Memory switching of ZnGa2Te4 thin films

Electrical and switching property of amorphous defect chalcopyrite ZnGa₂Te₄ thin films prepared by thermal evaporation technique has been studied. The elemental chemical compositions of the prepared bulk as well as the as-deposited film were determined by means of energy dispersive X-ray spectrometry. X-ray diffraction pattern revealed that the powder compound is polycrystalline and the as-deposited and the annealed films at t _a ≤ 548 K have the amorphous phase, while that the annealed at t ≥ 573 K are polycrystalline with a single phase of a defect chalcopyrite structure similar to that of the synthesized material. The great advantage of this material is the capability to appear in two different phases, the amorphous and the crystalline phases, with rather different electrical properties. Both dynamic and static I–V characteristics and the switching phenomenon at 601 nm are investigated. The threshold switching mechanism was explained by a thermal model of switching, i.e., joule heating with an electrically conducting channel. ZnGa₂Te₄ is good candidate in phase change memory device.     

Electrodeposition of As2Se3 thin films

Semiconducting As₂Se₃ thin films have been prepared from an aqueous bath at room temperature onto stainless steel and fluorine-doped tin oxide (F.T.O.)-coated glass substrates using an electrodeposition technique. It has been found that As₂O₃ and SeO₂ in the volumetric proportion as 4:6 and their equimolar solutions of 0.075 M concentration forms good quality films of As₂Se₃. The films are annealed in a nitrogen atmosphere at temperature of 200 °C for 2 h. The films are characterised by scanning electron microscopy, X-ray diffraction and optical absorption techniques. Studies reveal that asdeposited and annealed thin films are polycrystalline in nature. The optical band gap has been found to be 2.15 eV for the above-mentioned composition and concentration of the film.     

Characterization of phase transition in silver photo-diffused Ge2Sb2Te5 thin films

Surface activity of thermally evaporated amorphous chalcogenide films of Ge₂Sb₂Te₅ has been investigated. Silver (Ag) is readily deposited on such films from appropriate aqueous ionic solution and Ag diffuses into the films upon irradiation with energetic photons. The composition of Ge₂Sb₂Te₅ thin films and the amount of Ag photo-diffused has been gathered from electron probe micro-analyzer having a wavelength dispersive spectrometer. The composition of the films was found to be very close to the bulk used to deposit films and the amount of Ag photo-diffused was ∼ 0.38 at. %. X-ray diffraction and temperature dependent sheet resistance studies have been used for the structural analysis of the bulk alloy, as-deposited, Ag photo-diffused and annealed films at different temperatures. The films remain amorphous after Ag photo-diffusion into the amorphous Ge₂Sb₂Te₅ films. The reflectivity, reflectivity contrast and extinction coefficient of the crystalline and amorphous photo-diffused thin films are presented. The optical band gaps of the amorphous and crystalline photo-diffused (Ge₂Sb₂Te₅)_100−xAg_x=0.38 phase change thin films have also been calculated from absorption data using UV-VIS spectroscopy.     

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.     

Structural, electrical and optical properties of Bi2Se3 and Bi2Se(3−x)Tex thin films

Thin films of Bi₂Se₃, Bi₂Se_2.9Te_0.1, Bi₂Se_2.7Te_0.3 and Bi₂Se_2.6Te_0.4 are prepared by compound evaporation. Micro structural, optical and electrical measurements are carried out on these films. X-ray diffraction pattern indicates that the as-prepared films are polycrystalline in nature with exact matching of standard pattern. The composition and morphology are determined using energy dispersive X-ray analysis and scanning electron microscopy (SEM). The optical band gap, which is direct allowed, is 0.67 eV for Bi₂Se₃ thin films and the activation energy is 53 meV. Tellurium doped thin films also show strong optical absorption corresponding to a band gap of 0.70-0.78 eV. Absolute value of electrical conductivity in the case of tellurium doped thin film shows a decreasing trend with respect to parent structure.     

Structural, electrical and optical studies of SILAR deposited cadmium oxide thin films: Annealing effect

Successive ionic layer adsorption and reaction (SILAR) method has been successfully employed for the deposition of cadmium oxide (CdO) thin films. The films were annealed at 623 K for 2 h in an air and changes in the structural, electrical and optical properties were studied. From the X-ray diffraction patterns, it was found that after annealing, H₂O vapors from as-deposited Cd(O₂)_0.88(OH)_0.24 were removed and pure cubic cadmium oxide was obtained. The as-deposited film consists of nanocrystalline grains of average diameter about 20-30 nm with uniform coverage of the substrate surface, whereas for the annealed film randomly oriented morphology with slight increase in the crystallite size has been observed. The electrical resistivity showed the semiconducting nature with room temperature electrical resistivity decreased from 10⁻² to 10⁻³ Ω cm after annealing. The decrease in the band gap energy from 3.3 to 2.7 eV was observed after the annealing.     

