The influence of growth patterns on the transmission properties of nonabrupt GaAs/AlxGa1−xAs heterojunctions

The influence of growth patterns in the transmission properties of nonabrupt GaAs/Al_xGa_1−xAs heterojunctions is investigated. Five interfacial growth patterns, representative of interfacial alloy variations generated by different growth techniques, are used. It is shown that carrier transmission depends on the type of the aluminum molar fraction variation through the interface. A study of the role of the interface width in carrier transmission is done for each compositional growth profile.     

Interface effects in the high electric field resonances of single AlxGa1-xAs non-abrupt barriers in GaAs

The influence of nonabrupt interfaces in the high electric field resonances of single Al_xGa_1-xAs barriers in GaAs is studied. The resonances are considerably smoothed when interfacial widths are as small as two GaAs lattice parameters. Several resonances in the transmission coefficient of a 0.154 eV electron through a non-abrupt Al_xGa_1-xAs single barrier in GaAs, with height of 240 meV and 200 Å of width, can even disappear if interfacial widths of four GaAs lattice parameters are considered. Interface effects are shown to be more important for heavy holes than for electrons.     

Energy levels of single nonabrupt GaAs/AlxGa1-xAs quantum wells

Energy levels of electrons in nonabrupt GaAs/Al_xGa_1-xAs single quantum wells are calculated with and beyond the constant interfacial effective mass approximation (CIEMA), and compared with those of abrupt GaAs/Al_xGa_1-xAs quantum wells. For a given interface width, the energy levels calculated with the CIEMA are higher than those calculated beyond it, but both are higher than those of the abrupt semiconductor quantum well. The shifts of the energy levels increase with the interfacial width of the nonabrupt quantum well, as well as with the degree of interfacial asymmetry.     

Small-signal ac response of Ba1−xLaxF2+x crystals

The small-signal ac response of single crystalline Ba_1?xLa_xF_2+x solid solutions has been studied in the temperature region 300–820 K, and in the frequency range 4×10^?2–5×10⁴ Hz. At low temperatures the frequency dispersion is dominated by processes in the bulk. Apart from the bulk conductance these processes include a dielectric response determined by broad distributions of dipole-like relaxations. The increase of the static dielectric constant with increasing solute content is explained. At higher temperatures the frequency dispersion reflects interfacial phenomena. In addition to an almost ideal interface capacitance, phenomena are observed which indicate the growth of a surface layer due to a reaction between the electrolyte, and residual oxygen and/or water vapour from the ambient.     

Influence of annealing temperature on structural, optical and magnetic properties of Mn-doped ZnO thin films prepared by sol-gel method

Thin films of Zn_1−xMn_xO (x=0.01) diluted magnetic semiconductor were prepared on Si (1 0 0) substrates by the sol-gel method. The influence of annealing temperature on the structural, optical and magnetic properties was studied by X-ray diffraction (XRD), atom force microscopy (AFM), photoluminescence (PL) and SQUID magnetometer (MPMS, Quantum Design). The XRD spectrum shows that all the films are single crystalline with (0 0 2) preferential orientation along c-axis, indicating there are not any secondary phases. The atomic force microscopy images show the surfaces morphologies change greatly with an increase in annealing temperature. PL spectra reveal that the films marginally shift the near band-edge (NBE) position due to stress. The magnetic measurements of the films using SQUID clearly indicate the room temperature ferromagnetic behavior, and the Curie temperature of the samples is above room temperature. X-ray photoelectron spectroscopy (XPS) patterns suggest that Mn²⁺ ions were successfully incorporated into the lattice position of Zn²⁺ ions in ZnO host. It is also found that the post-annealing treatment can affect the ferromagnetic behavior of the films effectively.     

Molecular beam epitaxy and optical evaluation of AlxGa1-xAs

The epitaxial growth of Al_xGa_1-xAs on GaAs by the molecular beam vaporization method was investigated. The ratio of Al to Ga incorporated into the film was monitored by a mass spectrometer. Observations by reflection of high energy electron diffraction during growth and measurements of optical reflectivity suggest that it is possible to produce high quality Al_xGa_1-xAs single crystal thin films by the molecular beam method. Films grown with x varying from 0.1 to 0.47 were studied.     

