Configurations and local vibrations of differently charged oxygen vacancies in quartz crystal

We have studied local configurations and vibration states of oxygen vacancies in different charge states in α-quartz. The local atomic structure and the lattice dynamics have been simulated using first-principles potentials of the Buckingham type. The effect of defects on the vibration spectrum of α-quartz has been studied by calculating the local densities of vibrational states.     

Studies on the crystal growth,crystal structure,optical and thermal properties of an organic crystal: Benzophenone hydrazone

Single crystals of benzophenone hydrazone (BH) were successfully grown by slow evaporation method at constant temperature, 30 °C. The crystal structure of BH has been determined, and it belongs to the noncentrosymmetric space group P2₁. The grown crystal has been characterized by FT-IR). The optical transition and the lower cutoff wavelength of the BH have been identified by UV–vis–NIR studies. Thermo gravimetric analysis and differential thermal analysis have been carried out; the BH was found to be thermally stable up to 104 °C. Powder second harmonic generation (SHG) was investigated to explore its nonlinear optical (NLO) properties. The high SHG is due to one dimensional charge transfer between amine and phenyl group. Both the real ε_r and imaginary ε_i components of the dielectric constant have been calculated as functions of photon energy.     

Investigation of the thermal conditions during silicon carbide crystal growth

In the present work an analysis of the thermal conditions during silicon carbide crystal growth from the vapour phase by the sublimation method is carried out. On the basis of the obtained results from the calculation of the temperature distribution along the length of the growing crystal it was drawn a conclusion that in order to decrease the density of dislocations in the growing crystals it is necessary to decrease the temperature gradients in the crucible for growing.     

Optical and laser properties of Nd:LuVO4 crystal

Polarized absorption spectra of Nd:LuVO₄ crystal were measured at room temperature. The optical parameters of π polarization and σ polarization were calculated by Judd‐Ofelt theory. Meanwhile, the phenomenological intensity parameters: Ω₂, Ω₄, and Ω₆ were obtained, then the parameters were used to calculate the luminescence parameters of Nd:LuVO₄ crystal, including oscillator strengths, fluorescence branching ratio, radiative lifetime and integrated emission cross‐sections. Experiments of a‐cut and c‐cut Nd:LuVO₄ lasers were also performed, and opt‐opt conversion efficiency and slope efficiency were 40.3%, 50.5% for a‐cut and 23.6%, 30.9% for c‐cut, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reversible LED controlled optical activity of a cholesteric liquid crystal layer

Abstract

A thin layer of the photosensitive cholesteric liquid crystal possessing a high value of the optical activity is studied. Reversible change in the optical activity controlled by two light emission diodes (LEDs) with different emission wavelengths is applied for information recording. The behavior of this structure under exposure to UV and blue light is investigated. A smooth decrease in optical activity caused by the UV light resulting in a change in the color of the structure between crossed polarizers is obtained. The reverse process occurs under exposure to the blue light. On the basis of the obtained results, two methods of optical information recording in such a layer have been demonstrated.     

Writing of crystal line patterns in glass by laser irradiation

We examined the laser-induced crystallization to form the fresnoite type Ba₂TiGe₂O₈ crystal line patterns in transition metal ion doped BaO–TiO₂–GeO₂ glass. Ba₂TiGe₂O₈ crystal line was written in 0.6FeO–33.3BaO–16.7TiO₂–50GeO₂ glass by continuous wave yttrium–aluminum–garnet (YAG) laser irradiation. We obtained polarization dependence of Raman spectra in crystal line pattern. Second harmonic generation (SHG) indicated unique fringe patterns from Ba₂TiGe₂O₈ crystal lines.     

Shape of thermal etch pits and crystal structure in tellurium crystals

During the systematic study of thermal etching of tellurium crystals, various shaped thermal etch pits were observed on the {101 0} cleavage faces of this crystal. An attempt has been made to explain the shape of non-dislocation and dislocation etch pits. A simple model based on crystal structure and bonding of atoms has been suggested to explain the shape of etch pits.     

Femtosecond-laser-encoded distributed-feedback color center laser in lithium fluoride single crystal

Focused infrared femtosecond laser pulses (wavelength ∼800 nm, emission pulse duration 100 fs) were employed to fabricate optoelectronic devices such as waveguides, micro-gratings and laser active centers in LiF crystals. F₂ color centers of about 2 × 10¹⁸ cm⁻³ and refractive index change of about 1% at 633 nm were induced by the fs-laser irradiation. This technique was applied to fabricate a distributed-feedback (DFB) F₂ color center laser structure inside LiF single crystal. The LiF DFB laser exhibited laser oscillation at 707 nm at room temperature. The slope efficiency of ∼10% and beam divergence of ∼20 mrad were achieved.     

