Epitaxial layers of CuInSe2 on GaAs

CuInSe₂ single-crystal films with (112) orientation and with thicknesses in the range 800–1200 Å were deposited onto semi-insulating (111)A-oriented GaAs substrates by flash evaporation under controlled growth conditions. Epitaxial growth began at a substrate temperature T_sub = 720 K and twins in the 〈221〉 direction were detected in reflection high energy electron diffraction investigations. The twin concentration decreased with increasing growth temperature. At T_sub = 870 K a change of the structure from chalcopyrite to sphalerite was observed. Films produced at T_sub ? 720 K showed n-type conductivity whereas at higher growth temperatures the films were always p type and showed a rapid increase in hole concentration with increasing substrate temperature. Two different acceptor levels with ionization energies of 92 meV and about meV were found.     

Epitaxial growth of SnO2 film on Sn-doped TiO2(110)

Using a simple vacuum deposition-and-annealing method, Sn-doped TiO₂ (110) single crystals were prepared. Compared with the case of a pure TiO₂ substrate, the lattice mismatch between SnO₂ and the Sn-doped TiO₂ was reduced to −2.85% for the a-axis from −3.25 to −2.85% and for the c-axis from −7.35 to −6.62%. Surface morphologies of deposited SnO₂ films were compared on the Sn-doped TiO₂ (110) and on the pure TiO₂ (110). Results showed that the use of Sn-doped TiO₂ (110) single crystal as a substrate was favorable for growing epitaxial SnO₂ (110) films.     

Epitaxial growth of ZnO on CuInS2(112)

We report on epitaxial growth of ZnO on (112) orientated CuInS₂ thin films. Step-by-step growth and investigation by photoelectron spectroscopy (PES) and low energy electron diffraction (LEED) provided information on the growth mode and the electronic structure of the ZnO-CuInS₂-interface. During the initial growth no ZnO is deposited. Instead a monolayer of ZnS is formed by depletion the CuInS₂ surface of excess sulfur. Thereafter, the ZnO growth starts on the ZnS buffer layer. The band alignment derived from PES shows that the ZnS buffer layer is thin enough to provide a beneficial band alignment for photovoltaic applications.     

CuInSe2 monograin growth in the liquid phase of potassium iodide

The results of monograin CuInSe₂ synthesis from Cu-In alloy and Se in liquid KI are presented. The amounts of CuInSe₂ and KI were nearly equal to fulfil the criterion for the monograin growth (all free volume between the particles has to be filled with liquid). All the grown powder materials with narrow-disperse granularity were chalcopyrite CuInSe₂. The grown crystallites had tetrahedral shapes and homogeneous composition. Particle size distribution was used to describe the growth process. The activation energy of linear growth of crystals was E_d = 0.25 ± 0.05 eV, and the power of time dependence of the crystal growth was l/n = 0.26 ± 0.06. The solubility of CuInSe₂ in KI at 990 K was 0.17 ± 0.05 wt. %. The solubility of potassium and iodine in CuInSe₂ at 990 K was 0.094 wt. %, and 0.0086 wt. %, respectively. As a result, homogeneous p-type CuInSe₂ monograin materials were synthesised in KI solvent.     

Epitaxial growth of ferromagnetic CrO2 films in air

Epitaxial growth of CrO₂ films under atmospheric pressure has been investigated. Single crystal films of CrO₂ were obtained by the thermal decomposition of gaseous CrO₃ onto the substrates of rutile single crystals in air. The optimum temperature of the substrate for the pure CrO₂ epitaxial films was found to be 390°C. At the substrate temperature of 380°C, the obtained film included Cr₂O₅ as impurities, and Cr₂O₃ appeared at 400°C. Magnetic domain patterns of these films were observed by longitudinal Kerr effect. The growth patterns of domain were obtained with applied field.     

