Iridium Crucible Research Articles

Modeling of dopant segregation in sapphire single crystal fibre growth by Micro-Pulling-Down method

Experiments and numerical simulations are conducted in order to study the causes and solutions for the Ti inhomogeneity problem in Ti doped sapphire Micro-Pulling-Down (μ-PD) growth. The measurement and modeling of the thermal and flow fields, electromagnetic field, Ti concentration in the molten zone and along the fibre axis are compared. For the mean Ti concentration along the fibre and temperature along the iridium crucible, the modeling results are consistent with experiments. Results showed that for high pulling rate, the mass transfer in the capillary is dominated by convection. Marangoni convection is strong in the meniscus due to the large temperature gradient, which has great impact on the Ti distribution for different fibre radii. For high pulling rate, Ti concentration increases quickly from the seed along the fibre axis, and reaches a constant value after about 0.5–2mm. Radial segregation is high for large diameter fibres. The constant Ti concentration along the fibre axis is increasing when increasing the fibre radius from 0.2 to 0.6mm. For 0.8mm, it decreases due to the change of the vortex. At low growth rate, the transport in the capillary is diffusive, back to the crucible, which leads to a Scheil-like Ti distribution, in full agreement with the experimental results.

Improvement of dopant distribution in radial direction of single crystals grown by micro-pulling-down method

Two iridium crucibles with one and five capillaries at a nozzle were developed for the growth of Ce-doped Y3Al5O12 [Ce:YAG] single crystals by the micro-pulling-down [μ-PD] method. The purpose of the study was to examine the effect of the capillary number on the Ce distribution in the radial direction of the crystals. The crystals grown from 2mol% and 5mol% Ce-doped YAG melts were then cut perpendicular to the growth direction and polished to produce specimens suitable for the measurement of Ce distribution. The Ce:YAG crystals grown using the crucible with one capillary [Ce:YAG(1C)] had greater Ce content in the central portion of the crystal when compared to its peripheral parts. On the other hand, in the case of the Ce:YAG crystal grown using the crucible with five capillaries [Ce:YAG(5C)], the increased Ce concentration was detected in five specimen sections positioned just below the capillaries. The results indicated that increase of the number of capillaries is effective practice that results in an improvement of the radial homogeneity of the single crystals grown by the μ-PD method.

New Forensic Insight into Carl Auer von Welsbach's 1910 Observation of Induced Radioactivity: Theoretical, Experimental and Historical Approaches

In 1910, Carl Auer von Welsbach noted that he had made an observation of a radioactive substance inducing radioactivity to an inactive substance. From today's point of view, this could have been the first observation of neutron activation. Herein, we present new insights into our investigation of this ‘mysterious observation’ as Auer von Welsbach termed it. We believe that one of the activated objects was a platinum–iridium crucible. The dominating activation product of the crucible could have been iridium-194. We have discovered several platinum crucibles from Auer's heritage and investigated them by gamma-spectrometry and, one of them, by SEM/EDX. In the EDX investigations, however, no iridium was found in the most promising crucible. Hence this particular crucible was probably not the activated object. In any case, gamma-ray spectrometry revealed very low but detectable amounts of natural radionuclides. This indicated that these crucibles were used by Auer von Welsbach for his radioactive work and that these crucibles were bought prior to World War I. Hence Auer von Welsbach somehow managed to save these crucibles from the noble metal collections during the war. Auer's 1910 publication carried the suffix ‘Part I’, however, Part II was thought to be lost. In our recent work, however, we rediscovered a hand-written manuscript of Part II, in which the peculiar observation is mentioned vaguely. Lastly, we converted Auer's uranium standard into becquerels. Based on this conversion, we estimated that Auer von Welsbach observed an 194Ir activity of the crucible of 500 kBq. It is further estimated that a (thermal) neutron flux density of approximately 8E+4 cm−2 s−1 was needed to activate the crucible in a way to meet Auer's description.