Photoluminescence of strained-layer superlattices

The strained-layer superlattices (SLS's) of In_xGa_1?xAsGaAs and the single hetero-structure of In_xGa_1?xAs on GaAs were grown by MBE method. The samples obtained have a perfect surface morphology. The alloy composition of In_xGa_1?xAs layer and the growth rate were determined with high accuracy by in situ observation of the intensity oscillation of RHEED pattern. Photoluminescence peak energies of SLS's are in agreement with the calculated value by the Kronig-Penny analysis.     

The dependence of the structural and optical properties on the Te mole fraction in ZnS1−xTex/GaAs heterostructures

Lattice-mismatched ZnS_1−xTe_x epilayers with various Te mole fractions on GaAs (100) substrates were grown by double well temperature gradient vapor deposition. X-ray diffraction patterns showed that the grown ZnS_1−xTe_x layers were epitaxial films. The photoluminescence spectra showed that the peak position of the acceptor-bound exciton (A⁰, X) varied dramatically with changing the Te mole fraction and that the behavior of the (A⁰, X) peak position of the ZnS_1−xTe_x epilayers with a small amount of the Te mole fraction was attributed to a bowing effect. The reflectivity and ellipsometry spectra showed that the absorption energy peak was significantly affected due to the Stoke's effect. These results provide important information on the structural and optical properties of ZnS_1−xTe_x/GaAs heterostructures for improving optoelectronic device efficiencies operating in the spectral range between near ultraviolet and visible regions.     

The band structure and optical properties of the CdxHg1-xTe mixed crystals

The band structure of the Cd_xHg_1?xTe mixed crystals have been computed on the basis of tight-binding interpolation scheme and virtual crystal approximation. The bowing parameters of main energy gaps have been calculated as well as composition dependence of position of E₁ and E₁ + Δ₁ maxima in reflectivity spectra. Good agreement with previously reported experimental data has been found.     

Synthesis and characterization of Ni-Zn ferrite nanoparticles

Nickel zinc ferrite nanoparticles Ni_xZn_1−xFe₂O₄ (x=0.1, 0.3, 0.5) have been synthesized by a chemical co-precipitation method. The samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, electron paramagnetic resonance, dc magnetization and ac susceptibility measurements. The X-ray diffraction patterns confirm the synthesis of single crystalline Ni_xZn_1−xFe₂O₄ nanoparticles. The lattice parameter decreases with increase in Ni content resulting in a reduction in lattice strain. Similarly crystallite size increases with the concentration of Ni. The magnetic measurements show the superparamagnetic nature of the samples for x=0.1 and 0.3 whereas for x=0.5 the material is ferromagnetic. The saturation magnetization is 23.95 emu/g and increases with increase in Ni content. The superparamagnetic nature of the samples is supported by the EPR and ac susceptibility measurement studies. The blocking temperature increases with Ni concentration. The increase in blocking temperature is explained by the redistribution of the cations on tetrahedral (A) and octahedral (B) sites.     

Vapor–solid–solid growth of Ge nanowires from GeMn solid cluster seeds

We describe the fabrication of Ge nanowires during a single co-deposition step of Ge and Mn at high temperature. In these experimental conditions, a phase separation occurs and two different phases Ge and Ge_1 ? xMn_x are formed with Ge_1 ? xMn_x in the shape of small clusters distributed randomly in the Ge matrix. Because of the high deposition temperature, a new Ge_1 ? xMn_x phase with low eutectic point is stabilized; this phase is different from the one (commonly Ge₃Mn₅) stabilized at lower temperature. During the growth process at 350 °C, the crystalline clusters remain solid but they are highly mobile and can float at the surface, serving as seeds to direct the growth of crystalline Ge nanowires from the vapor. The sketch steps of NWs formation are first the phase separation with formation of specific Ge_1 ? xMn_x critical nuclei with low eutectic point and second the growth of Ge NWs directed by the Ge_1 ? xMn_x solid cluster seeds. Ge NWs growth is forced along particular crystalline axis by the cluster seeds that lower the interfacial energy Ge/Ge_1 ? xMn_x and the energy formation of the germanium crystal stabilizes the cluster position at the tip of the NWs. The density of NWs can be tuned by varying the nominal Mn concentration since this density is related to the number of clusters with the specific Ge_1 ? xMn_x phase (with low eutectic point). The single step MBE process presented here has the main advantage to fully avoid any incorporation of unintentional impurity into Ge nanowires (apart from Mn atoms) and could be applied to several other systems. This work also provides new insights into the vapor–solid–solid growth mechanisms of Ge NWs.     