Oxidative annealing of ZnSe/GaAs heterostructures

We annealed ZnSe/GaAs heterostructures in the oxygen atmosphere and investigated structural and optical properties of the forming films using X-ray diffraction and photoluminescence. While highly textured ZnO films were synthesized via low-temperature processing (~ 500 °C), high temperature processing (~ 800 °C) promoted reaction at the film/substrate interface and Zn loss from the film surface resulting in the polycrystalline ZnGa₂O₄ and ZnO₂.     

Crystal structure and optical absorption investigations on β-In2S3 thin films

The crystal structure of annealed β-In₂S₃ thin films with different thickness was investigated by X-ray diffraction technique. Lattice parameters, crystallite size and microstrain were calculated. It was found that the lattice parameters are independent on film thickness, while annealing temperatures increase them. Crystallite sizes were increased with the increase of the film thickness and improved by annealing temperatures. In all cases, the microstrains were decreased gradually with the increase in both film thickness and annealing temperatures. Optical properties of β-In₂S₃ thin films were performed in the spectral range from 400 to 2500 nm to determine the optical constants (n and k), the high frequency dielectric constant, ε_∞, the lattice dielectric constant, ε_L, and the energy gap. The optical constants were found to be independent on film thickness in the range from 200 to 630 nm. The high frequency dielectric and lattice dielectric constants of the as-deposited film increased by annealing temperatures. The energy gap for the as-deposited In₂S₃ was found to be 2.60 eV and increased to 2.70 and 2.75 eV by annealing at 423 and 473 K for 1 h, respectively.     

Spray pyrolyzed β-In2S3 thin films: Effect of postdeposition annealing

Indium sulfide thin films prepared using spray pyrolysis, with In/S ratio 2/3 in the solution, were annealed in vacuum at 300 and 400 °C. The effect of this treatment on properties of the films was studied using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, optical absorption, transmission and electrical measurements. Optical constants of the films were calculated using the envelope method. Annealing did not affect the optical properties of the film much, but the resistivity of the films showed a drastic decrease and the grain size increased. In₂S₃ thin films have potential use as buffer layer in photovoltaic heterojunction devices.     

Chemical bath deposition of MoS2 thin film using ammonium tetrathiomolybdate as a single source for molybdenum and sulphur

Molybdenum disulphide, MoS₂, thin films have been deposited by chemical bath deposition method on glass and quartz substrate using ammonium tetrathiomolybdate as a single source precursor for Mo and S and subjected to vacuum heat treatment at different temperatures. X-ray diffraction of as-deposited film indicated its amorphous character and showed the development of poorly crystalline MoS₂ thin film on increasing annealing temperature. The film has been characterized by energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, scanning electron micrograph and the optical properties also have been studied.     

Fabrication and characterization of n-CdSe0.7Te0.3/p-CdSe0.15Te0.85 solar cell

Hot wall deposited CdSe_xTe_1−x where 0 ≤ x ≤ 1 thin films for solar cell applications have been prepared from a compound synthesized by direct reaction of high purity Cd, Se and Te elements. Crystal structure and composition of the films were analyzed by X-ray diffraction, scanning electron microscope and EDAX. X-ray diffraction studies carried out on pseudo-binary system revealed that the films are polycrystalline in nature with CdSe_0.7Te_0.3 film exhibiting hexagonal structure and CdSe_0.15Te_0.85 film exhibiting cubic zinc blende structure. The type of conduction was determined by Hall studies. A novel solar cell with structure n-CdSe_0.7Te_0.3/p-CdSe_0.15Te_0.85 has been fabricated and the efficiency was found to be 3.13%.     

La-substitution Bi2Ti2O7 thin films grown by chemical solution deposition

(La_0.05Bi_0.95)₂Ti₂O₇ (LBTO) thin films had been successfully prepared on P-type Si substrate by chemical solution deposition method. The structural properties of the films were studied by X-ray diffraction. The phase of (La_0.05Bi_0.95)₂Ti₂O₇ is more stable than the phase of Bi₂Ti₂O₇ without La substitution. The films exhibited good insulating properties with room temperature resistivities in the range of 10¹²-10¹³ Ω cm. The dielectric constant of the film annealed at 550 °C at 100 kHz was 157 and the dissipation factor was 0.076. The LBTO thin films can be used as storage capacitors in DRAM.     

