Cs2LiGdCl6 (Ce): New scintillation material

We investigated the scintillation properties of Cs₂LiGdCl₆:Ce³⁺ as a function of the Ce concentration. X-ray excited luminescence spectra of the scintillation material showed broad emission bands between 360 and 460 nm, with two overlapping peaks, due to the d→f transitions on Ce³⁺ ions. The samples provide good scintillation results. The energy resolution was found to be 5.0% (FWHM) at 662 keV for 10% Ce sample. Under γ-ray excitation, Cs₂LiGdCl₆:Ce³⁺ showed three exponential decay time components of about 130–200 ns decay time constant. The light output of the investigated samples was 20,000 photons/MeV for a 10% Ce concentration. The light output deviation from the linear response is within 7% between the energy range of 31 and 1333 keV. Overall, the scintillation properties confirm that Cs₂LiGdCl₆:Ce³⁺ single crystal is a promising candidate for medical imaging and radiation detection.     

Sublimation crystal growth of yttrium nitride

The sublimation–recombination crystal growth of bulk yttrium nitride crystals is reported. The YN source material was prepared by reacting yttrium metal with nitrogen at 1200 °C and 800 Torr total pressure. Crystals were produced by subliming this YN from the source zone, and recondensing it from the vapor as crystals at a lower temperature (by 50 °C). Crystals were grown from 2000 to 2100 °C and with a nitrogen pressure from 125 to 960 Torr. The highest rate was 9.64×10⁻⁵ mol/h (9.92 mg/h). The YN sublimation rate activation energy was 467.1±21.7 kJ/mol. Individual crystals up to 200 μm in dimension were prepared. X-ray diffraction confirmed that the crystals were rock salt YN, with a lattice constant of 4.88 Å. The YN crystals were unstable in air; they spontaneously converted to yttria (Y₂O₃) in 2–4 h. A small fraction of cubic yttria was detected in the XRD of a sample exposed to air for a limited time, while non-cubic yttria was detected in the Raman spectra for a sample exposed to air for more than 1 h.     

Hydrothermal crystal growth of fresnoite

The growth of fresnoite, Ba₂TiSi₂O₈, by hydrothermal synthesis has led to spontaneous generation of large, (4-5 mm) optically clear crystals from 6 M KF mineralizer solutions. Growth was achieved at relatively low synthesis temperatures (575 °C) comparative to fresnoite synthesis by Czochralski or flux methods. Bulk crystal growth possibilities were explored by transport reactions performed in both fluoride and hydroxide mineralizers with 25-45 °C temperature gradients. Growth rates of 0.14×0.19×0.22 mm³/week were established in 6 M KOH, which is significantly slower than standard hydrothermal rates of 1 mm/week. Although relatively slow, the hydrothermal method has been demonstrated as a synthesis route to high quality single crystals of fresnoite.     

AlN bulk single crystal growth on 6H-SiC substrates by sublimation method

Large and thick AlN bulk single crystals up to 43 mm in diameter and 10 mm in thickness have been successfully grown on 6H-SiC (0 0 0 1) substrates by the sublimation method using a TaC crucible. Raman spectrum indicates that the polytype of the grown AlN single crystals is a Wurtzite-2H type structure, and the crystals do not include any impurity phases. The quality at the top of the crystal improves as crystal thickness increases along the 〈0 0 0 1〉 direction during growth: a low etch pit density (7×10⁴ cm⁻²) and a small full width at half maximum for a 0002 X-ray rocking curve (58 arcsec) have been achieved at a thickness of ∼8 mm. The possible mechanism behind the improvement in the AlN crystal quality is also discussed.     

