Sapphire: Relation between luminescence of starting materials and luminescence of single crystals

A relation between photoluminescence (PL) characteristics of different starting materials used for crystal growth and un-doped sapphire single crystals manufactured using various methods of crystal growth (Kyropolus, HEM, Czochralski, and EFG) was found. The crystals grown using the Verneuil starting material exhibited significant PL when any method of crystal growth was used. On the contrary, sapphire samples grown by the same technologies wherein the starting material was EMT HPDA^R revealed very low PL. (HPDA^R is produced by EMT, Inc., with proprietary and patented technology.)     

Fibers and square sapphire shaped single crystals grown from the melt and optical characterization

The micro-pulling down technique has been used to grow fibers (? = 1 mm) and square (2 × 2 mm²) sapphire shaped single crystals with controlled sizes. The hot zone construction and fiber pulling conditions have been optimized. As a result, growth of transparent and homogeneous sapphire shaped crystals has been performed. The crystals have been grown under 1 bar of argon atmosphere with pulling rate varied in the range of 0.5–3 mm/min. 100% of the melt has been continually transformed into crystalline sapphire with empty crucible at the end of the growth operation. The as grown crystals show high chemical homogeneities and good optical properties.     

Growth of controlled profile crystals from the melt: Part I - Sapphire filaments

A crystal growth technique is described in which a tubular, annular, or ribbon-shaped capillary orifice is used to maintain the liquid level constant during growth and to determine the shape of the growing crystal. The orifice material must be wetted by, but chemically nonreactive to the molten phase of the crystal to be grown. Using molybdenum orifices and crucibles, single crystal sapphire filaments several hundred feet in length and diameters of .01 - .05 cm have been grown at speeds of 2.5 - 5 cm/min; c-axis crystals have been grown at rates up to 20 cm/min, and a variety of other crystal orientations has been grown at speeds up to 2.5 cm/min. Sapphire tubes, ribbons and multiple filaments have also been grown.     

Metal organic chemical vapor deposition and investigation of ZnO thin films grown on sapphire

A new type of large area metal organic chemical vapor deposition (MOCVD) system for the growth of high quality and large size ZnO materials is introduced. Materials properties of the un-doped, n- and p-doped ZnO epi-films grown on sapphire substrates by this MOCVD system are studied by various techniques, including high resolution X-ray diffraction (XRD), UV-Visible optical transmission (OT), photoluminescence (PL) and photoluminescence excitation (PLE), synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS). The wurtzite (w) ZnO crystal structures grown with primary (0002) orientation were identified. Results have shown the high crystalline quality of MOCVD-grown ZnO films, indicated by the narrow XRD, PL and Raman line widths, strong PL signals, sharp OT edge and smooth surface. In particular, high p-type carrier concentration of > 10¹⁷ cm^− 3 have been achieved besides the good n-type doping in ZnO.     

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)     

Photoluminescence properties of Nd:YVO4 single crystals by multi-die EFG method

Nd-doped YVO₄ crystal is an excellent material for making the diode-pumped solid lasers. In this study, Nd-doped YVO₄ multi-crystal with high quality was grown simultaneously by developed edge-defined film-fed growth (EFG) method. All crystals appeared homogenous Nd distribution and were found the stable step-faceting phenomena. The up-conversion properties on the Nd:YVO₄ multiple-crystal have been measured by the observations of transmission and photoluminescence (PL) spectra.     

Optical investigations on Tb3+ doped l-Histidine hydrochloride mono hydrate single crystals grown by low temperature solution techniques

