Band gap engineering in PbSe thin films from near-infrared to visible region by photochemical deposition method

Lead selenide (PbSe) thin films have been synthesized by the established photochemical deposition technique using lead nitrate and lead acetate as sources for the metal ions and sodium seleno sulphate as the selenium source along with triethanolamine, ammonia and hydrazine hydrate as complexing agents. A comprehensive study of the effect of substrate materials on physical properties of as deposited PbSe thin films is reported in this work. Two substrates were used in this investigation, namely soda lime glass slides and gold coin corning glass slides. The solution is irradiated with UV light and the photochemical reactions in the aqueous solution resulted in highly adherent metallic thin films. X-ray diffraction (XRD), scanning electron microscopy, optical and electrical measurement techniques were used for film characterization. The XRD analysis confirmed that all films were cubic, regardless of the cationic precursors and substrates used. The scanning electron microscope micrographs showed variations in morphology. The optical studies revealed that the films have good absorption in the visible region. The remarkable success of our effort was that we have been able to modify optical band gap of PbSe thin films over a wide spectral range by a cost effective route. The band gaps estimated from the transmission spectra were in the range 1.32–1.40 eV for films deposited on soda lime glass substrates and 1.46–1.55 eV for corning glass substrates. The room temperature conductivity of the PbSe films were in the range of 3.71 × 10^?7–513 × 10^?7 (Ω cm)^?1. The as deposited PbSe thin films with low transmittance in the visible region coupled with an appreciable reflectance in infrared region were found to satisfy the basic requirements for solar control coatings for window glazing applications in warm climates. Through this work we established that irrespective of metal salts, soda lime glass substrate was superior to corning glass substrate.     

Diffusion of Mn2+ impurities in alkali halides and its influence in epitaxy phenomena

A previous paper reported that Mn²⁺ impurities contained in NaCl tend to migrate from the volume to the surface when the crystals are heated to temperatures between 400° and 550°C in air. Once on the surface, the impurities react with oxygen and an epitaxial oxide film is produced on the NaCl substrate. In the present work new data about this oxide film are reported. It has been found that diffusion to the surface can also be observed for KCl:Mn²⁺, LiF:Mn²⁺ and LiCl:Mn²⁺ crystals, at several impurity concentrations, and when heating is carried out in a vacuum of 10^-6 torr. It is shown that this effect has a strong influence in the epitaxy of metal films on alkali halide substrates.     

Effect of implanted phosphorus ions on the crystallization of amorphous silicon films under the action of pulsed excimer laser radiation

We have studied the effect of implanted phosphorus ions on the crystallization of thin amorphous silicon films under the action of nanosecond radiation pulses of a XeCl excimer laser. The amorphous silicon films with a thickness of 90 nm, obtained by plasmachemical deposition on glass substrates, were implanted with phosphorus ions at a dose of 3 × 10¹⁴ and 3 × 10¹⁵ cm⁻². The subsequent laser treatments were performed using energies both above and below a threshold corresponding to the fusion of amorphous silicon. The structure of the silicon films was studied using Raman scattering spectroscopy. An analysis of the experimental data shows that implanted phosphorus stimulates nucleation, especially in the case of liquid phase crystallization. The results are of interest for the development of the technology of thin-film transistors on nonrefractory substrates.     

Electrical properties of thin PbTe films on Si substrates

An attempt was made to reduce the carrier concentration in thin PbTe films on Si substrates by optimizing deposition conditions. A modified hot-wall method was used for reproducible growth ofp-type films with 5 × 10¹⁵ < p(77 K) < 5 × 10¹⁷ cm^-3 andn-type films with 3 × 10¹⁵ < n(77 K) < 5 × 10¹⁶ cm^-3. The IR irradiation was found to have a significant effect on the temperature variation of film resistance. The activation energy of the IR-sensitivity centers was determined to be 0.11 ± 0.005 eV at room temperature and 0.18 ± 0.005 eV between 150 and 180 K.     

