Structural and optical properties of hexagonal MgxZn1−xO thin films

High-quality epitaxial magnesium zinc oxide (Mg_xZn_1-xO) alloy thin films were grown on sapphire (α-Al₂O₃ (0001)) substrates using pulsed laser deposition. The structural and optical properties of these hexagonal films were determined using transmission electron microscopy (TEM), x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), absorption, and photoluminescence measurements. XRD and TEM data reveal that magnesium zinc oxide alloy films, grown by domain matching epitaxy, exhibited the following relationships: MgZnO[0001] ∥ α-Al₂O₃ [0001] and MgZnO[01 0] ∥ α-Al₂O₃ [2 0]. RBS data demonstrate that a maximum magnesium content of x=0.34 can be obtained in hexagonal Zn_xMg_1-xO thin films. This value is significantly higher than the thermodynamic limitation of x=0.04. The absorption spectra of magnesium zinc oxide alloy films obtained at room temperature demonstrate significant excitonic behavior. The exciton binding energies have been extracted from the absorption data. Values of the exciton bandgap as a function of magnesium content were determined by fitting the bandgap energies using polynomial fitting. The Zn_xMg_1−xO alloy thin films demonstrate bright room-temperature luminescence and significant excitonic behavior. A shift in the excitonic emission peak as a function of magnesium content was observed.     

Structural,optical and electrical properties of MgxZn1−xO ternary alloy films

The structural, optical and electrical properties of Mg_xZn_1−xO (x=0.05–0.3) ternary alloy thin films deposited by the sol–gel method on the glass substrate were investigated. The presence of Mg in deposited samples was confirmed through EDAX. XRD spectra revealed that the deposited Mg doped ZnO films were polycrystalline in nature. The optical band gap of the films was tailored between 3.2 and 3.45 eV by varying Mg mole concentration between 0.05 and 0.3. I–V characteristics showed decrease in current with increase in the Mg mole concentration. These results explore the applicability of MgZnO to form effective and efficient heterostructures with ZnO as an active layer for efficient carrier confinement in light emitting devices.     

Optical studies of TiO2–lead phthalocyanine nanocomposite thin films prepared by electron beam evaporation

The nanocomposite thin films of titanium dioxide (TiO₂)–lead phthalocyanine (PbPc) have been prepared on glass substrates by the electron beam evaporation technique. The optical properties of TiO₂/PbPc nanocomposite thin films have been investigated using a spectrophotometric measurement of the absorbance and transmittance at normal incident of light in the wavelength region 300–800 nm. Surface morphology of thin films has been characterized using field emission scanning electron microscopy (FESEM). The UV–vis analysis has been performed to determine the type of electronic transition and the optical energy band gap. The analysis of the spectral behavior of the absorption coefficient in the intrinsic absorption region reveals that the absorption mechanism is due to direct transition. Moreover, by studying the absorption coefficient spectra just below the fundamental absorption edge, the width of band tails of localized states (Urbach energy), steepness parameter and width of the defect states have been evaluated. The obtained results of this novel nanocomposite (TiO₂/PbPc) support the desirable features for the optoelectronic devices.     

Preparation of CuInxGa1−xSe2 solar cells by electron beam evaporation of powdered evaporants

Cu(In,Ga)Se₂ (CIGS) thin films were deposited by electron beam evaporation of ball-milled powders containing various amounts of gallium. The effects of the gallium concentration in the Cu(In,Ga)Se₂ on the structure, surface morphology and optical properties of the films were investigated using X-ray diffraction, energy-dispersive X-ray analysis, atomic force microscopy and optical spectroscopy. All of the films, which were deposited at 450 °C, were polycrystalline and exhibited a chalcopyrite structure with a (112) preferred orientation. The optical constants of the films were calculated. The grain size, the roughness and the band gap increased with increasing amounts of gallium in the films. A glass/TCO/CdS/CIGS/Au solar cell with 12.87% efficiency was prepared directly from the powdered material.     

