Vertical Zone Melting Research Articles

Characteristics of structural defects of Cd0.9Zn0.1Te crystals grown by vertical zone melting

Large-size Cd0.9Zn0.1Te crystals were grown using high-pressure vertical zone melting. It has been found that Te inclusions ranging 70–150 μm with density 3.5·102 cm−2 are formed during crystal growth under conditions specified in this work. Optical spectra display the transmission of the CZT crystals in near- and medium infrared range, which is close to theoretical value and does not degrade despite presence of Te inclusions. Based on the obtained data on sizes and distribution of the inclusions, diffusion coefficient of tellurium in CZT was estimated for Te particles entrapped by growing interface. The calculated value (1.22–3.2)·10−8 cm2/s most probably indicates vacancy mechanism of diffusion. Density of in-grown dislocations revealed by chemical etching is 4·104 cm−2. Branched 3D dendrites are found in CZT crystals.

Морфология поверхности и структурные свойства кристаллов GaTe после ионно-плазменной обработки

The effect of ion-plasma treatment on the physical properties of the surface of GaTe crystals is investigated. Gallium telluride crystals were grown by vertical zone melting under the pressure of an inert argon gas of 10.0 MPa at a temperature of 1000 °C and a zone displacement velocity of 9 mm/hr. The treatment was carried out in argon plasma in a high-density low-pressure radio frequency (RF) inductively coupled plasma reactor at an argon ion energy of 100-200 eV for 15-120 s. Using scanning electron microscopy methods, it was shown that the formation of nano- and submicron structures of various architectures (nanohillocks, nanocones, droplet structures) occurred on the surface during processing. It is shown that the sputtering processes are accompanied by enrichment of the near-surface layer with metal atoms and a decrease in oxygen content. The formation of nano- and submicron gallium droplets on the surface has been proved by X-ray diffractometry. The analysis of the raman scattering spectra showed a decrease in the oxide phases of tellurium after plasma treatment. It is established that modification of the GaTe surface leads to suppression of specular optical reflection in the range of 0.4-6.2 eV.

Cr2＋ distribution in ZnSe crystals grown from melt

ZnSe : Cr2＋ crystals were grown using a high-pressure vertical zone melting technique with concentrations of 5⋅1017, 1018, and 1019 cm−3. Infrared transmittance measurements showed a noticeable change in the Cr2＋ ions distribution along the as-grown crystals. The effective segregation coefficients of Cr2＋ ions were calculated and their dependence on concentration was approximated.

Variation of Surface Nanostructures on (100) PbS Single Crystals during Argon Plasma Treatment

The nanostructuring of the (100) PbS single crystal surface was studied under varying argon plasma treatment conditions. The initial PbS single crystals were grown by high-pressure vertical zone melting, cut into wafer samples, and polished. Subsequently, the PbS single crystals were treated with inductively coupled argon plasma under varying treatment parameters such as ion energy and sputtering time. Plasma treatment with ions at a minimum energy of 25 eV resulted in the formation of nanotips with heights of 30–50 nm. When the ion energy was increased to 75–200 eV, two types of structures formed on the surface: high submicron cones and arrays of nanostructures with various shapes. In particular, the 120 s plasma treatment formed specific cruciform nanostructures with lateral orthogonal elements oriented in four <100> directions. In contrast, plasma treatment with an ion energy of 75 eV for 180 s led to the formation of submicron quasi-spherical lead structures with diameters of 250–600 nm. The nanostructuring mechanisms included a surface micromasking mechanism with lead formation and the vapor–liquid–solid mechanism, with liquid lead droplets acting as self-forming micromasks and growth catalysts depending on the plasma treatment conditions (sputtering time and rate).

