Czochralski-grown Crystals Research Articles

Extension of the scope of the photoluminescence method after electron irradiation for quantifying low-level carbon in silicon

We have extended the applicability of the photoluminescence method after electron irradiation for quantifying low-level C impurities in Si crystals. The intensity ratio of the G-line to the intrinsic emission normalized by the ratio of the reference sample is used as an index of the C concentration. The calibration curves have already been established for Czochralski-grown crystals with resistivity higher than 50 Ω·cm (n-type) and higher than 5 kΩ·cm (p-type). We showed that the method was extendable to the resistivity range down to 30 Ω·cm in n-type samples with the O concentration in the range 1–6 × 1017 cm−3. The extension to float-zone (FZ) crystals was realized by using the theoretical relationship between the C concentration and the G-line intensity ratio normalized by the ratio of the FZ reference sample. Regarding the extension to conductive p-type B-doped samples, the formation of B-related radiation-induced defects was found to be an obstacle.

Interconfigurational d-f luminescence of Pr3+ ions in praseodymium doped Li6Y(BO3)3 single crystals

Luminescence properties of undoped Li6Y(BO3)3 (LYB) crystals grown by the Bridgman method as well as undoped and praseodymium doped LYB crystals grown by the Czochralski method were studied under excitation by ultraviolet light, synchrotron radiation and electron beam in the temperature region of 5–300 K. The emission bands peaking at 4.8, 3.76 and 2.95 eV were found in undoped Czochralski-grown crystals, while the band at 3.34 eV was dominant in the Bridgman-grown crystals. These bands are presumably ascribed to excitons localized at regular and defect-perturbed lattice sites. Intense emission bands peaking at 4.75, 4.50, 4.17 and 3.98 eV were discovered in Pr doped crystals at low temperatures and related to the interconfigurational 4f15d1 → 4f2 radiative transitions in Pr3+ ions. The structure of the emission spectrum is shown to be determined by the 3H4, 3H5, 3H6, 3F2, 3F3, 3F4 terms of the Pr3+ ion. The characteristic decay time of the 4f15d1 → 4f2 emission is τ = 24 ns at low temperatures, however, it shortens to 9 ns by room temperature due to a strong thermal quenching. The study of glow curves and spectra of thermostimulated luminescence in Li6Y(BO3)3:Pr crystal irradiated by 10-keV electron beam shows that Pr3+ ions do not participate in an electron-hole recombination process.

Parametric numerical study of dislocation density distribution in Czochralski-grown germanium crystals

Dislocations strongly affect the quality of Czochralski-grown germanium crystals used for various applications, however, the influence of different growth parameters on the dislocation distribution has not been extensively studied in the literature. In the present work, a simplified parametric crystal growth model is introduced which is suitable for large-scale numerical studies. The time-dependent temperature and dislocation density distributions in the crystal are calculated and analyzed. The Alexander–Haasen model is applied for dislocation modeling. Model parameters are taken from previously published discrete dislocation dynamics simulations, establishing a novel coupling between mesoscopic and macroscopic simulations. A W-shaped radial distribution of the dislocation density is obtained in the simulations. It is shown that the effective ambient temperature and crystallization front deflection have the strongest influence on the dislocation distribution. The results are compared to the experiment, showing a qualitative agreement, but also several deviations. Limitations of the considered dislocation modeling approach and the simplified thermal model are discussed.

Transition-metal ions in β-Ga2O3 crystals: Identification of Ni acceptors

Transition-metal ions (Ni, Cu, and Zn) in β-Ga2O3 crystals form deep acceptor levels in the lower half of the bandgap. In the present study, we characterize the Ni acceptors in a Czochralski-grown crystal and find that their (0/−) level is approximately 1.40 eV above the maximum of the valence band. Both Ni2+ (3d8) and Ni3+ (3d7) acceptors are present in the as-grown crystal. Also present are unintentional Ir3+ (5d6) and Ir4+ (5d5) donors. The neutral Ni3+ acceptors have a low-spin S = 1/2 ground state and are easily monitored with electron paramagnetic resonance (EPR). Principal values of the g matrix for these acceptors are 2.131, 2.138, and 2.233. Although paramagnetic, the singly ionized Ni2+ acceptors are not seen with EPR at X band (9.4 GHz). The Ir4+ donors are monitored with EPR and with infrared absorption spectroscopy. Exposing the Ni-doped β-Ga2O3 crystal to 275 nm light at room temperature increases the concentration of Ni3+ ions and reduces the concentration of Ir4+ ions as electrons move from the acceptors to the donors. After illumination, heating the crystal above 375 °C restores the initial concentrations of the Ni3+ and Ir4+ ions. Broad optical absorption bands peaking near 303 and 442 nm are attributed to the Ni3+ acceptors.