Scaling-Up of Bulk β-Ga2O3 Single Crystals by the Czochralski Method

We present a new approach for scaling-up the growth of β-Ga2O3 single crystals grown from the melt by the Czochralski method, which has also a direct application to other melt-growth techniques involving a noble metal crucible. Experimental and theoretical results point to melt thermodynamics as the crucial factor in increasing the volume of a growing crystal. In particular, the formation of metallic gallium in the liquid phase in large melt volumes causes problems with crystal growth and eutectic or intermetallic phase formation with the noble metal crucible. The larger crystals to be grown the higher oxygen concentration is required. The minimum oxygen concentration ranges from about 8 to 100 vol.% for 2 to 4 inch diameter cylindrical crystals, challenging the use of iridium crucibles in a combination with such high oxygen concentrations. A specific way of oxygen delivery to a growth furnace with the iridium crucible allows to minimize the formation of metallic gallium in the melt and thus obtaining large crystal volumes while decreasing the probability of the eutectic formation.

Large-lattice-parameter perovskite single-crystal substrates

The pseudobinary system LaLuO3–LaScO3 was explored in hopes of discovering new perovskite-type substrates with pseudocubic lattice parameters above 4Å. A complete solid solution of the type (LaLuO3)1−x(LaScO3)x forms between the end members LaLuO3 and LaScO3, enabling large single crystals of (LaLuO3)1−x(LaScO3)x to be grown from the melt. A single crystal with x≈0.34 was demonstrated. Considering the maximum thermal load of the iridium crucibles appropriate for Czochralski growth of this solid solution, the theoretically maximum achievable x-value is ~0.67. Based on the phase diagram determined, it is anticipated that single crystals with pseudocubic lattice constants between 4.09 and 4.18Å can be grown in this system by the Czochralski method.

New OSL detectors based on LiMgPO4 crystals grown by micro pulling down method. Dosimetric properties vs. growth parameters

Rod-shape crystals of the LiMgPO4 compound were grown by micro pulling down technique under the different growth conditions. Influence of the different growth rates, thermal setups and gaseous atmospheres on the crystals dosimetric properties was investigated. Samples were irradiated with 90Sr/90Y β particles and optically stimulated luminescence spectra were measured with the automatic Risø TL/OSL-DA20 reader. The sensitivity level, repeatability, dose–response dependence and short-time fading were compared for all grown crystals. It was found that the crystal grown from the iridium crucible was about three times more sensitive to radiation as compared to the crystal grown from the graphite crucible. Also the radio-sensitivity measured for the crystals grown from the graphite crucible was higher in case of higher growth rates. It was also shown that the residual OSL signal measured one and two weeks after the irradiation was higher for the crystals grown with higher growth rates. There was also no correlation observed between the growth conditions and the level of dose–response nonlinearity. Over the studied dose range the response nonlinearity of the studied samples fluctuated around over a dozen percent, regardless of the applied growth parameters. The obtained results tend to suggest that LiMgPO4 crystals may be considered as promising dosimeters in different fields of research.

The growth of Ho:YAG single crystals by Czochralski method and investigating the formed cores

Ho:YAG single crystals were grown by Czochralski technique, and investigated by the X-ray diffraction (XRD) and optical methods. The crystals were cut and polished in order to observe and analyze their cores. It was found that the deviation of the cores formed in the Czochralski grown Ho:YAG single crystals are resulted from non-symmetrical status of thermal insulation around the Iridium crucible.