Properties of V–Ce mixed-oxide thin films deposited by RF magnetron sputtering

Thin films of vanadium cerium mixed oxides are good counter-electrodes for electrochromic devices because of their passive optical behavior and very good charge capacity. We deposited thin films of V–Ce mixed oxides on glass substrates by RF magnetron sputtering under argon at room temperature using different power settings. The targets were pressed into pellets of a powder mixture of V₂O₅ and CeO₂ at molar ratios of 2:1, 1:1, and 1:2. For a molar ratio of 2:1, the resulting crystalline film comprised an orthorhombic CeVO₃ phase and the average grain size was 89 nm. For molar ratios of 1:1 and 1:2, the resulting films were completely amorphous in nature. Scanning electron microscopy images and energy-dispersive X-ray spectroscopy data confirmed these results. The optical properties of the films were studied using UV-Vis-NIR spectrophotometry. The transmittance and indirect allowed bandgap for the films increased with the RF power, corresponding to a blue shift of the UV cutoff. The average transmittance increased from 60.9% to 85.3% as the amount of CeO₂ in the target material increased. The optical bandgap also increased from 1.94 to 2.34 eV with increasing CeO₂ content for films prepared at 200 W. Photoacoustic amplitude (PA) spectra were recorded in the range 300–1000 nm. The optical bandgap was calculated from wavelength-dependent normalized PA data and values were in good agreement with those obtained from UV-Vis-NIR data. The thermal diffusivity calculated for the films increased with deposition power. For thin films deposited at 200 W, values of 53.556×10⁻⁸, 1.069×10⁻⁸, and 0.2198×10⁻⁸ m²/s were obtained for 2:1, 1:1, and 1:2 V₂O₅/CeO₂, respectively.     

Characterization of ZrTiO4 thin films prepared by sol–gel method

The electrical properties, memory switching behavior, and microstructures of ZrTiO₄ thin films prepared by sol–gel method at different annealing temperatures were investigated. All films exhibited ZrTiO₄ (111) and (101) orientations perpendicular to the substrate surface, and the grain size increased with increasing annealing temperature. A low leakage current density of 1.47×10^?6 A/cm² was obtained for the prepared films. The I–V characteristics of ZrTiO₄ capacitors can be explained in terms of ohmic conduction in the low electric field region and Schottky emission in the high electric field region. An on/off ratio of 10² was measured in our glass/ITO/ZrTiO₄/Pt structure with an annealing temperature of 600 °C. Considering the primary memory switching behavior of ZrTiO₄, ReRAM based on ZrTiO₄ shows promise for future nonvolatile memory applications.     

Cadmium(1−x)zinc(x)telluride thin films deposited by sequential pulsed laser deposition

In this work, sequential pulsed laser deposition was used for the deposition of cadmium zinc telluride (CZT) thin films. CZT is a ternary II–VI compound semiconductor with a tunable band gap between 1.51 and 2.26 eV. In this work, three different CZT film compositions were achieved at room temperature by sequential deposition of nanometric layers with a precise number of laser shots on the cadmium telluride (CdTe) and zinc telluride (ZnTe) targets. XPS, XRD and UV–vis transmittance techniques were used to characterize the CZT films. The atomic content of zinc ranged from 60% down to 13%. This represents an enlargement of the lattice constant from 6.19 to 6.41 Å, and a band gap decrement from 1.94 to 1.55 eV. In addition, the CZT film resistivity can be modulated between the CdTe (4.1×10⁷ Ω-cm) and ZnTe (2.8×10⁵ Ω-cm) values. Our results demonstrated that the sequential pulsed laser deposition can be used to obtain several CZT film compositions with precise control of its stoichiometry and can be extended to the production of other ternary compounds.     