Mechanical characteristics of gallium sulfide crystals measured using micro- and nanoindentation

Gallium sulfide single crystal is a potential material for nonlinear optics. Due to its high non-linearity it can be used as a material for IR and THz frequency converters. In the present work GaS bulk crystals of precise stoichiometry and high quality were melt grown using vertical zone melting. Elementary content of the crystals was tested using electron probe microanalysis, phase composition and unit cell parameters were studied using X-ray powder diffraction (XRD), morphology of the cleavage plane and layered structure of a cross section normal to the growth axis were studied using light microscopy, morphology of impression under an indenter tip was imaged by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Various mechanical properties, such as hardness, Young modulus of nanoindentation, residual indentation depth, contributions of elastic and plastic parts to total deformation were studied in the nanoindentation experiments. Experiments on microindentation of GaS single crystals were carried out for comparison. It was found that nanoindentation hardness and elasticity modulus depend non-monotonously on peak load. The results confirm that despite of their layered structure, GaS crystals have high micro- and nanohardness and plastic deformation ability, which provide their suitability to be processed for optical elements.

Growth and characterization of directionally solidified eutectic systems for scintillator applications

Fabrication of scintillating eutectic composites by various directional solidification techniques is introduced and discussed in some details. Number of methods widely practiced for the single crystal growth including Czochralski, Bridgman or vertical zone melting, and micro-pulling-down are also acceptable for the production of eutectic materials. Two types of such materials are considered here. These are (1) composites used for neutron scintillators; eutectic materials that consist of high neutron cross section material and efficient scintillator (they do not require regular micro-structure) and (2) high resolution scintillators; eutectic materials that have well ordered rod/fiber structure of one of the phases immersed into matrix of second phase (one of them should be efficient scintillator). In later one, the wave guiding can be established in one of the phases (either matrix or fibers). The composites with fiber structure are supposed to be very promising substances for detecting initial irradiation with high spatial resolution. LiF/LiYF4 and LiF/CaF2:Eu eutectic composites represent first group of the scintillators, but CsI/NaCl and GdAlO3/α-Al2O3 correspond to the second one.

Synthesis and Growth of GaSe1 – xS x (x = 0–1) Crystals from Melt. Phase Composition and Properties

Vertical zone melting under argon pressure is used to grow crystals of GaSe and GaS binary compounds, as well as solid solutions in the GaSe–GaS system. The crystals of these compounds are applied in laser optics owing to a high transmission coefficient in the IR region, but they are also of a special interest in nonlinear optics, where they are used as frequency converters in the terahertz range. In this context, interest grows in alloying with isovalent and non-isovalent dopants as in a method aimed at increasing efficiency of optical frequency converters. In this work, it is shown that crystals of solid solutions are characterized by unit cell parameters which decrease linearly as the sulfur content increases. It is shown that doping of GaSe with sulfur leads to a regular shift of the edge of the transmission spectrum to the short wavelength region and to a shift of low temperature photoluminescence peaks to higher energies. Measurements of microhardness show high magnitudes depending on composition of the crystals.

Improved mechanical properties of thermoelectric (Bi0.2Sb0.8)2Te3by nanostructuring

Temperature-dependent strength of Bi-Sb-Te under uniaxial compression is investigated. Bi-Sb-Te samples were produced by three methods: vertical zone-melting, hot extrusion, and spark plasma sintering (SPS). For zone-melted and extruded samples, the brittle-ductile transition occurs over a temperature range of 200-350 °C. In nanostructured samples produced via SPS, the transition is observed in a narrower temperature range of 170-200 °C. At room temperature, the strength of the nanostructured samples is higher than that of zone-melted and extruded samples, but above 300 °C, all samples decrease to roughly the same strength.

Mechanical properties of (Bi,Sb)2Te3 solid solutions obtained by directional crystallization and spark plasma sintering

We have studied the temperature dependence of the mechanical strength at uniaxial compression for solid solutions based on bismuth and antimony chalcogenides, which were prepared by three methods: (i) vertical zone melting (VZM), (ii) hot extrusion, and (iii) spark plasma sintering (SPS). In the samples of solid solutions obtained by VZM and extrusion, a brittle–ductile transition was observed in a wised temperature interval of 200–350°C. In nanostructured SPS samples, transition from brittle to plastic fracture was observed within 170–200°C. The room-temperature strength of nanostructured samples was eight to nine times as large as that of VZM samples, and the stress–strain curves of these materials were significantly different. At a temperature of about 300°C, the strength of nanostructured solid solutions decreases to nearly zero.

