Effect of the sign of misfit strain on the formation of a dislocation structure in SiGe epitaxial layers grown on Si and Ge substrates

Comparative analysis of the specific features of the formation of a dislocation structure in the single-layer epitaxial heterostructures Si_1?xGe_x/Si and Ge_1?ySi_y/Ge is performed. It is ascertained that, at a relatively low lattice mismatch between an epitaxial layer and a substrate, the sign of misfit strain at the interface significantly affects the processes of defect formation. The most probable reasons for the observed phenomena are analyzed with allowance for the specific features of the state of the ensemble of intrinsic point defects in epitaxial layers subjected to elastic strains of a different sign.     

Water-soluble ferroelectric crystals with inhomogeneous impurity distribution

Stabilization of the unipolar state via the formation of inhomogeneous impurity distribution in crystal bulk is considered. Possible growth of crystals with stable characteristics is demonstrated on triglycine sulfate (TGS) crystals with a regular inhomogeneous impurity distribution. The properties of TGS crystals with an inhomogeneous distribution of chromium ions grown above and below the Curie temperature T_C are studied. Inhomogeneous TGS crystals of three types are obtained: type-I crystals with a smooth variation of the concentration gradient along the growth direction, type-II crystals with a periodic layer variation of the impurity concentration, and type-III crystals with a sawtooth-like variation of the impurity concentration along the sample length. The TGS crystals with the regular inhomogeneous impurity distribution in the ferroelectric phase are characterized by higher values of the internal bias field E _b , unipolarity coefficient k, and pyroelectric coefficient γ than the inhomogeneous crystals in the paraelectric phase and the crystals with the statistic impurity distribution grown by the conventional method.     

Distribution of Al and in impurities along homogeneous Ge-Si crystals grown by the Czochralski method using Si feeding rod

A distribution of Al and In impurities in Ge_{1 ? x} Si _x crystals (0 ≤ x ≤ 0.3) grown by a modified Czochralski method (with continuous feeding of melt using a Si rod) have been studied experimentally and theoretically. Experimental Al and In concentrations along homogeneous crystals have been determined from Hall measurements. The problem of Al and In impurity distribution in homogeneous Ge-Si single crystals grown in the same way is solved within the Pfann approximation. A set of dependences of Al and In concentrations on the crystal length obtained within this approximation demonstrates a good correspondence between the experimental and theoretical data.     

Electrical and nonlinear optical properties of KTiOPO<Subscript>4</Subscript> single crystals doped with niobium,antimony, and tantalum

Single crystals of K_{1 ? x}Ti_{1 ? x}Nb_xOPO₄ (KTP: Nb), K_{1 ? x}Ti_{1 ? x}Sb_xOPO₄ (KTP: Sb), and K_{1 ? x}Ti_{1 ? x}Ta_xOPO₄ (KTP: Ta) solid solutions are grown and their dielectric, conducting, and nonlinear optical properties are investigated. The maximum contents x of niobium, antimony, and tantalum impurities in the crystals are equal to 0.11, 0.23, and 0.25, respectively. The doping of the KTiOPO₄ crystals with niobium, antimony, and tantalum brings about the formation of additional potassium vacancies and additional potassium positions and, as a consequence, an increase in the ionic conductivity σ₃₃. An increase in the doping level leads to a smearing of the ferroelectric phase transitions and a decrease in the phase transition temperatures. The permittivity of the doped crystals exhibits a broad relaxation peak in the temperature range 200–600°C.     

