Interface transitions in Czochralski growth of garnets

A source of variation in boule diameter, that has not been previously described, has been observed to occur during automated or weight regulated pulling of garnets from the melt. The effects are related to refractive index variations observed in longitudinal cross section under polarized light. Solution at the tip of boule simultaneously with compensatory lateral growth higher on the crystal melt interface can take place under low temperature gradient conditions. This occurs when heat flow or radiation loss through the crystal is insufficient to preserve the initial, faceted or approximately conical, interface shape.     

Crystalline perfection and optical properties of rapid grown KH2PO4 crystal with chromate additive

Potassium dihydrogen phosphate (KDP) crystals were grown in the presence of a series of chromate (CrO $_{4}^{2-}$ ) additive concentrations via rapid growth method. CrO $_{4}^{2-}$ made KDP crystals were coloured by yellow-green, suggesting CrO $_{4}^{2-}$ had entered into the crystal lattice. The elemental analysis indicated that Cr element in KDP crystal was at ppm level. High resolution X-ray diffraction data revealed that the crystalline perfection of these as-grown KDP crystals was destroyed after CrO $_{4}^{2-}$ entered into crystal lattice, embedded in the full width at half maximum was broadened and satellite peaks appeared. Additionally, the extinction ratio was decreased with rise of CrO $_{4}^{2-}$ concentration. CrO $_{4}^{2-}$ introduced two absorption peaks centred at 360 and 280 nm and enhanced the intrinsic absorption near 220 nm, which were at the same band positions compared with the CrO $_{4}^{2-}$ or HCrO $_{4}^{-}$ transmittance spectra. Additionally, CrO $_{4}^{2-}$ could increase the size of light scattering, which was attributed to the point defects and microscopic defects by the replacement by CrO $_{4}^{2-}$ at PO $_{4}^{3-}$ position.     

The elimination of defects in Czochralski grown lithium niobate

A method is described for growing large LiNbO₃ single crystals from the melt, completely free of low-angle grain boundaries. This crystalline perfection was achieved by eliminating localized cellular structures, which were introduced by thermal supercooling due to faceted growth. These defects were distributed only near the developed (012)_h and (0 –1 –2)_h planes when the growth of these planes as facets could be reduced rapidly in the conical part of the boule. Low-angle grain boundaries along the z-axis and polygonization of dislocations were induced by the stresses around the cellular structure. Voids and a corrugated interface have also been observed as a cell-boundary groove trail. The observation of cellular structures indicated that their formation was strongly dependent on growth in the radial direction and the pulled rate. Furthermore, to eliminate these structures, it was found most effective to keep the crystal growth rate, G, at less than 10 mm h^–1.     

Thermal-stress reduction for a Czochralski grown single crystal

In this paper an optimal-control approach for thermal-stress reduction inside a Czochralski-grown single crystal is presented. Using the lateral heat flux as a control variable, an optimal-control formulation for minimizing thermal stress with a given crystal shape is derived. Since the thermal stress is also affected by the lateral shape of crystals during growth, the level of the stress can be reduced by growing crystals into a suitable shape. Using the lateral shape as a control variable, a similar optimal-control formulation for stress reduction is derived. In both cases, the von Mises stress is used as an objective function for the constrained optimization problem. Euler–Lagrange equations are derived using the calculus of variations and Lagrange multipliers. Various stress-reduction strategies are explored by solving the Euler–Lagrange equations numerically.     

The structural,mechanical and optical properties of NaCl:Eu2+ crystal grown by the Czochralski method

Journal of Materials Science: Materials in Electronics - The pure and Eu2+-doped NaCl crystal were successfully grown by the melt (Czochralski) method. The structural, mechanical and optical...     

Titanium distribution in Ti-sapphire single crystals grown by Czochralski and Verneuil technique

The distributions of Ti³⁺ and Ti⁴⁺ ions were evaluated by photoluminescence measurement in the wafers cut from different positions of the ingots grown by Czochralski and Verneuil techniques. Particular radial distributions of Ti⁴⁺ as function of the position in the ingot were observed in the crystals grown by Verneuil technique different than the crystals grown by Czochralski method.     

Twinning structure of LaGaO3 grown by the Czochralski method

Crystals of lanthanum gallate were grown by the Czochralski method and their quality was tested by microscopic observation in polarized light. This material is used as a substratum for the epitaxial growth of highT _c films of YBCO, particularly for certain high-frequency device applications. Two types of twins were observed and the difference in their temperature behaviour is discussed. The best quality single crystals (practically without twins) were obtained with the 〈101〉 and 〈100〉 seeds.     

