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.     

Characterization of vapor grown (001) GaAs1−xPx layers by selective photo-etching

The technique of selective photo-etching known for GaAs was successfully applied to (001)GaAs_0.6P_0.4 and it can also be used for the characterization of phosphorus-rich GaAs_1?xP_x layers and for GaP. Using this technique misfit dislocations in GaAs_1?xP_x up to now mainly examined with the transmission electron microscope (TEM) in the GaAs_1?xP_x transition layer could be examined in a density range which is hardly accessible for observation with the TEM. Misfit dislocations are also observed in GaP and GaAs_1?xP_x layers when these are heavily doped with nitrogen. The dislocation density of GaAs_1?xP_x layers of constant composition is compared with predictions from a model of Abrahams et al. for the dislocation density of GaAs_1?xP_x as a function of the rate of grading. Within this model our results can only be explained when the asymmetry factor m between the length to distance ratio of misfit dislocations in the transition layer is about two orders of magnitude greater than the value previously derived from TEM studies of GaAs_1?xP_x layers.     

Electron-beam microprobe analysis of epitaxial GaxIn1−xP solid solutions

Ga_xIn_1–xP epitaxial layers were investigated by means of a scanning electron microscope X-ray microanalyser. The conditions for quantitative X-ray microprobe analysis are discussed. The growth of Ga_xIn_1–xP layers is possible on GaP substrates with x > 0,8 and on GaAs with 0,3 < x < 0,6. For the deposition of layers with 0,5 < x < 0,8 two substrate materials are possible: GaAs_1–xP_x or Ga_xIn_1–xP with suitable compositions. These materials must be epitaxially deposited by step by step layer growth or by vapour phase epitaxy, respectively.     

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.     

Characterization of defects in GaP and GaAsP graded heterojunctions by transmission electron microscopy

Misfit dislocations are observed in graded heterojunctions GaAs_1?xP_x by electron microscopy. Results are in agreement with previous work concerning the nature of the dislocations (Lomer and 60° dislocations) and their density dependence on the phosphorus gradient. The discussion concerns the formation of Lomer dislocations and the possibility of reducing the density of inclined dislocations which reach the surface of the epitaxial layer. GaP substrates, S-doped, are examined by transmission electron microscopy. Numerous defects such as Frank loops, perfect loops, helical dislocations and precipitates are observed. A GaP homoepitaxial layer realized on this substrate is free from these defects but exhibits stacking faults. A zinc diffusion does not produce additional defect but a 1000 Å thick amorphous layer is observed a at the surface.     

Das quaternäre Schmelzdiagramm Ga?As-P Sn

The pseudoternary cut Sn GaAs GaP of the quarternary system Ga As P Sn has been investigated in a temperature range from 860 to 1200°C. The calculation based on Vieland's method by use of Darken's quadratic formalism. A comparision between the solvents tin and gallium shows a higher solubility in tin for the mixed crystal Ga_xPAs_1−x and a stronger variation of the composition along the growth direction.     

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.     

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%.     

Thermally stimulated relaxation of misfit strains in Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>x</Subscript>/Si(100) heterostructures with different buffer layers

The regularities of the defect formation in Si_1−x Ge_x/Si heterostructures (x = 0.15 and 0.30), consisting of a low-temperature Si buffer layer and a SiGe solid solution, during their growth and subsequent annealings at temperatures 550–650°C are investigated by the methods of optical and transmission electron microscopy and X-ray diffraction. It is shown that the misfit-strain relaxation by plastic deformation under the conditions studied occurs most intensively in heterostructures with low-temperature SiGe buffer layers. The maximum degree of misfit-strain relaxation (no higher than 45%) is observed in the heterostructures with x = 0.30 after annealing at 650°C. The results obtained are explained by the effect of the nature and concentration of dislocation-nucleation centers, existing in low-temperature buffer layers, on the characteristics of the formation of a dislocation structure in the heterostructures under consideration.     

A complex method for structural investigation of GaAs1−xPx epitaxial layers grown on GaAs substrates

A universal X-ray diffractometer is used for the structural complex investigation of GaAs_1−xP_x epitaxial layers, grown on the (100) face of GaAs substrate. Information is obtained from the analyses of diffraction patterns for some qualities of the epitaxial layers: crystallographical orientation, composition, thickness, as well as structure of the transition region. The suggested complex method has important advantages against the standard Laue method. It is far easier express and convenient for serial investigations.     

Polytypes of the system Fe1−xS

The structures of Fe_1−xS (0 < x < 0.135) are members of a family of derivative structures in the sense of Megaw. The NiAs structure as aristotype is the simplest and most symmetrical member of this family. The other polytypes of Fe_1−xS may be derived as hettotypes by lowering the symmetry. The loss of symmetry in changing from the aristotype to the hettotype may be of various kinds: small displacements of Fe and S atoms from the NiAs positions and/or Fe vacancies in some atomic planes occur. The structures of the system Fe_1−xS are interpreted as OD structures consisting of OD layers. The symmetry relations of the structures of FeS and Fe₇S₈ are described by OD groupoid families. Polymorphism, twinning and stacking faults are explained on this basis.     

AlxGa1−xAs LPE growth

Al_xGa_1−xAs LPE growth was studied within the temperature range of 930–900°C with Al concentrations in solutions from 0.04 to 2.4 at.%. AlAs concentration in layers has been shown to grow with the cooling rate increase of solution. Interface and volume nucleation parameter dependence of K_i and K_v and formation time t_f on Al concentration in Ga solution have been found. Addition of Al to Ga solution increases critical values of As supersaturation (supercooling) and, as a result, increase in thickness of Al_xGa_1−xAs layers compared with GaAs layers have been determined in spite of As concentration lowering in Ga solution.     