Mössbauer studies of Fe x Mn1 − x S single crystals

Single crystals of iron manganese sulfides Fe _x Mn_{1 ? x} S (0.25 ≤ x ≤ 0.29) are experimentally investigated using Mössbauer spectroscopy and x-ray diffraction. The Mössbauer spectra measured at 300 K exhibit a single broadened line characteristic of paramagnets. The isomer shift of this line is equal to 0.92–0.94 mm/s, which is typical of Fe²⁺ ions in the octahedral position. The quadrupole splitting (0.18–0.21 mm/s) suggests a distortion of the coordination polyhedron of iron ions in the Fe _x Mn_{1 ? x} S compounds.     

Formation of a system of InGaAs quantum wires in a gallium arsenide matrix

The formation of a system of one-dimensional quantum conductors in porous multilayer In_xGa_1?xAs/GaAs structures with a two-dimensional charge-carrier gas in the In_xGa_1?xAs layers is discussed. The transition from the single-crystalline to porous matrix is studied with scanning atomic force microcopy. A decrease in the dimensionality of the electron-hole gas in the objects, i.e., a transition from the two-dimensional to a one-dimensional system, is established by analyzing the dependences of the position and width of a spectral line in the photoluminescence spectra on the etching time. Both multilayer periodic superlattices and a structure with a single In_xGa_1?xAs layer located near the surface of gallium arsenide are studied. The electrophysical characteristics of electrons in the porous superlattices are measured as functions of temperature. They confirm the formation of a new structure and indicate a change in the mechanism of electron scattering in the quasi-one-dimensional transport channels formed in the system.     

Critical interface in two dimensions

The interface separating two-dimensional phases close to a critical point is observed in the ferroelectric BaZr_xTi_1−xO₃ by transmission electron microscopy. The temperature dependence of the interfacial thickness is determined and compared with theoretical predictions.     

Morphological evolution of SiGe layers

Extensive research activity has been devoted to self-assembly of very small coherent islands. However, while island formation is commonly described by a widely used S-K growth scheme, more complex mechanisms based on competitive effects of kinetics and thermodynamics take place during the epitaxy of Si_1−xGe_x on Si(0 0 1). The aim of this paper is to explain the formation and the evolution of Si_1−xGe_x islands on Si(0 0 1). The paper presents a comprehensive investigation of the different growth modes of Si_1−xGe_x films (with x varying from 0 to 1) on Si(0 0 1) and Si(1 1 1). The results are presented in the form of kinetic morphological growth diagrams of as-grown samples. Two and four growth regimes are distinguished on (1 1 1) and (0 0 1) respectively. These growth regimes correspond to different levels of relaxation. In particular the four regimes observed on Si(0 0 1) correspond to (i) no relaxation in regime I (2D layer), (ii) 15-20% relaxation in regime II (“huts” islands with (1 0 5) facets), (iii) 20% and 50% relaxation in regime III (in “huts” and “domes” respectively) and (iv) 50% and 80% relaxation in regime IV (“domes” with bimodal size distribution). Every growth regime characteristic of as-grown sample is also associated with a specific equilibrium steady state morphology which is obtained after long-term annealing of the as-grown samples. In the two first regimes (no or small strain relaxation) the equilibrium morphology of highly strained Si_1−xGe_x deposits corresponds to (1 0 5) faceted islands. We show that these islands are stabilised by the compressive stress. As soon as strain is released, (1 0 5) facets disappear at the expense of the (1 1 3) and (1 1 1) facets and first-order transition occurs between “huts” and “domes” islands.     

Electron energy spectrum and the specific features of the optical absorption of solid solutions of magnesium,aluminum, and boron oxides

A calculation scheme based on density functional theory, generalized for the case of periodic structures, is used to calculate the electron energy spectrum of binary and ternary oxides of magnesium, aluminum, and boron with MgB _x Al_2?x O₄ spinel structure. The dependences of the electron energy and geometric characteristics of the studied structures on the degree of substitution x are examined. It is shown that, by changing this parameter (i.e., the composition of the ternary oxide), it is possible to adjust the bandgap width and the position of the chemical potential for boron-magnesium-aluminum spinel. The dependence of the imaginary part of the relative permittivity on the energy of absorbed photons is obtained and analyzed.     