Structural perfection of laterally overgrown GaN layers grown in polar- and non-polar directions

The structural quality of GaN overgrown layers was evaluated using Transmission Electron Microscopy methods. Growth on polar and non-polar substrates was compared. Independent from growth polarity much better structural quality of the overgrown areas compared to the seed areas was obtained, but overgrowth on non-polar substrates is more difficult. For the latest samples, two wings on the opposite sites of the seed area grow in two different polar directions with different growth rates. Wings grown with Ga polarity are much wider than wings grown with N-polarity making coalescence of these layers difficult. Defects formed in the overgrown wings were characterized and their density was compared. It is shown that two-step growth (using two different temperatures) lead to much smaller misorientation between the wings than one step growth     

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.     

Characterization of CdS nanocrystals embedded in KCl single crystal matrix grown by Czochralski method

The growth of CdS nanocrystals (NCs) embedded in bulk KCl single crystal matrix is performed using the Czochralski method. The X-ray diffraction reveals the incorporation of the CdS NCs with a cubic structure inside the KCl matrix. The optical density measurements of the CdS NCs embedded in KCl single crystal show a shift of the absorption edge towards higher energies. The optical band-gap is estimated to be about of 2.60 eV. The photoluminescence (PL) spectrum of the studied samples presents four emission bands in the range of 2.20–2.56 eV.     

Study of the line intensity in the optical and magnetooptical spectra in holmium-containing paramagnetic garnets

Studies of line intensity in the optical and magneto-optical spectra in the holmium-containing paramagnetic garnet Ho³⁺:YAG were carried out within the visible spectrum at T = 85 K. Detailed investigation of the magnetic circularly polarized luminescence spectra at 85 and 300 K on ⁵S₂ → ⁵I₈ emission transition in Ho³⁺:YAG was carried out. A quasi-doublet state in the energy spectrum of the Ho³⁺ ions was observed, characterized by a significant magneto-optical activity, which is caused by a large Zeeman splitting of the quasi-doublet. The measurement of the magnetic circular polarized luminescence spectrum carried out within one of the emission lines of the luminescence band ⁵S₂ → ⁵I₈ in Ho³⁺:YAG at 85 K shows significant magneto-optical effects of the intensity change of the emitted light, compared to that measured for the other emission lines in the same luminescent band.     

Features of radiation damage of BGO crystals grown by the low-thermal-gradient Czochralski technique

We examined the effect of the radiation damage on optical and scintillation properties of BGO crystals produced by the low-thermal-gradient Czochralski growth technique. Irradiation tests were performed for γ-ray doses from 1 krad to 10 Mrad. Optical absorption and light output were measured upon irradiation and self-recovery. Based on the shape of radiation-induced absorption, degradation of transparency and light output upon radiation damage as well as their self-recovery, two distinct types of BGO crystals were established.     

Study of the colouring process in copper ruby glasses by optical and EPR spectroscopy

The mechanism responsible for the development of the ruby colour in silica glasses containing copper have been investigated by means of optical and EPR spectroscopy. During the striking treatment, the evolution of the concentration of Cu⁺ and Sn²⁺ has been followed through their luminescence bands, whereas the Cu²⁺ concentration has been monitored by the EPR spectra. To account for the data, a model for the colouring which involves two simultaneous redox reactions: (i)2Cu⁺Cu²⁺+Cu⁰ (ii)2Cu²⁺+Sn²⁺2Cu⁺+Sn⁴⁺ has been proposed. According to it, the role of tin is to act as a redox buffer to regenerate Cu⁺, therefore favouring the coalescence of Cu₂O colloids.     

Structural perfection of silicon carbide crystals grown on profiled seeds by sublimation method

The distribution of extended defects in silicon carbide (SiC) crystals grown on profiled seeds by the sublimation sandwich method (physical vapor transport) has been studied by optical microscopy in combination with chemical etching. It is established that free lateral overgrowth of protruding relief elements (mesas) is accompanied by a sharp decrease in the density of threading dislocations and micropores. The decreased density of dislocations is retained after growing a thick layer that involves the overgrowth of grooves that separated individual mesas.     