Microstructure and magnetic properties of In1−xCrxN thin films

Microstructure and magnetic properties of In_1−xCr_xN thin films grown on GaN-on-sapphire templates by molecular beam epitaxy are investigated. Optimized growth conditions are identified for the In_1−xCr_xN thin films at reduced growth temperature. The In_1−xCr_xN thin films on the top of the InGaN buffer layers exhibit high crystalline-quality. The magnetic properties of In_1−xCr_xN thin films show a ferromagnetic behavior even at room temperature.     

Heterojunctions utilizing CuInxGa1−xTe2 thin films

Polycrystalline CuIn_xGa_1−x Te₂ thin films are prepared by pulsed laser evaporation. The room-temperature hole densities and mobilities of the films are determined. It is established that direct optical contact of the postgrowth surface of such films with the surface of a cleaved InSe wafer exhibits the photovoltaic effect. The spectra of the relative quantum efficiency of photoconversion of the heterojunctions are investigated as a function of the composition of the CuIn_xGa_1−x Te₂ films and the photodetection geometry. It is concluded that the fabricated heterojunctions have potential applications in photodetectors of unpolarized radiation. Fiz. Tekh. Poluprovodn. 33, 824–827 (July 1999)     

Properties of ITO：Zr films deposited by co-sputtering

ITO：Zr films were deposited on glass substrate by co-sputtering with an ITO target and a Zirconium target. Substrate temperature and oxygen flow rate have important influences on the properties of ITO：Zr films. ITO：Zr films show better crystalline structure and lower surface roughness. Better optical-electrical properties of the films can be achieved at low substrate temperature. The certain oxygen flow rates worsen the electrical properties but can enhance the optical properties of ITO：Zr films. The variation in optical band gap can be explained on the basis of Burstin-Moss effect.     

Physical,structural and topographical aspects of Zn1−xCoxSe thin films

In the present study, Zn_1−xCo_xSe (0≤x≤0.275) thin films were synthesized via a chemical route and characterized through the physical, compositional, structural and morphological properties. The change in colour appearance from ash-grey to charcoal-black suggested integration of Co²⁺ into ZnSe host lattice. Similar conclusions on the colour appearance were drawn from colorimetric studies. The hydrophobic nature of the as-obtained sample surface was revealed in wettability measurements. Zn²⁺, Co²⁺ and Se^2- states of constituents in the thin films were found in the elemental analysis. Formation of ternary alloy was confirmed by shift in (111) X-ray diffraction peak. The surface topography was analysed by an atomic force microscopy (AFM). A variety of AFM parameters were determined to study the effect of Co²⁺ addition onto the surface topography. Magnetic mapping of the surface topography concluded the existence of magnetic domains of irregular sizes and shapes.     

Band gap engineering by substitution of S by Se in nanostructured CdS1−xSex thin films grown by soft chemical route for photosensor application

Ternary alloys of CdS_1−xSe_x (x=0, x=0.2, x=0.4, x=0.6, x=0.8, x=1) thin films were prepared on to glass substrates by a simple and economical soft chemical route (chemical bath deposition) at 50° to 80 °C in air. The as-grown films were characterized by optical and electrical measurement systems, X-ray diffraction (XRD), Energy dispersive X-ray analysis (EDAX) and SEM (scanning electron microscopy). The optical study reveals shift in the absorption edge towards the higher wavelengths, i.e. the band gap decreases as a function of ‘x’ (quantity of selenium in the bath). I–V measurement of CdS (resulted when x=0), CdS_1−xSe_x (x=0.2, x=0.4, x=0.6, x=0.8) and CdSe (resulted when x=1) thin films under dark and illumination conditions (60 W) were measured. Increase in photoconductivity is observed, suggesting its applicability in photosensor devices. Electrical resistivity shows semiconducting behavior and activation energy decreases. The XRD patterns reveals that deposited thin films have hexagonal mixed structure. EDAX confirmed the compositional parameters. SEM images showed uniform deposition of the material over the entire glass substrate.     