Use of a fluorine-containing solvent in the synthesis of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> single crystals

A laboratory technological procedure has been developed for the synthesis of high-temperature superconducting YBa₂Cu₃O_{7 ? δ} single crystals (T _c ～ 90 K, ΔT _c ～ 1.0 K) up to 0.25 cm² in size from a nonstoichiometric fluorine-containing flux of (YO_1.5)(BaO)_{4 ? x}(BaF₂)_x(CuO)₁₀, where 2 ≥ x ≥ 0, using a combination of enhanced nucleation and directional crystallization by the Czochralski method. Studies using differential thermal analysis demonstrated that the addition of BaF₂ decreased the eutectic-crystallization temperature and increased the crystal-growth rate. The optimum concentration of BaF₂ in the starting melt composition was found (x = 0.2). The single-crystal surface was studied by atomic-force nanoscopy. The morphology of single crystals that have been synthesized from a melt of their own components differs substantially from that of crystals grown from a BaF₂-containing melt.     

Crystal-structure refinement of Sr<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript>

The accurate X-ray diffraction study of a Sr₃Ga₂Ge₄O₁₄ crystal was performed based on two X-ray diffraction data sets collected on a diffractometer equipped with a CCD area detector (a = 8.2776(2), c = 5.0415(1) Å, sp. gr. P321, Z = 1, R/wR = 0.78/0.69%, 3645 independent reflections). The structure of Sr₃Ga₂Ge₄O₁₄ is characterized by the presence of two mixed cation sites, which is accompanied by the anharmonic motion not only of cations, but also of two oxygen atoms in general positions. The structures and electromechanical characteristics of Sr₃Ga₂Ge₄O₁₄ and Sr₃TaGa₃Si₂O₁₄ were compared. The Sr₃Ga₂Ge₄O₁₄ structure is characterized by a larger elongation of the Sr polyhedron along the a axis and, simultaneously, by the smaller unit-cell parameter a compared with Sr₃TaGa₃Si₂O₁₄, which correlates with the ratio of the piezoelectric coefficients d ₁₁. The absence of thermally stable directions in the crystals of Sr₃Ga₂Ge₄O₁₄ and Sr₃TaGa₃Si₂O₁₄ is consistent with the absence of the anomalous temperature dependence of the dielectric constant ?₃₃.     

Investigation of the possibility of obtaining homogeneous Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>x</Subscript>Sb crystals under weak flow conditions

The possibility of growing macrohomogeneous Ga_{1 ? x}In_xSb crystals with x = 0.2 was studied using the axial heating process close to the melt/crystal interface. The grown ingots were analyzed by a JSM-5300 scanning electron microscope and the experimental results were compared with the results of numerical simulation. It was shown that, as a whole, the mathematical model adequately describes the processes of the steady-state heat and mass transfer. It was found that, at the crystallization of Ga_{1 ? x}In_xSb by the axial heating process under conditions of weak laminar flows, longitudinal homogeneity is observed when a dead zone is formed in the melt and that the crystallization regime is similar to diffusion. It was shown that the composition of the grown crystals strongly depends on the structure of a melt flow and its dynamics.     

Behavior of ammonium ion in β-LiRb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>(NH<Subscript>4</Subscript>)<Subscript>x</Subscript>SO<Subscript>4</Subscript> mixed crystals (0.5 ≤ <Emphasis Type="Italic">x</Emphasis> ≤ 1.0)

The influence of replacement of an ammonium ion by rubidium on the x-T phase diagram in the concentration range 0.50 ≤ x ≤ 1.0 has been studied by X-ray and neutron powder diffraction over a wide temperature range. It is shown that a decrease in the ammonium concentration is accompanied by an increase in the temperature of the II ? I phase transition and stabilization of phase II up to low temperatures (20 K). The changes occurring in the dynamics of mixed crystals are studied by inelastic incoherent neutron scattering. The spectra of the generalized vibrational density of states obtained allowed one to establish the difference in the phonon modes corresponding to phase III of β-LiNH₄SO₄ and phase II of β-LiRb_1?x(NH₄)_xSO₄ mixed crystal with x = 0.91 and x = 0.77 at 20 K. It is shown that a mixed crystal with x = 0.77 at 20 K is in the orientational glass state.     

