Qualitative and quantitative bubbles defects analysis in undoped and Ti-doped sapphire crystals grown by Czochralski technique

Small bubbles can be observed in both sapphire and Ti-sapphire bulk crystals grown by Czochralski technique (Cz). Various thickness of wafers cut from the grown ingots were studied by optical microscopy. Bubbles distribution has different regulation in sapphire and Ti-sapphire crystals. All bubbles are spherical and have a diameter range from 2 μm to 5 μm in sapphire crystals while 10–45 μm in Ti-doped sapphire crystals. Some adjacent bubbles congregated into defects present various sizes and shape. The bubbles density increases as a function of pulling rate and rotation rate. The effect of bubbles on optical characterizations of Ti-sapphire crystals has been studied.     

Growth and spectroscopic properties of Ti-doped sapphire single-crystal fibers

Titanium doped sapphire (Ti:Al₂O₃) crystal fibers have been grown by the micro-pulling-down (μ-PD) method using different pulling rate in the range (0.1–0.5 mm/min). The present work has investigated the crystal growth, including diameter, bubbles defects and segregation properties. The fiber’s absorption and IR emission experiments are analyzed. The lifetime of Ti³⁺ in the IR range obtained at room temperature is 3.1 μs. Moreover, the two blue emissions at 420 nm and 470 nm bands have been discussed.     

Yb-doped NaBi(WO4)2 fibre single crystals grown by the micro-pulling down technique and emission spectroscopic characterization

NaBi_1−xYb_x(WO₄)₂ fibres single crystals were successfully grown by micro-pulling down technology (MPD). The Yb³⁺-doped NaBi(WO₄)₂ fibres single crystals have been pulled using MPD technique with controlled diameter and stationary stable growth conditions corresponding to flat crystallization interface with meniscus length equal to the fibre radii and pulling rate range [6-48 mm h⁻¹]. We have determined the monophased field of NaBi_1−xYb_x(WO₄)₂ for x ≤ 0.3. The lattices parameters decrease as a function of Yb³⁺ substitution in Bi³⁺ sites. The melt behaviour has been study by DTA/TG analysis. We have found that the stoichiometric compounds NaBi(WO₄)₂ melt congruently at 935 °C. The fibre diameters varied from 0.5 to 1 mm depending on the capillary die diameter, pulling rate and the molten zone temperature. Complementary Yb³⁺ spectroscopic characterization in the NaBi(WO₄)₂ lattice has been done by IR emission measurements under laser pumping at room temperature.     

Eutectic solidification of the pseudo binary system of polyethylene and 1, 2, 4, 5-tetrachlorobenzene

This paper describes a study on the solidification of the pseudo binary eutectic system, consisting of unfractionated linear polyethylene and the faceted growing diluent 1, 2, 4, 5-tetrachlorobenzene. Crystallization under eutectic conditions resulted in very fine-grained structures, which were found to depend on the growth rate. This rate of solidification was varied by pulling the polymer solutions through a fixed temperature gradient of 3° C mm^–1 at different speeds ranging from 1 to 216 mm h^–1. Fibrillar polymer crystals with lateral dimensions of about 0.5 m remained after removal of the solid diluent. At rates of solidification in the region of 2 to 20 mm h^–1, the fibrils appeared to be aggregated in domains of well oriented structures, closely resembling the complex regular structures of the eutectic Sn-Bi system. At higher speeds the fibrillar crystals formed an irregular three-dimensional network. The polymeric structures grown from more dilute mixtures were characterized by rectangular holes originating from the growth of faceted primary diluent crystals. Despite the complexity of the crystallization of the highly entangled polymer solutions there appears to be quite some similarities between the eutectic polymeric system investigated and faceted/non-faceted atomic or small molecular eutectics.     

Crystal growth of sapphire filaments by a laser-heated floating zone technique

A laser-heated floating zone technique is employed to grow a filamentary c-axis sapphire. Crystal growth is directly recorded during the process by photography, and crystals grown under various conditions are examined by means of a transmission optical microscope to obtain the relationships between microvoid formation and growth conditions. Observations of filaments indicated that the microvoid results from shrinkage due to dendritic growth. Microvoids are eliminated by choosing a growth rate less than 4.7 cm h^–1 or by focusing the laser beam. By using this method, a void-free filament of 1.4 mm diameter is obtained.     

