Annealing effect on electrical and photoconductive properties of Si implanted GaSe single crystal

GaSe single crystals grown by Bridgman method have been doped by ion implantation technique. The samples were bombarded in the direction parallel to c‐axis by Si ion beam of about 100 keV to doses of 1 × 10¹⁶ ions/cm² at room temperature. The effects of Si implantation with annealing at 500 and 600 °C on the electrical properties have been studied by measuring the temperature dependent conductivity and photoconductivity under different illumination intensities in the temperature range of 100–320 K. It is observed that Si implantation increases the room temperature conductivity 10⁻⁷ to 10⁻³ (Ω‐cm)⁻¹ depending on the post annealing temperature. The analysis of temperature dependent conductivity shows that at high temperature region above 200 K, the transport mechanism is dominated by thermal excitation in the doped and undoped GaSe samples. At lower temperatures, the conduction of carriers is dominated by variable range hopping mechanism in the implanted samples. Annealing of the samples at and above 600 °C weakens the temperature dependence of the conductivity and photoconductivity. This indicates that annealing of the implanted samples activates Si‐atoms and increases structural deformations and stacking faults. The same behavior was observed from photoconductivity measurements. Hence, photocurrent‐illumination intensity dependence in the implanted samples obeys the power low I_pc ∝ Φⁿ with n between 1 and 2 which is an indication of continuous distribution of localized states in the band gap. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Czochralski growth and characterization of Dy: La3Ga5.5Nb0.5O14 single crystals

Dysprosium (Dy) doped La₃Ga_5.5Nb_0.5O₁₄ single crystals were grown by the traditional Czochralski method along z‐axis. The structure of the crystal has been studied by X‐ray powder diffraction method, and the unit‐cell parameters are calculated to be a=8.22070 Å, c=5.12533 Å and V=299.965 ų. The segregation coefficient of Dy³⁺ in La₃Ga_5.5Nb_0.5O₁₄ crystal was measured by X‐ray fluorescence analysis. For 1 mol% doping level in the melt, the distribution coefficient of Dy³⁺ was determined to be 0.341 wt%. Specific heat, thermal expansion and transmission spectrum of Dy: La₃Ga_5.5Nb_0.5O₁₄ single crystals have been measured. The fluorescence spectra of Dy³⁺: La₃Ga_5.5Nb_0.5O₁₄ crystals were measured at room temperature, and there were four emission transitions occurring at 479, 576, 662 and 754 nm, respectively. The fluorescent lifetimes measurement results show 1.0% Dy: La₃Ga_5.5Nb_0.5O₁₄ possesses shorter fluorescence decay time (303.4 μs) than does 1.0%Dy:LGS (436.12 μs). (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural characterization and magnetic properties of the spinel compound CoIn0.5Cr1.5S4

Single crystals of the magnetic semiconductor CoIn_0.5Cr_1.5S₄, belong to the system CoIn_(2‐2X)Cr_(2X)S₄ with x = 0.75, was grown by the chemical transport method. X‐ray powder diffraction characterization by the Rietveld method indicated that CoIn_0.5Cr_1.5S₄ crystallizes in the space group Fd‐3m, Z = 8, with a = 10.0700(6) Å and V = 1021.2(1) ų, in a normal spinel structure. The temperature dependence of the DC magnetization suggests that the studied compound presents a ferromagnetic behavior with a Curie temperature Tc = 220 K. Sharp spin‐glass like behavior was found also. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation,structure, and photoluminescence properties of Ga2O3/SnO2 coaxial nanowires

