Stoichiometry of Thin Layers of CuInSe2 Investigated by ESCA

The stoichiometry of thin epitaxial layers of CuInSe₂ on CaF₂ and GaAs substrates was determined by the ESCA method. Substrate temperature was varied. It was found, that layers of exact stoichiometry are got at substrate temperatures around 700 K.     

Structural and electrical properties of CuIn0.7Ga0.3Se2 epitaxial layers on GaAs substrates

Epitaxial layers of CuIn_0.7Ga_0.3Se₂ could be prepared by flash evaporation onto (111)A-oriented GaAs substrates in the substrate temperature range T_sub = 745 … 870 K. At T_sub = 745 … 820 K the films had the chalcopyrite structure, at T_sub = 820 … 845 K an additional pseudohexagonal phase was found. Indications to the presence of a sphalerite phase were found at high substrate temperatures. Films grown at T_sub ≦ 750 K were always n-type conducting and showed a largely pronounced impurity band conduction effect. At T_sub ≧ 860 K the films were always p-type conducting and two acceptor states with ionization energies of some 10⁻³ eV and of 125 … 140 meV were found.     

Influence of substrate orientation on the electrical properties of CuInSe2 epitaxial layers on GaAs substrates

The electrical properties of CuInSe₂ epitaxial layers on (111)A-, (110)-, and (100)-oriented GaAs substrates are investigated. At substrate temperatures T_sub ≧ 820 K the p-type conductivity observed is governed by an acceptor state with an ionization energy of about 110 meV independent of the substrate orientation. At T_sub ≦ 770 K three different acceptor states are found in dependence on the substrate orientation: 390 meV for (111)A-, 110 meV for (110)-, and a very shallow acceptor for (100)-oriented substrates. A possible correlation between these acceptor states and intrinsic defects is proposed.     

Optical and structural properties of Ge–Se bulk glasses and Ag–Ge–Se thin films

The optical and structural properties of Ge₂₀Se₈₀, Ge₂₅Se₇₅ and Ge₃₀Se₇₀ bulk glasses and Ag_x(Ge_0.20Se_0.80)_100−x thin films, where x = 0, 6, 11, 16, 20 and 23 at.% were studied. All samples were confirmed as amorphous according to XRD. The Raman spectra showed increase in 260 cm⁻¹ and 237 cm⁻¹ and decrease in 198 cm⁻¹ and 216 cm⁻¹ bands with different Se content in the bulk samples. The optical bandgap energy of bulk samples decreased (2.17–2.08 eV) and refractive index increased (2.389–2.426 at 1550 nm) with increasing Se content in bulk glasses. The Ge₂₀Se₈₀ thin films were prepared by vacuum thermal evaporation from Ge₃₀Se₇₀ bulk glasses. The Raman spectra of the films showed that peaks at 260 cm⁻¹ and 216 cm⁻¹ decreased their intensities with increasing Ag content in the thin films. The significant red shift of bandgap energy occurred upon different Ag content. The optically induced dissolution and diffusion resulted in graded refractive index profile along the film thickness caused by different Ag concentration. The refractive index increased from the substrate side to the top of thin films. The graded profile was getting more uniform with increasing content of silver in the thin film.     

A Gamma-ray Diffraction Study of MBE-Grown CaF2/Si(111) Heterostructures

MBE grown epitaxial films of CaF₂ onto Si(111) substrates were investigated by gamma ray diffraction to obtain assertions about the real structure and the strain situation in the epitaxial systems. It were measured the integral reflection coefficient R_i and the angle distribution of the reflected intensity of both (111) and (333) reflections. It was found that (i) a high temperature annealing step during the wafer preparation (1200 °C) causes a drastical increase of real structure defects in the substrate material, (ii) expitaxial layers of 18 nm thickness are grown pseudomorphically, layers having a thickness of 30 nm are relaxed, (iii) the misfit dislocation network formed during the relaxation process is localized not in the deposit but in the substrate material.     