Calculations of homogeneous region in Ga1−xAlxAs and GaAs1−xPx ternary solid solutions

Ternary solid solutions of A^IIIB^V compounds are considered as pseudobinary A(x)^IIIB(x)^v compounds, where the behaviour of A(x)^III and B(x)^v pseudoatoms is quite similar to A^III and B^v atoms in a binary A^IIIB^v crystal. Weak dependence of point defect contribution into Gibb's energy of A^IIIB^v crystal on its defect nature, random character of ternary solid solutions of A^IIIB^v compounds allow to use already for binary compounds developed formalism in the determination of component thermodynamic potentials of solid solution. Basing on literature data for the equilibrium solidus of AlAs the approximation for the temperature dependence of thermodynamic potential of an AB quasimolecule in A^IIIB^v crystal is revised. This result together with the well-known parameters for the equilibrium liquidus in Ga–P, Ga–As, and Al–As systems were used for calculations of the nonstoichiometric factor at the boundary of a homogeneous region in Ga_1−xAl_xAs and GaAs_1−xP_x ternary solid solutions. The results are compared with the known literature data.     

The dependence of surface morphology of GaSb- and AlxGa1–xSb epitaxial layers on the conditions of LPE growth

The present work shows the dependence of surface morphology of the GaSb and Al_xGa_1–xSb epitaxial layers on the conditions of LPE growth. It is found that for LPE growth at 500 °C a supersaturation of 5—10 °C and a cooling rate of 0.24—0.4°C/min for GaSb epitaxial layers and 0.8—1.2 °C/min for Al_xGa_1–xSb epitaxial layers is necessary to obtain a flat and smooth surface.     

Extention of the “three partial rates” model to layer growth and composition of the ternary Si1–(x+y)GexBy system

The previously published “three-partial rates” model of Si_1–xGe_x layer growth (Kühne 1992) has been extended for describing Si_1–(x+y)Ge_xB_y ternary systems by taking additionally into account a partial rate of boron and a partial rate of boron-originated silicon deposition. Silicon deposition from a silane-germane-diborane-helium gas mixture at T = 500°C and P_total = 0.2 Torr is clearly enhanced by the presence of germane though to a low degree only, but not by diborane. Germanium partial rate as well as boron partial rate depends linearly on the partial pressure of the respective source gas. For illustration the data published by Murase have been used.     

Effective electro‐optic coefficient of (1–x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 single crystals

Refractive indices and effective electro‐optic coefficient γ_c of (1–x)Pb(Zn_1/3Nb_2/3)O₃–xPbTiO₃ (PZN‐xPT, x = 0.05, 0.09 and 0.12) single crystals were measured at 532 nm wavelength. Orientation and temperature dependences of the electro‐optic coefficient were investigated. Large electro‐optic coefficient (γ_c = 470 pm/V) was observed in [001]‐poled PZN‐0.09PT crystal. More importantly, γ_c of tetragonal PZN‐0.12PT is almost unchanged in a temperature range −20 ∼ 80 °C. The γ_c of PZN‐xPT single crystals are much higher than that of widely used electro‐optic crystal LiNbO₃ (γ_c = 20 pm/V). These results show that PZN‐xPT single crystals are very promising materials for electro‐optic modulators in optical communications.     

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.     

Phase Equilibrium of the Ga–Al–Sb system

Employing the method of liquid-phase epitaxy (LPE) solid-solutions of Ga_1–xAl_xAl_xSb (0 ≦ x ≦ 0.8) have been obtained. The dependence of Sb solubility on Al concentration in the liquid phase at 403°C, 452°C, 500°C has been established. The dependence of AlSb concentration in the solid phase on the composition of the liquid phase has been investigated at 452°C. Using the chemical constants equilibrium method, the phase equilibrium of the Ga–Al–Sb system in the region of liquid phase composition near the Ga-rich corner of the phase diagram has been calculated. The comparison of experimental and calculated data for the liquid and solid phases shows their agreement within the limits of experimental error.     

Crystal and Molecular Structure of 1-Ethyl-3-[tris(trimethylsiloxanyl)silyl]pyrrolinium Hydrochloride

The crystal and molecular structure of 1-Ethyl-3[tris(trimethylsiloxyl)silyl]pyrrolinium hydro-chloride (C₁₅H₃₈N⁺O₃Si₄ · C¹⁻) has been determined by direct methods. The title compound crystallizes in the monoclinic space group C2/c with a = 20.640(3), b = 19.494(2), c = 27.34(3) Å, β = 90.60(4)°, V = 11000(13) ų, Z = 16, D_x = 1.034 Mg m⁻³. There are two molecules with different conformations in the crystal. The pyrroline rings are non-planar.-The Si O Si angles range from 149(1)° to 163(1)°. Two of the SiMe₃ groups are disordered. All molecules are connected by C¹⁻ – N⁺ contacts and C¹⁻ - HN⁺ hydrogen bonds to form double chains.     

Untersuchung der Kinetik der Kristallisation unterkühlter Schmelzen

Experimental arrangement and a method are given to investigate the kinetics of solidification of supercooled liquids in thin layers. The kinetics of isothermal phase transformation of supercooled melts of betol are investigated between 0° and 85°C. Kinetic curves present the course of phase transformation in this range. It is stated that the process of isothermal solidification can sufficiently be described by the equation S_π = S₀ [1 − exp (− a τ^b)], b lying between 2 and 3. Compared with the theoretical value the experimental one is lowered, this being explained by the presence of active impurities and their exhaustion during the process of solidification.