Preparation and characterization of amorphous manganese sulfide thin films by SILAR method

Manganese sulfide thin films were deposited by a simple and inexpensive successive ionic layer adsorption and reaction (SILAR) method using manganese acetate as a manganese and sodium sulfide as sulfide ion sources, respectively. Manganese sulfide films were characterized for their structural, surface morphological and optical properties by means of X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and optical absorption measurement techniques. The as-deposited film on glass substrate was amorphous. The optical band gap of the film was found to be thickness dependent. As thickness increases optical band gap was found to be increase. The water angle contact was found to be 34°, suggesting hydrophilic nature of manganese sulfide thin films. The presence of Mn and S in thin film was confirmed by energy dispersive X-ray analysis.     

Influence of thermal annealing on microstructural,morphological, optical properties and surface electronic structure of copper oxide thin films

In this study, effect of the post-deposition thermal annealing on copper oxide thin films has been systemically investigated. The copper oxide thin films were chemically deposited on glass substrates by spin-coating. Samples were annealed in air at atmospheric pressure and at different temperatures ranging from 200 to 600°C. The microstructural, morphological, optical properties and surface electronic structure of the thin films have been studied by diagnostic techniques such as X-ray diffraction (XRD), Raman spectroscopy, ultraviolet–visible (UV–VIS) absorption spectroscopy, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The thickness of the films was about 520 nm. Crystallinity and grain size was found to improve with annealing temperature. The optical bandgap of the samples was found to be in between 1.93 and 2.08 eV. Cupric oxide (CuO), cuprous oxide (Cu₂O) and copper hydroxide (Cu(OH)₂) phases were observed on the surface of as-deposited and 600 °C annealed thin films and relative concentrations of these three phases were found to depend on annealing temperature. A complete characterization reported herein allowed us to better understand the surface properties of copper oxide thin films which could then be used as active layers in optoelectronic devices such as solar cells and photodetectors.     

The optical properties of Bi3.25La0.75Ti3O12 thin films with different thickness prepared by chemical solution deposition

Bi_3.25La_0.75Ti₃O₁₂(BiLT) thin films with different thickness were successfully deposited onto fused quartz by chemical solution deposition. X-ray diffraction analysis shows that BiLT thin films are polycrystalline with (0 0 2)-preferred orientation. The dispersion of refractive indices of the BiLT thin films was investigated by the optical transmittance spectrum. The optical band gap energy was estimated from the graph of (hνα)² versus hν. The results show that the refractive index and band-gap energy of the BiLT thin films decrease with the films thickness.     

Preparation and luminescence properties of Mn-doped ZnGa2O4 nanofibers via electrospinning process

One-dimensional Mn²⁺-doped ZnGa₂O₄ nanofibers were prepared by a simple and cost-effective electrospinning process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectrum (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. SEM results indicated that the as-formed precursor fibers and those annealed at 700 °C are uniform with length of several tens to hundred micrometers, and the diameters of the fibers decrease greatly after being heated at 700 °C. Under ultraviolet excitation (246 nm) and low-voltage electron beams (1–3 kV) excitation, the ZnGa₂O₄:Mn²⁺ nanofibers presents the blue emission band of the ZnGa₂O₄ host lattice and the strong green emission with a peak at 505 nm corresponding to the ⁴T₁–⁶A₁ transition of Mn²⁺ ion.     

Comparative study of physical properties of vapor chopped and nonchopped Al2O3 thin films

Aluminium oxide being environmentally stable and having high transmittance is an interesting material for optoelectronics devices. Aluminium oxide thin films have been successfully deposited by hot water oxidation of vacuum evaporated aluminium thin films. The surface morphology, surface roughness, optical transmission, band gap, refractive index and intrinsic stress of Al₂O₃ thin films were studied. The cost effective vapor chopping technique was used. It was observed that, optical transmittance of vapor chopped Al₂O₃ thin film showed higher transmittance than the nonchopped film. The optical band gap of vapor chopped thin film was higher than the nonchopped Al₂O₃, whereas surface roughness and refractive index were lower due to vapor chopping.     

Compositional dependence of absorption coefficient and band-gap for Nb2O5-SiO2 mixture thin films

The absorption coefficient of composite films consisting of niobia (Nb₂O₅) and silica (SiO₂) mixtures is studied for photon energies around the band gap. The films were deposited by co-evaporation and their composition was varied by changing the ratio of deposition rates of the two materials. Both, as-deposited and thermally annealed films were characterized by different techniques: the absorption coefficient was determined by spectrophotometric measurements and the structural properties were investigated using infrared spectroscopy, transmission electron microscopy and X-ray diffraction. The correlation between the variations of absorption properties and film composition and structure is established. The absorption coefficients determined experimentally are compared with the results derived from effective medium theories in order to evaluate the suitability of these theories for the studied composites.     

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.