Thermoelectric properties of <Emphasis Type="Italic">n</Emphasis>-Bi<Subscript>2</Subscript>Te<Subscript>2.7</Subscript>Se<Subscript>0.3</Subscript>〈I,In〉 crystals

n-Bi₂Te_2.7Se_0.3〈I〉 single crystals additionally doped with indium (0.1–2.0 mol % In₂Te₃) have been grown by Czochralski pulling through a floating crucible. As the In₂Te₃ content of the growth charge increases to 0.2 mol %, the thermoelectric figure of merit (Z) of the crystals increases from 0.0029 to 0.0031 K⁻¹ and Z _max shifts to higher temperatures by ∼30 K. Increasing the In₂Te₃ content to 2 mol % sharply reduces Z _max to 0.002 K⁻¹ and shifts the maximum to lower temperatures. This behavior of the thermoelectric figure of merit can be understood in terms of the nonmonotonic variation of electron mobility with indium and iodine concentrations in the crystals.     

Single-crystal growth of La2−2x Sr1+2x Mn2O7 under pressure

Large single crystals of La_2–2x Sr_1+2x Mn₂O₇ (x=0.3 to 0.525) have been prepared under controlled atmospheric conditions. The crystals were grown by the floating-zone technique in an image furnace under a mixed oxygen/argon atmosphere pressurized to 6–8×10⁵ Pa. Rectangular single crystals with sizes up to 50×9×4 mm³ have been obtained. The phase-purity, composition, and quality of the crystals were analyzed by X-ray diffraction and electron probe microanalysis. The magnetic behavior is found to be sensitive to the composition of the atmosphere during growth.     

Monophase domain, fibers single crystals grown by the micro-pulling down technique and optical characterisation of LiGd1−xYbx(WO4)2

We have determined the single phase domain of LiGd_1−xYb_x(WO₄)₂. The lattices parameters decrease as a function of Yb³⁺ substitution in Gd³⁺ sites. Transparent LiGd_1−xYb_x(WO₄)₂ fibers single crystals were successfully grown by the micro-pulling down technique (μ-PD). The Yb³⁺-doped LiGd(WO₄)₂ fibers single crystals have been pulled under stationary stable growth conditions corresponding to flat crystallization interface with meniscus length equal to 120 μm. The fibers diameters varied from 0.5 to 1 mm depending on the capillary die diameter, pulling rate and the molten zone temperature. Fibers single crystals free of defects are observed for Ytterbium concentration in the melt up to 5 at%. Above this limit, inclusions and cracks appear and the optical quality of the fibers were deteriorated. The emission spectra of Yb³⁺-doped LiGd(WO₄)₂ were investigated.     

Formation of a two-dimensional electron gas in ZnO/MgZnO single heterostructures and quantum wells

The properties of ZnO/MgZnO heterostructures grown by pulsed-laser deposition on sapphire (112?0) and ZnO (0001?) have been compared. Electron accumulation layers have been observed for ZnO/MgZnO heterostructures grown on sapphire by capacitance-voltage (C-V) spectroscopy. The formation of a two-dimensional electron gas (2DEG) in these structures has been confirmed by temperature dependent Hall effect measurements. From C-V measurements the sheet carrier density in a Zn_0.8 Mg_0.2O/ZnO/Zn_0.8 Mg_0.2O quantum well (QW) structure with a well width of about 5 nm is calculated to be only about 9.0 × 10¹⁰ cm^− 2. For the films deposited on sapphire 2D growth is observed in the Burton-Cabrera-Frank mode, as confirmed by atomic force microscopy. Step flow growth mode was achieved for the homoepitaxial thin films. Quantum confinement effects have been confirmed by photoluminescence (PL) measurements. Homoepitaxial QWs are more homogeneous (smaller inhomogeneous recombination broadening) than heteroepitaxial QWs.     

Etch pits in flux-grown corundum

Procedures have been developed for chemically polishing and etching {0001}, {10¯11}, {10¯12}, {11¯20}, and {1¯100} planes in crystals of ruby and sapphire grown from a PbF₂ flux. The shape and the orientation of the etch pits were found to be characteristic for each plane and the density of the pits was 10² to 10⁴/cm². Similar pits were produced in flame-fusion material, but the density was 10⁶ to 10⁸/cm². Ruby and sapphire crystals grown by the same process behaved similarly. There is evidence that etch pits reveal dislocations which emerge normally to the basal or to the prismatic planes, since similar patterns of pits were produced after the removal of successive layers of material parallel to these planes, and a correlation was found between the pit patterns on opposite {0001} faces. Inconclusive evidence on this point was obtained for the rhombohedral planes.     

