High-temperature conductivity in chemical bath deposited copper selenide thin films

This paper reports high-temperature (305–523 K) electrical studies of chemical bath deposited copper (I) selenide (Cu_2−xSe) and copper (II) selenide (Cu₃Se₂) thin films. Cu_2−xSe and Cu₃Se₂ have been prepared on glass substrates from the same chemical bath at room temperature by controlling the pH. From X-ray diffraction (XRD) profiles, it has been found that Cu_2−xSe and Cu₃Se₂ have cubic and tetragonal structures, respectively. The composition of the chemical constituent in the films has been confirmed from XRD data and energy-dispersive X-ray analysis (EDAX). It has been found that both phases of copper selenide thin films have thermally activated conduction in the high-temperature range. In this paper we also report the variation of electrical parameters with film thickness and the applied voltage.     

Preparation and the growth mechanism of zinc blende structure tin sulfide films by successive ionic layer adsorption and reaction

Zinc blende structure tin sulfide (SnS) films have been prepared by successive ionic layer adsorption and reaction (SILAR) method. It is found that both the annealing and addition of NH₄Cl or NaCl to cation solution during the SILAR process can promote the crystallization of SnS films. The growth mechanism for this novel structure considered that the initial distribution of cations on the substrate surface during the cation adsorption process is the crucial factor that determines the structure of the final production. Because Cl⁻ anions can complex with Sn²⁺ cations, when the concentration of Cl⁻ in the cation precursor solution increases, more [SnCl]⁺ complex ions are adsorbed on the substrate. These [SnCl]⁺ ions assemble more orderly than Sn²⁺ ions because of the polarity of ions, so crystallized SnS films can be obtained when NH₄Cl or NaCl is added to the cation solution.     

Growth and optical property characterization of textured barium titanate thin films for photonic applications

We have investigated the growth of barium titanate thin films on bulk crystalline and amorphous substrates utilizing biaxially oriented template layers. Ion beam-assisted deposition was used to grow thin, biaxially textured, magnesium oxide template layers on amorphous and silicon substrates. Growth of highly oriented barium titanate films on these template layers was achieved by molecular beam epitaxy using a layer-by-layer growth process. Barium titanate thin films were grown in molecular oxygen and in the presence of oxygen radicals produced by a 300 W radio frequency plasma. We used X-ray and in situ reflection high-energy electron diffraction (RHEED) to analyze the structural properties and show the predominantly c-oriented grains in the films. Variable angle spectroscopic ellipsometry was used to analyze and compare the optical properties of the thin films grown with and without oxygen plasma. We have shown that optical quality barium titanate thin films, which show bulk crystal-like properties, can be grown on any substrate through the use of biaxially oriented magnesium oxide template layers.     

Enhancement of near-band edge photoluminescence of ZnO thin films by employing MgF2 buffer layer

ZnO/MgF₂/ZnO sandwich structure films were fabricated. The effects of a buffer layer on structure and optical properties of ZnO films were investigated by X-ray diffraction, photoluminescence, optical transmittance and absorption measurements. Measurement results showed that the buffer layer had the effects of improving the quality of ZnO films and releasing the residual stresses in the films. The near-band edge emissions of ZnO films deposited on the MgF₂ buffer layer were significantly enhanced compared with those deposited on bare substrate due to the smaller lattice mismatch between MgF₂ and ZnO than that between fused silica and ZnO.     

Optical and electrical properties of lithium doped nickel oxide films deposited by spray pyrolysis onto alumina substrates

Non-doped and lithium doped nickel oxide crystalline films have been prepared onto quartz and crystalline alumina substrates at high substrate temperature (600 °C) by the pneumatic spray pyrolysis process using nickel and lithium acetates as source materials. The structure of all the deposited films was the crystalline cubic phase related to NiO, although this crystalline structure was a little bit stressed for the films with higher lithium concentration. The grain size had values between 60 and 70 nm, almost independently of doping concentration. The non-doped and lithium doped films have an energy band gap of the order of 3.6 eV. Hot point probe results show that all deposited films have a p-type semiconductor behavior. From current–voltage measurements it was observed that the electrical resistivity decreases as the lithium concentration increases, indicating that the doping action of lithium is carried out. The electrical resistivity changed from 10⁶ Ω cm for the non-doped films up to 10² Ω cm for the films prepared with the highest doping concentration.     