Growth of PbI2 single crystal by the top seed vertical zone melting method

PbI2 is a type of syntectic compound, and its single crystal is one of the room temperature semiconductor nuclear radiation detector materials. A new method for the growth of the PbI2 single crystal is proposed in this article, which was named the top seed vertical zone melting method (TSVZMM), directly from the synthesis of polycrystal with analytically pure lead and iodine, by controlling the decomposition and stratification of the melt and the stoichiometry of the PbI2 crystal. Impurities in the crystal and the coagulating droplets of lead were eliminated during the PbI2 crystal growth process from the top to bottom by TSVZMM. The PbI2 single crystal was successfully grown with the size of Φ15 mm×15 mm, an infrared transmittance of more than 40%, a resistivity of 2×1012 Ω cm magnitude and stoichiometry close to its theoretical value.

Structure and microstructure of GaTe crystals grown by High-Pressure vertical zone melting

GaTe crystals of 20mm in diameter and up to 70mm in length have been grown from melt using the high-pressure vertical zone melting (HPVZM) method. Real defect structure has been evaluated by XRD, SEM, EPMA, optical microscopy, microhardness and transparency measurements. The phase transformation of hexagonal GaTe single crystal to monoclinic form by grinding has been detected. Such local defects as dendrites and dendrite clusters have been found at cleaved surface (0001), basal dislocations with density of 2×105cm−2 have been detected by the etch-pit technique.

Numerical and Experimental Investigation of Arsenic Removal Process from Phosphoric Acid by Vertical Zone Melting Technique

A combined numerical and experimental approach was undertaken to investigate the arsenic removal process from phosphoric acid by vertical zone melting technique. The numerical model, which predicts...

Determination of Equilibrium Distribution Coefficients of Impurities in Phosphorus by Vertical Zone-melting Technique

The equilibrium distribution coefficients of 12 impurities (As, Fe, Ca, Co, Al, Cr, Cu, Mg, Mn, Ni, Pb, Zn) in phosphorus were obtained by measuring their effective distribution coefficients at zone travel rate of 3, 5, 10, 15, and 20 mm·h −1 in the purification process with vertical zone-melting technique. The results indicate that the method is reliable. The equilibrium distribution coefficients are below 0.3 except arsenic.

Semiconductor Thermoelectric Converters of Alternating Current Signals

Measuring converters have been developed based on Bi2Te3 solid solutions optimized for temperature and temporal stability of the basic thermoelectric parameters (thermal conductivity, electrical conductivity, and the Seebeck coefficient). Semiconductor material was grown by the vertical zone melting method. High efficiency of the optimized semiconductor materials made it possible to reduce the operating temperature considerably, as well as facilitating selection of structural materials and the converter fabrication technique. The influence of the Peltier and Thomson effects on the accuracy of direct current conversion has been reduced, and the operating frequency range of measurements has been expanded. Thermoelectric converters with sensitivity up to 30 V W−1 have been created without evacuation of the working space, which is filled with an inert gas mixture. The engineered converters offer the advantages of high sensitivity and a wide operating frequency range (up to 30 MHz).

Mechanism of formation of texture and its influence on the strength of thermoelectric p-Bi0.5Sb1.5Te3

It is established that, in preparing p-Bi0.5Sb1.5Te3 by vertical zone melting, in addition to the directional texture (characteristic of materials exhibiting a highly anisotropic growth rate) in which the cleavage planes of crystal grains are parallel to the direction of propagation of the crystallization front, other texture types can arise, in which the orientation of grain cleavage planes is ordered in a cross-sectional plane of the ingot. Two types of such textures, “radial” and “circular,” were observed. In a radial texture, the lines of intersection of grain cleavage planes with a cross-sectional plane of the ingot are oriented along radii of this cross section and, in a circular texture, these lines of intersection are oriented approximately perpendicular to a radius crossing the grain. The formation of a radial texture is associated with rotation of the ampoule with the crystallizing substance about its vertical axis causing centrifugal flows of the melt. The formation of a circular texture is associated with the orientation effect of the ampoule walls and with circular motion of the melt during torsional oscillations of the ampoule about the vertical axis. Ingots with a radial texture exhibit much lower resistance to splitting along their axis than ingots with a circular texture do. An explanation is provided for this fact.

