X-Ray double crystal rocking curves of Ga1−x Al x As/GaAs laser structures

Summary The double crystal X-ray rocking curves of Ga_1−x Al _x As/GaAs laser structures, with both a single and double confinement, have been calculated on the basis of the Takagi-Taupin dynamica theory. It has been demonstrated that very small changes in the thickness and composition of the active and the internal confining layers give rise to dramatic modifications of the rocking curves; this offers in principle a very powerful tool for measuring very precisely thickness and composition of these layers. However, the shape of the Bragg peak of the external confining layers exhibits a nearly period behaviour as a function of the thickness of the active or the internal confining layer; a simple relation between the thickness period and the composition difference of the considered layers has been obtained for the first time. Finally, the effect of the interchange of the confining layers on the rocking curves has been discussed.     

A thermodynamic lattice theory on melting curve and eutectic point of binary alloys. Application to fcc and bcc structure

A thermodynamic lattice theory has been developed for determination of the melting curves and eutectic points of binary alloys. Analytical expressions for the melting curves of binary alloys composed of constituent elements with the same structure have been derived from expressions for the ratio of root mean square fluctuation in atomic positions on the equilibrium lattice positions and the nearest neighbor distance. This melting curve provides information on Lindemann’s melting temperatures of binary alloys with respect to any proportion of constituent elements, as well as on their eutectic points. The theory has been applied to fcc and bcc structure. Numerical results for some binary alloys provide a good correspondence between the calculated and experimental phase diagrams, where the calculated results for Cu_1−x Ni _x agree well with the measured ones, and those for the other alloys are found to be in a reasonable agreement with experiment.     

Ionic potential analysis of RHEED rocking curves from fluoride structures

Rocking curves of reflection high energy electron diffraction from MnF₂ and CaF₂(1 1 1) surfaces have been calculated with ionic scattering potentials. The potentials were derived from tabulated X-ray scattering factors and expressed in the Doyle-Turner representation. The Coulomb potential of the ions was introduced into the calculations of diffracted intensity using dynamical diffraction theory. Comparison of the calculated and measured rocking curves for CaF₂(1 1 1) surface confirmed that it is bulk terminated; the correction to volume average potential was found to be 1.1 eV (much less than when using atomic potentials). Analysis of the rocking curves from ultra-thin MnF₂ layers on CaF₂(1 1 1) indicated that MnF₂ inherits the cubic lattice of fluorides up to a thickness of three molecular layers.     

2p3/2?13x?1?3x?13d?1 X-ray satellites spectra in the Lα1 region of 4d transition elements

The X-ray satellite spectra arising due to 2p _3/2 ⁻¹3x ⁻¹−3x ⁻¹3d ⁻¹ (x ≡ s, p, d) transition array, in elements with Z = 40 to 48, have been calculated, using available Hartree-Fock-Slater (HFS) data on 1s ⁻¹−2p ⁻¹ 3x ⁻ and 2p _3/2 ⁻¹−3x ⁻¹,3x ⁻¹ Auger transition energies. The relative intensities of all the possible transitions have been estimated by considering cross — sections for the Auger transitions simultaneous to a hole creation and then distributing statistically the total cross sections for initial two hole states 2p _3/2 ⁻¹−3x ⁻¹ amongst various allowed transitions from these initial states to 3x ⁻¹ 3d ⁻¹ final states by Coster-Kronig (CK) and shake off processes. In both these processes initial single hole creation is the prime phenomenon. Each transition has been assumed to give rise to a Gaussian line and the overall spectrum has been computed as the sum of these Gaussian curves. The calculated spectra have been compared with the measured satellite energies in Lα₁ spectra. Their intense peaks have been identified as the observed satellite lines. The peaks in the theoretical satellite spectra were identified as the experimentally reported satellites α₃, α₄ and α₅, which lie on the high-energy side of the Lα₁ dipole line.     

Raman scattering and hot luminescence spectra of Zn1 − x Mn x Te quantum wires

The Raman scattering and luminescence spectra of Zn_{1 − x} Mn _x Te (0 ≤ x ≤ 0.6) quantum wires have been investigated. The quantum wires have been grown by molecular-beam epitaxy on the (100)GaAs substrate with Au used as a catalyst. The spectrum of optical phonons in ZnMnTe quantum wires varies with a variation in x in accordance with an intermediate (between one- and two-mode) type of transformation. The optical phonon spectrum has been analyzed in terms of the microscopic theory. It has been demonstrated that the experimental data can be brought in accord with the theory by properly modifying the calculated density of phonon states for ZnTe. The spatial confinement has been found to affect the electronic states in Zn_{1 − x} Mn _x Te quantum wires.     