Ga₂O₃/SnO₂ coaxial nanowires were synthesized by thermal evaporation of GaN powders and then atomic layer deposition of SnO₂. Transmission electron microscopy (TEM) and X‐ray diffraction (XRD) analysis results indicate that the Ga₂O₃ cores and the SnO₂ shells of the coaxial nanowires after thermal annealing are single crystals with monoclinic and simple orthorhombic structures, respectively, although the SnO₂ shells are amorphous before annealing. Our results also show that photoluminescence (PL) emission can be enhanced by thermal annealing in an H₂/N₂ atmosphere. EDX concentration profile suggests that the enhancement in the bluish green emission is due to the increase in the concentration of the Ga vacancies in the cores during the H₂/N₂ annealing. On the other hand, a red emission is newly formed while the bluish green emission is degraded by annealing in an oxygen or nitrogen atmosphere (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical and photoconductive properties of GaS0.75Se0.25 mixed crystals

The conductivity, mobility, photoconductivity and photo response measurements in GaS_0.75Se_0.25 mixed crystals were carried out in the temperature range of 150‐450 K. The room temperature conductivity, mobility and electron concentration values were 10^‐9 (Ω‐cm)^‐1, 48 cm²V^‐1s^‐1 and ∼10⁹ cm^‐3, respectively. Two donor levels were obtained from temperature‐dependent conductivity and carrier concentration, located at energies of about 755 and 465 meV below the conduction band. Single donor‐single acceptor analysis yields the same donor level at 465 meV with donor and acceptor concentrations of 8.7 × 10¹⁴ and 5.3 × 10¹³ cm^‐3, respectively. The mobility‐temperature dependence shows that ionized impurity scattering dominates the conduction up to the temperature 310 K with different temperature exponent, while above this critical temperature; the phonon scattering is dominant conduction mechanism. From the photo‐response spectra, the maximum photocurrent was observed for all the samples at 2.42 eV, and varied slightly with temperature. Moreover, the photocurrent‐light intensity dependence in these crystals obeys the power law, I_ph ∝ ϕ^γ with γ between 1.7 and 2.0 for various applied fields and temperatures. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of thermoannealing on crystallinity and optical properties of Sb2S3 thin films

Sb₂S₃ thin films are obtained by evaporating of Sb₂S₃ powder onto glass substrates maintained at room temperature under pressure of 2×10^‐5 torr. The composition of the thin films was determined by energy dispersive analysis of X‐ray (EDAX). The effect of thermal annealing in vacuum on the structural properties was studied using X‐ray diffraction (XRD) technique and scanning electron microscopy (SEM). The as‐deposition films were amorphous, while the annealed films have an orthorhombic polycrystalline structure. The optical constants of as‐deposited and annealed Sb₂S₃ thin films were obtained from the analysis of the experimental recorded transmission spectral data over the wavelength range 400‐1400 nm. The transmittance analysis allowed the determination of refractive index as function of wavelength. It was found that the refractive dispersion data obeyed the single oscillator model, from which the dispersion parameters (oscillator energy, E₀, dispersion energy, E_d) were determined. The static refractive index n(0), static dielectric constant, ε_∞, and optical band gap energy, E_g, were also calculated using the values of dispersion parameters. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and thermal characterization of methylammonium tetrachloro zincate (II) monohydrate crystals

Single crystals of methylammonium tetrachloro zincate (II) monohydrate were grown by slow evaporation of saturated aqueous solutions at room temperature. The grown crystals were colourless, bright and transparent. The crystals were characterized through elemental analysis, powder X‐ray diffraction, thermogravimetric (TG‐DTA) and low temperature differential scanning calorimetric (DSC) techniques. While the powder XRD pattern shows the crystallinity of the compound, the elemental analysis and the TG‐DTA confirm the stoichiometry of the compound. The low temperature DSC indicates first order structural phase transition at ‐8°C during heating cycle. The FTIR and far IR spectra of the compound show characteristic vibrational frequencies due to CH₃NH₃ and ZnCl₄^2‐ ions and other chemical bonds. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of passivation and postannealing on the photoluminescence properties of MgO/SiO2 core‐shell nanorods