Growth and optical absorption spectra of ZnO films grown by pulsed laser deposition

ZnO films on Al₂O₃ substrate were grown by using a pulsed laser deposition method. Through photoluminescence (PL) and X-ray diffraction (XRD) measurements, the optimum growth conditions for the ZnO growth were calculated. The results of the XRD measurement indicate that ZnO film was strongly oriented to the c-axis of hexagonal structure and epitaxially crystallized under constraints created by the substrate. The full-width half-maximum for a theta curve of the (0 0 0 2) peak was 0.201°. Also, from the PL measurement, the grown ZnO film was observed to be a free exciton, which indicates a high quality of epilayer. The Hall mobility and carrier density of the ZnO film at 293 K were estimated to be 299 cm²/V sec and , respectively. The absorption spectra revealed that the temperature dependence of the optical band gap on the ZnO films was − .     

Growth and energy band gap of CaSeS thin films prepared by hot-wall epitaxy

This paper investigates preparation of CaSeS thin films using hot-wall epitaxy. These films can be grown epitaxially on cleaved BaF₂(1 1 1) at a substrate temperature of 873 K by tailoring the VI/II flux ratio vaporized from Ca and SeS resources. The optical absorption edge of these films thus tailored can be observed clearly, shifting toward higher photon energy with increasing S content. In particular, the energy band gap of CaSe_0.66S_0.34, capable of lattice-matching to InP was found to be 4.69 eV, producing considerably large band gap difference of 3.34 eV between the CaSe_0.66S_0.34 and InP.     

Growth of epitaxial ZnO films on Si (1 1 1) substrates with Cr compound buffer layer by plasma-assisted molecular beam epitaxy

Single crystalline ZnO film was grown on (1 1 1) Si substrate through employing an oxidized CrN buffer layer by plasma-assisted molecular beam epitaxy. Single crystalline characteristics were confirmed from in-situ reflection high energy electron diffraction, X-ray pole figure measurement, and transmission electron diffraction pattern, consistently. Epitaxial relationship between ZnO film and Si substrate is determined to be (0 0 0 1)_ZnO‖(1 1 1)_Si and [1 1 2¯ 0]_ZnO‖[0 1 1]_Si. Full-width at half-maximums (FWHMs) of (0 0 0 2) and (1 0 1¯ 1) X-ray rocking curves (XRCs) were 1.379° and 3.634°, respectively, which were significantly smaller than the FWHMs (4.532° and 32.8°, respectively) of the ZnO film grown directly on Si (1 1 1) substrate without any buffer. Total dislocation density in the top region of film was estimated to be ∼5×10⁹ cm⁻². Most of dislocations have a screw type component, which is different from the general cases of ZnO films with the major threading dislocations with an edge component.     

Influence of oxygen/argon ratio on structural,electrical and optical properties of Ag-doped ZnO thin films

Ag-doped ZnO (ZnO:Ag) thin films were deposited on quartz substrates by radio frequency magnetron sputtering technique. The influence of oxygen/argon ratio on structural, electrical and optical properties of ZnO:Ag films has been investigated. ZnO:Ag films gradually transform from n-type into p-type conductivity with increasing oxygen/argon ratio. X-ray photoelectron spectroscopy measurement indicates that Ag substitutes Zn site (Ag_Zn) in the ZnO:Ag films, acting as acceptor, and being responsible for the formation of p-type conductivity. The presence of p-type ZnO:Ag under O-rich condition is attributed to the depression of the donor defects and low formation energy of Ag_Zn acceptor. The I–V curve of the p-ZnO:Ag/n-ZnO homojunction shows a rectification characteristic with a turn-on voltage of ∼7 V.     