On the structure of ion-implanted bicrystalline gold films

Bicrystalline thin films of gold containing 4°, 16° and 18° [001] tilt boundaries were implanted at room temperature with cobalt, chromium, gold and lead at an energy of 40 keV with integrated fluxes ranging from 5 × 10¹³ to 100 × 10¹³ ions cm^-2 and were subsequently examined by transmission electron microscopy. In all cases, small dislocation loops are produced by the implantation process; in addition, the discrete strain contrast pattern associated with dislocations in low angle boundaries is obliterated by implantation with cobalt or chromium but not by implantation with gold or lead under identical conditions. It is concluded that cobalt and chromium segregate at grain boundaries, thereby distorting the lattice and obliterating the discrete strain contrast pattern associated with individual dislocations, whereas gold cannot distort the lattice and lead, being totally immiscible in gold, segregates at free surfaces. Moreover, under large integrated fluxes, new grains are nucleated at and expand parallel to the original grain boundary during implantation with cobalt. Such nucleation and anisotropic growth is attributed to intrinsic grain boundary migration in the depletion zone surrounding the original boundary.     

Structural and wettability investigation of titanium dioxide coating: influence of dopant concentration (Si and Sr)

In this study, different samples of Si and Sr co-doped TiO₂ thin films (5 mol% Si and 5-10-15-20 mol% Sr) were prepared via simple sol–gel synthesis method by using strontium acetate as strontium precursor, tetraethoxysilane as silicon precursor, and titanium (IV) isopropoxide (TTIP) as titanium precursor. The effects of co-doping process on the structural, optical, and wettability properties of thin films were studied by X-ray diffraction (XRD), Scanning electron microscope (SEM) and contact angle measurement. XRD results showed that, the anatase phase formation was promoted by Si⁺⁴ and Sr⁺² co-doped TiO₂ samples at higher percentage of Sr.Accordingly, the titania thin films showed various water contact angles, the water contact angles first decreased from 55.1° to 8.1° then increased to 40.7° by changing the content of Sr dopant from 5 to15 mol% and then to 20% mol.     

Nitrogen ion implantation effects on the structural,optical and electrical properties of CdSe thin film

In the present study, thin films of cadmium selenide (CdSe) are deposited on ITO substrate by electrodeposition method using aqueous solution of 3CdSO₄·8H₂O and SeO₂. These films are implanted with 40 keV N⁺ ions with different fluencies i.e. 1?×?10¹⁵, 5?×?10¹⁵, 1?×?10¹⁶ and 5?×?10¹⁶ ions/cm² using a beam current of 0.9 µA. The structural, morphological, optical and electrical properties of pristine and nitrogen ion-implanted CdSe thin films are analyzed using XRD, SEM, AFM, UV-PL Spectrophotometer and I–V four probes setup. XRD analysis revealed the effects of nitrogen ions on the structural parameters such as grain size, FWHM, micro strain and dislocation density etc. Crystallanity of the material increased with increase in implantation dose. SEM and AFM analysis show decrease in the surface roughness with implantation. From the optical studies, band gap value decreased from 2.50 to 2.29 eV with increase in N⁺ implantation doses. Noticeable changes in the electrical properties are also reported. The effect of N⁺ ion implantation on the properties of CdSe thin films are discussed on the basis of lattice disorder.     

Electrical properties and extension mechanism of Ohmic region of sol–gel derived Ba0.7Sr0.3TiO3 thin films by Zn doping

Undoped and 3 mol% Zn-doped barium strontium titanate thin films were deposited on Pt/Ti/SiO₂/Si substrates using a sol–gel method. The microstructure and morphology of the films were characterized by X-ray diffraction and atomic force microscopy. It showed that both films are polycrystalline with a perovskite structure and smaller grains were observed for the Zn-doped thin films. Dielectric measurements showed that the dielectric loss at 500 kHz was reduced from 0.042 to 0.019 by Zn doping, which was accompanied by a slight decrease of the dielectric constant from 303 to 273. At an applied electric field of 60 kV/cm, the leakage current density of the Zn-doped Ba_0.7Sr_0.3TiO₃ thin films was 2.5 × 10⁻⁸ A/cm², which was by two orders of magnitude lower than that of the undoped films. The leakage current characteristics also indicated that the Ohmic conduction region of barium strontium titanate thin films was extended by Zn dopant. The microstructure, electrical properties and extension mechanism of Ohmic conduction region of the Zn-doped barium strontium titanate thin films were discussed in relation to the effect of Zn doping.     