On the effect of structural and symmetrical features of potassium titanyl phosphate crystals with different contents of niobium,antimony, and zirconium on the second-harmonic intensity

A model is proposed, which shows that, at small deviations from the centrosymmetric state of the atomic structure, the quadratic nonlinear susceptibility of a crystal monotonically decreases with approach of the degree of central symmetry \(\eta _{\overline 1 } \)[φ(r)] of the electric potential function of the crystal structure to unity. The quadratic nonlinear susceptibility of K_{1 ? x} Ti_{1 ? x} Nb _x OPO₄ (x = 0, 0.02, 0.04, 0.11), K_{1 ? x} Ti_{1 ? x} Sb _x OPO₄ (x = 0.01, 0.07, 0.17), and KTi_{1 ? x} Zr _x OPO₄ (x = 0.03, 0.04) crystals has been measured. The degree of central symmetry \(\eta _{\overline 1 } \)[φ(r)] has been calculated for the structures of K_{1 ? x} Ti_{1 ? x} Nb _x OPO₄ (x = 0, 0.04, 0.11), K_{1 ? x} Ti_{1 ? x} Sb _x OPO₄ (x = 0.01, 0.07, 0.17), and KTi_{1 ? x} Zr _x OPO₄ (x = 0.03, 0.04) crystals. It is shown that, at \(\eta _{\overline 1 } \)[φ(r)] > 0.7, the relationship between the quadratic nonlinear susceptibility of the investigated crystals and the degree of their central symmetry \(\eta _{\overline 1 } \)[φ(r)] is in qualitative agreement with the proposed model.     

Synthesis and properties of zirconium-doped RbTiOPO<Subscript>4</Subscript> single crystals

Single crystals of the solid solutions RbTi_{1 ? x} Zr _x OPO₄ (0.015 < x < 0.034) were grown and their physical properties were studied. In the presence of zirconium in the crystals with the maximum content x = 0.034, the ferroelectric phase transition and the high-temperature transition from the orthorhombic to the cubic phase are shifted to lower temperatures by 100 and 50°C, respectively. In the temperature range from 700°C to room temperature, the conductivity of doped crystals decreases compared to that of the undoped crystals. It is of particular interest that the intensity of the second-harmonic generation of the doped crystals is substantially higher than that of RbTiOPO₄.     

Synthesis,physical properties,and atomic structure of niobium-doped RbTiOPO<Subscript>4</Subscript> crystals

Single crystals of solid solutions Rb_1?xTi_1?xNb_xOPO₄(RTP: Nb) were grown and the temperature dependences of their dielectric and nonlinear optical properties and electric conductivity were studied. The maximum possible niobium content in these crystals is close to x = 0.1. The niobium impurities decelerate growth of {100} faces, and crystals take a plate-like habit. With increasing doping level, ferroelectric phase transitions diffuse and their temperature decreases. A specific feature of the dielectric properties of RTP: Nb crystals is the appearance of a broad relaxation maximum ε₃₃ in the temperature range 200–600°C caused by the formation of vacancies in the rubidium cation sublattice. The intensity of second-harmonic generation under laser irradiation decreases with increasing niobium content. The atomic structure of a crystal with x = 0.01 is studied and it is established that niobium substitutes for titanium only in Ti(1) positions.     

Synthesis and electrical properties of Aurivillius phases in the Bi<Subscript>2</Subscript>MoO<Subscript>6</Subscript>-Bi<Subscript>2</Subscript>VO<Subscript>5.5</Subscript> system

To search for new oxygen conductive phases, a series of Bi₂V_{1 ? x} Mo_xO_{5.5 + x/2} ceramic samples have been synthesized in which the extreme compositions Bi₂VO_5.5 (x = 0) and Bi₂MoO₆ (x = 1) are single-layer Aurivillius phases having ferroelectric properties and high ionic oxide conductivity. It is established that single-layer Aurivillius phases exist in the composition ranges 0 ≤ x ≤ 0.1 and 0.9 ≤ x ≤ 1. Additional phases with BiVO₄ and Bi₆Mo₂O₁₅ structures are found in the range 0.2 ≤ x ≤ 0.8. The presence of molybdenum stabilizes the orthorhombic β-Bi₂VO_5.5 phase at room temperature. The conductivity of solid solutions based on Bi₂VO_5.5 differs only slightly from that of pure bismuth vanadate. Conductivity of solid solutions based on Bi₂MoO₆ is half an order of magnitude higher than that of pure bismuth molybdate.     