Properties of Si-rich SiO2 films by RF magnetron sputtering

Si-rich silicon oxide (SiO_x, 1<x<2) films were prepared by RF magnetron reactive sputtering or co-sputtering on the Si(1 1 1) substrates. X-ray diffraction patterns showed that the peak of silicon nanocrystals (NCs), separated from SiO_x films, had (1 1 1) preferred orientation. The results of scanning electron microscopy indicated the Si NCs uniting into clusters. We demonstrated that the photoluminescence (PL) peaks at 650 nm was caused by defect center. In particular, we discussed the correlation between the PL and the structure of SiO_x films. The mean size of the Si NCs was estimated to be about 3 nm by the PL peak position.     

Growth of epitaxial thin films of scandium nitride on 100-oriented silicon

A series of 100-oriented ScN films was grown under N-rich conditions on 100-oriented Si using different Sc fluxes. The ScN films grew in an epitaxial cube-on-cube orientation, with [0 0 1]_ScN//[0 0 1]_Si and [1 0 0]_ScN//[1 0 0]_Si, despite the high (11%) lattice mismatch between ScN and Si. The film grain size increases and the film ω-FWHM decreases with increasing Sc flux, but the film roughness increases. Films grown under similar conditions on 111-oriented Si resulted in mixed 111 and 100 orientations, indicating that the 100 orientation is favoured both due to texture inheritance from the substrate and due to the growth conditions used.     

Electron microscopy studies of epitaxial MgB2 superconducting thin films grown by in situ reactive evaporation

The morphology and chemistry of epitaxial MgB₂ thin films grown using reactive Mg evaporation on different substrates have been characterized by transmission electron microscopy methods. For polycrystalline alumina and sapphire substrates with different surface planes, an MgO transition layer was found at the interface region. No such layer was present for films grown on MgO and 4-H SiC substrates, and none of the MgB₂ films had any detectable oxygen incorporation nor MgO inclusions. High-resolution electron microscopy revealed that the growth orientation of the MgB₂ thin films was closely related to the substrate orientation and the nature of the intermediary layer. Electrical measurements showed that very low resistivities (several μΩ cm at 300 K) and high superconducting transition temperatures (38 to 40 K) could be achieved. The correlation of electrical properties with film microstructure is briefly discussed.     

The photoluminescence of ZnO thin films grown on Si (1 0 0) substrate by plasma-enhanced chemical vapor deposition

High-quality ZnO thin films have been grown on a Si(1 0 0) substrate by plasma-enhanced chemical vapor deposition (PECVD) using a zinc organic source (Zn(C₂H₅)₂) and carbon dioxide (CO₂) gas mixtures at a temperature of 180°C. A strong free exciton emission with a weak defect-band emission in the visible region is observed. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption peak in the absorption spectra, are closely related to the gas flow rate ratio of Zn(C₂H₅)₂ to CO₂. Full-widths at half-maximum of the free exciton emission as narrow as 93.4 meV have been achieved. Based on the temperature dependence of the PL spectra from 83 to 383 K, the exciton binding energy and the transition energy of free excitons at 0 K were estimated to be 59.4 meV and 3.36 eV, respectively.     

Growth and Raman spectroscopic characterization of As4S4 (II) single crystals

As described by Kutoglu (1976 [16]), single crystals of As₄S₄ (II) phase have been grown using a new two-step synthesis that drastically increases the reproducibility that is attainable in synthetic experiments. First, through photo-induced phase transformation, pararealgar powder is prepared as a precursor instead of AsS melt. Then it is dissolved and recrystallized from CS₂ solvent. Results show that single crystals of the As₄S₄ (II) phase were obtained reproducibly through the dissolution–recrystallization process. Single crystals of As₄S₄ (II) obtained using this method were translucent and showed a uniform yellow-orange color. The crystal exhibits a platelet-like shape as a thin film with well-developed faces (0 1 0) and (0 1¯ 0). The grown crystals are as large as 0.50×0.50×0.01 mm. They were characterized using powder and single crystal X-ray diffraction techniques to confirm the phase identification and the lattice parameters. The As₄S₄ (II) phase crystallizes in monoclinic system with cell parameters a=11.202(4) Å, b=9.954(4) Å, c=7.142(4) Å, β=92.81(4)°, V=795.4(6) Å³, which shows good agreement with the former value. Raman spectroscopic studies elucidated the behavior of the substance and the relation among phases of tetra-arsenic tetrasulfide.     

