 nm, and they are identified as Z₁₂ and I₁ exciton lines, respectively. The free exciton emission is extremely stable at room temperature and it was found to be less influenced by the exciton-phonon interaction compared to the bound exciton emission which dissociates after 40 K. The Z₃ free exciton line exhibits a slight blue-shift on increasing the temperature. The temperature dependence of exciton line shift, line broadening and emission intensity are discussed in detail.     

Structural and optical properties of Zn3P2 thin films

Conditions have been developed for the deposition of a stoichiometric thin film of zinc phosphide (Zn₃P₂) using electron beam evaporation. Structural properties of as-deposited and annealed thin films of zinc phosphide have been studied using electron and X-ray diffraction. The as-deposited film is non-crystalline, structural ordering starts on annealing at 200° C and the film becomes crystalline at 300° C with the structure matching that of the bulk material. Optical absorption has been investigated over the range 1 to 3 eV with emphasis on the region of interband absorption. The thin film absorption edge is found to be exponential for lower values of absorption coefficient. Analysis of thin film data showed that Zn₃P₂ is a direct-band-gap material. On annealing there is a shift in the band edge towards higher energy. NCL Communication No. 3571.     

Optical and photoelectric properties of monoclinic Zn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>2</Subscript> crystals

The first data are presented on the band structure of monoclinic Zn_{1 ? x} Cd _x As₂ crystals. The fundamental absorption edge of Zn_{1 ? x} Cd _x As₂ for the E ‖ c polarization is shown to be dominated by an indirect allowed transition for absorption coefficients α^‖ < 6 cm^?1 and by a direct allowed transition for α^‖ > 6 cm^?1, both transitions involving excitonic levels. The absorption edge for E ⊥ c is due to a direct forbidden transition with the participation of excitonic levels. We have determined the band gap values for these transitions in the temperature range 80–300 K, the exciton binding energy, and the ionization energies of four deep acceptor levels produced in the band gap by structural defects.     

The optical properties of CuInS2 thin films

The optical absorption in flash-evaporated CuInS₂ thin films was studied in the photon energy range from 0.5 to about 4.2 eV. CuInS₂ was found to be a direct gap semiconductor with a gap energy of 1.524±0.005 eV at room temperature. The ground state energy of the free exciton was found to be about 8 meV. An indirect allowed transition was observed at 1.565±0.005 eV and was ascribed to an optical transition from the valence band maxima at the boundary of the Brillouin zone to the lowest conduction band minimum at the zone centre. Three further optical transitions which were probably due to the copper d states in the valence band were found at energies well above the fundamental edge.     

Temperature-dependent integral exciton absorption in semiconducting InP crystals

The temperature dependence of the fundamental absorption edge in free-standing “epitaxial” InP layers has been experimentally studied. The integral exciton absorption coefficient K(T) exhibits an increase at low temperatures, which is explained in terms of the exciton-polariton mechanism of light transfer in semi-conductor crystals with spatial dispersion. A critical temperature (T _c = 200 K), above which the integral absorption becomes constant, has been experimentally determined, and the corresponding critical decay parameter (Γ_c = 0.341 meV), longitudinal-transverse splitting (ħω_LT = 0.175 meV), and oscillator strength of the exciton transition (β = 0.237 × 10⁻⁴) have been calculated. The temperature dependence of the true dissipative decay has been determined.     

Some optical properties of Se-Ge-As amorphous chalcogenide glasses

The effects of composition, film thickness, substrate temperature, and annealing of amorphous thin films of Se₇₅Ge_25−x As _x (5⩽x⩽20) on their optical properties have been investigated. X-ray diffraction revealed the formation of amorphous films. The absorbance and transmission of vacuum-evaporated thin films were used to determine the band gap and refractive index. Optical absorption measurements showed that the fundamental absorption edge is a function of glass composition and the optical absorption is due to indirect transition. The energy gap increases linearly with increasing arsenic content. The optical band gap,E _opt, was found to be almost thickness independent. The shapes of the absorption edge of annealed samples displayed roughly the same characteristic as those of the unannealed films, but were shifted towards shorter wavelengths; as a result,E _opt increased andE _e, the width of the band tails, decreases. The increase inE _opt is believed to be associated with void removal and microstructural re-arrangement during annealing. The influence of substrate temperature on the optical parameters is discussed.     