Investigation of the crystallization features,atomic structure,and microstructure of chromium-doped monticellite

A series of Cr⁴⁺:CaMgSiO₄ single crystals is grown using floating zone melting, and their microstructure, composition, and crystal structure are investigated. It is shown that regions with inclusions of second phases, such as forsterite, akermanite, MgO, and Ca₄Mg₂Si₃O₁₂, can form over the length of the sample. The composition of the single-phase regions of the single crystals varies from the stoichiometric monticellite CaMgSiO₄ to the solid solution Ca_{(1 ? x)}Mg_{(1 + x)}SiO₄(x = 0.22). The Cr:(Ca_0.88Mg_0.12)MgSiO₄ crystal is studied using X-ray diffraction. It is revealed that, in this case, the olivine-like orthorhombic crystal lattice is distorted to the monoclinic lattice with the parameters a = 6.3574(5) Å, b = 4.8164(4) Å, c = 11.0387(8) Å, β = 90.30(1)^o, Z = 4, V = 337.98 ų, and space group P2₁/c. In the monoclinic lattice, the M(1) position of the initial olivine structure is split into two nonequivalent positions with the center of symmetry, which are occupied only by Mg²⁺ cations with the average length of the Mg-O bond R _av = 2.128 Å. The overstoichiometric Mg²⁺ cations partially replace Ca²⁺ cations (in the M(2) position of the orthorhombic prastructure) with the average bond length of 2.347 Å in the [(Ca,Mg)-O₆] octahedron. The average distance in SiO₄ distorted tetrahedra is 1.541 Å.     

Growth and structure of La<Subscript>3</Subscript>Zr<Subscript>0.5</Subscript>Ga<Subscript>5</Subscript>Si<Subscript>0.5</Subscript>O<Subscript>14</Subscript> crystals

The complete X-ray structure determination of Czochralski grown La₃Zr_0.5Ga₅Si_0.5O₁₄ single crystals with the Ca₃Ga₂Ge₄O₁₄ structure is performed (sp. gr. P321, a = 8.226(1) Å, c = 5.1374(6) Å, Z = 1, Mo K_α1 radiation, 1920 crystallographically independent reflections, R = 0.0166, R_w = 0.0192). The absolute structure is determined. It is shown that possible transition of some of La atoms (～1.2%) from the 3e to 6g position may give rise to the formation of structural defects.     

Stabilization of lattice period in quinary solid solutions of the <Emphasis Type="Italic">A</Emphasis><Superscript>3</Superscript><Emphasis Type="Italic">B</Emphasis><Superscript>5</Superscript> compounds

The stabilizing effect of elastic strains on the lattice period of a quinary solid solution is considered. The expression for the stabilization factor for quinary solid solutions of the A_xB_1?xC_yD_zE_1?y?z type is derived. It is shown that the stabilizing influence of the substrate sharply increases in the vicinity of the region of the chemical spinodal. The stabilization factors are calculated for the Ga_xIn_1?xP_yAs_zSb_1?y?z and A1_xGa_yIn_1?x?yP_zAs_1?z quinary solid solutions isoperiodic to InAs, GaSb, and GaAs. It is shown that in the region of thermodynamic instability the stabilization factor has negative values. The changes in the composition of the above elastically strained quinary solid solutions are analyzed with respect to the equilibrium composition. It is also shown that stabilization of the lattice period does not signify the stabilization of its composition.     

