Investigation of multiple layer heterojunction structures by means of an electron-beam microprobe-analyzer

Multiple layer heterojunction structures in the AlAs GaAs system are investigated by means of an X-ray microanalyzer. The quantitative determination of Ga and Al is extensively discussed. The correction for atomic number, absorption and secondary fluorescence are considered. For Al_xGa_1–xAs layers it is favourable for quantitative analysis the AlK_α-emission for x < 0,3 and the GaK_α-emission for x > 0.3. Some multilayer heterojunction structures of laser and luminescence diodes are shown in form of their AlK_α microprobe diagrams.     

On the Density of Synthetic Glasses in the System Soda Melilite – Gehlenite/Ákermanite

The solid density has been determined at 25°C by hydrostatic weighing along the binary systems Sm–Ge₅₀Åk₅₀ and Sm–Ge₇₀Åk₃₀. The molar volume V decreases linearly with increasing mole fraction of Ge_mÅk_n, x_B, according to δV/δx_B = −5.08 cm³ mol⁻¹ and −4.40 cm³ mol⁻¹ for Ge₅₀Åk₅₀ and Ge₇₀Åk₃₀, respectively. The results enable the molar fraction x_B to be determined from density measurements with an absolute error of ±5 mol%.     

Germanium Incorporation in Ga1–xInxAs LPE Layers and GaAs Bulk Crystals

Atomic absorption spectroscopy (AAS) is used to determine the Ge concentration in Ga_1–xIn_xAs and GaAs. The orientation dependence of Ge incorporation in (111)A-and (111)B oriented samples has been studied. The distribution coefficients for both the orientations were determined to be k_Ge^(111)A = 2.6 · 10⁻² and k_Ge^(111B = 1.4 · 10⁻² for Ga_1–xIn_xAs with 0 ≦ x ≦ 0.13. The differences between the two orientations are explained with the aid of the band bending model. Doping gradients in thick epitaxial layers and along crystal length of polycristalline TGS-grown GaAs ingots have been investigated too. In Ga_1–xIn_xAs layers any Ge concentration gradient couldn't be observed, but in TGS compact crystals Ge concentration increases with crystal length because the melt composition changes significantly during solidification. The results are compared with those of electrical measurements.     

Electroactive complex in thermally treated Ge-Si 〈Cu,Al〉 crystals

It is shown by Hall measurements that quenching complexly doped Ge_{1 − x} Si _x 〈Cu, Al〉 (0 ≤ x ≤ 0.20) crystals from 1050–1080 K leads to the formation of additional electroactive acceptor centers in them. The activation energy of these centers increases linearly with an increase in the silicon content in the crystal and is described by the relation E _k ^x = (52 + 320x) meV. Annealing these crystals at 550–570 K removes the additional acceptor levels. It is established that the most likely model for the additional electroactive centers is a pair composed of substituent copper and aluminum atoms (Cu _s Al _s ) or interstitial copper and substituent aluminum atoms (Cu _i Al _s ). It is shown that the generation of additional deep acceptor levels must be taken into account when using the method of precise doping of Ge_{1 − x} Si _x 〈Al〉 crystals with copper.     

Bandstruktur und thermoelektrische Eigenschaften von Wismut-Antimon-Legierungen

The authors investigate the influence of the electronic band structure on the thermoelectrical properties of Bi_xSb_1–x-alloys (0 ≦ (1–x) ≦ 1). For alloys with a high figure of merit the differential thermoelectric power and the ratio σ/k (σ: electrical conductivity, k: thermal conductivity) give an equal share to the increase of the figure of merit (in relation to pure bismuth). It become evident that the descrease of ratios of mobility for free carriers with different sign is overcompensated by means of modifications of the partial Seebeck coefficients. This is a result of changes in the electronic band structure and in the position of the Fermi level in an m-band-model with m > 2, impairing the electrical conductivity and the thermal conductivity in different wise.     

Optical absorption spectra of epitaxial layer - gradient layer - substrate systems

Theoretical relations are derived for the photon energy dependence of the absorption coefficient of epitaxial layer — gradient layer — substrate systems assuming a spatially linear variation of the gap energy in the gradient layer. It is shown that optical absorption measurements on such systems can be used to determine the gap energy of the epitaxial layer. Numerical calculations for the system GaAs_0.3P_0.7—GaAs_1—xP_x—GaP are presented.     

Quantitative analysis of Bi1−xSbx

The chemical composition of Bi_1-xSb_x alloys is determined by means of square-wave polarography, density measurements, measurements of the electron backscatter coefficient, and electron probe microanalysis. These experimental methods are discussed with respect to their results. At 10 keV, the electron probe microanalysis allows the determination of any antimony concentration without corrections (c_Sb = k_Sb).     

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.     