The potential nonlinear optical material of Terbium (Tb³⁺) ion doped l-Histidine hydrochloride monohydrate (LHHC) single crystals were successfully grown. Tb³⁺:LHHC crystals of 7 mm × 5 mm × 3 mm and 59 mm length and 15 mm diameter have been grown by the slow solvent evaporation and Sankaranarayanan-Ramasamy (SR) techniques respectively. The grown crystals were characterized by single crystal X-ray diffraction analysis to confirm the crystalline structure and morphology. High resolution X-ray diffraction (HRXRD) studies revealed that the SR grown sample shows relatively good crystalline nature with 9″ full-width at half-maximum (FWHM) for the diffraction curve. Functional groups were identified by Fourier transform infra-red spectroscopy (FTIR). The optical transparency and band gaps of grown crystals were measured by UV–Vis spectroscopy. Thermogravimetric and differential thermal analysis (TG/DTA) studies reveal that the crystal was thermally stable up to 155 °C in SR grown crystal. Surface morphology of the growth plane was observed using scanning electron microscopy (SEM). The incorporation of Tb ion was estimated by EDAX. The frequency-dependent dielectric properties of the crystals were carried out for different temperatures. Vickers hardness study carried out on (1 0 0) face at room temperature shows increased hardness of the SR method grown crystal. Second harmonic generation efficiency of SEST and SR grown crystals are 3.2 and 3.5 times greater than that of pure KDP. The Photoluminescence (PL) studies of Tb³⁺ ions result from the radiative intra-configurational f-f transitions that occur from the ⁵D₄ excited state to the ⁷F_j (j = 6, 5, 4, 3) ground states. The decay curve of the ⁵D₄ level of emission was observed with a long life time of 319.2041 μs for the SR grown Tb³⁺:LHHC crystal.     

Growth and characterisation of ferroelectric potassium lithium niobate crystals

Abstract

Potassium lithium niobate crystals have been grown by the resistance heating Czochralski technique. The optical transmission spectrum of the crystal has been determined. The results showed that the spectrum properties of potassium lithium niobate crystals grown from a melt with a higher Li₂O content are better than that of crystals grown from the melt with a lower Li₂O content. The crystals grown by this method have good quality and second harmonic generation properties. Frequency doubling results of a quasi cw-Ti:sapphire laser using crystal samples showed that potassium lithium niobate can be used to double the frequency of near infrared quasi cw-lasers in the 890–960 nm wavelength range.     

Compositional variation of photoluminescence from Mn doped MgAl2O4 spinel

Spinel (MgAl₂O₄) crystals doped with 1.0% Mn have been grown by floating zone (FZ) technique with various Mg compositions, x = MgO/Al₂O₃, from 0.2 to 1.0. Compositional variations of photoluminescence are evaluated for a fluorescence thermometer application using crystals grown. Strong photoluminescence (PL) peak is observed at λ from 512 to 520 nm from the crystals grown from compositions, x, from 0.3 to 1.0. Peak wavelength of PL increases linearly from 512 to 520 nm with x. Weak PL peaking at λ = 750 nm is also observed from the specimens. Compositional variations of PL are considered to be due to the variation of crystal field surrounding the Mn²⁺ ions. The variation of crystal field strength agrees with the compositional variation of lattice constant.     

泡生法生长掺碳钛宝石激光晶体的研究

钛宝石激光晶体在现代高功率激光领域具有重要的应用价值, 但大尺寸、高品质的晶体生长仍是当前钛宝石应用面临的重大难题. 本文研究了泡生法技术生长大尺寸掺碳钛宝石激光晶体, 结果显示, 泡生法生长得到的直径180 mm、30 kg的钛宝石没有出现应力集中的开裂等宏观缺陷现象, 钛离子在晶体中分布均匀接近理论值, 晶体的FOM值达到200. 该研究对低红外残余吸收, 高品质因素、大尺寸钛宝石激光晶体的生长应用具有重要的现实意义.     

Investigation of SR method grown 0 0 1 directed KDP single crystal and its characterization by high-resolution X-ray diffractometry (HRXRD), laser damage threshold, dielectric, thermal analysis, optical and hardness studies

0 0 1 directed potassium dihydrogen orthophosphate (KDP) single crystal was grown by Sankaranarayanan–Ramasamy (SR) method. The 0 0 1 oriented seed crystals were mounted at the bottom of the platform and the size of the crystals were 10 mm diameter, 110 mm height. Two different growths were tried, in one the crystal diameter was the ampoule's inner diameter and in the other the crystal thickness was less than the ampoule diameter. In the first case only the top four pyramidal faces were existing whereas in the second case the top four pyramidal faces and four prismatic faces were existing through out the growth. The crystals were grown using same stoichiometric solution. The results of the two growths are discussed in this paper. The grown crystals were characterized by high-resolution X-ray diffractometry (HRXRD), laser damage threshold, dielectric, thermal analysis, UV–vis spectroscopy and microhardness studies. The HRXRD analysis indicates that the crystalline perfection is excellent without having any very low angle internal structural grain boundaries. Laser damage threshold value has been determined using Nd:glass laser operating at 1054 nm. The damage threshold for the KDP crystal is greater than 4.55 GW cm⁻². The dielectric constant was higher and the dielectric loss was less in SR method grown crystal as against conventional method grown crystal. In thermal analysis, the starting of decomposition nature is similar in SR method grown KDP crystal and conventional method grown crystal. The SR method grown KDP has higher transmittance and higher hardness value compared to conventional method grown crystals.     