Czochralski growth and characterization of the multicomponent garnet (Lu1/4Yb1/4Y1/4Gd1/4)3Al5O12

This work demonstrates the potential for practical scalable growth of complex garnets and evaluates the implications of a multicomponent composition in the optical quality and elemental distribution of a Czochralski-grown crystal. Our experimental approach was designed to elucidate the relation between a complex garnet composition ${({\mathrm{Lu}}_{1/4}{\mathrm{Yb}}_{1/4}{\mathrm{Y}}_{1/4}{\mathrm{Gd}}_{1/4})}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12}$, crystal growth parameters, crystal structural, and elemental homogeneity. Our hypothesis is that combining multiple rare earths (REs) that will fractionally occupy the dodecahedral site in the aluminum garnet structure will result in a stable, single garnet compound that can be grown by the Czochralski method. Single-crystal and powder x-ray diffraction indicated a single garnet phase with an increasing unit cell volume from seed to tail. In addition, we propose that the pattern of elemental segregation will be based on the deviation of the ionic radius of each constituent RE from the average RE ionic radius of the multicomponent garnet. Electron probe microanalysis revealed that ions that are smaller than that average (${\mathrm{Lu}}^{3+}$ and ${\mathrm{Yb}}^{3+}$) are preferentially incorporated in the crystal, while elements that are larger than that average (${\mathrm{Gd}}^{3+}$) are rejected. The ionic radius of ${\mathrm{Y}}^{3+}$ is close to that average and yttrium segregation was minimal. The concentrations of the four REs are closer to stoichiometric on the tail end of the boule. Scanning electron microscopy and energy-dispersive x-ray spectroscopy analysis reveal Gd-rich inclusions with eutectic microstructures in the tail end of the boule.

Incongruent Melting La xY ySc4-x-y(BO3)4: LYSB Nonlinear Optical Crystal Grown by the Czochralski Method.

Nonlinear optical (NLO) crystals with incongruent melting of La xY ySc z(BO3)4 ( x + y + z = 4) (LYSB)-type were grown for the first time, to the best of our knowledge, by the Czochralski method. A special thermal assembly was used and the melt composition, growth direction, and the pulling and rotation rates have been optimized. Good optical quality LYSB crystal with a diameter of about 13 mm and a length of 25 mm has been grown from the La0.765Y0.485Sc2.75(BO3)4 starting melt composition, along the c-axis direction, using a slow rotation rate of 8-10 rpm and a high pulling rate of 2 mm/h. The grown crystal has an acentric huntite-type structure (space group R32, Z = 3) with cell dimensions a = 9.8098(4) Å and c = 7.9802(3) Å, and its chemical composition was determined to be La0.78Y0.32Sc2.90(BO3)4. The optical transmission and the refractive indices were determined, and the second harmonic generation (SHG) and sum frequency generation (SFG, ω + 2ω) properties were reported. The laser damage threshold was also determined to be ∼2.0 GW/cm2 at 1064 nm (6 ns pulses). The main nonlinear properties of Czochralski-grown LYSB crystal were found to be similar to those of flux-grown LYSB, and comparable to YAl3(BO3)4 (YAB) nonlinear properties. The big advantage of Czochralski-grown LYSB crystals is that they can be grown with large size and high quality, making them promising candidates for various NLO applications, including frequency conversion of high-average power radiation sources.