Global heat loss and thermal stress analysis in Czochralski crystal growth

Based on our invention of an energy‐efficient Czochalski crystal growth furnace, a 2D‐axisymmetric numerical simulation model of LiNbO₃ crystal growth is developed. The heat transfer, melt and gas flow, radiation and the interface deflection have been examined. Heat losses in the furnace and the insulator, as well as the heating power and thermal stress distribution at three stages of crystal growth are calculated in detail. It is found that a large proportion of heat dissipates through the water‐cooling system, and at the steel shell of the furnace, gas convection heat transfer is the major cooling mechanism. Less heat dissipation by radiation and more heat flux by gas convection to the crystal sidewall results in a larger concentrated thermal stress, which may induce large crystal cracks in the growth process. The simulation results of heating power are in coincidence with the actual power of our furnace, which verifies the feasibility of our model. The detailed information with respect to the device obtained from simulation can help to optimize the energy‐saving design and growth process.     

Synthesis,thermal analysis,and crystal structure of potassium triiodomercurate monohydrate

Single crystals of potassium triiodomercurate monohydrate KHgI₃ · H₂O are synthesized and studied by X-ray diffraction: space group Pna2₁; a = 8.57961(11), b = 9.23621(13), and c = 11.38977(16) Å, Z = 4; R= 0.0574. Thermal conditions of decomposition of the compound are studied by differential scanning calorimetry.     

Numerical simulation of thermal and mass transport during Czochralski crystal growth of sapphire

The thermal and flow transport in an inductively heated Czochralski crystal growth furnace during a crystal growth process is investigated numerically. The temperature and flow fields inside the furnace, coupled with the heat generation in the iridium crucible induced by the electromagnetic field generated by the RF coil, are computed. The results indicate that for an RF coil fixed in position during the growth process, although the maximum value of the magnetic, temperature and velocity fields decrease, the convexity of the crystal‐melt interface increases for longer crystal growth lengths. The convexity of the crystal‐melt interface and the power consumption can be reduced by adjusting the relative position between the crucible and the induction coil during growth. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of cadmium mercury thiocyanate dimethylsulphoxide single crystal for laser frequency doubling

This paper reports the growth of an organometallic nonlinear optical (NLO) complex crystal, Cadmium mercury thiocyanate dimethyl-sulphoxide (CdHg(SCN)₄(H₆CP₂OS)₂, CMTD), which was grown from aqueous solution by a temperature lowering method. Its powder SHG intensity is higher than that of CMTC. The crystal size of CMTD can reach 25×23×15mm³. The solubility curve, and solubility variation with pH value are also reported. The growth habits of crystals of CMTD grown under different conditions are discussed and the optimized condition for the growth CMTD using a temperature-lowering method from aqueous solution are also discussed.     

Numerical optimization of thermal field in CdTe crystal growth by travelling heater method

Cadmium telluride (CdTe) and his compounds play a leading role in X‐ray and γ‐ray detector technology. One of the most used methods for growing these crystals is the travelling heater method (THM). The ingots obtained by using this technique show excellent composition uniformity, but the structural quality is affected by the presence of large grains which appear because of large curvatures of the solid‐liquid interface during the solidification process. This numerical work investigate the thermal field and melt convection in CdTe processing by THM in order to elucidate the mechanism of growing these crystals. The influence of the furnace geometry on the interface shape and temperature gradient in liquid is analyzed for samples with small (1 cm) and large (5 cm) diameters. The computations include flow effects on thermal field in the melted zone. The thermal conditions are optimized for THM growth of CdTe crystals at high solidification temperatures. A new multi‐zone furnace configuration for growing crystals of large diameter and flattened interface is proposed in this work.     

Mechanism for generating interstitial atoms by thermal stress during silicon crystal growth