Atomic force microscopy study of Gd(Co,Cu)<Subscript>5</Subscript> single-crystal surface relief

A comprehensive study of the magnetic properties, magnetic viscosity, and microstructure of Gd(Co,Cu)₅ intermetallic compounds has been carried out using vibrating-coil magnetometry and atomic force microscopy. High-quality images of liquation inhomogeneities at the basal planes of Gd(Co_{1 − x} Cu _x )₅ single crystals with x = 0.1−0.6 have been obtained. Their size distribution curves have been plotted, and a correlation between the size of the structure inhomogeneities and the effective width of domain walls has been established.     

Photoluminescence properties of Eu3+-doped Cd1−x Zn x S quantum dots

Eu³⁺-doped Cd_1−x Zn _x S (0 ≤ x ≤ 0.5) quantum dots (QDs) have been synthesized using wet chemical precipitation method. X-ray diffraction and transmission electron microscope have been used for the crystallographic and morphological characterization of synthesized nanomaterials. In order to understand the spectral characteristics of doped QDs, N₂-laser induced time resolved spectra have been recorded. Excited state lifetime values for dichromatic emission (red and violet) attributed to ⁵D₀ → ⁷F_J (J = 1, 2) transitions of Eu³⁺ and host lattice transitions have been calculated from the recorded luminescence decay curves. Decay time dependence on the dopant concentration (0.01–10 at. wt% of Cd²⁺) has been studied in detail.     

Spin-orbit coupling in the ground and low-lying excited states of HF<Superscript>+</Superscript> and HF<Superscript>-</Superscript>

Electronic structure and spectroscopic properties B _e, ω_e, ω_e x _e, α_e, T _e of ground state and the low-lying excited states of HF⁺ and HF^- molecular ions were investigated within scalar relativistic multireference configuration interaction with single and double excitations framework using the GAMESS-US program package. All potential energy curves (PECs) were calculated using the relativistic complete active space self-consistent field/spin-orbit multi-configuration quasi-degenerate perturbation theory (CASSCF/SO-MCQDPT). The curves are all fitted to the analytical potential energy function (APEF), from which accurate spectroscopic constants are derived. The spin-orbit splitting was also been studied, the split value of X²P^{2}{\rm \Pi} state of HF⁺ is determined to be 288.38 cm^-1. The calculated properties are in good agreement with the available experimental value. Spectroscopic constants of the ground states of HF^- that have never been observed in experiment are obtained. These curves provide an interpretation of the known experimental observations on this system and suggest a number of further experiments which possible provide a critical test of this data.     

Elastic properties of TeO<Subscript>2</Subscript>–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–Ag<Subscript>2</Subscript>O glasses

A series of glasses [(TeO₂) _x (B₂O₃)_1−x ]_1−y [Ag₂O] _y with x = 70 and y = 10, 15, 20, 25 and 30 mol% were synthesised by rapid quenching. Longitudinal and shear ultrasonic velocity were measured at room temperature and at 5 MHz frequency. Elastic properties, Poisson's ratio, microhardness, softening temperature and Debye temperature have been calculated from the measured density and ultrasonic velocity at room temperature. The experimental results indicate that the elastic constants depend upon the composition of the glasses and the role of the Ag₂O inside the glass network is discussed. Estimated parameters based on Makishima–Mackenzie theory and bond compression model were calculated in order to analyse the experimental elastic moduli. Comparison between the experimental elastic moduli data obtained in the study and the calculated theoretically by the mentioned above models has been discussed.     

Simulation of the electronic structure of simple oxides BeO and SiO2 and complex oxides Be2SiO4 and Be2SixGe1 ? xO4 with the phenacite structure

The ab initio numerical calculations of the electronic structure of simple oxides BeO and SiO₂ and complex oxides Be₂SiO₄ and Be₂Si _x Ge_{1 − x} O₄ with the phenacite structure have been performed using the electron density functional theory. The calculations indicate that the main feature of the systems under investigation is the presence of oxygen states in both the valence and conduction bands. The splitting of the bottom of the conduction band has been revealed in the electronic structure of the Be₂Si _x Ge_{1 − x} O₄ system. The splitting width is about 1.5 eV. The main contribution to the formation of a narrow subband of the conduction band comes from the 2s and 2p states of oxygen and the 4d state of germanium. Microscopic models of the spatial localization of the electron density on lower energy states of the conduction band of oxide crystals have been developed using the Wannier function technique. The reflection spectra of BeO, SiO₂, and Be₂SiO₄ have been analyzed. The reported calculations of the electronic structure imply the exciton nature of the 9.7-eV reflection peak in the Be₂SiO₄ crystal.     