Characterization of bismuth tri-iodide single crystals for wide band-gap semiconductor radiation detectors

Bismuth tri-iodide is a wide band-gap semiconductor material that may be able to operate as a radiation detector without any cooling mechanism. This material has a higher effective atomic number than germanium and CdZnTe, and thus should have a higher gamma-ray detection efficiency, particularly for moderate and high energy gamma-rays. Unfortunately, not much is known about bismuth tri-iodide, and the general properties of the material need to be investigated. Bismuth tri-iodide does not suffer from some of the material issues, such as a solid state phase transition and dissociation in air, that mercuric iodide (another high-Z, wide band-gap semiconductor) does. Thus, bismuth tri-iodide is both easier to grow and handle than mercuric iodide. A modified vertical Bridgman growth technique is being used to grow large, single bismuth tri-iodide crystals. Zone refining is being performed to purify the starting material and increase the resistivity of the crystals. The single crystals being grown are typically several hundred mm³. The larger crystals grown are approximately 2 cm³. Initial detectors are being fabricated using both gold and palladium electrodes and palladium wire. The electron mobility measured using an alpha source was determined to be 260±50 cm²/Vs. An alpha spectrum was recorded with one of the devices; however the detector appears to suffer from polarization.     

Crystal growth of Bi2Sr2CuO6 from flux system KCl-KF and its superconductivity

Single crystals of Bi₂Sr₂CuO₆ (BSCO) were grown from high-temperature solution using KCl-KF as flux. Differential thermal analysis (DTA) of the starting materials enabled an appropriate temperature programme to be introduced for crystal growth by spontaneous nucleation. By means of the improved thermogravimetric (TG) method, Bi₂Sr₂CuO₆ crystals were grown after determination of the crystallization temperature. A single crystal in the form of a thin rectangular platelet up to 6×4 mm² was obtained and was identified by X-ray powder pattern and by scanning electron microscopy (SEM). Analyses were performed by using high-resolution electron probe microanalysis (EPMA), which indicated that it is the nonstoichiometric compound Bi_2+xSr_2–xCuO₆. The superconducting properties are strongly dependent on the composition of Bi and Sr in the crystals and on heat treatment. The transition temperature of 7 K with 0.5 K width for as-grown crystals, has been obtained.     

Nature of intrinsic and impure luminescence of MAlP2O7 crystals

The results of the photoluminescence (PL) investigation of pure and chromium-doped MAlP₂O₇ (M = Na, K, Cs) compounds are presented. The spectra of the intrinsic luminescence of MAlP₂O₇ crystals consist of a separated UV band at a peak position near 330 nm and a complex wide band which covers the region of visible light up to 750 nm at excitation by VUV synchrotron radiation. The “red” band in 600–1000 nm diapason appears in the PL spectra of crystals doped with chromium ions. The effect of the temperature on the structure, the peak positions and intensities of the luminescence bands was studied. An assumption about the nature of the intrinsic PL was made. The “red” luminescence was considered as a result of the ⁴Т₂ → ⁴А₂ radiation transitions in the impurity Cr³⁺ ions located in the intermediate crystal field.     