Deep level defects in β-Ga2O3 pulsed laser deposited thin films and Czochralski-grown bulk single crystals by thermally stimulated techniques

Thermally stimulated techniques—thermally stimulated current (TSC) spectroscopy and thermally stimulated depolarization current (TSDC) spectroscopy—were used to comparatively study the electrical properties and deep level defects in β-Ga2O3 pulsed laser deposited thin films and Czochralski-grown bulk crystals. It was found that the samples are highly resistive and each sample may have different dark current activation energy. Deep level defects revealed by the thermally stimulated techniques vary from sample to sample. In addition to the common traps E1 (∼0.56 eV), E2 (∼0.84 eV), and E3 (∼0.99 eV), reported in the literature and revealed by DLTS studies of Ga2O3 bulk crystals, that were also found in our samples by the thermally stimulated techniques, a trap at ∼110 meV and several other traps are revealed specifically by TSDC between 105 and 225 K.

Optimization of the Growth Technology for Crystals with Garnet Structure Based on X-Ray Diffraction Data

High efficiency of the methods of double-crystal X-ray diffractometry (DCXRD) and topography for improving the growth technology of highly homogeneous crystals has been demonstrated on the example of gadolinium gallium garnet (GGG) single crystals. The main types of structural defects observed in Czochralski-grown GGG crystals are found to be macroscopic inhomogeneity of composition distribution, caused by the facet effect manifestation; microinhomogeneous distribution of impurity and main components of the composition in striations; dislocations; and second-phase inclusions. The relationship between the type and density of newly formed defects and the technological conditions for crystal growth are considered. Optimization of the composition of crystals and their growth technology made it possible to obtain high-quality dislocation-free crystals of GGG and complex-substituted garnets on its basis for magneto-optical and microwave devices, elements of solid-state lasers, and other applications.

Structure and thermal expansion of Ca 9 Gd(VO 4 ) 7 : A combined powder-diffraction and dilatometric study of a Czochralski-grown crystal

Abstract Materials of the Ca 9 RE(VO 4 ) 7 (CRVO) formula (RE = rare earth) and whitlockite-related structures are considered for applications in optoelectronics, e.g. , in white-light emitting diodes and lasers. In the CRVO structure, the RE atoms are known to share the site occupation with Ca atoms at two or three among four Ca sites, with partial occupancy values depending on the choice of the RE atom. In this work, the structure and quality of a Czochralski-grown crystal of this family, Ca 9 Gd(VO 4 ) 7 (CGVO), are studied using X-ray diffraction methods. The room-temperature structure is refined using the powder diffraction data collected at a high-resolution synchrotron beamline ID22 (ESRF, Grenoble); for comparison purposes, a laboratory diffraction pattern was collected and analyzed, as well. The site occupancies are discussed on the basis of comparison with literature data of isostructural synthetic crystals of the CRVO series. The results confirm the previously reported site-occupation scheme and indicate a tendency of the CGVO compound to adopt a Gd-deficient composition. Moreover, the thermal expansion coefficient is determined for CGVO as a function of temperature in the 302–1023 K range using laboratory diffraction data. Additionally, for CGVO and six other single crystals of the same family, thermal expansion is studied in the 298–473 K range, using the dilatometric data. The magnitude and anisotropy of thermal expansion, being of importance for laser applications, are discussed for these materials.

Green luminescence from Cu-diffused LiGaO2 crystals

An intense green luminescence is observed from single crystals of LiGaO2 doped with copper. Czochralski-grown undoped crystals are wrapped in thin copper foil and then held at 900°C for 1h in a flowing nitrogen atmosphere. Large concentrations of Cu+ ions enter the crystals during this process and occupy Li+ sites. These copper-diffused crystals are characterized with optical absorption, photoluminescence (PL), photoluminescence excitation (PLE), thermoluminescence (TL), and electron paramagnetic resonance (EPR). An optical absorption band peaking near 350nm is assigned to the Cu+ ions at Li+ sites and represents an excitation from a 3d10 ground state to a 3d94s1 excited state. A broad PL emission from these excited Cu+ ions has a peak near 523nm and the related PLE band has a peak near 356nm (this PLE band links the emission to the optical absorption band). Illuminating a Cu-diffused crystal at room temperature with 325nm laser light converts a portion of the Cu+ ions to Cu2+ ions. EPR spectra from these 3d9 ions are easily seen at low temperatures and their angular dependence is used to determine the g matrix and the 63Cu hyperfine matrix. Subsequent heating produces a TL peak near 122°C with a maximum in its spectral dependence near 535nm. Correlated EPR measurements show that this TL peak occurs when trapped electrons are thermally released from unintentionally present transition-metal ions (most likely Fe) and recombine with holes at the Cu2+ ions.