Growth and characterization of laser-deposited Ag-doped YBa2Cu3O7−x thin films on bare sapphire

Microstructural and superconducting properties of YBa₂Cu₃O_7−x thin films grownin situ on bare sapphire by pulsed laser deposition using YBa₂Cu₃O_7−x targets doped with 7 and 10 wt% Ag have been studied. Ag-doped films grown at 730°C on sapphire have shown very significant improvement over the undoped YBa₂Cu₃O_7−x films grown under identical condition. A zero resistance temperature of 90 K and a critical current density of 1·2×10⁶ A/cm² at 77 K have been achieved on bare sapphire for the first time. Improved connectivity among grains and reduced reaction rate between the substrate and the film caused due to Ag in the film are suggested to be responsible for this greatly improved transport properties.     

Effects of pulling speed and hot zone temperature on the directional growth of YBa2Cu3Ox superconductor

The effects of pulling speed and hot zone temperature on the microstructure and the superconducting properties of YBa₂Cu₃O_x sample textured using directional growth of Y₂BaCuO₅, BaCuO₂, and CuO powder mixture were studied. The grain size, the alignment, and the critical current density of the sample were increased as the pulling speed decreased. The sample grown directionally at 1.5 mm h^–1 pulling speed consisted of a single grain. The zero resistivity temperature and the critical current density of the sample increased as the hot zone temperature increased up to 1120°C beyond which the sample consisted of Y₂O₃ and impurities and showed resistivity at 77 K. The sample grown directionally at 1.5 mm h^–1 pulling speed and at 1120 °C hot zone temperature showed sharp resistivity transition of 91 K zero resistivity and over 6000 Acm^–2. The sample showed well aligned microstructure. Compared to data from another study, the hot zone temperature required to produce maximum critical current density is lower due to low liquid forming temperature.     

Preparation of single crystals of MoB2 by the aluminium-flux technique and some of their properties

Crystal growth of MoB₂ (R-3m;a _hex. = 0.30136(6) nm,c _hex. = 2.0961 (4) nm) from high-temperature aluminium solutions (1000 to 1500°C) has been investigated, and the optimum conditions for obtaining single crystals of this compound were examined. Starting with about 33 to 53 g of a mixture of molybdenum, boron and aluminium, in which the atomic ratio is 1 : 2.5 : 53.3 to 88.9, MoB₂ single crystals of about 1.5 mm in size were grown without being accompanied by any other crystal phases. The stoichiometry and the crystal structure were corroborated by chemical analysis and single-crystal X-ray diffractometry. The Vickers microhardness measured in the (001) plane is in the range 2170 to 2470 kg mm^–2. The oxidation of the crystal in air is examined by thermogravimetric and differential thermal analyses up to 1200°C.     

Growth crystallography of Bi12SiO20 single crystals solidified by a floating zone method

Relations between the growth directions of Bi₁₂SiO₂₀ (BSO) single-crystal rods solidified by a floating zone method and the pulling parameters were studied using an X-ray back-reflection Laue technique. It was found that when a platinum wire is used as a seed, the growth direction of the produced BSO single-crystal rod is related to the pulling rate. Statistically, the probability of the growth direction approaching 〈0 1 1〉, 〈1 1 2〉 or 〈0 0 1〉 appears to increase in this order with increase of the pulling rate. In addition when a BSO crystal is used as a seed, the growth direction of the produced BSO crystal rod has the same orientation as the seed crystal if the pulling rate is less than 30 mm h^-1. If the pulling rate is higher than 30 mm h^-1, the growth appears to incline mostly to 〈1 1 2〉 if the seed orientation is near 〈0 1 1〉 or 〈1 1 1〉 but far from 〈0 0 1〉, or to 〈0 0 1〉 if the seed orientation is near 〈0 0 1〉 but far from both 〈0 1 1〉 and 〈1 1 1〉. The angle of inclination increases with the pulling rate, and also with the difference in angle between the orientation of seed crystal and 〈1 1 2〉 or 〈0 0 1〉. The facet morphology of the BSO single-crystal rod is related to its growth direction. The cross-section of the BSO single-crystal rod grown along 〈0 0 1〉 is an octagon with tetrad-rotational symmetry, that grown along 〈0 1 1〉 is an ellipsoid with diad-rotational symmetry, and that along 〈1 1 1〉 is a hexagon with triad-rotational symmetry. The cross-sections of the BSO single-crystal rods grown in other directions are not regular, because there is no clear symmetry. This revised version was published online in November 2006 with corrections to the Cover Date.     