Epitaxial growth and relaxation of γ-Al2O3 on silicon

0.5-10 nm-thick single crystal γ-Al₂O₃ films was epitaxially grown, at high temperature, on Si(001) and Si(111) substrates using electron-beam evaporation techniques. Reflection High Energy Electron Diffraction studies showed that the Al₂O₃ films grow pseudomorphically on Si (100) up to thickness of 2 nm. For higher thicknesses, a cubic to hexagonal surface phase transition occurs. Epitaxial growth and relaxation were also observed for Si(111). The film surfaces are smooth and the oxide-Si interfaces are atomically abrupt without interfacial layers.     

Studies on chemically deposited CuInSe2 thin films

Nanocrystalline thin films of CuInSe₂ have been prepared by chemical bath deposition technique at temperatures below 60 °C. X-ray diffraction of the films confirmed the identity of CuInSe₂ and with largely broadened peaks indicated the nanocrystalline nature of the films. Images from scanning electron microscope represented spherical nanoparticles.     

Electrodeposition of CuInSe2 in citrate-containing electrolytes

CuInSe₂ is a frequent constituent of solar cells. Low-cost solar cells can be prepared by electrodeposition from a single bath, provided that the multiple variables affecting the process can be properly tuned. Our design includes an n-layer of TiO₂ (dense and/or nanoporous), on top of which a p-layer of CuInSe₂ is electrodeposited. The TiO₂ layer is supported on conductive glass. When CuInSe₂ is applied onto TiO₂, a p–n junction is formed by which sunlight could be transformed into electricity. CuInSe₂ films have been prepared by electrodeposition from a single bath aqueous solution. Citrate has been used as complexing agent, so as to shift the copper deposition potential in the negative direction, bringing it closer to the In deposition potential. Films are obtained potentiostatically at −0.8 V (vs. standard calomel electrode) for 1 h. The electrolyte consisted of 0.4 mol/L sodium citrate containing 3 mmol/L CuCl₂, 6 mmol/L InCl₃, and 5 mmol/L SeO₂. Different pH values between 4 and 6 are investigated. The samples are annealed in a furnace under flowing Ar at 350 °C for 15 min, which proves to be essential to improve the crystallinity of the CuInSe₂ films. TiO₂ and CuInSe₂ have been characterized using scanning electron microscopy, X-ray diffraction, and optical absorption spectroscopy. The morphology and the physical and optical properties are in good agreement with those reported in the literature. In order to investigate the photoresponse of the cells, current–voltage curves of prototype devices are performed in the dark and under light.     

Photoconductivity effects in CuInS2, CuInSe2 and CuInTe2 thin films

Photoconductivity response spectra for CuInS₂, CuInSe₂ and CuInTe₂ thin films are reported. The temperature dependences of the spectra are utilized to determine the bandgap energies as functions of temperature. Photoconductive response times are measured for these I–III–VI₂ ternary semiconductors and are discussed in terms of deposition parameters and post-deposition treatments. The photoconductive decay times are in the range 10^-3 – 10¹ ms.     

Characterization of stepwise flash-evaporated CuInSe2 films

CuInSe₂ (CIS) films were deposited by stepwise flash evaporation from polycrystalline powder source onto glass substrates held at various temperatures ranging from 100 to 560 K. The phase purity and microstructure were analyzed by transmission electron microscopy. The investigations show that films grown at 300 K and below were amorphous, whereas those grown at 370 K and above were polycrystalline in nature. The grain size in polycrystalline films were found to improve with increase in substrate temperature and during post-deposition annealing. The films had near stoichiometric composition as revealed by Rutherford backscattering spectrometry. Analysis of the optical transmittance spectra of CIS films deposited at 520 K yielded a value of ∼0.97 eV for the fundamental band gap.     