Nucleation data for gold on pure NaCl surfaces

New experimental results on the nucleation of gold on vacuum-eleaved NaCl substrates are given. The measurements cover the range of substrate temperatures from room temperature up to 250°C and deposition rates from 5 × 10¹¹ to 5 × 10¹⁴ atoms cm^-2 s^-1. In contrast with ealier investigations it now appears that in all cases the dependence of the cluster density on deposition rate and time can be expressed by root functions. The condensation is always incomplete, even at room temperature.     

Low temperature interdiffusion in copper-gold thin films analysed by helium back-scattering

The technique of back-scattering MeV helium ions has been applied to the study of the interdiffusion behaviour of bimetallic Cu-Au thin films. The films were annealed for up to 210 min at 170°–300°C. A rapid initial mixing was detected for all the temperatures investigated and was attributed to the large density of structural defects present in an evaporated film. The plateaus exhibited in the copper and gold energy spectra suggest the appearance of CuAu₃ as a first phase and then of CuAu and Cu₃Au phases. The interdiffusion coefficient was measured from the gold concentration profiles for gold atomic concentrations in the range 25-0% and a value of 2.5 × 10^-15 cm²/sec at 220°C was obtained. An activation energy of 1.4 eV was estimated for this process.     

Influence of CeO2-Cap Layer on the Texture and Critical Current Density of YBCO Film

YBa₂Cu₃O₇ (YBCO) thin films have been fabricated on different textured CeO₂-cap layers by pulsed laser deposition (PLD). The texture and critical current density J _c of YBCO thin films have been systematically investigated. Both in-plane and out-of-plane textures of YBCO films and CeO₂-cap films were characterized by X-ray diffraction (XRD). And the critical currents of YBCO films were measured by the conventional four-probe method. It was found that the texture and J _c of YBCO films were largely dependent on the texture of CeO₂-cap layers under the optimized deposition conditions. With increasing the degree of in-plane and out-of-plane texture of CeO₂-cap layers, J _c of YBCO films decreased from 4.23×10⁶ A/cm² to 0.47×10⁶ A/cm². The FWHM values of the omega scan rocking curves of YBCO films decreased from 3.71 to 1.84° and the phi scan rocking curves from 6.68 to 4.91° with improvement of CeO₂-cap layer texture. Our results indicated that the fabrication of high texture quality of CeO₂-cap layer was necessary for the epitaxial growth of high-J _c YBCO films. The high-quality YBCO films which are comparable with those grown on single crystal substrates could be achieved on high textured CeO₂-cap layers buffered metal substrates.     

Effect of growth temperature on structural, electrical and optical properties of dual ion beam sputtered ZnO thin films

ZnO epitaxial thin films were grown on p-type Si(100) substrates by dual ion beam sputtering deposition system. The crystalline quality, surface morphology, optical and electrical properties of as-deposited ZnO thin films at different growth temperatures were studied. Substrate temperature was varied from 100 to 600 °C at constant oxygen percentage O₂/(O₂ + Ar) % of 66.67 % in a mixed gas of Ar and O₂ with constant chamber pressure of 2.75 × 10^?4 mBar. X-Ray diffraction analyses revealed that all the films had (002) preferred orientation. The minimum value of stress was reported to be ?0.32 × 10¹⁰ dyne/cm² from ZnO film grown at 200 °C. Photoluminescence measurements demonstrated sharp near-band-edge emission (NBE) was observed at ~375 nm along with deep level emission (DLE) in the visible spectral range at room temperature. The DLE Peak was found to have decrement as ZnO growth temperature was increased from 200 to 600 °C. The minimum FWHM of the NBE peak of 16.76 nm was achieved at 600 °C growth temperature. X-Ray photoelectron spectroscopy study revealed presence of oxygen interstitials and vacancies point defects in ZnO film grown at 400 °C. The ZnO thin film was found to be highly resistive when grown at 100 °C. The ZnO films were found to be n-type conducting with decreasing resistivity on increasing substrate temperature from 200 to 500 °C and again increased for film grown at 600 °C. Based on these studies a correlation between native point defects, optical and electrical properties has been established.     