Growth of A3N whiskers and plate-shaped crystals by molecular-beam epitaxy with the participation of the liquid phase

It is shown that InN and GaN whiskers and plate-shaped crystals can be grown by molecular-beam epitaxy (MBE), and the growth mechanism on gallium arsenide and sapphire substrates is investigated. A comparison is made with the theory. It is proved that the growth mechanism corresponds to the vapor-liquid-solid (VLS) mechanism, and the parameters of the crystallization process are determined. The nanometer sizes of the crystals grown give hope that the crystals and the VLS growth method itself can be used to obtain quantum-size objects (quantum dots and wires) by MBE in the promising system of elements A³B⁵-AlGaInN. Pis’ma Zh. Tekh. Fiz. 25, 55–63 (September 26, 1999)     

Flux growth and characterization of TiC crystals

Single crystals of TiC were grown using nickel and cobalt metal as a flux at soaking temperatures of 1500 to 1800° C and at compositions of 12.5 to 30 mol% TiC. The use of cobalt flux produced crystal sizes less than 0.5 mm under all conditions. With a nickel flux, a maximum crystal size of 1.5 mm was obtained at 1700 and 1800°C and at 20 and 25 mol% TiC composition, using a cooling rate of 3° C min^–1. A slower cooling rate of 0.2° C min^–1 also gave crystals of 1.5 mm at 1600° C. The crystals were cubic and metallic-lustre silver grey in colour. The lattice parameter of the crystal was measured to bea ₀ = 0.432 75 ± 0.000 05 nm, with nearly stoichiometric composition. The grown faces were of the {1 0 0} family with a dislocation density around 10⁷ cm^-2. The Vickers' microhardness on these faces was in the range 2600 to 2800 kg mm^–2. The nickel impurity and free carbon contents in the crystals were 700 to 1000 p.p.m. and 0.8 to 4 wt%, respectively.     

Growth of CuInTe2 single crystals by iodine transport and their characterization

The single crystals with stoichiometry close to 1:1:2 of CuInTe₂ (CIT) have been grown by chemical vapor transport (CVT) technique using iodine as the transporting agent at different growth temperatures. Single crystal X-ray diffraction studies have confirmed the chalcopyrite structure for the grown crystals and the volume of unit cell is found to be the same for the crystals grown at different conditions. Energy dispersive X-ray (EDAX) analysis of CIT single crystals grown shows almost the same stoichiometric compositions. Scanning electron microscope (SEM) analysis reveals kink, step and layer patterns on the surface of CIT single crystals depending on the growth temperatures. The optical absorption spectra of as-grown CIT single crystals grown at different conditions show that they have same band gap energies (1.0405 eV). Raman spectra exhibit a high intensity peak of A₁ mode at 123 cm^?1. Annealed at 473 K in nitrogen atmosphere for 40 h CIT single crystals have higher hole mobility (105.6 cm²V^?1s^?1₎ and hole concentration (23.28 × 10¹⁷ cm^?3) compared with values of hole mobility (63.69 cm² V^?1 s^?1) and hole concentration (6.99 × 10¹⁵ cm^?3) of the as-grown CIT single crystals.     

Hydrothermal synthesis of improved ZnO crystals for epitaxial growth of GaN thin films

ZnO single crystals with thickness up to 12 mm, 2 inches in “diameter” and weight of about 150 g have been grown from KOH, NaOH, and K₂CO₃ based hydrothermal solutions on the seeds of (0001) orientation. The addition of LiOH up to 3.0–4.5 mol/L allowed to decrease the growth rate of ZnO crystals along the 〈0001〉 crystallographic direction. For positive and negative monohedra, it was achieved 0.12 and 0.01 mm/day, respectively, at temperature 340 °С and ΔТ = 10 °С. The best ZnO etching agent was found to be the solutions 25 mol% HCl + 3 mol% NH₄F at room temperature, and etching time 5 min. The dislocation density of ZnO crystals varied from 240 cm⁻² to 3,200 cm⁻² in the case of growth rates 0.04 mm/day to 0.11 mm/day, respectively. It was also found that ZnO crystals grown are stable in air, oxygen, nitrogen, and argon atmosphere as well as in vacuum at the temperatures up to 1,000 °C under thermal treatment during 4 h.     

Growth and properties of CuInS2 single crystals

CuInS₂ single crystals 14–16 mm in diameter and up to 40 mm in length were grown by the traveling solvent method. The crystals were found to ben-type, with a conductivity of 0.1–10 S/cm, carrier concentration of 10¹⁶–10¹⁷ cm⁻³, and carrier mobility of 150–220 cm²/(V s). The anisotropic thermal expansion of the crystals was measured.     

Growth and spectroscopic characteristics of Yb3+-Doped NaBi(MoO4)2 crystals

We have determined the growth charge composition and low thermal gradient (< 1°C/cm) Czochralski pulling parameters for the growth of bulk homogeneous Yb³⁺-doped NaBi(MoO₄)₂ crystals and assessed the main spectroscopic characteristics of the Yb³⁺:NaBi(MoO₄)₂ crystals as laser materials.