Properties of Ge-doped,high-quality bulk GaN crystals fabricated by hydride vapor phase epitaxy

We investigated the properties of Ge-doped, high-quality bulk GaN crystals with Ge concentrations up to 2.4×10¹⁹ cm⁻³. The Ge-doped crystals were fabricated by hydride vapor phase epitaxy with GeCl₄ as the dopant source. Cathodoluminescence imaging revealed no increase in the dislocation density at even the highest Ge concentration, with values as low as 3.4×10⁶ cm⁻². The carrier concentration, as determined by Hall measurement, was almost identical to the combined concentration of Ge and unintentionally incorporated Si. The electron mobilities were 260 and 146 cm² V⁻¹ s⁻¹ for n=3.3×10¹⁸ and 3.35×10¹⁹ cm⁻³, respectively; these values are markedly larger than those reported in the past for Ge-doped GaN thin films. The optical absorption coefficient was quite small below the band gap energy; it slightly increased with increase in Ge concentration. Thermal conductivity, estimated by the laser-flash method, was virtually independent of Ge concentration, maintaining an excellent value around 2.0 W cm⁻¹ K⁻¹. Thermal expansion coefficients along the a- and m-axes were approximately constant at 5.0×10⁻⁶ K⁻¹ in the measured doping concentration range.     

Czochralski-growth of germanium crystals containing high concentrations of oxygen impurities

Oxygen-containing germanium (Ge) single crystals with low density of grown-in dislocations were grown by the Czochralski (CZ) technique from a Ge melt, both with and without a covering by boron oxide (B₂O₃) liquid. Interstitially dissolved oxygen concentrations in the crystals were determined by the absorption peak at 855 cm⁻¹ in the infrared absorption spectra at room temperature. It was found that oxygen concentration in a Ge crystal grown from melt partially or fully covered with B₂O₃ liquid was about 10¹⁶ cm⁻³ and was almost the same as that in a Ge crystal grown without B₂O₃. Oxygen concentration in a Ge crystal was enhanced to be greater than 10¹⁷ cm⁻³ by growing a crystal from a melt fully covered with B₂O₃; with the addition of germanium oxide powder, the maximum oxygen concentration achieved was 5.5×10¹⁷ cm⁻³. The effective segregation coefficients of oxygen in the present Ge crystal growth were roughly estimated to be between 1.0 and 1.4.     

Growth and Raman spectroscopic characterization of As4S4 (II) single crystals

As described by Kutoglu (1976 [16]), single crystals of As₄S₄ (II) phase have been grown using a new two-step synthesis that drastically increases the reproducibility that is attainable in synthetic experiments. First, through photo-induced phase transformation, pararealgar powder is prepared as a precursor instead of AsS melt. Then it is dissolved and recrystallized from CS₂ solvent. Results show that single crystals of the As₄S₄ (II) phase were obtained reproducibly through the dissolution–recrystallization process. Single crystals of As₄S₄ (II) obtained using this method were translucent and showed a uniform yellow-orange color. The crystal exhibits a platelet-like shape as a thin film with well-developed faces (0 1 0) and (0 1¯ 0). The grown crystals are as large as 0.50×0.50×0.01 mm. They were characterized using powder and single crystal X-ray diffraction techniques to confirm the phase identification and the lattice parameters. The As₄S₄ (II) phase crystallizes in monoclinic system with cell parameters a=11.202(4) Å, b=9.954(4) Å, c=7.142(4) Å, β=92.81(4)°, V=795.4(6) Å³, which shows good agreement with the former value. Raman spectroscopic studies elucidated the behavior of the substance and the relation among phases of tetra-arsenic tetrasulfide.     

Extended defects in CdZnTe crystals: Effects on device performance

We explored some unique defects in a batch of cadmium zinc telluride (CdZnTe) crystals, along with dislocations and Te-rich decorated features, revealed by chemical etching. We extensively investigated these distinctive imperfections in the crystals to identify their origin, dimensions, and distribution in the bulk material. We estimated that these features ranged from 50 to 500 μm in diameter, and their depth was about ∼300 μm. The density of these features ranged between 2×10² and 1×10³ per cm³. We elaborated a model of them and projected their effect on charge collection and spectral response. In addition, we fabricated detectors with these defective crystals and acquired fine details of charge-transport phenomena over the detectors’ volume using a high-spatial resolution (25 μm) X-ray response mapping technique. We related the results to better understand the defects and their influence on the charge-transport properties of the devices. The role of the defects was identified by correlating their signatures with the findings from our theoretical model and our experimental data.     