Growth and applications of superconducting and nonsuperconducting oxide epitaxial films

Growth and characterization of high-temperature-superconducting YBa₂Cu₃O₇ and several metallic-oxide thin films by pulsed laser deposition is described here. An overview of substrates employed for epitaxial growth of perovskite-related oxides is presented. Ag-doped YBa₂Cu₃O₇ films grown on bare sapphire are shown to giveT _c=90 K, critical current >10⁶ A/cm² at 77 K and surface resistance =450μΩ. Application of epitaxial metallic LaNiO₃ thin films as an electrode for ferroelectric oxide and as a normal metal layer barrier in the superconductor-normal metal-superconductor (SNS) Josephson junction is presented. Observation of giant magnetoresistance (GMR) in the metallic La_0·6Pb_0·4MnO₃ thin films up to 50% is highlighted.     

Effect of 80 keV Ar+ implantation on the properties of pulse laser deposited magnetite (Fe3O4) thin films

Highly oriented thin films of Fe₃O₄ were deposited on (100) LaAlO₃ substrates by pulsed laser ablation. The structural quality of the films was confirmed by X-ray diffraction (XRD). The films showed a Verwey transition near 120 K. The films were subjected to 80 keV Ar⁺ implantation at different ion doses up to a maximum of 6 × 10¹⁴ ions/cm². Ion beam induced modifications in the films were investigated using XRD and resistance vs temperature measurements. Implantation decreases the change in resistance at 120 K and this effect saturates beyond 3 × 10¹⁴ ions/cm². The Verwey transition temperature,T _V, shifts towards lower temperatures with increase in ion dose.     

Exciting Dilute Magnetic Semiconductor: Copper-Doped ZnO

The present study is focused on the copper-doped ZnO system. Bulk copper-doped ZnO pellets were synthesized by a solid-state reaction technique and used as target material in pulsed laser deposition. Thin films were grown for different Cu doped pellets on sapphire substrates in vacuum (5×10^?5 mbar). Thin films having (002) plane of ZnO showed different oxidation states of dopants. M–H curves exhibited weak ferromagnetic signal for 1–3 % Cu doping but for 5 % Cu doped thin film sample showed the diamagnetic behavior. For deeper information, thin films were grown for 5 % Cu doped ZnO bulk pellet in different oxygen ambient pressures and analyzed. PL measurement at low temperature showed the emission peak in thin films samples due to acceptor-related transitions. XPS results show that copper exists in Cu²⁺ and Cu⁺¹ valence states in thin films and with increasing O₂ ambient pressure the valence-band maximum in films shifts towards higher binding energy. Furthermore, in lower oxygen ambient pressure (1×10^?2 mbar) thin films showed magnetic behavior but this vanished for the film grown at higher ambient pressures of oxygen (6×10^?2 mbar), which hints towards the decrease in donor defects.     