Growth of high quality YbBa2Cu3O7−δ thin films by pulsed laser deposition

YbBa₂Cu₃O_7−δ (Yb-123) films are deposited for the first time using Pulsed Laser Deposition (PLD) method at three different substrate temperatures, viz. 675°C, 700°C and 725°C. Films are characterized using XRD, dc electrical resistivity, critical current density (J_c) and microstructural study by Atomic Force Microscopy (AFM) techniques. It is found that 700°C is the optimum growth temperature for growing high quality Yb-123 films. The best T_c and J_c values obtained at optimum growth conditions are 88 K and 2.6×10⁶ A cm⁻² at 77 K, respectively. AFM photographs provide evidence in confirming the relation between growth temperature and superconducting properties.     

The surface morphology,structural properties and chemical composition of Cd1−xZnxTe polycrystalline thick films deposited by close spaced vacuum sublimation

Thick polycrystalline Cd_1−xZn_xTe films with x ranged from 0.37 to 0.80 were obtained by the close spaced vacuum sublimation method. In order to investigate properties of the films structural, PIXE and Raman studies were carried out. Determination of chemical composition of the films by EDS, PIXE and XRD has shown good correlation of results. Raman spectroscopy reveals the relation between zinc concentration and vibrational properties of the films. Studies of the spatial distribution of the chemical elements on the film surface by micro-PIXE and micro-Raman spectroscopy have shown that films are uniform and free of secondary phases such as CdTe, ZnTe and Te inclusions.     

Structural and optical properties of (Al,K)-co-doped ZnO thin films deposited by a sol–gel technique

Thin films of Al-doped ZnO (AZO) and (Al, K)-co-doped ZnO (AKZO) were synthesized by sol–gel spin coating and their structural and optical properties were investigated. All the films had a preferential orientation in which the c-axis was perpendicular to the substrate. The optical bandgap increased after Al doping, but decreased after K doping at a given Al doping concentration. UV emission and a broad visible emission band were observed in photoluminescence (PL) spectra. The intensity of both emission bands decreased after Al and K co-doping. PL excitation (PLE) spectra of the blue emission band indicate that the initial state is possibly the same for all the samples and a similar case occurs for the orange–red emission band. The green emission can be attributed to electronic transitions involving oxygen vacancies. A possible process for the orange–red emission of the thin films is radiative recombination of an electron trapped in a zinc interstitial defect with a hole deeply trapped in interstitial oxygen.     

Photoresponse characteristics of p-Si/n-CuxIn1−xO heterojunction diode prepared by sol-gel spin coating

In the present work, we report on the fabrication and detailed electrical characterization of p-Si/n-Cu_xIn_1−xO heterojunction prepared via the deposition of nanocrystalline Cu_xIn_1−xO thin films on p-type silicon substrate by sol-gel method using spin coating technique. X-ray diffraction and Raman spectroscopy results revealed the polycrystalline nature of Cu_xIn_1−xO thin films consisting diffractions peaks and vibration modes, respectively, corresponding to CuO and In₂O₃. Field-emission scanning electron microscopy showed compact surface morphology while UV–vis absorption spectra exhibited sharp absorption between 300 and 425 nm along with a long tail extending in the visible region. The current-voltage (I-V) characteristics of the fabricated heterojunction demonstrated obvious rectifying behavior in the dark and under illumination. The heterojunction exhibited low reverse leakage current (~10⁻⁷), and upon illumination, the forward current and rectification ratio of the junction was improved while the forward threshold voltage lowered. By fitting the experimental data we have observed that the forward current conduction is dominated by the space charge limited current mechanism.     