Theory of Systems of RodIike Particles: II. Thermotropic systems with orientation-dependent interactions

Abstract

A system of rigid, inpenetrable, rodlike molecules subject to orientation-dependent mutual attractions is treated by extension of the theory presented in the preceding paper. This energy is formulated, for a system at constant volume, by considering interactions between pairs of segments in contact, rather than in terms of interactions between entire molecules. The orientation-dependent energy between a pair of segments is taken to be proportional to cos² Ψ_ij, where Ψ_ij is the angle between the principal axes of their polarizability tensors, assumed to be cylindrically symmetric with respect to the molecular axis. A characteristic temperature T? measures the intensity of these interactions. The orientational energy of the system as a whole is of the form derived by Maier and Saupe. The orientation distribution with respect to the domain axis and the partition function are formulated with T? and the axial ratio x as parameters, Steric effects of molecular shape asymmetry, embodied in x, are of foremost importance. The reduced temperature [Ttilde]_ni = T_ni/xT? at which the nematic-isotropic transition takes place in the neat liquid decreases with decrease in x below its athermal limit x_erit = 6.417 for [Ttilde]^–1 _ni = 0. Both the entropy difference between isotropic and nematic phases and the orientational heat capacity C^p, are monotonic through the transition; C_p, diverges at a temperature appreciably above T_nl, where the metastable anisotropic state becomes unstable. Comparison of theory with experiments on those nematogens whose molecules can be approximated by rigid rods lends encouragement to the prospect of relating characteristics of the nematic-isotropic transition to molecular structure.     

Elasticity and Orientational Order in Some trans-p-n-Alkoxy-α-methyl Cyanophenyl Cinnamates

The splay (k ₁₁), twist (k ₂₂) and bend (k ₃₃) elastic constants determined by the Freedericksz method and the orientational order parameters (s) derived from optical measurements in the nematic phase of six homologues of trans-p-n-alkoxy-α-methyl cyanophenyl cinnamates (n OMCPC) are reported. The data close to the nematic-isotropic transition point (T _NI) are compared with T _NI and the heats of transition (ΔH). The temperature-variation of elastic constants is discussed in terms of existing theories. The pretransitional increase in the twist and bend constants near the nematic-smectic A transition point (T _NA ) of 10 OMCPC has also been analysed.     

Optical Reflection Spectra of Silicon Clathrate Compounds Ba8AgxSi46−x

Abstract

Optical reflection spectra are measured for Ba₈Ag_xSi_46?x (0 ≤ x ≤ 6) at room temperature. A systematic decrease in the plasmon energy is found with increasing x, indicating that the carrier concentration decreases with increasing x. When x increases, the superconducting transition temperature T_c decreases. An origin of the decrease in T_c is assigned to the decrease in the density of states at the Fermi level due to the decrease in the carrier concentration.     

Thermodynamic functions of (InP)x(InAs)1-x from 5 to 300 K

Temperature dependence of the heat capacity of the solid solutions (InP)_x(InAs)_1-x has been investigated over the temperature range from 5 to 300 K. Solid solution samples with x = 1; 0.8; 0.6; 0.4; 0.2; 0.1; 0 produced by means of crystallization from the melt the following annealing and diffusion in the powder at temperature close to solidus have been studied. The absolute values of enthalpy H, entropy S, Gibbs free energy Z as well as their deviations from the additive values have been obtained for the (InP)_x(InAs)_1-x system from the analysis of the thermodynamic properties of the solid solutions of the III–V compounds at low temperatures. The curves for the characteristic temperatures (T) of the solid solutions studied show that the change of the character of the phonon spectrum is negligible, the thermodynamic functions are close to additive, the influence of the enthalpy and entropy of mixing on the deviations from the additive enthalpy and entropy of the alloys is insignificant.     

X-ray diffraction studies of interdiffusion in InAs—GaAs powder blends

Interdiffusion in the pseudobinary system In_xGa_1−xAs is investigated by means of X-ray diffractometry of annealed powder blends. The interpretation of the experimental results by means both of the concentric spheres and concentric cubes model yields for In_0.43Ga_0.57As an activation energy of (5.39 ± 0.11) eV. This value shows that the diffusion mechanism could be a vacancy one as in the pure compounds. According to the lower atomic radius the Ga atoms within the temperature interval of 550–625 °C diffuse more easily than the In atoms.     

LPE Growth of Ga1 – xInxSb Multi-grading Layers

Electronegativity difference approach (ENDA) has been successfully employed to obtain good prediction of the In atomic fraction, energy bandgap and lattice constant of the Ga_{1 – x}In_xSb/Ga_{1 – y}In_ySb/GaSb system. Nearly lattice-matched GalnSb epilayers with In atomic fraction of 0.42 have been obtained by the liquid phase epitaxy. The cut-off wavelength was 2.2 μm at room temperature.     