MgO nanorods were grown by the thermal evaporation of Mg₃N₂ powders on the Si (100) substrate coated with a gold thin film. The MgO nanorods grown on the Si (100) substrate were a few tens of nanometers in diameter and up to a few hundreds of micrometers in length. MgO/SiO₂ core‐shell nanorods were also fabricated by the sputter‐deposition of SiO₂onto the MgO nanorods. Transmission electron microscopy (TEM) and X–ray diffraction (XRD) analysis results indicated that the cores and shells of the annealed core‐shell nanorods were a face‐centered cubic‐type single crystal MgO and amorphous SiO₂, respectively. The photoluminescence (PL) spectroscopy analysis results showed that SiO₂ coating slightly decreased the PL emission intensity of the MgO nanorods. The PL emission of the MgO/SiO₂ core‐shell nanorods was, however, found to be considerably enhanced by thermal annealing and strongly depends on the annealing atmosphere. The PL emission of the MgO/SiO₂ core‐shell nanorods was substantially enhanced in intensity by annealing in a reducing atmosphere, whereas it was slightly enhanced by annealing in an oxidative atmosphere. The origin of the PL enhancement by annealing in a reducing atmosphere is discussed with the aid of energy‐dispersive X‐ray spectroscopy analyses. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excitation intensity and temperature‐dependent photoluminescence and optical absorption in Tl4Ga3InSe8 layered crystals

Photoluminescence (PL) spectra of Tl₄Ga₃InSe₈ layered crystals grown by Bridgman method have been studied in the wavelength region of 600‐750 nm and in the temperature range of 17‐68 K. A broad PL band centered at 652 nm (1.90 eV) was observed at T = 17 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.13 to 55.73 mW cm^‐2 range. Radiative transitions from donor level located at 0.19 eV below the bottom of conduction band to shallow acceptor level located at 0.03 eV above the top of the valence band were suggested to be responsible for the observed PL band. From X‐ray powder diffraction and optical absorption study, the parameters of monoclinic unit cell and the energy of indirect band gap were determined, respectively. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Properties of AgGa1‐xInxSe2 single crystals grown by Bridgman method

High‐pure and single‐phase AgGa_1‐xIn_xSe₂ (x=0.2) polycrystalline was synthesized by the mechanical and temperature oscillation method. Adopting the modified Bridgman method an integral AgGa_1‐xIn_xSe₂ single crystal with diameter of 14 mm and length of 35 mm has been obtained at the rate of 6 mm/day. It was found that there is a new cleavage face which was (101), and observed the four order X‐ray spectrum of the {101} faces. By the method of DSC analysis the melting and freezing points of the AgGa_1‐xIn_xSe₂ (x=0.2) single crystal were about 828°C and 790°C. The transmission spectra of the AgGa_1‐xIn_xSe₂ (x=0.2) sample of 5×6×2 mm³ were obtained by means of UV and IR spectrophotometer. The limiting frequency was 774.316nm and the band gap was 1.6eV. It can be found in the infrared spectrum that the infrared transmission was above 60% from 4000cm^‐1 to 600cm^‐1. The value of α in 5.3µm and 10.6µm were 0.022cm^‐1 and 0.1cm^‐1 respectively. All results showed that the crystal was of good quality. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal growth and investigation of CuAlxGa1−xTe2 solid solutions

Alloys of the CuGaTe₂‐CuAlTe₂ system have been synthesized by the one‐temperature method and homogenized by annealing. It was established for the first time by X‐ray and DTA methods that a continuous row of solid solutions is formed in the system. Large block crystals of CuAl_xGa_1−xTe₂ solid solutions have been grown by the Bridgman method (horizontal version). Measurements of density, microhardness and thermal expansion have been carried out on the grown crystals. It has been established, that the density and thermal expansion coefficients are changed linearly with composition, microhardness has a maximum at x = 0.6. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dislocation and microindentation analysis of vapour grown Bi2Te3‐xSex whiskers