Phase evolution during CuInSe2 electrodeposition on polycrystalline Mo

Using structural analyses means of ex-situ Raman spectroscopy and X-ray diffraction combined with electrical measurements, we study the phase evolution in the growth by electrodeposition technique of CuInSe₂ on polycrystalline Mo. For this purpose the growth was stopped at different stages, and then the different layers were analysed. First growth steps seem to be controlled by the deposition of secondary phases, like elemental Se and Cu₂Se binary. After the deposition of approximately 300 nm of material, CuInSe₂ ternary and ordered vacancy compounds start to adequately form. At a thickness close to 2000 nm, the formation of binary Cu_xSe is observed, remaining up to the final growth process (4350 nm). All these results are compared with the kinetic model of the system under the consideration of the experimental composition evolution.     

Epitaxial growth of CdS on ionic substrates

Transmission electron microscopy has been used to study the structure of films of CdS evaporatedin vacua in the 10^–5 torr range on to (100) cleavage faces and cut and polished (110) and (111) faces of NaCl, and also on to (111) cleavage faces of BaF₂, (111) substrate faces were found to produce wurtzite structure (hexagonal) films with great structural perfection over wider ranges of epitaxial growth temperature than (100) substrate faces. The (100) and (110) substrates produced sphalerite structure (cubic) films. Electron beam evaporation and generally clean growth conditions were found to produce good quality films at low substrate temperatures. The films were in general free of any included grains and the diffraction patterns free of satellite spots. 111 or 10¯10 streaking was present in the diffraction patterns, however, except in the case of films grown on BaF₂ above 170° C, and on NaCl (111) above 250° C. These films were also free of planar defects and only contained of the order of 10¹⁰ dislocations per cm².     

Preparation and crystal growth of materials in the pseudo-binary CuInSe2ZnSe and CuGaSe2ZnSe systems

Solid solution formation in the systems CuM^IIISe₂ (chalcopyrite)-ZnSe (sphalerite) system (where M^III = In, Ga) have been studied. It was found that these systems form extensive terminal regions of solid solutions. Large crystals of some of these new compositions can be prepared from the melt by directional freezing in horizontal boats or by the Bridgeman technique.     

The electrical properties of CuInSe2 thin films deposited onto CaF2 substrates

CuInSe₂ thin films were deposited onto (111)-oriented CaF₂ substrates by flash evaporation in the substrate temperature range T_s = 650–890 K. Epitaxial growth was found at T_s = 770–800 K; at lower and higher substrate temperatures the films were partly polycrystalline. Films produced at T_s ? 675 K showed n-type conductivity due to a donor with an ionization energy of 78 ± 5 meV ascribed to indium interstitials. At T_s ? 725 K the films exhibited p-type conductivity due to a shallow acceptor with an ionization energy characteristic of indium vacancies. The important role of the substrate material in establishing the electrical parameters of CuInSe₂ thin films is proved by a comparative study of thin films deposited onto GaAs and CaF₂ substrates.     

Optical transitions in laser-evaporated thin CuInSe2 films

The optical absorption coefficients of highly oriented laser-evaporated thin films were determined from the measured reflectance R(λ) and transmittance T(λ) in the wavelength range 400–1700 nm. The optical absorption spectrum of CuInSe₂ thin films shows three energy gaps, which are associated with the fundamental edge and valence band splitting by the tetragonal crystal-field and spin-orbit effects, and four optical transitions from the copper d levels to the conduction bands.     

Epitaxial intergrowth of the layered crystals SnTaS2 and SnS2

Layered crystals were obtained, with an overall composition of about Sn₃Ta₂S₆, which gave unusual electron diffraction patterns. The patterns are explained in terms of an intergrowth of thin, alternating layers of SnTaS₂ and epitaxially deformed SnS₂. The SnS₂ has a monoclinic structure with $\text{a}\text{= 15.78}\text{A}\text{?}$, $\text{b}\text{= 5.67}\text{A}\text{?}$ and β = 95°. Similar crystals based on Nb rather than Ta have also been obtained.     