In-situ doping of LPCVD poly silicon (II). Temperature influence

Poly silicon deposition by pyrolysis of silane under low pressure conditions has been investigated with respect to the influence of temperature when simultaneously in-situ doping of the deposited layer takes place. The growth rate of poly silicon is retarded in the presence of phosphine provided that a certain lower PH₃/SiH₄-ratio has been exceeded. It has been shown how that lower ratio depends on temperature. Increasing PH₃/SiH₄-ratio not only slows down layer growth rate but also the apparent activation energy of the layer forming reaction. An empirical equation describing the temperature dependence of that activation energy has been derived. Phosphine adsorption has been discussed as a cause of both layer growth rate and activation energy reduction. Additionally, incorporation of phosphorus during layer growth has been investigated with respect to the total amount and the electrically active concentration, the latter measured after a postdeposition anneal at 1000 °C.     

The cation distributions in the spinel phases of the mixed vanadium oxides LiZnx V2-xO4 and LiMgx V2-xO4

Cation distributions and values of the oxygen parameter in the spinel phases of mixed vanadium oxides with the general formula Li_xMeV_2–xO₄, where Me is Zn or Mg, respectively, were studied by X-ray diffraction measurements. The distribution of cations on the sublattices in the zinc series can be expressed by the formula Li_1–x–δ Zn_x V_δ [Li_x+δ V_2–x-δ] and for the magnesium series by the formula Li_1–δ V_δ [Li_δ Mg_x V_2–x-δ]. For the zinc series a general uniform increase of the oxygen parameter was determined. The substitution of magnesium ions leads to an anomal course of this parameter, which is possible to explain by substitution of magnesium ions on intersticial sites. The results are discussed using the values of the Madelung energy. The recently observed tetragonal deformation of the spinel lattice for the bulk composition Li Mg_0.375 V_1.625 O₄ is explained by a cooperative Jahn Teller effect of V⁴⁺ ions.     

Experimental and first‐principle investigation of Cu‐doped ZnO ferromagnetic powders

Zn_1‐xCu_x O powders were synthesized by using sol‐gel method. Electronic band structure and ferromagnetic properties of Zn_1‐xCu_x O powders were studied experimentally and theoretically. The simulations are based upon the Perdew‐Burke‐Ernzerhof form of generalized gradient approximation within the density functional theory. Zn_1‐xCu_x O shows dilute ferromagnetism, as a saturated magnetization of 0.9×10^‐3emu/g was observed for Zn_0.95Cu_0.05O powders. The strong p ‐d hybridization between Cu and its four neighbouring O atoms is responsible for the ferromagnetism. Comparing with ZnO whose Fermi level locates at the valence band maximum, the Fermi level of the Zn_1‐xCu_x O shifts upward into the valence band and hence the Zn_1‐xCu_x O system exhibits theoretically a p ‐type metallic semiconducting property. The Zn_1‐xCu_x O system may be a potential candidate in spintronics. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Some crystallographic trends into the Na1+xZr2SixP3-xO12 System

This paper gives a contribution to the crystallographic characterization of the Na_1+xZr₂Si_xP_3-xO₁₂ system, useful e.g. to analyze the right stoichiometric composition of a synthesized sample concerning this family of compounds, by powders X-ray diffraction analysis. In the paper relations are in fact displayed to exist among d_hkl, unit cell parameters and the stoichiometric parameter x. Such relations show to be linear for d_hkl and a₀, while the relation trend involving c₀ let one think the system is isostructural in the atomic arrangement for the ranges x < 2 and x<2, respectively, to the end terms with x=0 and x=3.     

Crystal and magnetic structures of the Bi1 ? xSrxFeO3 ? δ and Bi1 ? xAxFe1 ? xMnxO3 (A = Sr, Ca) solid solutions

Bi_{1 − x} Sr _x FeO_{3 − x/2} (I), Bi_{1 − x} Sr _x Fe_{1 − x} MnxO₃ (II), and Bi_{1 − x} Ca _x Fe_{1 − x} Mn _x O₃ (III) solid solutions have been obtained. Their magnetization has been measured by X-ray and neutron diffraction and M?ssbauer spectroscopy. According to the M?ssbauer spectroscopy data, iron ions are in the trivalent state in system I. Near the concentration x ≈ 0.2, rhombohedral distortions (sp. gr. R3c) are transformed into tetragonal (P4/mmm). The symmetry of system II changes at x > 0.2 (R3c → R3c), whereas orthorhombic distortions (R3c → Pbnm) arise in system III at x > 0.2. The magnetic structure is antiferromagnetic (of G type). The samples of systems II and III exhibit weak ferromagnetism at x > 0.2 due to the Dzyaloshinski-Moriya interaction.     

Growth and Properties of Trigonal Aluminium Gallium Orthophosphate Single Crystals Al1-xGaxPO4

The new piezoelectric solid solution aluminium gallium orthophosphate single crystals have been successfully developed for the first time in our laboratory by hydrothermal method. It has been demonstrated that these crystals are homogeneous, and isomorphous with AIPO₄ and GaPO₄. The solid solution crystals Al_1-xGa_xPO₄ are easy to grown and have better optical quality than those of AlPO₄ crystals under the same conditions. A phase transition temperature T_α-β is 587 ± 3 °C for x = 0.12. Cell parameters, and elastic, piezoelectric, and dielectric constants are given.