Characterization of bulk GaN crystals grown from solution at near atmospheric pressure

The properties of GaN crystals grown from solution at temperatures ranging from 780 to 810 °C and near atmospheric pressure ∼0.14 MPa, have been investigated using low temperature X-band (∼9.5 GHz) electron paramagnetic resonance spectroscopy, micro-Raman spectroscopy, photoluminescense spectroscopy, and photoluminescence imaging. Our samples are spontaneously nucleated thin platelets of approximate dimensions of 2×2×0.025 mm³, or samples grown on both polycrystalline and single crystal HVPE large-area (∼3×8×0.5 mm³) seeds. Electron paramagnetic resonance spectra consists of a single Lorentzian line with axial symmetry about the c-axis, with approximate g-values, g_∥=1.951 and g_⊥=1.948 and a peak-to-peak linewidth of∼4.0 G. This resonance has been previously assigned to shallow impurity donors/conduction electrons in GaN and attributed to Si- and/or O impurities. Room temperature photoluminescence and photoluminescence imaging data from both Ga- and N-faces show different dominant emission bands, suggesting different incorporation of impurities and/or native defects. Raman scattering and X-ray diffraction show moderate to good crystalline quality.     

Ca3N2 as a flux for crystallization of GaN

In this work Ca₃N₂ was investigated as a potential flux for crystallization of GaN. Melting temperature of the potential flux at high N₂ pressure evaluated by thermal analysis as 1380 °C is in good agreement with the theoretical prediction. It is shown that Ca₃N₂ present in the liquid gallium in small amount (1 at%) dramatically accelerates synthesis of GaN from its constituents. On the other hand, it does not influence significantly the rate of GaN crystallization from solution in gallium in temperature gradient for both unseeded and seeded configurations. However the habit and color of the spontaneously grown GaN crystals change drastically. For 10 mol% Ca₃N₂ content in the liquid Ga it was found that the GaN thick layer and GaN crystals (identified by micro-Raman scattering measurements) were grown on the substrate. For growth from molten Ca₃N₂ (100%) with GaN source, the most important observations were (i) GaN source material was completely dissolved in the molten Ca₃N₂ flux and (ii) after experiment, GaN crystals were found on the sapphire substrate.     

Reduction of the dislocation density in molecular beam epitaxial CdTe(2 1 1)B on Ge(2 1 1)

The high dislocation density (2×10⁷/cm² for a thickness of 7 μm) in CdTe(2 1 1)B on Ge(2 1 1) has become a roadblock for the technological exploitation of this material. We present a systematic study of in situ and post-growth annealing cycles aimed at reducing it. An etch pit density of 2×10⁶/cm² was achieved by optimizing the growth conditions and annealing the samples in situ. This finding was corroborated by high-resolution X-ray diffraction, atomic force microscopy, photoluminescence and ellipsometry measurements.     

Chemically assisted vapour transport for bulk ZnO crystal growth

A chemically assisted vapour phase transport (CVT) method is proposed for the growth of bulk ZnO crystals. Thermodynamic computations have confirmed the possibility of using CO as a sublimation activator for enhancing the sublimation rate of the feed material in a large range of pressures (10⁻³ to 1 atm) and temperatures (800–1200 °C). Growth runs in a specific and patented design yielded single ZnO crystals up to 46 mm in diameter and 8 mm in thickness, with growth rates up to 400 μm/h. These values are compatible with an industrial production rate. N type ZnO crystals (μ=182 cm²/(V s) and n=7 10¹⁵ cm⁻³) obtained by this CVT method (Chemical Vapour Transport) present a high level of purity (10–30 times better than hydrothermal ZnO crystals), which may be an advantage for obtaining p-type doped layers ([Li] and [Al] <10⁺¹⁵ cm⁻³). Structural (HR-XRD), defect density (EPD), electrical (Hall measurements) and optical (photoluminescence) properties are presented.     