The property of transformation strain in the texturedNi50550.5Mn 2424Ga25525.5 alloy

A large strain of 14% during the martensitic transformation has been observed in the textured Ni50550.5Mn2424Ga25525 .5 polycrystal prepared by vertical zone melting unidirectional solidification. This martensitic transformation strain decreases to 08% at the ninth and tenth thermal cycles. Expansion and shrinkage phenomenon during the martensitic transformation has been obtained, indicating the existence of a preferential orientation of martensitic variants.

Phase transition strain and large magnetic field induced strain in Ni 50.5Mn 24Ga 25.5 unidirectionally solidified alloy

A large magnetic-field-induced strain (MFIS) of 1150 ppm has been obtained in textured Ni 50.5Mn 24Ga 25.5 polycrystal prepared by vertical zone melting unidirectional solidification. X-ray diffraction measurements of the cubic structure reveal that the preferential orientation along the growth direction is (2 2 0). Expansion and shrinkage phenomenon upon the martensitic transition has been observed at 223 K (martensitic transition temperature M s), indicating the existence of a preferential orientation of martensitic variants to minimize elastic energy. This martensitic transition strain can be enhanced six times, up to 1.5% shrinkage with a bias magnetic field of 1.5 T.

Suppressing three-dimensional unsteady flows in vertical zone-melting by steady ampoule rotation

Three-dimensional (3D) unsteady heat flow and molten zone induced by nonlinear bifurcations or imperfect growth conditions are typical in vertical zone-melting crystal growth or refining. Through flow modification, we find that the use of steady ampoule rotation is effective in suppressing these 3D characteristics. Even with only 10 RPM ampoule rotation, numerical simulation indicates that the steady axisymmetric (or nearly) mode can be easily retained for 2-in GaAs under a typical growth condition, in which the 3D unsteady flows prevail for no rotation. A model gallium zone is also examined, and its 3D bifurcations are significantly suppressed by rotation as well. However, if the rotation is not sufficient to suppress the unsteadiness, baroclinic waves may appear. The effects of ampoule rotation for imperfect growth conditions, including asymmetric heating and ampoule tilting, are further illustrated. The ampoule rotation is also found useful.

Effects of axial vibration on vertical zone-melting processing

Vibration can be an effective way for controlling heat flow and the growth interface during crystal growth. To better understand the role of axial vibration in vertical zone melting, an important process for crystal growth and purification, computer simulation is performed. For the zone melting of a 2-cm GaAs crystal in a quartz ampoule, simulation results show that an axial vibration, especially at high frequency, can significantly affect the flows and further the growth interface. The concave growth front due to buoyancy convection can be also inverted easily to a convex one. The effects of vibration frequency are further illustrated, and the validity of the high-frequency model is examined. For GaAs, the resonance frequency is found at about 0.5 Hz, where significant flow oscillation and severe periodic growth and remelting can be induced.

Effects of ampoule rotation on vertical zone-melting crystal growth: steady rotation versus accelerated crucible rotation technique (ACRT)

Computer simulation is performed to illustrate the significance of ampoule rotation on the flows, interface shapes, and the growth rate in vertical zone-melting (VZM) crystal growth. For the growth of a 2 cm GaAs crystal in a quartz ampoule, simulation results show that even low-speed ampoule rotation can significantly affect the flows and further the growth interface. The concave growth front due to buoyancy convection can be inverted easily to a convex one by steady ampoule rotation. The accelerated crucible rotation technique (ACRT) also has a similar effect on the growth interface, but less effective. In addition, severe periodic growth and remelting is induced by ACRT, and it is enhanced significantly by natural convection.