Spectroscopic characterization of Sm3+ in La3Ga5.5Ta0.5O14 single crystals

La3Ga5.5Ta0.5O14 single crystals with nominal Sm3+ concentrations of 0.5 at.% and 3 at.% were grown by the Czochralski method from iridium crucibles. Examination of chemical composition of manufactured crystals revealed that actual Sm3+ concentrations are 0.27 at.% and 1.96 at.%, respectively. Spectroscopic measurement encompassed acquisition of absorption spectra, luminescence spectra and luminescence decay curves at temperatures between 5K and 300K. Analysis of room temperature absorption spectra in the framework of the Judd–Ofelt treatment made it possible to determine radiative transition rates branching ratio values related to the 4G5/2 luminescence of Sm3+ ions. Crystal field splitting of Sm3+ multiplets involved in luminescence phenomena was inferred from absorption and emission spectra recorded at 5K. It was found that absorption and luminescence transitions exhibit strong anisotropy and their intensities for σ-polarized spectra is higher by a factor of two roughly than those for π-polarized spectra. Observed strong inhomogeneous broadening of spectral bands was attributed to certain structural disorder of the host resulting from the substitution of octahedrally coordinated Ga3+ ions by Ta5+ ions. It follows from calculated and experimental values of luminescence branching ratio that about 80% of excitation energy of the 4G5/2 metastable level is converted to visible luminescence distributed into three bands peaking at 550nm, 600nm and 646nm. It was concluded that the La3Ga5.5Ta0.5O14:Sm3+ system is promising for application as an orange–red emitting phosphor and deserves further investigation aimed at assessing its ability to show yellow laser emission.

Electrical properties of β-Ga2O3 single crystals grown by the Czochralski method

Electrical properties of nominally undoped β-Ga2O3 crystals grown by the Czochralski method from an iridium crucible under a carbon dioxide containing atmosphere were studied by temperature dependent conductivity and Hall effect measurements as well as deep level transient spectroscopy. All crystals were n-type with net donor concentrations between 6 × 1016 and 8 × 1017 cm−3. The Hall mobility of electrons was on average 130 cm2/Vs at room temperature and attained a maximum of 500 cm2/Vs at 100 K. The donor ionization energy was dependent on the donor concentration. Extrapolation of this dependence to zero concentration yielded a value of about 36 meV for isolated donors agreeing well with the ionization energy derived from effective-mass theory. Three deep electron traps were found at 0.55, 0.74, and 1.04 eV below the conduction bandedge. The trap at EC – 0.74 eV was detected in all samples with concentrations of 2 – 4 × 1016 cm−3. This concentration is comparable to that of compensating acceptors we have to take into account for an explanation of the temperature dependent electron density. Therefore, under the assumption that the electron trap at EC – 0.74 eV possesses acceptor character, this trap could be the dominating compensating acceptor in our crystals. Besides, a value of ΦB = (1.1 ± 0.1) V was determined for the Schottky barrier height of Ni on the (100) surface of n-type β-Ga2O3.

The surface band structure of β-Ga2O3

Ga2O3 belongs to the group of transparent conducting oxides (TCOs) with a wide band gap and electrical conductivity. It exhibits the largest band gap with Eg = 4.8 eV and thus a unique transparency from the visible into the UV region. The information on the electronic structure of β-Ga2O3 is very scarce. This is in part due to the challenging problem of growing high purity single crystals. Transparent semiconducting β-Ga2O3 single crystals were grown by the Czochralski method from an iridium crucible under a dynamic protective atmosphere to control partial pressures of volatile species of Ga2O3. The investigated samples were characterized by different techniques (LEED, Laue, and STM). The experimental valence band structure of the of β-Ga2O3 single crystals along Γ-Z and A-M symmetry directions of the (100)-surface of Brillouin zone was determined by high-resolution angle-resolved photoelectron spectroscopy (ARPES) utilizing synchrotron radiation. The experimental band structure is compared and discussed with the theoretical calculations. The effect of changing the temperature from 300K to 20K on the experimental band structure β-Ga2O3 was studied.

Bismuth valence states and emission centers in Mg-Al-silicate glass

Two Al-Mg-silicate glass sample sets with variable Bi2O3 content were prepared. One set was sintered by melting in an iridium crucible at 1850°C in nitrogen, the other—in alumina crucibles at 1550°C in air. Their absorption and emission properties were investigated and compared in the 200–1600 nm spectral range. It was determined that the visible range extinctions values of high-temperature melted samples were almost two orders of magnitude higher than those of the low-temperature melted samples with the same doping level. The concentration dependences of the extinction values at 500, 700 and 800 nm were nonlinear. The investigations allowed us to estimate the absolute concentrations of Bi3+ ions and NIR-emitting centers as well as the efficiency of the Bi3+ ions conversion into these centers. It was concluded that optical centers emitting at 1100 and 1300 nm contain a pair of bismuth ions. Speculations about some possible structures of Bi dimer NIR-emitting centers were made.