Surface and interfacial structure of epitaxial SrTiO3 thin films on (0 0 1) Si grown by molecular beam epitaxy

Effects of relaxation of interfacial misfit strain and non-stoichiometry on surface morphology and surface and interfacial structures of epitaxial SrTiO₃ (STO) thin films on (0 0 1) Si during initial growth by molecular beam epitaxy (MBE) were investigated. In situ reflection high-energy electron diffraction (RHEED) in combination with X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) techniques were employed. Relaxation of the interfacial misfit strain between STO and Si as measured by in situ RHEED indicates initial growth is not pseudomorphic, and the interfacial misfit strain is relaxed during and immediately after the first monolayer (ML) deposition. The interfacial strain up to 15 ML results from thermal mismatch strain rather than lattice mismatch strain. Stoichiometry of STO affects not only surface morphology but interfacial structure. We have identified a nanoscale Sr₄Ti₃O₁₀ second phase at the STO/Si interface in a Sr-rich film.     

Epitaxial growth of ZnO thin films on Si substrates by PLD technique

Epitaxial ZnO thin films have been grown on Si(1 1 1) substrates at temperatures between 550 and 700 °C with an oxygen pressure of 60 Pa by pulsed laser deposition (PLD). A ZnO thin film deposited at 500 °C in no-oxygen ambient was used as a buffer layer for the ZnO growth. In situ reflection high-energy electron diffraction (RHEED) observations show that ZnO thin films directly deposited on Si are of a polycrystalline structure, and the crystallinity is deteriorated with an increase of substrate temperature as reflected by the evolution of RHEED patterns from the mixture of spots and rings to single rings. In contrast, the ZnO films grown on a homo-buffer layer exhibit aligned spotty patterns indicating an epitaxial growth. Among the ZnO thin films with a buffer layer, the film grown at 650 °C shows the best structural quality and the strongest ultraviolet (UV) emission with a full-width at half-maximum (FWHM) of 86 meV. It is found that the ZnO film with a buffer layer has better crystallinity than the film without the buffer layer at the same substrate temperature, while the film without the buffer layer shows a more intense UV emission. Possible reasons and preventive methods are suggested to obtain highly optical quality films.     

Physical properties of Ag-doped cadmium telluride thin films fabricated by closed-space sublimation technique

Cadmium telluride (CdTe) thin films were prepared by the closed-space sublimation (CSS) technique, using CdTe powder as evaporant onto substrates of water-white glass. In the next step, the annealed films at 450 °C for 30 min were dipped in AgNO₃–H₂O solution at room temperature. These films were again annealed at 450 °C for 1 h to obtain silver-doped samples. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electrically i.e. DC electrical resistivity as well as photo resistivity by van der Pauw method at room temperature, dark conductivity, activation energy analysis as a function of temperature by two-probe method under vacuum, and spectrophotometry. The electron microprobe analyzer (EMPA) results showed an increase of Ag content composition in the samples by increasing the immersion time of films in solution. The Hall measurements indicated the increase in mobility and carrier concentrations of CdTe films by doping of Ag. A significant change in the shape and size of the CdTe grains were observed.     

High intense UV-luminescence of nanocrystalline ZnO thin films prepared by thermal oxidation of ZnS thin films

High quality zinc oxide (ZnO) films were obtained by thermal oxidation of high quality ZnS films. The ZnS films were deposited on a Si substrate by a low-pressure metalorganic chemical vapor deposition technique. X-ray diffraction spectra indicate that high quality ZnO films possessing a polycrystalline hexagonal wurtzite structure with preferred orientation of (0 0 2) were obtained. A fourth order LO Raman scattering was observed in the films. In photoluminescence (PL) measurements, a strong PL with a full-width at half-maximum of 10 nm around 380 nm was obtained for the samples annealed at 900°C at room temperature. The maximum PL intensity ratio of the UV emission to the deep-level emission is 28 at room temperature, providing evidence of the high quality of the nanocrystalline ZnO films.