Superconducting characteristics and epitaxial-growth analyses of YBa2Cu3O7−x thin films by direct-current magnetron sputtering

Using direct-current magnetron sputtering deposition, we have successfully prepared highquality epitaxial YBa₂Cu₃O_7–x thin films, on (1 0 0) and (1 1 0) ZrO₂, SrTiO₃ and LaAlO₃ substrates. The films reached zero resistance at about 90 K and had a critical current density, J _c (at 77 K, H=O), above 10⁶ Acm^–2. Electrical measurements showed that the films had a small microwave surface resistance. The epitaxial structure of the films was studied by X-ray diffraction (XRD), Rutherford backscattering (RBS) and channeling spectroscopy, X-ray double-crystalline diffraction and transmission electron microscopy (TEM). It was found that the c-axis of the film grown on the (1 0 0) substrates under optimum deposition conditions was perpendicular to the substrate surface. But on the (1 1 0) substrates, epitaxial growth was along the (1 1 0) or (1 0 3) direction. The experimental results indicate that the films had excellent superconducting properties and complete epitaxial structure.     

Effect of residual gas pressure on the epitaxial growth of vacuum-deposited chromium on Cu{111} substrates

Transmission electron microscopy, electron diffraction and Auger electron spectroscopy were used to study the effect of vacuum conditions on the epitaxial growth of vacuum-deposited chromium films of thicknesses less than 40 nm on Cu{111} substrates. The chromium films were condensed (i) in ultrahigh vacuum at 10⁻⁹ Torr, (ii) at total pressures of 10⁻⁵, 10⁻⁶ and 5 × 10⁻⁸ Torr and (iii) at oxygen partial pressures of 5 × 10⁻⁹, 10⁻⁷ and 10⁻⁶ Torr. The presence of contaminants during the chromium evaporation suppressed the formation of Kurdjumov-Sachs orientations. A low oxygen partial pressure was most effective in altering the orientation and morphology of the deposits.     

Cubic BN formation by ion implantation

Boron nitride was synthesized by nitrogen ion implantation. Boron films were prepared as implantation targets on single-crystal Si(100) substrates by 13.56-MHz radio frequency sputtering. The diatomic single 30-keV nitrogen ions were chosen for implantation. The implantation dose ranged from 1×10¹⁷ ions/cm² to 2×10¹⁸ ions/cm². The films were characterized using a transmission electron microscope. Several phases of boron nitride were found at the medium implantation dose. At the high dose of 2×10¹⁸ ions/cm², the pure c-BN phase was observed. It is believed that the transition from the low ordered phases to c-BN phase occurred during implantation. The films showed good adhesion to the Si substrate.     

Preparation and properties of epitaxial films of indium-doped lead tin telluride

Several hundred epitaxial films of indium-doped lead tin telluride were grown on BaF₂ substrates using a hot-wall method and a source material containing 0.5 at.%In. Films were grown from source materials containing SnTe at concentrations of 0, 22, 24.8 and 27.7 mol.%. The growth procedure, source material preparation and substrate preparation are described. The films were characterized by van der Pauw measurements of the mobility and carrier concentration at room temperature and 77 K. As finally developed, the growth procedures gave a high yield of films with mobilities at 77 K in the range of 3.5–4.5 m² V^-1s^-1.     

Structure transition and multiferroic properties of Mn-doped BiFeO3 thin films

Pure BiFeO₃ (BFO) and Mn-doped BiFe_1?yMn_yO₃ thin films were prepared on FTO/glass (SnO₂: F) substrates by using a sol–gel method. The effects of Mn-doping on the structure and electric properties of the BFO thin films were studied. The X-ray diffraction (XRD) analysis reveals a structure transition in the Mn-doped BiFe_0.96Mn_0.04O₃ (BFMO) thin film. The Rietveld refined XRD patterns conform the trigonal (R3c: H) and tetragonal (P422) symmetry for the BFO and BFMO thin films, respectively. The structure transition and the mixed valences of Mn ions substantially improve the electric properties of the BFMO thin film. The remnant polarization (P _r) of the BFMO thin film was 105.86 μC/cm² at 1 kHz in the applied electric field of 865 kV/cm. At an applied electric field of 150 kV/cm, the leakage current density of BFMO thin film is 1.42 × 10^?5 A/cm². It is about two orders of magnitude lower than that of the pure BFO thin film (1.25 × 10^?3 A/cm²). And the enhanced saturated magnetization of the BFMO thin film is 4.45 emu/cm³.