The structural defects and microhardness of Bi₂Te_3‐xSe_x whiskers (x = 0, 0.2 and 0.4 at% Se) grown by physical vapour deposition (PVD) method have been investigated. Concentric pairs of dislocation loops were observed on the as‐grown surfaces of short hexagonal prisms. A systematic study of dislocations in these crystals was carried out by chemical etching technique. The effects of Se doping, annealing and quenching on the mechanical properties have also been studied on the prism faces of Bi₂Te_3‐xSe_x whiskers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electro‐optic,Piezoelectric and Dielectric Properties of Langasite (La3Ga5SiO14), Langanite (La3Ga5.5Nb0.5O14) and Langataite (La3Ga5.5Ta0.5O14)

Refractive indices were determined of single crystals of La₃Ga₅SiO₁₄ (langasite, LGS), La₃Ga_5.5Nb_0.5O₁₄ (langanite, LGN) and La₃Ga_5.5Ta_0.5O₁₄ (langataite, LGT) in the wavelength region between 0.36 μm and 2.33 μm. While phase‐matched optical second harmonic generation is not possible in LGS it occurs in the isotypic compounds LGN and LGT. Temperature‐dependent examination of the dielectric properties of LGS up to 600 °C showed anomalous behaviour. For all three substances the electro‐optic [r^σ_ijk] (“unclamped”) and the piezoelectric [d_ijk ] tensors were determined at room temperature. In addition, the temperature‐dependence of these properties was studied for LGS between –200 °C and +200 °C by a Jamin interferometer in combination with a modified Sénarmont compensator.     

Growth and characterization of 4‐Aminopyridinium‐4‐nitro phenolate single crystals

Organic optical material 4‐Aminopyridinium‐4‐nitro phenolate (4AP4NP) has been synthesized, and single crystals of size 20 x 14 x 6 mm³ have been grown from acetone solvent at room temperature by solvent evaporation technique. The grown crystals have been characterized by X‐ray diffraction to determine the cell parameters, and by FT‐IR technique to confirm the formation of the expected compound. The crystal belongs to monoclinic crystal system with space group P2₁/a.The structural perfection of the grown crystals has been analyzed by high‐resolution X‐ray diffraction (HRXRD) rocking curve measurements. The thermal stability of the compound has been determined by TG‐DTA curves. The transmittance of 4AP4NP has been used to determine the refractive index n; the extinction coefficient K and both the real ε_r and imaginary ε_i components of the dielectric constant as functions of photon energy. The optical band gap of 4AP4NP is 2.4 eV. The dielectric and mechanical behavior of the specimen was also studied. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical characterization of thermally evaporated thin CdO films

Cadmium Oxide films have been prepared by vacuum evaporation method on a glass substrate at room temperature. Detailed structural, optical, and electrical properties of the films are presented at different annealing temperatures. The crystal structure of the samples was studied by X‐ ray diffraction. The spectral absorption coefficient of the CdO film at the fundamental absorption region (450‐650nm) was determined using the spectral data of transmittance. The direct and indirect band gap energies were determined and found to be 2.33 eV and 1.95 eV respectively. The third order optical nonlinearities χ(3) of CdO films has been measured used the z‐can technique. The real and imaginary parts of χ(3) have been measured at 514 nm and found to be 1.7x10^‐3 esu and 3.0x10^‐3 esu, respectively. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth,structure and thermal properties of CuAlxGa1‐xSe2 alloys

Ingots of the CuAl_xGa_1‐xSe₂ (0 ≤ x ≤ 1) alloys system were prepared by direct fusion of the stoichiometric mixture of the elements. The analysis of X‐ray Powder Diffraction data showed the presence of one single phase with chalcopyrite tetragonal structure at room temperature for all the studied compositions. The lattice parameters, a and c, and the bond lengths were calculated. The phase transition temperatures were obtained by the onset method from Differential Thermal Analysis measurements performed on samples sealed in evacuated quartz ampoules. Fusion or transition enthalpies were determined from the area of the corresponding DTA peak. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal Growth and Differential Thermal Analysis of AgGaSe2

