Preparation Of Single Crystals Research Articles

Growth of AlN and AlN-SiC Solid Solution by Sublimation Method

AlN is considered as the most suitable substrate material for further development of high quality and high performance nitride-based micro- and opto-electronics. AlN ingots are often grown on SiC seeds. To solve the formation of cracks due to the difference in lattice parameters between seed and crystal we chose to “adapt” the lattice mismatch by a buffer layer of the (AlN)x(SiC)1-x solid solution. This paper gives some inputs on the growth of AlN and the solid solution by the sublimation technique, in terms of materials compatibility, hetero- and homo-epitaxial growth of AlN and on the preparation of crack-free solid solution single crystals.

Preparation of Nanoleaf-like Single Crystals of Anatase-type TiO2 by Exfoliation and Hydrothermal Reactions

Abstract Nanoleaf-like single crystals of anatase were prepared by hydrothermal treatment of a layered titanate nanosheet colloidal solution at 135 °C. The single crystals have high crystallinity and prefer to expose (010) or (100) plane. The surface of the nanocrystals showed a high photocatalytic activity for degradation of Methylene Blue.

Preparation and optical properties of Co-doped ZnSe single crystals

ZnSe single crystals doped via Co diffusion are investigated. The diffusion was carried out from metal Co in He or Ar atmosphere. The spectra of optical density in the wavelength range 0.4–2 μm are investigated. It is found that the absorption edge shifts as the concentration of doping impurities increases. This shift is caused by the formation of the Zn1 − xCoxSe alloy. The diffusion profile of the Co dopant is determined via measurement of the relative optical density of the crystals in the visible spectral region. The Co diffusivities (D) in the ZnSe crystals at T = 1103–1273 K are calculated. The analysis of the temperature dependence D(T) made it possible to determine the coefficients in the Arrhenius equation, namely, D0 = 3.4 × 106 cm2/s and E0 = 3.8 eV.

Hochtemperatur‐Festkörperchemie: Der Niederdruck‐Hochfrequenz‐Plasmabrenner: Synthese stabiler und metastabile Oxometallate der Lanthanoide

AbstractHigh Temperature Solid State Chemistry. The Low Pressure High Frequency‐Plasmaburner: Synthesis of Stable and Metastable Oxometalates of Rare Earth OxidesReactions between high temperature refractory oxides (Alkaline Earth oxides and Rare Earth oxides) always require exceptionally high temperatures for the synthesis of new compounds, especially for the preparation of single crystals. It will be shown, that conventional electrical heaters on the base of resistance heating don't achieve the melting points of refractory oxides. Other methods like solar radiation, electron beam heating ore hot flames (oxyhydrogen / oxyacetylene welding torch) are discussed. In the middle of the sixties of last century the open high pressure and the closed low pressure HF‐plasma burners had been developed. The following contribution gives once attention to the discovery and application of the closed low pressure plasma burner. In this paper the synthesis of stable and metastable (entropy stabilized) compounds (> 2000 °C) between Alkaline Earth and Rare Earth Oxides is shown. In detail the contribution makes clear how to prepare stable (CaFe2O4‐type) and metastable compounds of the composition AM2O4 (SrCr2O4, α‐CaCr2O4, BaCr2O4). These prototypes are the basis for micro space disordered compounds like SrCa2Ln6O12, SrCa2Ln10O18 and BaSrCaLn22O23. Using high temperature synthesis the thermodynamic stable compound SrNd2O4 altered to the metastable Sr3Nd4O7‐type. On the other hand the formation of BaLn2O4 does not occur for the heavy Rare Earth elements. Instead of the well‐known CaFe2O4‐type a complicated new stable structure type of the composition Ba3Ln4O9 is created.The closed high frequency plasma burner makes possible to reinvestigate the “dissolution” of Alkaline Earth oxides in Rare Earth oxides aid of single crystals. A typical example is the formation of Ln2−xCaxO3−x. Starting with cubic Ho2O3 (C‐form) small values of x (x<0,2) stabilize the monoclinic form (B‐Form) of Rare Earth oxides (B‐Ho2−xCaxO3−x). For compositions 1.0 ≥ x ≥ 0.2 the monoclinic B‐form of Rare Earth oxides is left in favour of a new structure type Ca2Ho2O5. It may by regard as modified monoclinic B‐form with a statistically distributed deficiency of oxygen.In the end the importance of atomic hydrogen, produced by high temperatures of plasma torches, is shown in case of the reduction of TiO2 and in case of the formation of Eu2+, stabilized by compounds of the composition Eu2+BeLn2O5.

Surface Preparation of Single Crystals for ZnO Homoepitaxy

ABSTRACTFor the homoepitaxial growth of ZnO it is inevitable to obtain a regular crystalline single crystal surface prior to growth. Commercially available, hydrothermally grown ZnO single crystals show amorphous surfaces due to mechanical cutting and polishing. Here we present the results of a thermal treatment on these ZnO single crystals. After annealing, a regular crystalline oxygen terminated surface can be obtained. Changes in surface roughness, residual defect concentration and electrical properties can be shown. The bulk crystallinity though was not affected.

Preparation of bead metal single crystals by electron beam heating

For the fabrication of small metal bead crystals a gas flame is used to melt a wire forming a liquid droplet which solidifies upon cooling into a single crystal metal bead. Due to oxidation under ambient conditions bead crystals can be formed only from noble metals using this method. Here we describe a method how to fabricate bead crystals from a wide variety of metals and metal alloys (Cu, Mo, Ru, Rh, Pd, Ag, Ta, W, Re, Ir, Pt, Au, PtPd, Pd80Pt20, PtRh, AuAg, and PtIr) by electron beam heating under vacuum conditions. Narrow x-ray diffraction peaks confirm a high crystal quality of the bead crystals.

Preparation and Characterization of Synthetic Polypeptide Single Crystals with Controlled Thickness

Molecular crystallization provides an experimentally simple way to arrange molecules on the nanoscale. Synthetic polypeptides present the potential to control ultimate crystal structure by design o...

Modified Physical Vapor Transport Growth of SiC - Control of Gas Phase Composition for Improved Process Conditions

We review the development of a modified physical vapor transport (M-PVT) growth technique for the preparation of SiC single crystals which makes use of an additional gas pipe into the growth cell. While the gas phase composition is basically fixed in conventional physical vapor transport (PVT) growth by crucible design and temperature field, the gas inlet of the MPVT configuration allows the direct tuning of the gas phase composition for improved growth conditions. The phrase "additional" means that only small amounts of extra gases are supplied in order to fine-tune the gas phase composition. We discuss the experimental implementation of the extra gas pipe and present numerical simulations of temperature field and mass transport in the new growth configuration. The potential of the growth technique will be outlined by showing the improvements achieved for p-type doping of 4H-SiC with aluminum, i.e. [Al]=9⋅1019cm-3 and ρ<0.2Ωcm, and n-type doping of SiC with phosphorous, i.e. [P]=7.8⋅1017cm-3.

Preparation of polydiacetylene single crystals based on physical vapor growth technique

We have succeeded in obtaining 10-mm-size polydiacetylene (PDA) single crystals using the physical vapor growth technique followed by the irradiation of ultraviolet (UV) rays, for the first time. The variety of growth conditions, such as growth temperature, growth time, carrier gases, and flow rate of gases are examined. Especially, the growth temperature and growth time have clear effects on the quality of crystals. Morphologies and sizes with relation to the various growth conditions are also observed. They show only one type of plateletlike shape with a pair of large parallelopiped planes, regardless of variety of growth conditions. The crystal thickness was estimated as about 500 nm by the observation of atomic force microscopy. The solubility of diacetylene (DA) and PDA to some organic solvents, and the X-ray diffraction data indicate that the obtained crystals are single crystals.

Phase diagram of the Ag 2S–HgS–SnS 2 system and single crystal preparation, crystal structure and properties of Ag 2HgSnS 4

The phase equilibria in the quasi-ternary system Ag 2S–HgS–SnS 2 at 298 K were investigated using X-ray diffraction. The existence of the Ag 2HgSnS 4 compound that is the only one at this temperature was confirmed. The vertical section Ag 8SnS 6–HgS was constructed. It is of the eutectic type (eutectic point coordinates are 89 mol% HgS and 881 K), with the formation of a HT–Ag 8SnS 6 solid solution range extending to ∼57 mol% HgS at 881 K. The Ag 2HgSnS 4 crystal structure was refined using X-ray powder diffraction. Ag 2HgSnS 4 crystallizes in the orthorhombic space group Pmn2 1 (ST Cu 2CdGeS 4) with unit cell parameters a = 0.82074(4) nm, b = 0.70330(3) nm and c = 0.67180(3) nm. The atomic parameters were refined in the isotropic approximation to R = 0.0795. Ag 2HgSnS 4 single crystals ( Ø = 10–12 mm; l = 30 mm) were grown using the Bridgman–Stockbarger technique. The single crystals exhibit n-type semiconductor conductivity. The photoconductivity spectra at liquid nitrogen and room temperatures were investigated, and their non-elementary nature was shown.

Preparation of new ferroelectric glycine phosphite single crystals

An experimental procedure adopted to prepare the recent ferroelectric glycine phosphite single crystals is presented. As the crystal solubility in water depends on temperature, single crystals were grown from fine seed crystals in a saturated solution by employing the slow cooling technique. Transparent, colourless crystals with habit morphology weighing about 8 g were obtained within a month. The same procedure was used to grow single crystals with urea (UGPI). Powder X-ray diffraction studies as well as FTIR studies were carried out on both the crystals. The thermal properties of glycine phosphite (GPI) and UGPI across the ferroelectric phase transition temperature has been carried out. The results will be discussed in detail.

Improved crystal grinding and polishing holder for metal single crystal preparation

An automatic fixture for polishing a single crystal on a mechanical polishing machine has been designed and tested on a soft metal, aluminum. Crystal radii of the order of 3 m may be achieved without personal attention to the polishing work, minimizing the atomic step density due to crystal curvature.

Preparation of ultra-high purity CdTe single crystals

High-purity and quality CdTe single crystals are very important for their basic study, as well as practical applications. For this reason, Cd was purified by vacuum distillation (VD) and overlap zone-melting (OZM) method, and Te was purified by normal freezing method. Refined Cd was evaluated by glow discharge mass spectroscopy (GDMS) and residual resistivity ratio (RRR) measurement and Te was evaluated by low-temperature photoluminescence (PL) spectra. Results showed that refined Cd and Te have the purity of 6N-up. Using the refined Cd and Te as starting materials, extremely high-purity CdTe single crystals were prepared by the traditional vertical Bridgman technique. The crystals were characterized by low temperature high-resolution PL spectroscopy. Only a sharp peak at 1.5896 eV was detected in exciton emission region. The full width at half-maximum (FWHM) is less than 0.31 meV. These results indicate that the CdTe crystals are of extremely high-purity and quality.

Preparation of Single Crystals under Hydrothermal Conditions

The capability of preparing carbonates and hydroxides that decompose in air at high temperatures is one of the advantages of hydrothermal reactions as a preparation technique for inorganic materials. In this study, new hydrothermal techniques to prepare single crystals of calcite (carbonate) and hydroxyapatite (hydroxide) were described. For hydrothermal crystal growth of calcite, organic salt solutions were used as a mineralizer for the first time and a solution of ammonium acetate was found to be an excellent mineralizer to obtain high growth rate under mild hydrothermal conditions. Single crystals of hydroxyapatite were prepared from chlorapatite single crystals by ion exchange in alkaline solutions under hydrothermal conditions. The transformation mechanism was explained by dissolution and precipitation of clusters.

SiC single crystal growth by a modified physical vapor transport technique

We have developed a modified physical vapor transport (M-PVT) growth technique for the preparation of SiC single crystals which makes use of an additional gas pipe in order to control the gas phase composition of the conventional physical vapor transport (PVT) configuration. We discuss the experimental implementation of the extra gas pipe by comparing crystal growth runs under various gas flow conditions with numerical simulations. The potential of the M-PVT growth method will be demonstrated by showing the improved doping characteristics when performing nitrogen, phosphorus and aluminum doping of SiC using the additional gas pipe for dopant supply.

Preparation and Characterization of CdSe Single Crystal

Preparation and Characterization of CdSe Single Crystal

Preparation of novel optical-grade metanitroaniline and polymethylmethacrylate-coated single crystals and their optical properties

Preparation and optical properties of metanitroaniline (m-NA) and polymethylmethacrylate (PMMA)-coated m-NA single crystals were investigated. In this report, we developed relatively large-sized m-NA single crystals in methyl ethyl ketone (MEK) using solution growth techniques. The average size of the single crystals achieved is about 10×5×5 mm. The developed crystals were further coated with laboratory-developed optical-grade PMMA to study the effect of polymer coating on the second harmonic generation (SHG) properties of the single crystals. m-NA single crystals were characterized using DSC, FTIR and XRD measurement techniques. Optical characterization was done using UV-visible spectroscopy. Morphology was studied using scanning electron microscopy (SEM). Improved nonlinear optical (NLO) properties corresponding to SHG intensity values were also recorded for these coated and uncoated single crystals. PMMA-coated m-NA single crystal surfaces showed high SHG values.

Preparation and characterization of Ni-incorporated FeS2 single crystals

Nickel-incorporated FeS2 single crystals with various Ni compositions of Fe0.99S2:Ni0.01, Fe0.98S2:Ni0.02, Fe0.96S2:Ni0.04, and Fe0.9S2:Ni0.1 were grown by chemical vapor transport (CVT) method using ICl3 as a transport agent. Physical properties of the Ni-incorporated FeS2 single crystals were characterized using X-ray diffraction, Raman spectroscopy, electrical conductivity, and photoconductivity (PC) measurements. By means of the analyses of the X-ray diffraction patterns, the whole series of Ni-doped FeS2 single crystals were determined to be single-phase and isostructural. Raman spectroscopy of the Ni-doped FeS2 crystals was carried out at room temperature. Raman resonant peaks of the Ni-doped FeS2 crystals demonstrate an energy red-shift behavior with respect to the increase of the dopant densities. Conductivity measurements show the resistivity of the Ni-doped FeS2 decreased as the doping concentration of Ni is increased. Nickel is an n-type dopant, which behaves like a donor level existed near the conduction band edge of the synthetic FeS2. On the other hand, dopant effect of nickel on the synthetic FeS2 also destroys the photoconductive sensitivity in the photoconductivity measurements.

A new multi-purpose furnace for the preparation of compounds, alloys and single crystals

A new modular multi-purpose furnace has been designed and the prototype constructed. This furnace was a development utilizing more than two decades of experience at the JRC establishment, Karlsruhe, to bring together the possibility of several techniques that normally require separate, expensive facilities. With this new modular device, different functions are provided by exchanging the head of the furnace while leaving the base as a permanent fixture. The processes can be carried out in high vacuum (10 −6 Pa) or in the presence of high-purity gases, e.g., argon. The modules developed allow the following processes to be carried out: Arc melting, levitation melting, resistance and radio-frequency heating in a crucible, single-crystal growth by various techniques, and electron-beam heating. The rationale behind the development was to produce a device capable of many functions but at an acceptable cost so as to make the various techniques available to a wide range of research and development institutes. A full description of the apparatus is given, outlining the range of the methods which can be applied to the production of high-purity advanced materials for research purposes.

Electrical resistance measurements under pressure on NbTe2 single crystal

Transition metal dichalcogenides, because of their layered structure, are well suited for extreme pressure lubrication. These materials being semiconducting and of layered structure may undergo structural and electronic transitions under pressure. Here we report the details of the preparation and characterization of single crystals of NbTe2 and the results of electrical resistance measurements under pressure carried out on it to investigate this possibility. Single crystals were grown by the chemical vapor transport technique, using iodine as a transporting agent. The composition of the grown crystals was confirmed on the basis of Energy Dispersive Analysis by X-ray (EDAX) and remaining structural characterization was also accomplished by X-ray Diffraction (XRD) studies. Electrical resistance was measured employing a Bridgman anvil set up to 10 GPa and diamond anvil cell (DAC) assembly up to 25 GPa. A technique slightly modified from that described in the literature for carrying out electrical resistivity measurements in the diamond anvil cell (DAC) under pressure has been standardized.