Growth,structure, and stoichiometry of epitaxial Cu-III-VI2 films on CaF2 substrates

Thin films of CuInTe₂, CuInSe₂, CuInS₂, CuGaSe₂ and CuGaS₂ were deposited onto (111)-and (100)-oriented CaF₂ substrates by flash evaporation technique. Epitaxial growth occurs at substrate temperatures in the range from 750 to 900 K, dependent on the kind of film material. Contrary to the case of GaAs, GaP and Ge substrates there are always polycrystalline parts. The layers were found to be single phase and crystallizing in the chalcopyrite structure only. The composition of the films varied in dependence on the substrate temperature. In the range of about 700 K a nearly stoichiometric composition was found.     

Optical properties and synthesis of CuInSe2 thin films by selenization of Cu/In layers

In this paper, we report the effect of annealing temperature on the properties of copper indium diselenide (CuInSe₂) thin films. The CuInSe₂ thin films were fabricated at 500 °C for 2 h by annealing Cu‐In layers (as precursors) selenized in a glass tube with pure selenium powder. The structural and morphological properties of the CuInSe₂ thin films were characterized respectively by means of x‐ray diffraction (XRD) and field‐emission scanning electron microscope (FE‐SEM). The type of CuInSe₂ thin film has been identified as direct allowed and the band gap value was determined. The study of UV/Visible/NIR absorption shows that the band gap value of CuInSe₂ thin film is about 1.07 eV, which is within an optimal range for harvesting solar radiation energy. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Zum Wachstum von AIIIBV-Halbleitern aus nichtstöchiometrischen Schmelzen (II) Dotierung von GaAs und Ga1−xAlxAs mit Zink

Doping of GaAs and Ga_1−xAl_xAs with Zn in the LPE process was studied by a radioanalytical method. It is found that Zn diffuses from the Ga-(Al)-As-Zn-solution into the n-GaAs substrate before epitaxial growth starts. This “pre-diffusion” and the following diffusion of Zn out of the epitaxial layer into the substrate results in three regions with distinct Zn-graduation in the vicinity of the pn-junction. There are no striking differences for GaAs/GaAs and Ga_1−xAl_xAs/GaAs structures. The Zn concentration in GaAs epitaxial layers decreases exponentially from the substrate up to the layer surface. From this profile the temperature dependence of the Zn segregation coefficient is calculated. At 900°C a value k_eff = 5,2 · 10⁻² is found. The doping profile in Ga_1−xAl/As layers is more complex. It is influenced by the changings of temperature during the growth of the layer and by the nonuniform Aldistribution over the layer thickness.     

Preparation and Characterization of MBE-grown Si/CaF2 Heterostructures

Growth and fabrication of silicon-on-insulator structures, based on heteroepitaxial growth of insulating films on Si, is an area of research that has rapidly developed in recent years. Thin CaF₂ films and Si/CaF₂ multilayers were prepared on {111}-oriented Si substrates by molecular beam epitaxy (MBE) and were investigated by RHEED and RBS. For epitaxial growth the Si substrates were cleaned using the ultraviolet/ozone surface cleaning method which is an effective tool to remove contaminants from the surface by low-temperature in-vacuo preheating. The growth of CaF₂ on such Si{111} substrates provides epitaxial layers with a high structural perfection. When this layer system, however, is used as a substrate for epitaxial Si growth the Si layers show always twin formation. Si layers without twins could be obtained only after deposition of thin amorphous Si buffer layers at room temperature, immediately followed by Si growth at 700 °C.     

Morphology and crystal phase evolution of doctor-blade coated CuInSe2 thin films

CuInSe₂ (CIS) chalcopyrite thin films were prepared using a low-cost, non-vacuum doctor-blade coating and the thermal annealing method. An acetone-based precursor solution containing copper chloride, indium chloride, selenium chloride, and an organic binder was deposited onto a Mo-sputtered soda lime glass substrate using a doctor-blade coating method. After coating, the precursor films were annealed in a quartz tube furnace under low vacuum without the use of a Se atmosphere or reduction conditions. Evolution of the morphology, crystal structure, and thermal decomposition behavior of the films was analyzed by X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis, and the film formation mechanism was suggested. The as-deposited precursor film gradually decomposed with increase in temperature and formed Cu_2−xSe and In₂Se₃ nuclei on the surface of the film. Incorporation of Cu_2−xSe with In₂Se₃ yielded a chalcopyrite CIS phase, which crystallized on annealing above 400 °C. The obtained CIS film showed low-resistive ohmic behavior with a Mo electrode and a high absorption efficiency for visible–infrared (IR) light, making it suitable for use in photovoltaic applications.     