High quality AgGaSe₂ single crystals with 20 mm in diameter and 55 mm in length have been grown by Bridgman technique. It is found that there is a second phase of Ag-rich Ga₂Se₃ (or named AgGa₇Se₁₁) in the AgGaSe₂ crystal. It is concluded by discussions with the DTA results and the phase diagram that single crystals grown from polycrystalline materials with stoichiometric composition must contain a second phase of Ag-rich Ga₂Se₃. This result is helpful for the heat treatment, in which what and how much is added and what temperature is chosen for the heat treatment, are very important.     

Micro-structural studies of GeSe2–Ga2Se3–MX (MX = CsI and PbI2) glasses using Raman spectra

Microstructure of the chalcohalide glasses: GeSe₂–Ga₂Se₃–CsI and GeSe₂–Ga₂Se₃–PbI₂ ternary system were investigated by Raman spectra, lifetime of Dy³⁺ infrared emission and glass transition temperature (T_g). The evolution of the Raman spectra shows that the fundamental structural groups of these studied glasses consist of [Ge(Ga)Se₄] tetrahedral and some complex structure units [Ge(Ga)I_xSe_4?x](x = 1–4). The x value varied when the different iodide was added in Ge–Ga–Se matrix. For GeSe₂–Ga₂Se₃–CsI glasses, the [Ge(Ga)I_xSe_4?x](x = 1–4) mixed-anion tetrahedral and [Ga₂I₇]^? units occurred. For GeSe₂–Ga₂Se₃–PbI₂ glasses, the [Ge(Ga)I₂Se₂], [Ge(Ga)I₃Se] units can be formed. The changes of Dy³⁺ infrared emission lifetime and T_g support the results. Additionally, [PbI_n] structural units will be formed in GeSe₂–Ga₂Se₃–PbI₂ glasses due to high form-ability of these units when the PbI₂ content is high.     

A study of the vacancy-defect distribution in a GaAs/AlxGa1−xAs multi-layer structure grown at low temperature

We have grown a multilayer structure of GaAs and Al_xGa_1−xAs (x=0.25) by molecular beam epitaxy at low substrate temperature (250°C) in order to investigate the effects of annealing on arsenic precipitation and vacancy-defect formation. The as-grown heterostructure contained ∼5×10¹⁷ cm⁻³ gallium vacancies, but information about the individual layers was lost because the layer width (∼45 nm) was smaller than the average positron diffusion length (∼70 nm). Annealing at 500 and 600°C showed increases in the S-parameter (above the bulk value) of ∼2.5% and ∼1.5%, respectively, smaller than for a single LT-GaAs layer (∼3–4%). We explain this phenomenon by suggesting that the excess arsenic which migrates from the material with the higher precipitate/matrix energy (LT-Al_xGa_1−xAs) reduces the gallium vacancy concentration in the LT-GaAs and hence the S-parameter. This hypothesis is supported by SIMS data which shows a build-up of As in the LT-GaAs layers, and TEM images which indicate that arsenic precipitation occurs to a greater extent in the LT-GaAs than in the LT-Al_xGa_1−xAs.     

X‐ray powder diffraction study of the semiconducting alloy Cu2Cd0.5Mn0.5GeSe4

The structure of the semiconducting alloy Cu₂Cd_0.5Mn_0.5GeSe₄ was refined from an X‐ray powder diffraction pattern using the Rietveld method. The present alloy crystallizes in the wurtz‐stannite structure, space group Pmn2₁ (N^o 31), and unit cell parameters values of a = 8.0253(2) Å, b = 6.8591(2) Å, c = 6.5734(2) Å and V = 361.84(2) ų. The structure exhibits a three‐dimensional arrangement of slightly distorted CuSe₄, Cd(Mn)Se₄ and GeSe₄ tetrahedras connected by corners. © 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim