Flux growth and characterizations of NdPO4 single crystals

Neodymium phosphate single crystals, NdPO₄, have been grown by a flux growth method using Li₂CO₃-2MoO₃ as a flux. The as-grown crystals were characterized by X-ray powder diffraction(XRPD), differential thermal analysis (DTA) and thermogravimetric analysis (TG) techniques. The results show that the as-grown crystals were well crystallized. The crystal was stable over the temperature range from 26 to 1200 °C in N₂. The specific heat of NdPO₄ crystal at room temperature was 0.41 J/g °C. The absorption and the fluorescence spectra of NdPO₄ crystal were also measured at room temperature.     

Solution growth of single crystals of 4-hydroxycyanobenzene (4HCB) suitable for electronic applications

Squared, platelet-like single crystals of 4-hydroxycyanobenzene (4HCB) have been grown from solutions based on ethylic ether and petroleum ether. Properly modifying the growth conditions, in terms of both solvent used for the growth and concentration of 4HCB in the starting solution, allowed one to tune the planar dimensions of the platelets in the range 2–6 mm, and their thickness in the range 150–600 μm. In this way samples well suited for desired practical manipulation and electronic measurements may be obtained. Moreover, lowering the growth temperature resulted in larger but still thin 4HCB crystals. The ability to tailor crystal thickness has allowed one to study their Space-Charge Limited Current (SCLC) behaviour along that dimension, showing that the so-contacted samples exhibit intrinsic-like bulk conduction behaviour, and are hence well suitable for electronic studies and applications.     

Synthesis,growth and characterization of organic nonlinear optical bis-glycine maleate (BGM) single crystals

A new organic compound of bis-glycine maleate was synthesized in the alkaline medium of 10% ammonium hydroxide solution. The bulk single crystals of Bis-Glycine Maleate (BGM) have been grown by slow cooling method. The grown crystals were characterized by employing single crystal and powder X-ray diffraction, Fourier transform infrared, optical absorption spectral studies and thermo gravimetric analysis. The microhardness studies confirmed that the BGM has a fairly high Vicker’s hardness number value (41 kg mm⁻²) in comparison to other organic NLO crystals. Second harmonic generation efficiency of the crystal measured by Kurtz–Perry powder method using Nd:YAG laser is found to be comparable to that of potassium dihydrogen phosphate (KDP). Frequency dependent dielectric studies were carried out along the major growth axis.     

AlN bulk single crystal growth on 6H-SiC substrates by sublimation method

Large and thick AlN bulk single crystals up to 43 mm in diameter and 10 mm in thickness have been successfully grown on 6H-SiC (0 0 0 1) substrates by the sublimation method using a TaC crucible. Raman spectrum indicates that the polytype of the grown AlN single crystals is a Wurtzite-2H type structure, and the crystals do not include any impurity phases. The quality at the top of the crystal improves as crystal thickness increases along the 〈0 0 0 1〉 direction during growth: a low etch pit density (7×10⁴ cm⁻²) and a small full width at half maximum for a 0002 X-ray rocking curve (58 arcsec) have been achieved at a thickness of ∼8 mm. The possible mechanism behind the improvement in the AlN crystal quality is also discussed.     

GaN single crystals of different habit grown from solution at near atmospheric pressure

Near atmospheric pressure solution growth is one of the many developing methods for growing bulk GaN from solution. Apart from other approaches, this method holds certain advantages, such as relatively low growth pressure and temperature, and the ability to grow high quality GaN crystals with different orientations by varying the solvent composition. GaN whiskers of millimeter scale size with exceptional mechanical and optical properties were grown from solution. Crystals of near isotropic shape were also grown from solution by manipulating additives in the basic solvent.     

Solid mechanics and material strength studies on the melt growth of bulk single crystals

Microdefects such as dislocations and macrocracking should be controlled during the crystal growth process to obtain high-quality bulk single crystals. Solid mechanics and material strength studies on the single crystals are of importance to solve the problems related to the generation and multiplication of dislocations and the cracking of single crystals. The present paper reviews such research activities that comprise the thermal stress analysis during crystal growth process, the dislocation density estimation during crystal growth process, and the cracking of single crystal due to thermal stress.     

Solvent effects on the growth kinetics of subtilisin crystals

The effects of salts on subtilisin crystallization were investigated. Three salts—NaCl, NaNO₃ and NaSCN—were selected to study the effects of different anions on growth kinetics of three subtilisin mutants—Properase^®, Purafect^® and Purafect^®OX. The effectiveness of salts in decreasing the solubility of Properase^® and Purafect^® subtilisin followed the reverse order of the Hofmeister series: SCN⁻>NO₃⁻>Cl⁻. The average length and diameter of crystals were measured during crystallization. The nature of salt changed the length/diameter ratio of crystals, indicating the changes in the relative growth rate of different crystal faces. The required supersaturation, (c−s)/s, for a given growth rate increased in the order of NaCl, NaNO₃ and NaSCN. The observed trend in required supersaturation indicates a kinetic effect and was counter to the trend for the solubility data. A rationale is provided based on the influence of ion binding and kinetics on the energetics of crystal growth and growth rate is correlated to the molar Gibbs free energy of hydration of the anion.     

Compositional variation in Si-rich SiGe single crystals grown by multi-component zone melting method using Si seed and source crystals

The effect of the supply of depleted Si solute elements on the compositional variation in the Si-rich SiGe bulk crystals was studied using the method which was used to grow Ge-rich SiGe single crystals with a uniform composition. By selecting the proper pulling rate, we can obtain Si-rich Si_1−xGe_x bulk crystals with uniform composition of x=0.1 without using the supply mechanism of depleted Si solute elements. When the supply mechanism of Si solute elements was used, the initial composition in Si-rich SiGe crystals can be much more easily determined by controlling the growth temperature than that in Ge-rich crystals because the Si seed crystal is not melted down. The supply of Si solute elements is very effective to change the compositional variation even for Si-rich SiGe crystals.     

The influences of supersaturation on LPE growth of GaN single crystals using the Na flux method

The dependency of LPE growth rate and dislocation density on supersaturation in the growth of GaN single crystals in the Na flux was investigated. When the growth rate was low during the growth of GaN at a small value of supersaturation, the dislocation density was much lower compared with that of a substrate grown by the Metal Organic Chemical Vapor Deposition method (MOCVD). In contrast, when the growth rate of GaN was high at a large value of supersaturation, the crystal was hopper including a large number of dislocations. The relationship between the growth conditions and the crystal color in GaN single crystals grown in Na flux was also investigated. When at 800 °C the nitrogen concentration in Na–Ga melt was low, the grown crystals were always tinted black. When the nitrogen concentration at 850 °C was high, transparent crystals could be grown.     

A detailed study of gold single crystal growth in a silica gel

Gold single microcrystals have been fabricated by electrochemical growth in a silica gel. Structural characterization of the single crystals by backscatter electron diffraction showed a preferred orientation of Au (1 1 1) and a minor orientation of Au (1 0 0). In addition, the influence of additives on the nucleation and growth of gold microcrystals has been studied. It was found that the inclusion of chemical additives in the growth solutions altered the characteristics of the gold crystals. Possible mechanisms for nucleation and growth of these crystals are discussed.     

Growth and characterization of CdSe single crystals by modified vertical vapor phase method

Good quality, large single crystals of CdSe were grown by the modified growth method (i.e., vertical unseeded vapor phase growth with multi-step purification of the starting material in the same quartz ampoule without any manual transfer between the steps). Lower temperature gradients (8–9°C/cm) at the growth interface were used for the crystal growth. As-grown CdSe crystals was characterized by X-ray diffraction, scanning electron microscopy, energy dispersive analyzer of X-rays, high-resistance instrument measurement, and etch-pit observation. It is found that there are two cleavage faces of (1 0 0) and (1 1 0) orientations on the crystal, the resistivity is about 10⁸ Ω cm, and the density of etch pits is about 10^3–4/cm². The crystal was cut into wafers and was fabricated into detectors. The detectors were tested using an ²⁴¹Am radiation source. γ-ray spectra at 59.5 keV were obtained. The results demonstrated that the quality of the as-grown crystals was good. The crystals were useful for fabrication of room-temperature-operating nuclear radiation detectors. Therefore, the modified growth technique is a promising, convenient, new method for the growth of high-quality CdSe single crystals.     

Unidirectional seeded single crystal growth from solution of benzophenone

A novel crystal growth method has been established for the growth of single crystal with selective orientation at room temperature. Using volatile solvent, the saturated solution containing the material to be crystallized was taken in an ampoule and allowed to crystallize by slow solvent evaporation assisted with a ring heater. The orientation of the growing crystal was imposed by means of a seed fixed at the bottom of the ampoule. By selecting a suitable ring heater voltage and by controlling the ring heater voltage, nucleation and the growth rate of the crystal were controlled more effectively. By employing this novel method, benzophenone single crystal ingots of diameters 10 and 20 mm and length more than 50 mm were successfully grown using xylene as solvent. The ease in scaling up of diameter from 10 to 20 mm shows the vital advantage of this technique. It was possible to achieve solute–crystal conversion efficiency of 100 percent. The grown benzophenone crystal was characterized by FTIR, TG and DTA, powder X-ray diffraction, X-ray rocking curve, optical transmission study and powder SHG measurement. The results show that the crystal quality is at least as good as the quality of the crystal grown by other known methods. Also, microbial growth was naturally avoided in this method, as the fresh solution is constantly made available for the growing crystal.     

Synthesis and characterization of large single crystals of NpPd3 by flux method

Synthesis of the active [Ni–Fe]-hydrogenase in prokaryotes requires a series of ancillary maturation factors. Among them, the HypF maturation factor is a multidomain 82 kDa protein, whose N-terminal domain displays sequence and structural similarities to acylphosphatases. Acylphosphatases are small enzymes that are able to catalyze carboxyl-phosphate bond hydrolysis in acylphosphates, as well as in nucleoside di- and tri-phosphates and in arylphosphates. Here, we present a crystallographic comparison between microgravity and earth-grown crystals of the HypF N-terminal domain. Both crystals were of excellent quality, thereby allowing us to collect very high resolution datasets. A detailed analysis of data collection and refinement statistics, together with an analysis of the diffraction pattern showed that microgravity would appear to further improve the internal order of crystals.     

Large-size β-Ga2O3 single crystals and wafers

Large-size single crystals of β-Ga₂O₃ with 1 inc in diameter have been grown by the floating zone technique. The stable growth conditions have been determined by the examination of the crystal structure. Wafers have been cut and fine polished in the (1 0 0), (0 1 0) and (0 0 1) planes. These were highly transparent in the visible and near UV, as well as electrically conductive, indicating the potential use of β-Ga₂O₃ as a substrate for optoelectic devices operating in the visible/near UV and with vertical current flow.     

Improvement of crystal quality in ammonothermal growth of bulk GaN

Several key improvements in crystal quality of bulk GaN grown by the ammonothermal method are presented. Full width at half maximum of (0 0 2) X-ray rocking curve was reduced to 53 and 62 arcsec for Ga-side and N-side, respectively. Transparent bulk GaN crystal was also demonstrated. Oxygen and sodium concentrations were reduced to mid-10¹⁸ and mid-10¹⁵ cm⁻³, respectively. We are currently searching for a growth condition that produces transparent bulk GaN with high structural quality and low impurities. Small-sized, semi-transparent GaN wafers were fabricated by slicing the grown bulk GaN crystals, which demonstrate the high feasibility of ammonothermal growth for production of GaN wafers.     

Growth of CuS platelet single crystals by the high-temperature solution growth technique

Millimeter size CuS single crystals with a dark indigo blue color and a plate hexagonal shape have been successfully grown by the high-temperature solution growth technique using the KCl–LiCl eutectic as solvent. Surface microtopographic studies of the crystals indicated that the growth is made by the lateral spreading of the layers. Electrical resistivity measurements clearly show an anomaly at T55 K, related with the low-temperature structural transition, a high residual resistivity ratio of 400 and a sharp superconducting transition at T≈1.7 K confirming the very good quality of the crystals.     

Oriented growth of large size calcium fluoride single crystals for optical lithography

We report (1 1 1), (1 1 0) and (1 0 0) growth of CaF₂ by the vertical Bridgman method. Crystals up to 250 mm diameter were grown and various growth parameters such as growth rate, temperature gradient and post-growth cooling rate were studied. It was found that the growth rate and the cooling rate are slower for the larger diameter crystals with a fixed temperature gradient. These growth parameters were optimized for growing the crystals along specific orientation after realizing that CaF₂ has a tendency to grow along an orientation close to 1 1 0. Degradation in optical transmittance was evaluated by irradiating the crystal to γ-rays up to a dose of 10⁵ rad. Optimized scavenger addition resulted in crystals with better radiation resistance and excellent VUV transmittance.     

Improved growth of PbI2 single crystals

The phase equilibrium and the crystallization process of lead iodide (PbI₂) melt have been primarily investigated according to the lead–iodine phase diagram. It is found that the iodine evaporation and the segregated lead deposition are the two important factors that affect the PbI₂ crystal quality. The new method of Pulling U-type quartz growth ampoule has been made to impede the decomposition of PbI₂ and the vaporization and condensation of iodine. An orange and translucent PbI₂ single crystal of large size was obtained by the improved growth method, i.e. U-type ampoule pulling. Resistivity of the as-grown crystal is up to 4×10¹¹ Ω cm, and IR transmission is up to 45% in the region from 7800 to 450 cm⁻¹. Therefore, the improved growth method is a promising convenient new method for the growth of high quality PbI₂ crystals.     

Shape evolution of zinc oxide from twinned disks to single spindles through solvothermal synthesis in binary solvents

Shape evolution of ZnO crystals from twinned disks to single spindles was studied through solvothermal synthesis in binary solvents N,N-diethylformamide (DEF) and methanol (MeOH). The MeOH content in DEF had large influence on the morphology of the obtained ZnO crystals. In MeOH-free DEF, well-shaped ZnO twinned disks with perfect mirror symmetry could be formed through the assembly of ZnO₄⁶⁻–julolidinium–ZnO₄⁶⁻ growth units on the (0 0 0 1) growth interfaces. For small amounts of MeOH (MeOH/DEF=0.04), elongated twinned disks were formed since the growth along the polar c-axis was enhanced. With increasing MeOH content (MeOH/DEF=0.1), twinned rods with reduced mirror symmetry were formed. When a large amount of MeOH was added to DEF (MeOH/DEF=0.5), single spindles rather than twinned disks or twinned rods were obtained. A similar shape evolution of zinc oxide was observed in binary solvents DEF and N,N-dimethylformamide (DMF), suggesting that the growth of ZnO crystals with tuneable shape and size can be controlled by the composition of the binary solvent mixture.