Improvement of crystal quality in ammonothermal growth of bulk GaN

Several key improvements in crystal quality of bulk GaN grown by the ammonothermal method are presented. Full width at half maximum of (0 0 2) X-ray rocking curve was reduced to 53 and 62 arcsec for Ga-side and N-side, respectively. Transparent bulk GaN crystal was also demonstrated. Oxygen and sodium concentrations were reduced to mid-10¹⁸ and mid-10¹⁵ cm⁻³, respectively. We are currently searching for a growth condition that produces transparent bulk GaN with high structural quality and low impurities. Small-sized, semi-transparent GaN wafers were fabricated by slicing the grown bulk GaN crystals, which demonstrate the high feasibility of ammonothermal growth for production of GaN wafers.     

Synthesis,growth and characterization of organic nonlinear optical bis-glycine maleate (BGM) single crystals

A new organic compound of bis-glycine maleate was synthesized in the alkaline medium of 10% ammonium hydroxide solution. The bulk single crystals of Bis-Glycine Maleate (BGM) have been grown by slow cooling method. The grown crystals were characterized by employing single crystal and powder X-ray diffraction, Fourier transform infrared, optical absorption spectral studies and thermo gravimetric analysis. The microhardness studies confirmed that the BGM has a fairly high Vicker’s hardness number value (41 kg mm⁻²) in comparison to other organic NLO crystals. Second harmonic generation efficiency of the crystal measured by Kurtz–Perry powder method using Nd:YAG laser is found to be comparable to that of potassium dihydrogen phosphate (KDP). Frequency dependent dielectric studies were carried out along the major growth axis.     

Synthesis and characterization of strip-shape diamonds from Fe-based alloy and graphite system under high pressure and high temperature

The unusual strip-shape diamond crystals were successfully synthesized in the system of Fe-based powder alloy and graphite under pressures of 5.6–5.8 GPa and temperatures of 1600–1650 K in a cubic anvil high pressure high temperature (HPHT) apparatus (SPD-6×1200). The synthetic diamond crystals have a strip-shape with light green color. The lengths of the crystals are 0.6–0.8 mm with length to diameter ratios around 2.0–2.5. SEM photographs reveal that the crystals have two main kinds of shapes: one elongated along the (1 0 0) and the other along the (1 1 1) directions. The FTIR spectra of the diamond synthesized indicate that its nitrogen concentration is higher than the normal diamond crystal.     

Preparation,crystal structure,spectrographic characterization,thermal and third order nonlinear optical properties of benzyltriethylamine bis(2-thioxo-1,3-dithiole-4,5-dithiolato)nickel(III)

The preparation and crystal structure of a novel nonlinear optical organometallic crystal, benzyltriethylamine bis(2-thioxo-1,3-dithiole-4,5-dithiolato)nickel(III) (BTEANDT), are described. The crystal was characterized by elemental analyses, infrared and X-ray powder diffraction spectroscopy, thermal analysis and optical absorption. The third order nonlinear optical properties of crystal were investigated by using the Z-scan technique at 1064 nm with 20 ps. A self-focusing effect and a saturable absorption were observed. The nonlinear refraction coefficient (n₂) and the nonlinear absorption coefficient (β) have been found to be 7.311×10⁻¹⁸ m²/W and −6.064×10⁻¹¹ m/W when the on-axis irradiance at focus (I₀) is 3.025 GW/cm². The relationship between β and I₀ was studied, which has revealed that the former is proportional to the latter. The result has been explained with the theory of absorption cross-section.     