Studying the electronic structure of Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ti<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>2</Subscript> by X-ray resonance and absorption spectroscopy

Cr _x Ti_{1 − x} Se₂ (x = 0–0.83) solid solutions have been synthesized. Single crystals have been grown in the range of concentrations x = 0–0.83. Structural studies of samples have revealed that chromium atoms substitute titanium in the TiSe₂ matrix. The X-ray photoelectron spectra of the core levels, the resonance spectra of the valence bands in 1T-Cr _x Ti_{1 − x} Se₂, and the X-ray absorption spectra of titanium and chromium are studied. The titanium and chromium atoms are shown to have oxidation numbers of +4 and +3, respectively, in an identical octahedral environment. The local density of chromium states has been calculated. The results of the calculation agree well with the experimental data and indicate that the electronic 3d states of chromium substituting titanium in the matrix are spin-polarized and the density of chromium states is halfmetal magnet in behavior.     

Studies on electrical switching behavior and optical band gap of amorphous Ge–Te–Sn thin films

Amorphous thin film Ge₁₅Te_85−x Sn _x (1≤x≤5) and Ge₁₇Te_83−x Sn _x (1≤x≤4) switching devices have been deposited in sandwich geometry using a flash evaporation technique, with aluminum as the top and bottom electrodes. Electrical switching studies indicate that these films exhibit memory type electrical switching behavior. The switching fields for both the series of samples have been found to decrease with increase in Sn concentration, which confirms that the metallicity effect on switching fields/voltages, commonly seen in bulk glassy chalcogenides, is valid in amorphous chalcogenide thin films also. In addition, there is no manifestation of rigidity percolation in the composition dependence of switching fields of Ge₁₅Te_85−x Sn _x and Ge₁₇Te_83−x Sn _x amorphous thin film samples. The observed composition dependence of switching fields of amorphous Ge₁₅Te_85−x Sn _x and Ge₁₇Te_83−x Sn _x thin films has been understood on the basis of Chemically Ordered Network model. The optical band gap for these samples, calculated from the absorption spectra, has been found to exhibit a decreasing trend with increasing Sn concentration, which is consistent with the composition dependence of switching fields.     

Infrared lattice-reflection spectroscopy of Zn1−x CdxSe epitaxial layers grown on a GaAs substrate by molecular-beam epitaxy

Results are presented of the first measurements of infrared reflection spectra of Zn_1−x Cd_xSe films (x=0–0.55; 1) grown on a GaAs substrate by molecular-beam epitaxy. It is shown by a mathematical analysis of the experimental spectra that the investigated Zn_1−x Cd_xSe alloy system manifests a unimodal rearrangement of its vibrational spectrum as the composition is varied. Fiz. Tverd. Tela (St. Petersburg) 41, 982–985 (June 1999)     

Calculation of long-wavelength optical phonons and lattice reflection spectra of Al1−xGaxP mixed crystals

The two-mode behaviour of the infrared lattice vibrations of Al_1–x Ga _x P mixed crystals is described by the random-element-isodisplacement model with an effective electric field and phenomenological damping (EREI). The calculated long-wavelength transverse and longitudinal optical frequencies in dependence on the compositionx are in agreement with experimental measurements. The infrared lattice reflection spectra are calculated and fitted to the measured one by variation of damping parameters. The final values of the damping parameters are represented in dependence on the compositionx.This work has been done in the Arbeitsgemeinschaft AIII-BV-Halbleiter of the Karl-Marx-Universität Leipzig and was supported by the VEB Halbleiterwerk Frankfurt/Oder.     

Growth of wide band gap wurtzite ZnMgO layers on (0001) Al2O3 by radical-source molecular beam epitaxy

Wurtzite structure ZnMgO layers have been grown using radical-source molecular beam epitaxy on high-quality ZnO buffer layers grown on (0001) sapphire substrates. The thickness of the ZnO buffer layers is 300 nm, with full width at half maxim of the HR-XRD (0002) rocking curves as low as 25 arcsec. In-situ Reflection High-Energy Electron Diffraction (RHEED) was employed for the optimization of the ZnMgO growth. RHEED and X-Ray Diffractometry measurements did not reveal any phase change from the wurzite structure to the rocksalt structure. The C-lattice parameter of Zn_1−xMg_xO films decreased from 5.209 to 5.176 Å with increasing x to 0.2. The surface morphology of the samples was studied with atomic force microscopy. The root mean square roughness values of 200 nm thick ZnMgO (x=0.2) was less than 1 nm. The main photoluminescence peak of Zn_1−xMg_xO shifted to as high as 3.77 eV owing to the increasing Mg composition of up to x=0.2.     