Second-order optical effects in seed-mediated grown palladium nanoparticles

Palladium nanoparticles deposited on SnO₂-doped In₂O₃ substrate show substantial optical second harmonic generation (SHG) in the spectral range (λ?=?120–160?nm) which is a part of the vacuum ultraviolet (VUV) spectrum. A single crystalline Li₂ B₄ O₇ optical parametric generator (OPG) pumped by nanosecond xenon–fluorine excimer laser (EMG 500/218 (Lambda Physics)) with the wavelength 218?nm, pulse duration about 6–8?ns; pulse rate about 80?hz, average pulse power about 0.2?MW and beam diameter varying within the 1.3–7.5?nm was used to form the fundamental beam. The OPG Li₂ B₄ O₇ single crystal was cut in the XZ optical plane. We have tuned the fundamental wavelengths within the 250–320?nm spectral range varying the angle of the plane with respect to the incident pumping beam. Maximal SHG output (in the reflected SHG geometry) is observed for the incident angles 75^○–80^○ with respect to the surface normal and p-polarized incident fundamental ultraviolet beams. Spectral separation between the vacuum ultraviolet (VUV) SHG intensities and the fundamental beams was performed using a VUV Seya-Numioka vacuum monochromator with spectral resolution 6?nm in the investigated spectral range. We have found that decreasing mean average palladium nanoparticle sizes favour substantial enhancement of the output SHG within the 120–160?nm spectral range. A layer of platinum nanoparticles coated on a layer of palladium nanoparticles suppresses the SHG effect indicating a quenching of the surface plasmon excitation originating from the palladium nanoparticles. The observed effect allows utilizing the palladium nanoparticles as an efficient material for frequency transformation of the UV nanosecond pulses (spectral range 240–310?nm) into the nanosecond laser pulses with wavelengths 120–160?nm.     

Structural perfection of heteroepitaxial silicon layers grown on sapphire by sublimation-source molecular beam epitaxy

Structurally perfect, single-crystal silicon layers have been grown on \((1\overline 1 02)\) sapphire by sublimation-source molecular beam epitaxy. Electron and x-ray diffraction data demonstrate that silicon-on-sapphire epitaxy occurs at substrate temperatures from 550 to 850°C. As the thickness of the layers decreases from 1.0 to 0.2 μm, their structural perfection degrades. In the layers grown at 600°C, the density of nucleation sites in the initial stages of growth is ? 5 × 10⁹ cm^?2.     

Substitutions in vanadate garnets

Following the original synthesis of Durif of NaCa₂Cu₂V₃O₁₂ and later synthesis by Bayer of NaCa₂M⁺⁺V₃O₁₂ and Ca₃LiM⁺⁺V₃O₁₂, where M was Mg, Co, Ni, Cu or Zn, forty-one variations were prepared as phase-pure garnets. The general formulae of the garnets attempted were [A₂⁺⁺B⁺] M₂⁺⁺V₃O₁₂ (Type I) and A₃⁺ [B⁺M⁺⁺]V₃O₁₂ (Type II), where A⁺⁺ = Mg, Sr, Pb, Cd or Ba, M⁺⁺ = Mg, Co, Ni, Cu or Zn and B⁺ = Li, Ag, K or Rb.     

Effect of oxygen concentration on diffusion length in Czochralski and magnetic Czochralski silicon

Abstract

This paper reports some new results on the influence of oxygen aggregation phenomena in the 450-850°C range on the diffusion length of minorities in both Czochralski (Cz) and magnetic Czochralski silicon. Thefeatures of the oxygen aggregation phenomena were studied by infrared spectroscopy at 20 K and the effect of these processes on the diffusion length was monitored by surface photovoltage measurements at 300 K. The results show that in Cz silicon the formation of thermal donors at 450°C is not associated with a detectable increase of the recombination activity, but that thermal donors are the precursors of other oxygen species, which grow during any further heat treatment. It was shown, moreover, that selfinterstitials injected from a surface oxide inhibit the oxygen segregation, in good agreement with predictions in the literature concerning the ‘volume exigency’ of oxygen segregation processes. Marked evidence of the influence of the cooling rate on the diffusion length was observed at 850°C, where the lifetime of heat treated Cz silicon was shown to be longer for a slow cooling process.     

Numerical simulation of the crystallization front inversion in oxide single crystals grown from melt using the Czochralski method

The process of pulling a Czochralski grown oxide single crystal out from melt has been numerically simulated for the first time from the moment of seeding until attaining a stationary growth regime. It is established that the phase boundary inversion at the lateral (shouldering) growth stage is a rather complicated process accompanied by intense oscillations of the crystallization rate. Diagrams illustrating variations of the crystal shape during this process are presented.     

Structural and optical properties of yttria-stabilized-zirconia films grown by MOCVD

The morphology of yttria doped zirconia thin films deposited by metal organic chemical vapour deposition (MOCVD) in two different substrate materials, glassy quartz and sapphire single crystals has been examined. The Y₂O₃ doping concentration has been varied from 3 to 12 mol percent. Structural characterization has been realized by X-ray diffraction, raman spectroscopy and scanning electron microscopy. The structure of the films corresponds to that of bulk crystals of the same composition. Refractive index has been determined by the optical transmission method. Refractive index close to those of bulk crystals are obtained for epitaxially grown zirconia on sapphire substrates, whereas low refractive index values, related with low packing densities, are obtained for thin films in the glassy substrate.