Growth and property of Zn-doped near-stoichiometric LiTaO3 crystal

Near-stoichiometric LiTaO₃ (SLT) and Zn-doped near-stoichiometric LiTaO₃ (Zn:SLT) crystals with 10–15 mm in diameter and 10 mm in length were grown by using TSSG technique with K₂O as the flux. The effect of adding amount of K₂O was discussed in the growing process. The crystals were characterized by inductively coupled plasma-optical emission (ICP-OES), X-ray diffraction (XRD) and differential thermal analysis (DTA). The lattice constants of Zn:SLT were smaller than those of SLT and Curie temperature was higher than that of SLT. It was found that Zn doping is an efficient way to improve the optical damage resistance ability of SLT crystal. Compared with SLT crystal, Zn:SLT exhibited a much higher optical damage threshold, more than 500 MW/cm², which was attributed to Zn self-compensated effect that formed the charge compensated complexes, (Zn_Ta)³⁻–3(Zn_Li)⁺ in SLT crystal.     

Enlarged ferromagnetic hysteresis in InMnP:Be epilayers formed by thermal diffusion using MBE-grown Mn/InP:Be bilayers

The p-type InMnP:Be epilayers, which were prepared by thermal diffusion of Mn through in-situ deposition of Mn layer using molecular beam epitaxy (MBE) onto MBE-grown InP:Be epilayers and subsequent in-situ annealing at 300–350 °C, were investigated. InMnP:Be epilayers prepared by the above sequence clearly showed the Mn-related emission band at 1.1–1.2 eV, which indicates the effective incorporation of Mn²⁺ ions into the host layer InP:Be. The samples demonstrated very large ferromagnetic hysteresis loops with enhanced coercivity, and the ferromagnetic-to-paramagnetic transition of the samples was observed to occur at ∼85 K. These results suggest that InP-based ferromagnetic semiconductor layers having enhanced ferromagnetism can be effectively formed by the above-mentioned sequential in-situ processes.     

Recent achievements in AMMONO-bulk method

In this paper we present progress made recently in the development of the growth of truly bulk GaN crystals by the ammonothermal method in basic environment. High quality 2-in c-plane GaN seeds are shown. Non-polar wafers can also be cut out from thick GaN crystals grown by ammonothermal method. Perfect crystallinity manifests in very narrow peaks in X-ray rocking curves (the full width at half maximum equals about 15 arcsec). GaN epilayers deposited on these substrates exhibit intrinsic narrow exciton lines, which are very sensitive to the optical selection rules typical for hexagonal symmetry, proving the truly non-polar character of such AMMONO-GaN substrates. Other challenges like homogenous insulating properties or high p-type conductivity have been also accomplished by means of ammonothermal method. Semi-insulating crystals of resistivity up to 10¹¹ Ω cm and p-type conductivity within hole concentration up to 10¹⁸ cm⁻³ are already available in diameters up to 1.5-in.     

Investigation of void formation beneath thin AlN layers by decomposition of sapphire substrates for self-separation of thick AlN layers grown by HVPE

Void formation at the interface between thick AlN layers and (0 0 0 1) sapphire substrates was investigated to form a predefined separation point of the thick AlN layers for the preparation of freestanding AlN substrates by hydride vapor phase epitaxy (HVPE). By heating 50–200 nm thick intermediate AlN layers above 1400 °C in a gas flow containing H₂ and NH₃, voids were formed beneath the AlN layers by the decomposition reaction of sapphire with hydrogen diffusing to the interface. The volume of the sapphire decomposed at the interface increased as the temperature and time of the heat treatment was increased and as the thickness of the AlN layer decreased. Thick AlN layers subsequently grown at 1450 °C after the formation of voids beneath the intermediate AlN layer with a thickness of 100 nm or above self-separated from the sapphire substrates during post-growth cooling with the aid of voids. The 79 μm thick freestanding AlN substrate obtained using a 200 nm thick intermediate AlN layer had a flat surface with no pits, high optical transparency at wavelengths above 208.1 nm, and a dislocation density of 1.5×10⁸ cm⁻².