Improvement of laser-beam distortion in large-aperture PPMgLN device by using X-axis Czochralski-grown crystal.

Large-aperture periodically poled Mg-doped LiNbO3 device using X-axis Czochralski-grown MgLN crystal was proposed to avoid a laser-beam distortion problem. Availability of periodic poling in 5-mm-thick MgLN and compatibility of wavelength-conversion characteristics in QPM-OPO were evaluated by comparing with conventional arrangement using Z-axis-grown crystal.

Determination of the dislocation structure in paratellurite single crystals using the piezo-optic effect

It is shown theoretically and experimentally that dislocations in paratellurite single crystals can be revealed using the piezo-optic effect. The mechanical-stress distribution near dislocations is calculated. Dislocations in the samples are observed using the photoelastic method and chemical etching. Data on the spatial distribution of dislocations in Czochralski-grown paratellurite crystals are reported.

An analysis of and a model for spiral growth of Czochralski-grown oxide crystals with high melting point

The spiral morphology of Czochralski-grown high melting point oxides is an unwanted phenomenon because it diminishes the yield of useful material. We differentiate in our analysis of the spiral growth between (1) the growth conditions for spiral growth (boundary conditions) and (2) the events, which can trigger the onset of spiral growth. We present a detailed description of spiral growth (3), which is based on dominating radiative heat transport and the loss of the rotational symmetry of the melt meniscus. Once the symmetry breaking of cylindrical morphology has taken place, an asymmetric cooling of the meniscus of the crystal will enhance this event and the centre of the crystal interface will grow out of the crystal rotation axis. The rotation of the out-of-centre-interface on the melt surface further deforms the meniscus azimuthally by viscous shear. This leads to the beginning of spiral growth. In later growth stages, the asymmetric (“spiral”) cooling of the melt meniscus by the already grown spiral crystal above it is the main driver for the spiral growth. Our model description is supported by experimental results. We present a simplified quantitative model (4) in which the last complete spiral terrace is influencing the radiative cooling of the melt meniscus of the next growing spiral terrace. The model is based on sole radiative heat exchange between adjacent terraces and predicts a nonlinear dependence of the distance A of the spiral terraces on the pulling speed v z , namely A=const.• v z 1/2. This dependence indicates the trend, which was found experimentally. We propose the spiral growth of some high melting point oxides to be a kind of self-organisation.

Effect of necking on Czochralski-grown LiF crystals and its influence on crystalline perfection and the correlated physical properties

A bulk single crystal of lithium fluoride (LiF) was grown using an in-house-developed Czochralski crystal puller. The effect of necking on crystalline perfection, which in turn was found to influence the optical and dielectric properties, was studied. The crystalline perfection of the grown crystal was assessed along the length of the boule using high-resolution X-ray diffraction (HRXRD), and it was found that the grain boundaries that were present in the seed crystal were stopped gradually from propagating into the bulk crystal by necking. The UV–vis transparency of the crystal region having best crystalline perfection was found to be higher. Photoluminescence (PL) spectra revealed that a crystal region that was found to contain vacancy (point) defects by HRXRD yielded the maximum PL intensity because of color centers at the vacancies. The dielectric properties were also studied over a wide range of frequency.

Scintillation properties of Sc-, Pr-, and Ce-doped LuAG epitaxial garnet films

Lu3Al5O12 (LuAG) films were grown from the BaO–B2O3–BaF2 flux on LuAG and YAG substrates using the liquid phase epitaxy method (LPE). They were doped with Pr3+, Sc3+, and eventually still with Ce3+ ions. Photoelectron yield, its time dependence, and energy resolution were measured under α-particle excitation. A sample of the Czochralski-grown bulk LuAG:Pr single crystal was measured as a reference. The best performing epitaxial films had low Sc3+ concentration and appeared comparable or superior to the Czochralski-grown crystal in terms of photoelectron yield and relative intensity of the fast component of scintillation response. High concentration of Sc3+ ions results in slowdown of response and lower photoelectron yield. Some samples with lower photoelectron yield than that of the Czochralski-grown crystal did exhibit better energy resolution.