Circular dichroism of some Nd-doped crystals of the langasite family

The absorption and circular-dichroism spectra of neodymium-doped crystals of langasite La₃Ga₅SiO₁₄ and the compounds La₃Ga_5.5Nb_0.5O₁₄ and Pb₃Ga₂Ge_3.92Si_0.08O₁₄, isostructural with langasite, are investigated in the wavelength range 320–940 nm. Electronic transitions in Nd atoms, which substitute the main cation of the salts (La or Pb) in the positions with the local symmetry C₂, are observed in the spectra. All transitions observed in the absorption spectrum are also active in the circular-dichroism spectrum. Detailed analysis is performed for several (well separated) bands: Dipole strengths D _ok , rotation strengths R _ok , and the anisotropy factors G _ok are calculated. Some specific features of the spectra obtained are noted and their possible relationship with the structure of impurity centers of neodymium and effect on the intensity of the Stark components are discussed.     

Birefringence of (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals under the action of uniaxial pressures

The influence of uniaxial mechanical pressure σ _m ≤ 150 bar on the spectral (300–800 nm) dependence of the birefringence Δn _i of (NH₄)₂SO₄ crystals is studied. The dispersion Δn _i (λ) is shown to be normal and greatly increases when approaching the absorption edge. Uniaxial pressure changes the value of dispersion dΔn _i /dλ but not its character. It is found that the simultaneous action of pressures σ _x ～ σ y ～ 560 bar results in the occurrence of a uniaxial isotropic state. Piezoelectric constants of the crystals are estimated.     

Structural and magnetic phase transformations in La<Subscript>0.88</Subscript>MnO<Subscript>3 − <Emphasis Type="Italic">x</Emphasis></Subscript> crystals

The crystal structure and elastic, magnetic, and transport properties of the La_0.88MnO_{3 ? x} system are studied. It is established that, depending on the oxygen content at x > 0.13, the system is orbitally ordered by the LaMnO₃ type and consists of a mixture of orbitally ordered and disordered orthorhombic phases (0.11 ≤ x ≤ 0.13); at x < 0.11, the system is monoclinically distorted. The orbitally ordered samples show properties inherent in both antiferro-and ferromagnetics, whereas the orbitally disordered samples are ferromagnetics and possess extremely high magnetic resistance. Analysis of the structure and properties of the samples showed that no oxygen vacancies are formed in the La_0.88MnO_{3 ? x} system. It is assumed that the excessive Mn³⁺ is incorporated into interstitials in the vicinity of lanthanum vacancies.     

Growth and dielectric properties of KTiOPO<Subscript>4</Subscript> and K<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiOPO<Subscript>4</Subscript> crystals

Methods of growth of KTiOPO₄ and K_{1 ? x} Rb _x TiOPO₄ crystals of high optical quality have been optimized. The dielectric properties (permittivity and conductivity) of the crystals grown have been investigated at frequencies from 10² to 10⁶ Hz in the temperature range from 100 to 350 K, along the [001] crystallographic direction. It is established that partial substitution of K⁺ ions with Rb⁺ ions leads to a decrease in the permittivity and conductivity.     

Optical and electro-optical properties of ZnMgSe single crystals

The optical and electro-optical properties of large-sized (diameter, 20 mm; length, 70 mm) single crystals of Zn_{1 ? x}Mg_xSe (x ～ 0.5) semiconductor solid solutions are investigated. The crystals are grown by the vertical Bridgman-Stockbarger method. It is found that, for ZnMgSe single crystals with a magnesium content up to 25 at. %, the modulus of the difference between the electro-optical coefficients |r₁₃ ? r₃₃| at a wavelength of 0.63 μm is equal to (1.1 ± 0.22) × 10^?12 m/V, which is comparable in order of magnitude to the difference in the electro-optical coefficients for classical hexagonal A^IIB^VI compounds, such as CdS and CdSe. It is shown that single crystals of ZnMgSe solid solutions are promising materials for use in the fabrication of electro-optical modulators, λ/4-and λ/2-wavelength plates, and multifunctional optical elements operating in the range of high-intensity radiation of CO and CO₂ lasers.