Iodization of antimony thin films: XRD, SEM and optical studies of nanostructured SbI3

Antimony films of thicknesses of 25, 40 and 80 nm deposited using thermal evaporation technique were annealed at 200 °C for 6 h. Programmed iodization was carried out at room temperature for periods ranging from 5 min to 9 h on both as-deposited and annealed films. X-ray diffraction studies on iodized films reveal that antimony tri-iodide nanoparticles grow only on annealed antimony films. Surface morphology as revealed through SEM consists of antimony and antimony tri-iodide nanoparticles are 25 nm and 1 μm, respectively. Optical absorption of Sb and SbI₃ nanoparticles carried out at room temperature. As-deposited antimony film of thickness 25 nm exhibits a sharp rise in the absorption near ultraviolet region while post-deposition annealed films were characterized by red shifted absorption. Interestingly 45 nm thick Sb films exhibit a broad volume plasmon resonance peak around 500 nm with a width of 200 nm. Progressive iodization of 25 nm thick film reveals two absorption bands at 381.7 nm (A₂) and 458.3 nm (B₃) with photon energies 3.25 and 2.70 eV, respectively, due to the development of SbI₃ valence band structure. Last members of hydrogen-like series of absorption levels A₂ and B₃ due to halogen doublet (3/2, 1/2) splitting have been observed at room temperature.     

Influence of the self-buffer layer on ZnO film grown by atmospheric metal organic chemical vapor deposition

ZnO films with and without a self-buffer layer were grown on c-plane sapphire substrates by atmospheric metal organic chemical vapor deposition. The influence of the buffer layer thickness, annealing temperature and annealing time on ZnO films has been investigated. The full width at half maximum of the ω-rocking curve of the optimized self-buffer layer sample is only 395 arc sec. Its surface is composed of regular columnar hexagons. After the buffer layer was introduced, the A₁ longitudinal mode peak at 576 cm^− 1, related to the defects, disappears in Raman spectra. For the photoluminescence, besides the strong donor binding exciton peak at 3.3564 eV, an ionized donor binding exciton and a free exciton peak is respectively observed at 3.3673 and 3.3756 eV at the high-energy side in the spectrum of the sample with the buffer layer.     

Analysis of the shape of spectral dependence of absorption coefficient and stationary photoconductivity spectral response in nanocrystalline bismuth(III) sulfide thin films

Some aspects regarding the optical and photoelectrical properties of bismuth(III) sulfide nanocrystals deposited in thin film form were studied. The influence of electrostatic interaction between photogenerated charge carriers on the shape of the spectral dependence of absorption coefficient for as-deposited and thermally treated nanocrystalline Bi₂S₃ thin films was investigated. The experimentally obtained spectral dependencies of the optical absorption coefficient were analyzed using the models of Elliott and Toyozawa. It was shown that the sub-band gap optical absorption of the studied films is of exponential type. A modified Urbach rule function was employed to calculate the relevant parameters which determine the value of critical energy at which a transition between the two main absorption regimes occurs in studied semiconductor. Experimentally obtained changes of Urbach energies (i.e. tailing parameters) upon thermal treatment of the films were explained in terms of the differences in the degree of structural disorder of the as-deposited and annealed films according to the model of Cody. The experimental optical absorption data were also used to model the photoconductivity spectral response of thermally annealed films using various approaches.     

Annealing behaviors of quench-deposited (CsCl)1−x(PbCl2)x films studied by optical absorption spectroscopy

All the films of the (CsCl)_1−x(PbCl₂)_x system prepared by quench deposition exhibit a similar fundamental absorption spectrum over the full mixing range. On annealing the films, drastic changes occur in the spectra showing three characteristic types of spectral features depending on the mixing ratio. Type 1 (x≥0.7) suggests the formation of mixed crystals of a PbCl₂ base. Type 2 (0.5≥x≥0.3) exhibits the spectrum of CsPbCl₃ crystal. Type 3 (x=0.1, 0.05), the most characteristic, is the spectrum of Cs₄PbCl₆ crystal, which was observed for the first time. The spectrum of the Cs₄PbCl₆ crystal exhibits novel structures. Despite the crystalline entity of the compound, it shows oscillator-like fundamental absorption and a wide (≈1 eV) window just above the first absorption peak, suggesting the peculiarity of the associated energy band structure. To a first approximation, the spectrum is explainable in terms of Pb²⁺-ion excitation of the Pb²⁺(Cl⁻)₆ quasi-complexes.     

The effect of composition and substrate temperature on the optical energy gap of SiOx/SnO amorphous thin films

The fundamental absorption edge of co-evaporated SiO _x /SnO thin films of various compositions has been investigated. The results have been analysed by assuming optical absorption by non-direct electronic transitions. The optical energy gap is seen to decrease with increase in the molar content of SnO which increases the level of disorder of the complex system. The effect of substrate temperature during deposition on the optical absorption is also studied. The optical energy gap increases with the substrate temperature. This is attributed to the annealing of some of the dangling bonds and hence to an increase in the order of the system. The Urbach tail is related to the broadening of the absorption edge as a result of charged defects present in the films and related to the Franz-Keldysh effect.