Growth of Heterostructures Based on InAs–AlxGa1–xSb

It is reported on the liquid phase epitaxial (LPE) growth of heterostructures on the base of InAs–Al_xGa_1–xSb. The paper includes the investigation of epitaxial layers of Al_xGa_1–xSb alloys on InAs substrates and results of experiments for the determination of optimum growth regimes.     

Structure and features of the surface morphology of <Emphasis Type="Italic">A</Emphasis><Superscript>4</Superscript><Emphasis Type="Italic">B</Emphasis><Superscript>6</Superscript> chalcogenide epitaxial films

The structure and features of the surface morphology of Pb_{1 − x} Mn _x Se (x = 0.03) epitaxial films grown on freshly cleaved BaF₂(111) faces and PbSe_{1 − x} S _x (100) (x = 0.12) single-crystal wafers were investigated by molecular beam condensation and the hot-wall method. It is shown that the epitaxial films, in accordance with the data in the literature for other chalcogenides, grow in the (111) and (100) planes, repeating the substrate orientation. Black aggregates are observed on the film surface of the films grown. The results obtained are compared with the data in the literature and generalized for other chalcogenides: A ⁴ B ⁶:Pb (S, Se, Te); Pb_{1 − x} Sn _x (S, Se, Te); and Pb_{1 − x} Mn (Se, Te). It is established that the formation of black aggregates, which are second-phase inclusions on the surface of epitaxial films obtained by vacuum thermal deposition, is characteristic of narrow-gap A ⁴ B ⁶ chalcogenides.     

Zur Heteroepitaxie von AIBIIIC2VI-Verbindungen – Interpretation von RHEED-Diagrammen

Semiconductors of the A^IB^IIIC₂^VI type, crystallizing in the chalcopyrite structure, grow epitaxially with their {112}-planes on monocrystalline substrates with a three-fold symmetry of faces. It is shown that application of the RHEED technique permits definitively to decide what kind of epitaxial overgrowth takes place, supposing the atomic scattering factors of the I and III atoms are sufficiently different and/or the c/a axis ratio differs markedly from 2. CuInSe₂/GaAs and CuGaSe₂/GaAs show one-dimensional epitaxy     

Growth and characterization of nanostructured CdS/Polyaniline/CuInSe2 thin films for solar cell applications

In the present paper, we report about synthesis of nanostructured organic–inorganic heterojunction of CdS/Polyaniline/CuInSe₂ thin films by cost effective chemical route at room temperature, for solar cell application. As such obtained thin films are characterized for structural, compositional, morphological, optical and electrical properties by X-ray diffraction (XRD) pattern, energy dispersive X-ray (EDAX) analysis, scanning electron microscopy (SEM), optical absorbance spectra and I–V response respectively. The XRD reveals the polycrystalline nature of the thin films having tetragonal crystal structure and a crystallite size of 19 nm. The presence of observed and expected elements in the EDAX spectra confirms the elemental compositions in CdS/Polyaniline/CuInSe₂ thin films. From SEM images it can be inferred that the surface morphology of the Polyaniline thin films exists like clothing fibers, while CdS/CuInSe₂ shows granular shape particles distributed over the substrate and the SEM of CdS/Polyaniline/CuInSe₂ represents mixing and attachment of circular particles to fiber like structure. The optical absorbance spectra have shown red shift in absorbance strength and energy band gap value of CdS/CuInSe₂ from ~ 1.36 eV to ~ 1.62 eV upon formation of CdS/Polyaniline/CuInSe₂ thin film. The I–V response of CdS/CuInSe₂ and CdS/Polyaniline/CuInSe₂ measured under dark and illumination to 100 mW/cm² light, exhibited the solar characteristics from these graphs and the conversion efficiency calculated is observed to be 0.26 and 0.55% for CdS/CuInSe₂ and CdS/Polyaniline/CuInSe₂ thin films respectively.     

Infrared lattice vibrations of CdIn2Se4

Infrared reflectivity spectra of CdIn₂Se₄ single crystals are measured at room temperature in the wavenumber range from 180 to 4000 cm⁻¹ for the polarization directions E ⟂ c and E ‖[112]. The spectra reveal two vibrational modes with nearly the same frequencies for both polarization directions. The frequencies of the modes with highest energy compare well with the corresponding mode frequencies in ZnIn₂Se₄, CuInSe₂ and AgInSe₂. It is concluded that these modes are sphalerite-like in nature and that their frequencies are essentially determined by the properties of the In Se bond.     