Czochralski growth and characterization of β‐Ga2O3 single crystals

AbstractTransparent semiconducting β‐Ga2O3 single crystals were grown by the Czochralski method from an iridium crucible under a dynamic protective atmosphere to control partial pressures of volatile species of Ga2O3. Thermodynamic calculations on different atmospheres containing CO2, Ar and O2 reveal that CO2 growth atmosphere combined with overpressure significantly decreases evaporation of volatile Ga2O3 species without any harm to iridium crucible. It has been found that CO2, besides providing high oxygen concentration at high temperatures, is also acting as a minor reducing agent for Ga2O3. Different coloration of obtained crystals as well as optical and electrical properties are directly correlated with growth conditions (atmosphere, pressure and temperature gradients), but not with residual impurities. Typical electrical properties of the n‐type β‐Ga2O3 crystals at room temperature are: ϱ = 0.1 – 0.3 Ωcm, µn,Hall = 110 ‐ 150 cm2V‐1s‐1, nHall = 2 ‐ 6 × 1017 cm‐3 and EIonisation = 30 ‐ 40 meV. A decrease of transmission in the IR‐region is directly correlated with the free carrier concentration and can be effectively modulated by the dynamic growth atmosphere. Electron paramagnetic resonance (EPR) spectra exhibit an isotropic shallow donor level and anisotropic defect level. According to differential thermal analysis (DTA) measurements, there is substantially no mass change of β‐Ga2O3 crystals below 1200 °C (i.e. no decomposition) under oxidizing or neutral atmosphere, while the mass gradually decreases with temperature above 1200 °C. High resolution transmission electron microscopy (HRTEM) images at atomic resolution show the presence of vacancies, which can be attributed to Ga or O sites, and interstitials, which can likely be attributed to Ga atoms. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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

AbstractThe 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)

Zone-leveling Czochralski growth and characterization of undoped and MgO-doped near-stoichiometric lithium tantalate crystals

Undoped and MgO-doped near-stoichiometric lithium tantalate (SLT) crystals were grown by the zone-leveling Czochralski (ZLCz) technique for the first time from Li-rich solutions (mixed with 1.2 mol% MgO for MgO-doped SLT crystal) having Li-excess feed supplied from below in an iridium crucible. Inclusion-free crystals were obtained. The transmittance along the growth axis was found to be satisfactory (75±1%) having an absorption edge at 264.1±0.5 nm for undoped and 264.8±0.4 nm for MgO-doped SLT crystals. The refractive indices of the crystals were found to be n o=2.1768 and n e=2.1750±0.0002 for undoped and n o=2.1768 and n e=2.1784±0.0006 for MgO-doped SLT crystals, respectively. The Curie temperature and coercive field for the crystals were measured to be 680±3 °C and 1.95±0.65 kV/mm for undoped and 689±0.7 °C and 3.55±0.95 kV/mm for MgO-doped SLT crystals, respectively. ICP measurement gives the Li/Ta ratios as 0.995±0.002 and 0.9755±0.0145 in undoped and MgO-doped SLT crystals, respectively. The Mg/Ta ratio in MgO-doped crystal was measured to be 0.001235±0.00035. The Li/Ta ratio was found to vary along the crystal length in undoped and MgO-doped SLT crystals. The variation of the Mg/Ta ratio in MgO-doped SLT crystals is explained by mass balance equation.

Inductively heated Bridgman method for the growth of zinc oxide single crystals

A Bridgman method using induction heating has been developed for growing zinc oxide crystals. Bulk single crystals were prepared in an iridium crucible, where the crucible is directly heated via eddy currents. A high-pressure vessel was used in order to suppress evaporation at the melting point and a carbon dioxide atmosphere is applied to prevent decomposition. Typically, single crystals of 33 mm in diameter and 40 mm in length have been obtained. The as-grown boules exhibit different transmission properties dependent on the purity of the starting material as well as the cooling rate after crystallization.