The use of parallel and monochromatic beams for determining the composition of pseudobinary alloys by X-ray fluorescence

A simple X-ray fluorescence method using nearly parallel and monochromatic beams is analyzed and discussed in connection with the determination of the alloy fraction in pseudobinary A_1−x B _x C solid solutions. Applications to optoelectronic materials are considered. The reliability of the method has been analyzed and discussed for both bulk and thin film samples. Errors due to small angular divergence in the accepted beam are seen to be neglibile. Deviations from a symmetrical alignment of the sample can strongly influence the measured fluorescence intensity. The experimental analysis of this effect can be used for a perfect symmetrical orientation of thick samples. The measurement of the fluorescence intensity as a function of the glancing angle can give both thickness and composition in solid-solution thin-film samples. The contributions of the fluorescence radiation excited by the characteristic X-ray lines of atoms within the substrate is shown to be important in thin epitaxial layers. An approximate formula for calculating these contributions is given and discussed in the light of experimental results. Bulk crystals and thin-film samples, vapourphase grown Cd_1−x Zn _x S solid solutions, have been used for experiments in which the ZnK _α radiation excited by a MoK _α primary beam was measured.     

Magnetic spin excitations in Mn doped GaAs: a model study

We provide a quantitative theoretical model study of the dynamical magnetic properties of optimally annealed Ga_{1− x} Mn _x As. This model has already been shown to reproduce accurately the Curie temperatures for Ga_{1− x} Mn _x As. Here we show that the calculated spin stiffness are in excellent agreement with those which were obtained from ab-initio based studies. In addition, an overall good agreement is also found with available experimental data. We have also evaluated the magnon density of states and the typical density of states from which the“mobility edge”, separating the extended from localized magnon states, was determined. The power of the model lies in its ability to be generalized for a broad class of diluted magnetic semiconductor materials, thus it bridges the gap between first principle calculations and model based studies.     

Relationship between the magnetoresistance and the magnetocaloric effect in La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> manganites

A generalized expression relating the magnetoresistance of manganites La_{1 − x} Ag _x MnO₃ with the change in the magnetic entropy has been proposed. The correct inclusion of the acting mechanisms of appearance of the magnetoresistance is shown to lead to adequate agreement between the experimental and calculated values of ΔS _M .     

Thermal expansion and permittivity of (Ba<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript>2<Emphasis Type="Italic">x</Emphasis>/3</Subscript>)TiO<Subscript>3</Subscript> solid solutions

The strain, the thermal expansion coefficient, and the permittivity of ceramic samples of (Ba_{1 − x} Bi_2x/3)TiO₃ solid solutions with x = 0, 0.01, 0.03, and 0.05 have been studied in the temperature range 120–700 K. Based on an analysis of the results, the temperature-composition phase diagram has been refined, and the temperature dependence of the polarization has been calculated.     

Angular dependence of surface acoustic wave characteristics in AlN thin films on a-plane sapphire substrates

AlN thin films have been grown on a-plane sapphire (Al₂O₃(112̄0)) substrates. X-ray diffraction measurements indicate the films are fully c-plane (0001) oriented with a full width at half maximum of the AlN(0002) rocking curves of 0.92. The epitaxial growth relationships have been determined by the reflection high energy electron diffraction analysis as AlN[11̄00]//Al₂O₃[0001] and AlN[112̄0]//Al₂O₃[11̄00]. Angular dependence of important surface acoustic wave (SAW) characteristics, such as the phase velocity and electromechanical coupling coefficient, has been investigated on the AlN(0001)/Al₂O₃(112̄0) structure. While the SAW is excited at all propagation angles with an angular dispersion of the phase velocity in the range of 5503–6045 m/s, a higher velocity shear-horizontal (SH) mode is observed only at 0°, 105° and 180° off the reference Al₂O₃[11̄00] over a 180° angular period. The phase velocity of the SH mode shows dispersion (6089–6132 m/s) as a function of the SAW wavelength. Temperature coefficients of frequency are also demonstrated for both modes. PACS 81.15.Hi; 77.84.-s; 77.65.Dq