X-ray and IR analysis of Cu-Si ferrite

Polycrystalline soft ferrite, Cu_1+xSi_xFe_2−2xO₄ (x=0.0, 0.05, 0.1, 0.15, 0.2 and 0.3) were prepared by standard ceramic technique. The X-ray analysis confirmed the single phase formation of the samples up to x=0.1 beside a second phase for x?0.15. The lattice parameter was found to decrease with composition(x), which is attributed to ionic size difference of cations involved. The bulk density measurements shows two different behavior for x< >0.15. The IR spectra of Cu-Si ferrite system have been analyzed in the frequency range 200-1200 cm⁻¹. It revealed two prominent bands ν₁ and ν₂ which are assigned to tetrahedral and octahedral metal complexes, respectively. The position of the highest frequency band is around 575 cm⁻¹ while the lower frequency band is around 400 cm⁻¹.     

Fine structure of the cubic semiconductor compound Zn0.9Ni0.1S

The fine structure of a single crystal of the Zn_0.9Ni_0.1S cubic compound synthesized by the chemical transport method has been investigated at room temperature using thermal neutron diffraction. It has been found that the diffraction patterns of this compound, along with strong Bragg reflections of the facecentered cubic phase, include a system of diffuse maxima, which indicate the occurrence of a local deformation of the cubic structure by the Jahn-Teller nickel ions. Results of this experiment have been compared with the previously obtained data for the lightly doped Zn_{1 ? x} Ni _x Se single crystal (x = 0.002).     

Effect of Si substitution on electronic structure and magnetic properties of Heusler compounds Co2TiAl1−xSix

The electronic structures of Co-based Heusler compounds CoTiAl_1−xSi_x (x=0, 0.25, 0.5, 0.75 and 1) are calculated by first-principles using the full potential linearized augmented plane wave (FP-LAPW) method within GGA and LSDA+U scheme. Particular emphasis was put on the role of the main group elements. In recent years, the GGA calculations of Co₂TiAl (x=0) and Co₂TiSi (x=1) indicated that they are half-metallic, but the electronic structure of this compound with x=0.25, 0.5 and 0.75 has not been reported yet, neither theoretically nor experimentally. The calculated results reveal that these are half-metallic and exhibit an energy gap in the minority spin state and also show 100% spin polarization. The substitution of Al by Si leads to an increase in the number of valence electrons, with increasing x. Our calculated results clearly show that with the Si doping, the lattice parameter linearly decreases; bulk modulus increases, and the total magnetic moment increases. The calculated energy gap in the minority spin state, using GGA scheme, was smaller than that obtained by using LSDA+U scheme. The outcomes of this research also show that the Co-3d DOS and therefore, the magnetic properties of compounds are dependent on electron concentration of the main group elements and it will affect the degree of p-d orbital occupation.     

Impedance-based interpretations in 2-dimensional electron gas conduction formed in the LaAlO3/SrxCa1−xTiO3/SrTiO3 system

Frequency-dependent impedance spectroscopy was applied to the 2-dimensioanl conduction transport in the LaAlO₃/Sr_xCa_1−xTiO₃/SrTiO₃ system. The 2-dimensional conduction modifies the electrical/dielectric responses of the LaAlO₃/Sr_xCa_1−xTiO₃/SrTiO₃ depending on the magnitude of the interfacial 2-dimensional resistance. The high conduction of the 2-dimensional electron gas (2DEG) layer can be described using a metallic resistor in series with two parallel RC circuits. However, the high resistance of the 2-dimensional layer drives the composite system from a finite low resistor in parallel with the surrounding dielectrics composed of LaAlO₃ and SrTiO₃ materials to a dielectric capacitor. This change in the resistance of the 2-dimensional layers modifies the overall impedance enabled by the presence of the interfacial layer due to Sr_xCa_1−xTiO₃, which alters the charge transport of the 2-dimensional layer from metallic to semiconducting conduction. A noticeable change is observed in the capacitance Bode plots, indicating highly amplified dielectric constants compared with the pristine SrTiO₃ substrates and Sr_xCa_1−xTiO₃ with a greater Ca content.