Growth of 12CaO·7Al 2O 3 single crystal with tetragonal symmetry by Czochralski method

Single crystals of 12CaO·7Al 2O 3 (C12A7) have been grown by the Czochralski method with three different growth directions of <100>, <110>, and <111>. Employment of a 2.0% oxygen-containing nitrogen atmosphere and use of an iridium crucible were the critical factors for obtaining the single crystal. The crystal exhibited orange color resulting from the incorporation of Ir 4+ ions of ∼ 5 × 10 17 cm − 3 , presumably occupying at Ca 2+ sites. High resolution X-ray diffraction analyses indicated that the C12A7 single crystal shows the tetragonal symmetry belonging to the space group I 4 ― 2 m .

Bridgman-grown zinc oxide single crystals

Zinc oxide single crystals were obtained by solidification of a melt contained in an iridium crucible. To the authors knowledge this is the first report on crystal growth of zinc oxide from the melt using an inductively heated metal crucible. This proves that under particular conditions metals are capable of withstanding liquid zinc oxide at high temperatures. Optical microscopy as well as X-ray rocking curve measurements reveal the high crystalline quality of the wafers with full-width at half-maximum values of 22 arcsec. Annealing of as-grown samples was observed to have a big influence on optical properties.

Growth and thermal properties of SrWO4 single crystal

A strontium tungstate (SrWO4) single crystal with dimensions of Φ 25×40mm2 has been grown by the Czochralski method using an iridium crucible. X-ray powder diffraction results show that as grown the SrWO4 crystal belongs to the tetragonal system in the scheelite structure. The effective segregation coefficients of elemental W and Sr in the crystal growth are measured by x-ray fluorescence, and the respective values are close to 1. The thermal properties of the SrWO4 crystal were systemically studied by measuring the thermal expansion, specific heat, and thermal diffusion coefficient. These results show that the crystal possesses a large anisotropic thermal expansion with thermal-expansion coefficients αa=8.61×10−6∕K, αb=8.74×10−6∕K, αc=18.78×10−6∕K over the temperature range of 303.15–773.15K. The measured value of the specific heat is 0.30–0.34Jg−1K−1 when the temperature is increased from 323.15to1073.15K. The thermal diffusion coefficient was measured over the temperature range of 303.15–543.15K. The thermal conductivities of the SrWO4 crystal along the [100] and [001] directions are calculated to be 3.1326Wm−1K−1 and 2.9477Wm−1K−1, respectively, at 303.15K. Then the thermal properties of some other Raman crystals were compared with these data, and the thermal focal lengths for these crystals were also estimated.

Czochralski Growth of 12CaO·7Al2O3 Crystals

Single crystals of nanoporous 12CaO·7Al2O3 (C12A7) as large as 27 mm in diameter, 120 mm in length, and 207 g in weight have been successfully grown by the Czochralski method. We found that a 2.0% oxygen-containing nitrogen atmosphere and an iridium crucible are the critical factors for success. The crystal ingot exhibited an orange color resulting from the incorporation of Ir4+ ions of ca. 5 × 1017 cm-3, which presumably occupy Ca2+ sites. X-ray diffraction analysis indicated that the ingot obtatined was a single crystal with few domains.

Investigation on defects in Lu2SiO5:Ce crystals grown by Czochralski method

AbstractCerium‐doped lutetium oxyorthosilicate crystals (Lu2SiO5:Ce) with dimension of ∅︁50 × 60 mm were grown by Czochralski method from an inductively heated iridium crucible. The vaporized substance during growth was examined with XRD and proved to be SiO2. The vertical and the screw strips existing on the surface of the boule were observed with optical microscope and tested with electron microprobe. They are confirmed to be iridium from the crucible and harmful to the crystal growth. The cleavage orientation of LSO was proved to be (110) and it is one of factors to cause crystal cracking. The scattering particles in LSO crystals are analyzed to be mainly composed of Lu2O3 inclusions. Two possible origins on these inclusions are proposed. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)