钙钛矿型氧化物SrTiO3/BaTiO3多层膜的激光分子束外延生长研究

用激光分子束外延技术在SrTiO3(001)衬底上外延生长SrTiO3/BaTiO3多层膜,通过反射式高能电子衍射(RHEED)原位实时监测并结合原子力显微镜(AFM),研究了不同基片温度下所生长薄膜的表面平整度,利用X射线衍射(XRD)对外延薄膜进行了结构分析,结果表明薄膜具有二维生长模式,在基片温度为380～470℃之间生长的薄膜具有原子级光滑,并且具有完全C轴取向.同时运用X射线光电子能谱(XPS)研究了薄膜界面的互扩散,结果表明降低制备薄膜时的基片温度有利于减少互扩散.     

Composition and Morphology of Electrodeposited CuInSe2 Precursor Films

CuInSe₂ (CIS) precursor films have been prepared by electrodeposition in aqueous solution. The electrodeposited films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) for structural, morphological and componential properties. The influence of deposition potential and Na-citrate concentration on composition and morphology of electrodeposited films was studied in detail. It is found that the film morphology is strongly influenced by deposition potential and Na-citrate concentration. Films with large and homogenous grain size and ratio of Cu/In approaching 1 were obtained at deposition potentials of -0.7 and -0.75 V vs the saturated calomel electrode (SCE) and Na-citrate concentration of 500 mmol/L. Chalcopyrite phase CuInSe₂ is contained in precursor films that have poor crystallinity.     

Epitaxial growth of RuO2 thin films by metal-organic chemical vapor deposition

Conductive RuO₂ thin films were epitaxially grown on LaAlO₃(100) and MgO(100) substrates by metal-organic chemical vapor deposition (MOCVD). The deposited RuO₂ films were crack-free, and well adhered to the substrates. The RuO₂ film is (200) oriented on LaAlO₃ (100) substrates at deposition temperature of 600°C and (110) oriented on MgO(100) substrates at deposition temperature of 350°C and above. The epitaxial growth of RuO₂ on MgO and LaAlO₃ is demonstrated by strong in-plane orientation of thin films with respect to the major axes of the substrates. The RuO₂ films on MgO(100) contain two variants and form an orientation relationship with MgO given by RuO₂(110)//MgO(100) and RuO₂[001]//MgO[011]. The RuO₂ films on LaAlO₃(100), on the other hand, contain four variants and form an orientation relationship with LaAlO₃ given by RuO₂(200)//LaAlO₃(100) and RuO₂[011]//LaAlO₃[011]. Electrical measurements on the RuO₂ thin films deposited at 600°C show room-temperature resistivities of 40 and 50 μΩ cm for the films deposited on the MgO and LaAlO₃ substrates, respectively.     

Flexible CuInSe2 photovoltaic cells fabricated by non-vacuum techniques

In this work, CuInSe₂ based flexible photovoltaic cells have been fabricated completely using non-vacuum low-cost techniques. Thin films were deposited on molybdenum thin foil substrates by electrodeposition using a buffered aqueous electrolyte with the deposition of subsequent layers performed by spray pyrolysis. In addition, the buffer layer CdS was replaced with a wider bandgap ZnS (3.7 eV) and analysis undertaken of the fabrication pathway, morphological and compositional changes resulting from the different precursor route. The deposited films were annealed in a Se atmosphere at 450 °C. The influence of annealing temperature and time on the properties of the films are briefly discussed. Characterisation of thin films was performed using aqueous electrolyte contacts. Capacitance measurements were made as a function of applied bias on thin films deposited on metal substrates with blocking electrolyte contacts where analysis of the impedance gave values of the space charge capacitance from which the doping density and flat band potential were derived. The structural characterisation was carried out using X-ray diffraction and Raman spectroscopy. The structure and device properties of Mo (SS)/CuInSe₂/ZnS/n⁺-ZnO/Ni were characterized using current-voltage technique and photocurrent spectroscopy.