Characteristics of Growing of Heterostructures in the System GaSb–AlSb

In the present work, some factors are pointed out hindering the growing of good-quality epitaxial layers and heterostructures in the system GaSb–AlSb. Their influence is studied on the morphology of the surface and the cleavage of the epitaxial structures GaSb–Ga_1–xAl_xSb. The characteristics of the optimal technological regime are shown.     

Liquid-phase epitaxy of ZnSe from Zn-Ga solution

Undoped or Ga doped ZnSe single crystal layers with thicknesses between 3 and 13 μm were grown on ZnS_xSe_1?x (0 ? x ? 1) single crystal substrates by liquid phase epitaxy in a sealed tube system from a Zn or Zn-Ga alloy solution. Smooth and uniform epitaxial layers with high crystal perfection were obtained on the (111)- and (100)-oriented substrates. All the epitaxial layers were n-type and highly conductive (0.1 < ? < 10 Ω cm). A remarkable enhancement of the blue part of the photoluminescence spectra at room temperature was observed for the epitaxial layers grown from the Zn-Ga alloy solution.     

On the capability of revealing the pseudosymmetry of the chalcopyrite‐type crystal structure

The tetragonal crystal‐structure type of chalcopyrites (chemical formula A^IB^IIIC^VI₂) is a superstructure of sphalerite type. The c /a ratio differs generally from the ideal value 2, i.e., the crystal structure is pseudocubically distorted. For CuInSe₂ and CuGaSe₂ thin films, simulations demonstrate that it is theoretically possible to reveal the tetragonality in electron backscatter‐diffraction (EBSD) patterns for CuGaSe₂, whereas it may not be possible for CuInSe₂. EBSD experiments on CuGaSe₂ thin films using the ”Advanced Fit” band‐detection method show that it is possible to extract accurate misorientation‐angle distributions from the CuGaSe₂ thin film. Pole figures revealing the texture of the CuGaSe₂ thin film are shown, which agree well with X‐ray texture measurements from the same layer. (© 2008 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

LPE growth and characterization of InP/InGaAsP ridge-waveguide lasers at 1.3 μm-wavelength

This paper presents the fabrication and the lasing characteristics of 1.3 μm-wavelength ridge-waveguide laser. The epitaxial material used in this study was grown applying the step-cooling technique of liquid phase epitaxy (LPE). The growth conditions for InGaAsP layers lattice-matched to the (001) InP-substrate are reported for lattice compositions corresponding to photoluminescence peak wavelengths of 1.07, 1.14, and 1.31 μm. We have used a conventional multiple-bin sliding boat to grow the LPE layers and a second apparatus for achieving batches of melts of uniform compositions. In the LPE apparatus the various batch melts (In–Sn In–Zn; In–Ga–As of different composition) were saturated with phosphorus using the seed dissolution technique. The epitaxial layers were grown by a single phase technique at a constant temperature. This LPE growth technique is useful for the fabrication of double-heterostructure wafers with an uniform alloy composition and a well-defined layer thickness. Using these epitaxial materials, metal-clad ridge-waveguide (MCRW) lasers have been prepared with stripe widths of 3.5 μm. CW threshold currents of 18 mA at room temperature are achieved for 200 μm long cavities. These lasers have T₀ values ranging from 50 to 70 K and well linear L-I-characteristics.     

Epitaxial layers of CuGaTe2 on GaAs

CuGaTe₂ thin films with thicknesses in the range from 0.1 to 0.2 μm were deposited onto semi-insulating (111) A-oriented GaAs substrates by flash evaporation under controlled growth conditions. Epitaxial growth begins at a substrate temperature of T_sub = 695 K. In the range T_sub = 695 … 820 K the films have the chalcopyrite structure, at T_sub = 820 to 840 K a second phase was observed besides the chalcopyrite phase. At T_sub > 840 K the films become polycrystalline. All epitaxial films were p-type conducting and showed an increase of the hole concentration with rising substrate temperature. The measured mobilities indicate to the presence of a high concentration of neutral impurities or defects.