Microstructure and electrical properties of nitrogen doped 3C–SiC thin films deposited using methyltrichlorosilane

The growth, microstructure and electrical properties of in-situ nitrogen doped 3C–SiC (111) thin films for sensor applications are presented in this paper. These thin films are deposited at a pressure of 2.5 mbar and temperature of 1040 °C on thermally oxidized Si (100) substrates from methyltrichlorosilane (MTS) precursor using a hot wall vertical low pressure chemical vapor deposition (LPCVD) reactor. Ammonia (NH₃) is used as the nitrogen doping gas. The sensor response depends on chemical composition, structure, morphology and operating temperature. The above properties are investigated for all in situ nitrogen doped (0, 9, 17 and 30 at% of nitrogen) 3C–SiC thin films using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and four probe method. The XRD patterns of the 3C–SiC thin films show a decrease in the crystallinity and intensity of the peak with increase in dopant concentration from 0 to 17 at%. AFM investigations show an improvement in the grain size of the nitrogen doped 3C–SiC thin films with increase in nitrogen concentration from 0 to 17 at%. The sheet resistance of nitrogen doped 3C–SiC thin films is measured by the four probe technique and it is found to decrease with increase in temperature in the range of 40–550 °C. The resistivity and average temperature coefficient of resistance (TCR) of doped 3C–SiC thin film deposited with 17 at% of nitrogen concentration are found to be 0.14 Ω cm and −103 ppm/°C, respectively and this can be used as a sensing material for high temperature applications.     

Effect of sodium doping on graded Cu(In1−xGax)Se2 thin films prepared by chemical spray pyrolysis

In the present work we have studied the effect of Na on the properties of graded Cu(In_1−xGa_x)Se₂ (CIGS) layer. Graded CIGS structures were prepared by chemical spray pyrolysis at a substrate temperature of 350 °C on soda lime glass. Sodium chloride is used as a dopant along with metal (Cu/In/Ga) chlorides and n, n-dimethyl selenourea precursors. The addition of Na exhibited better crystallinity with chalcopyrite phase and an improvement in preferential orientation along the (112) plane. Energy dispersive analysis of X-rays (line/point mapping) revealed a graded nature of the film and percentage incorporation of Na (0.86 at%). Raman studies showed that the film without sodium doping consists of mixed phase of chalcopyrite and CuAu ordering. Influence of sodium showed a remarkable decrease in electrical resistivity (0.49–0.087 Ω cm) as well as an increase in carrier concentration (3.0×10¹⁸–2.5×10¹⁹ cm⁻³) compared to the un-doped films. As carrier concentration increased after sodium doping, the band gap shifted from 1.32 eV to 1.20 eV. Activation energies for un-doped and Na doped films from modified Arrhenius plot were calculated to be 0.49 eV and 0.20 eV, respectively. Extremely short carrier lifetimes in the CIGS thin films were measured by a novel, non-destructive, noncontact method (transmission modulated photoconductive decay). Minority carrier lifetimes of graded CIGS layers without and with external Na doping are found to be 3.0 and 5.6 ns, respectively.     

Structural and optical properties of GaxIn1−xP layers grown by chemical beam epitaxy

Chemical beam epitaxial (CBE) Ga_xIn_1?xP layers (x≈0.5) grown on (001) GaAs substrates at temperatures ranging from 490 to 580°C have been investigated using transmission electron diffraction (TED), transmission electron microscopy, and photoluminescence (PL). TED examination revealed the presence of diffuse scattering 1/2{111}_B positions, indicating the occurrence of typical CuPt-type ordering in the GaInP CBE layers. As the growth temperature decreased from 580 to 490°C, maxima in the intensity of the diffuse scattering moved from ½{111}_B to ½{?1+δ,1?δ,0} positions, where δ is a positive value. As the growth temperature increased from 490 to 550°C, the maxima in the diffuse scattering intensity progressively approached positions of $\frac{1}{2}\{\bar 110\} $ , i.e., the value of δ decreased from 0.25 to 0.17. Bandgap reduction (～45 meV) was observed in the CBE GaInP layers and was attributed to the presence of ordered structures.