Structural and electrical properties of SiGe-on-insulator substrates fabricated by direct bonding

A new method for fabricating SiGe-on-insulator substrates, i.e., direct bonding of thermally oxidized Si wafers with Si1 − x Ge x wafers cut from Czochralski-grown crystals, is suggested. Si1 − x Ge x layers no larger than 10 μm thick in SiGe/SiO2/Si compositions were fabricated by chemical mechanical polishing. To increase the Ge content in the Si1 − x Ge x layer, thermal oxidation was used. It was shown that the increase in the Ge content and heat treatment procedures at 1250°C are not accompanied by degradation of structural and electrical characteristics of Si1 − x Ge x layers.

Studying kinetics of scintillation process in a CaMoO4 crystal

The kinetics of scintillation process in Czochralski-grown CaMoO4 crystals excited by α particles and γ rays from 238Pu and 137Cs radioactive isotopes, respectively, is investigated. Kinetics of scintillations is shown to be complex and multicomponent in character. Fast and slow scintillation components are observed, and their relative contributions to the total light yield in the scintillator are determined. The possibility is demonstrated of discriminating between scintillator signals due to α particles and γ rays.

Electron diffraction study of various lattice defects in a Bi4Ge3O12 crystal

Lattice defects in a scintillation detector made of Bi4Ge3O12 (BGO) could severely impact detector efficiency via non-radiative transfer of electron excitation, thus making thorough investigations of these defects highly important. Here we present a combined experimental and theoretical study of two- and three-dimensional defects in a Czochralski-grown BGO crystal. Upon examination by transmission electron microscopy the selected-area electron diffraction (SAED) patterns in two neighboring parts of the specimen reveal different kinds of two- and three-dimensional defects. Three sub-grains misoriented at 2.47° with reference to each other and probable presence of stacking faults lying in {011} planes were observed in the first examined local area. The SAED image taken from an area in the close neighborhood is much more complicated and is explained in terms of the superposition of reflections from: (i) a partially textured GeO2 second-phase inclusion; (ii) the basic lattice of BGO and (iii) a superlattice-like structure based on the BGO lattice. The atomic structure of such a superlattice-like structure was theoretically modeled and the corresponding simulated SAED patterns were found to be in good agreement with the experimentally observed one.

Reducing Melt Inclusion by Submerged Heater or Baffle for Optical Crystal Growth

Optical and laser crystals grown by the Czochralski technique from a solute-rich melt usually suffer defects of melt inclusion or bubble core, which severely affects optical, thermal, and mechanical properties of the material. The main purpose of this paper is to study the inclusion mechanism and reduce such defects. Two types of mechanism possibly responsible for inclusion are presented. Both of them show that solute pileup in front of the interface is the prerequisite for inclusion. The method to reduce inclusion is to prevent solute from pileup by controlling melt flow. Czochralski-grown optical crystals are examined to recognize the effects of crystal rotation and natural convection on the melt flow pattern and solidification interface shape. It is established that increasing the rotation rate of crystal or reducing natural convection in the melt will cause the solid–liquid interface change from the convex to concave shape and high concentration of species may be pushed away from the solidification interface. Simulations are performed to establish the relationships among the nondimensional parameter Gr/Re 2 , the change of growth interface shape, and the stagnant point location. A disk baffle or heater submerged into the melt is used to reduce natural convection. The effect of submerged baffle on enhancement of crystal rotation is demonstrated. Simulation results also demonstrate that melt flow near the solidification interface depends strongly on the baffle location. When the submerged heater is used, its temperature should be carefully designed. The value strongly depends on the ratio between crystal and crucible diameters, and a constant temperature may be not the best choice. An optimized temperature profile can be achieved by numerical simulations.

Influence of seed/crystal interface shape on dislocation generation in Czochralski Si crystal growth

Influence of interface shape between a seed and the grown crystal on dislocation generation behavior near the interface in Czochralski-grown Si crystal was investigated using heavily B-doped or heavily B and Ge codoped Si seeds. When the interface shape was convex toward the melt dislocations were generated near the interface, while dislocation generation was suppressed when the shape was planar. The seed with the convex interface sustains larger shear stress than that with the planar interface, and in the case dislocations are generated by thermal stress related to the interface shape at the edge of the interface.