Nonuniformities in free-standing GaN substrates

In this study, we report that free-standing GaN substrates grown by the hydride vapor-phase epitaxy (HVPE) are found to contain nonuniform regions with low crystal and optical quality located close to the top (near as-grown surface) and bottom (near interface between GaN/sapphire) regions of substrate cross-section. We considered that the origins of these nonuniformities were surface reconstruction by undesired residual gas reaction after crystal growth on the top regions and the individual columns forming an irregular layer in the bottom regions by lattice mismatch and difference of thermal expansion coefficient between GaN films and sapphire substrate. We used cathodoluminescence imaging and spectroscopy for analyzing these nonuniform regions. The low quality regions with high electron concentration are easily visualized using cathodoluminescence (CL). The coexistence of regions with low- and high quality allows us to explain the concurrent evidence of high substrate quality in double crystal X-ray diffraction and photoluminescence (PL).     

Time-resolved luminescence of defects and Cr3+ impurity centers in nanosized alumina crystals under vacuum-ultraviolet excitation

The photoluminescence (PL) of intrinsic defects and Cr³⁺ impurity centers in nanosized alumina (Al₂O₃) crystals of various phase compositions (?? + ?? and ?? phases) has been studied by the method of time-resolved spectroscopy with vacuum-ultraviolet (VUV) excitation at T = 7.5 and 295 K. It is established that the PL excitation spectrum and its kinetic parameters for ??-Al₂O₃ phase can change depending on the crystal grain size. In ??-Al₂O₃ samples with grain sizes below 100 nm, excitons localized on Cr³⁺ impurity centers and surface-modified defects of the crystal structure are observed even at T = 295 K. Manifestations of these exciton states in the PL spectra strongly depend on the grain size.     

Growth,morphology and structure of CsRP4O12 crystals

We have grown CsRP₄O₁₂ (R³⁺ is a rare-earth ion) crystals from 85% phosphoric acid solutions. Morphological, optical and X-ray studies have been carried out on all the crystals. CsRP₄O₁₂ crystallizes in four different structural modifications. The variation in growth temperature causes variation in their morphology and crystal structure.     

Thermal stability and annealing behavior of photoluminescence from Eu doped YAG

Eu₂O₃ doped Y₃Al₅O₁₂ (YAG) crystals have been grown using a floating zone technique and evaluated thermal stability and annealing behavior of PL for a fluorescence thermo-sensor application. Color of the crystals grown varies from deep purple to colorless with O₂ concentration of the growth atmosphere and annealing in air. Photoluminescence (PL) peaking at λ = 590, 607, 624, 647 and 709 nm due to Eu³⁺ ions are observed from the crystals under UV excitation. Anomalous temperature dependence of PL intensity, which is observed in as-grown crystals, is improved greatly by annealing through the heat cycle. From annealing behavior of optical absorption spectra, residual Eu²⁺ ions are suggested to be responsible for the de-coloration and the improvement of anomalous temperature dependence of Eu doped YAG crystals.     

Preparation of CaF2 precursors for laser grade crystal growth

The expansion of commercial excimer lasers has increased the need of high-quality optics manufactured from CaF₂. The key technical parameters, affecting the long-term quality of CaF₂ excimer laser optics, are controlled by the purity of the raw material. The new purifying techniques provide 99.99% purity of the starting material which rises up to 99.999% into the grown crystals. However, the techniques are rather expensive which leads to a significant rise in the price of the final crystal product. The present paper describes an alternative highly effective and relatively low-cost technique providing a precise control over a set of preparatory and purifying procedures and crystal growth. Using an original apparatus for electro-chemical grinding of the thermo-broken starting material, the grained product, uncontaminated by unintentional impurities, may be highly purified by two complementary methods to produce precursors used for UV- and laser-grade CaF₂ crystal growth. Received: 20 April 1999 / Reviewed and accepted: June 2, 1999     

Purification and crystal growth of LiY0.5Er0.5F4 crystals

The purification procedure of the LiY_0.5Er_0.5F₄ compound for Bridgman-Stockbarger and Czochralski single crystal growth has been investigated. Yttrium and rare-earth oxyfluorides were found to form during hydrofluoric heat-treatment of the starting fluoride mixture. Optically clear single crystals of purified LiY_0.5Er_0.5F₄ were grown by both Bridgman-Stockbarger and Czochralski methods. The stoichiometry was investigated by magnetic susceptibility measurements and the Er content in the single crystals grown by both Bridgman-Stockbarger and Czochralski methods were found to fit stoichiometry within the measurement error. No free (OH)^? ions were found by infrared absorption spectroscopy.