It has been known that, in growing silicon from melts, vacancies (Vs) predominantly exist in crystals obtained by high-rate growth, while interstitial atoms (Is) predominantly exist in crystals obtained by low-rate growth. To reveal the cause, the temperature distributions in growing crystal surfaces were measured. From this result, it was presumed that the high-rate growth causes a small temperature gradient between the growth interface and the interior of the crystal; in contrast, the low-rate growth causes a large temperature gradient between the growth interface and the interior of the crystal. However, this presumption is opposite to the commonly-accepted notion in melt growth. In order to experimentally demonstrate that the low-rate growth increases the temperature gradient and consequently generates Is, crystals were filled with vacancies by the high-rate growth, and then the pulling was stopped as the extreme condition of the low-rate growth. Nevertheless, the crystals continued to grow spontaneously after the pulling was stopped. Hence, simultaneously with the pulling-stop, the temperature of the melts was increased to melt the spontaneously grown portions, so that the diameters were restored to sizes at the moment of pulling-stop. Then, the crystals were cooled as the cooling time elapsed, and the temperature gradient in the crystals was increased. By using X-ray topographs before and after oxygen precipitation in combination with a minority carrier lifetime distribution, a time-dependent change in the defect type distribution was successfully observed in a three-dimensional manner from the growth interface to the low-temperature portion where the cooling progressed. This result revealed that Vs are uniformly introduced in a grown crystal regardless of the pulling rate as long as the growth continues, and the Vs agglomerate as a void and remain in the crystal, unless recombined with Is. On the other hand, Is are generated only in a region where the temperature gradient is large by low-rate growth. In particular, the generation starts near the peripheral portion in the vicinity of the solid–liquid interface. First, the generated Is are recombined with Vs introduced into the growth interface, so that a recombination region is always formed which is regarded as substantially defect free. Excessively generated Is after the recombination agglomerate and form a dislocation loop region. Unlike conventional Voronkov's diffusion model, Is hardly diffuse over a long distance. Is are generated by re-heating after growth.[In a steady state, the crystal growth rate is synonymous with the pulling rate. Meanwhile, when an atypical operation is performed, the pulling rate is specifically used.]This review on point defects formation intends to contribute further silicon crystals development, because electronic devices are aimed to have finer structures, and there is a demand for more perfect crystals with controlled point defects.     

Relationship between grown-in defects and thermal history during CZ Si crystal growth

An abrupt change of the crystal growth rate at temperatures in the range 1150–1080°C affects the annihilation or the agglomeration of grown-in defects such as flow pattern defects (FPD), crystal originated particles (COP), laser scattering defects (LSTD) and the defects measured by an optical precipitate profiler (OPPDs). Moreover, it is demonstrated that the densities of FPDs and LSTDs correlate with each other, and also with the cooling rate in such a temperature range. These relationships were investigated by growing several silicon single crystals in 10 kinds of hot-zone (HZ) configurations designed by using a numerical simulation. The cooling rate from 1412°C, the melting point of silicon, to 1150°C does not seem to be so important for the generation or the annihilation of these defects.     

Structure‐controlled growth of ZnO nanonails by thermal evaporation technique

Wurtzite ZnO nanonail structures have been grown on sapphire substrate by simple thermal evaporation of Zn powder in oxygen ambient. Growth parameters such as growth temperature and oxygen gas flow have been examined for the growth of nanonail structure. It is found that the nanonail structures repeatedly grow under a certain relation between the growth temperature and the oxygen flow. Also, at higher growth temperature, the nanonails grow in the form of branched‐structures. The grown ZnO nanonails have hexagonally well‐faceted cap and grow mostly perpendicular to the sapphire substrate. Excellent luminescence properties of a strong UV emission peak with negligible green band have been obtained at room temperature. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure of the polar (010) cleavage of a ferroelectric TGS crystal from the atomic-force microscopy data

The atomically smooth polar (010) cleavage of a ferroelectric triglycine sulfate (TGS) crystal has been studied by the method of atomic-force microscopy. It is shown that the rounded 0.6-nm-high (deep) protrusions and pits with nanometer lateral dimensions revealed on the surfaces of TGS crystals are characteristic of their microrelief. These microrelief details can be formed either as a result of crystal cleavage in the ferroelectric phase or the mechanical action of a cantilever onto the crystal surface. These two-dimensional formations are relatively stable and genetically related to the layer structure of the ferroelectric phase of TGS crystals.     

Hybrid observation of crystal growth using laser confocal microscope with atomic force microscope

We built up a hybrid microscope system that consists of an atomic force microscope (AFM) and a laser confocal microscope with differential interference contrast microscope (LCM-DIM) and investigated the combined imaging of a potassium dihydrogen phosphate (KDP) crystal surface and its growth. Using this integrated setup, we were able to approach a AFM cantilever tip to several-ten μm$\mathrm{\mu }\mathrm{m}$ crystals using an optical microscope with AFM and to observe steps with measuring the height using LCM-DIM/AFM. Evaluation of the accuracy of the setup was studied and resulted in less than 100 nm of the AFM tip accuracy using LCM-DIM/AFM. We also demonstrated an in-situ observation of a KDP crystal growth using LCM-DIM/AFM. The interference fringes at the cantilever and the movements of steps were simultaneously observed.