As-grown Single Crystals Research Articles

Fine-structure anomalies in EPR spectra ofGd3+ centresformed in TlCdF3 single crystals

Electron paramagnetic resonance (EPR) measurements have been made on as-grown single crystals ofperovskite-type TlCdF3 at room temperature in the cubic phase. Signals from two kinds of tetragonalGd3+ centre are observed. One centre observed in the Gd-only-doped crystal is ascribed to aGd3+ ionat a Cd2+ site being associated with a vacancy at the nearestCd2+ site. The other centre observed in the Gd, Li co-doped crystal is ascribed to aGd3+ ion at aCd2+ site beingassociated with a Li+ ion at the nearest Cd2+ site. The fine-structure terms are discussed by separating them into the uniaxial andcubic terms. Anomalous values of the axial and cubic parameters are obtained forTlCdF3 in contrast with the parameters for other host crystalsACdF3 (A = K,Rb, Cs).

Single crystal growth of lead vanado-chlorapatite Pb 5(VO 4) 3Cl using CsCl flux method

We report a crystal growth method of lead vanado-chlorapatite (vanadinite) Pb 5(VO 4) 3Cl single crystals using a CsCl flux. X-ray powder diffraction, scanning electron microscopy, electron microprobe analysis, and infrared spectroscopy characterized the crystals obtained in this study. We obtained only high-quality vanadinite single crystal phase from a two-step synthesis procedure. The as-grown single crystals with well-developed hexagonal prismatic shapes were as large as 6 mm. The crystals have an ideal chemical formula with highly homogeneous composition. Crystals grown from the CsCl flux contain no Cs impurities within the detection limit of EPMA. These results indicate that CsCl flux is a promising candidate for flux growth of single crystals in pyromorphite series.

Influence of ion implantation on structural and photoconductive properties of Bridgman grown GaSe single crystals

The structure and temperature dependent spectral photoconductivity of as-grown and N-and Si-implanted GaSe single crystals have been studied. It was observed that post-annealing results in a complete recovery of the crystalline nature that was moderately reduced upon implantation. The band edge is shifted in the implanted sample which is attributed to the structural modifications and continuous shallow levels introduced upon implantation and annealing. Our calculations showed that the trap density is increased upon implantation and annealing which confirms a possibility of explanation the phenomenon within a framework of continuous trap levels. Photocurrent measurements as a function of photo-excitation intensity also support continuous distribution of localized states in the band gap. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

An electron paramagnetic resonance study ofFe3+ centresin Tl2MgF4 and Tl2ZnF4 crystals

EPR measurements have been made on as-grown single crystals ofTl2MgF4 and Tl2ZnF4 doped only with Fe and co-doped with Fe and Li using anX-band spectrometer. Signals from the charge-uncompensatedFe3+ centre with tetragonal symmetry (centre I) were observed for both crystals. Signals from anFe3+ centre with monoclinic symmetry (centre II) were also observed forTl2ZnF4. On the basis of the spin-Hamiltonian separation analysis, centre II is ascribed to anFe3+ ion substitutingfor a Zn2+ ionwith a nearest Tl+ vacancy. The centre with orthorhombic symmetry (centre IV) observed in some Fe, Li co-dopedTl2ZnF4 crystals isascribed to an Fe3+ ionassociated with a Li+ ion at the nearest Zn2+ site. Anomalies were found in the fine structure parametersb20 for centres I and theseparated parameters b2a(1) for centres II and IV, which have about double the magnitude of the corresponding values inRb2ZnF4.

Single-crystal growth and superconductivity of (La 1−xSr x) 2CaCu 2O 6+δ

The effects of growth conditions and composition on the crystal growth of La-2126-phase superconducting materials have been studied by the floating-zone method. The composition of La-2126-phase single crystals was (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x = 0.0 7 5 – 0.2 5 ). The crystals were grown under oxygen pressures from 1 bar to 11 bar. A smooth planar solid–liquid growth interface of (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x = 0.0 7 5 ) tended to break down into a cellular interface when the growth velocity was higher than 0.5 mm/h, or the Sr concentration x > 0.0 7 5 . When the planar solid–liquid growth interface broke down into a cellular interface, the single-crystal size in an as-grown rod decreased abruptly and the as-grown rod was not of single phase. Various small sizes of CaO second phases were found inside the as-grown single crystals of (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x ⩾0.125). Based on the occurrence of these CaO second phases inside the as-grown crystals, the estimated solubility of as-grown single crystals is x = 0.0 7 5 for (La 1− x Sr x ) 2CaCu 2O 6+ δ . The melting point of the La-2126 materials increases with increasing oxygen pressure. Cubic-centimeter-size single crystals were successfully grown for (La 1− x Sr x ) 2CaCu 2O 6+ δ ( x = 0.0 7 5 ). The superconducting transition temperature of the as-grown (La 1− x Sr x ) 2CaCu 2O 6+ δ single crystals increases from T c = 2 8 K to 46 K with increasing the Sr concentration x from 0.075 to 0.25 at 11 bar oxygen pressure.

Crystal growth and superconductivity of (La1−xCax)2CaCu2O6+δ

Abstract The effects of growth conditions and composition on the crystal growth of (La1−xCax)2CaCu2O6+δ (x=0.05–0.15) superconducting materials have been studied by using the floating zone method. A smooth solid-liquid growth interface of (La1−xCax)2CaCu2O6+δ (x=0.05–0.06) tended to break down into a cellular interface when the growth velocity was higher than 0.5 mm/h. When the solid–liquid interface became cellular, the single crystal size in an as-grown rod decreased abruptly and the as-grown rod was not single phase. Various sized CaO inclusions were found inside the as-grown single crystals of (La1−xCax)2CaCu2O6+δ (x≥0.075). Cubic centimeter size single crystals were successfully grown for both (La0.95Ca0.05)2CaCu2O6+δ and (La0.94Ca0.06)2CaCu2O6+δ. The as-grown single crystals of (La1−xCax)2CaCu2O6+y under 1 bar pressure of oxygen were not superconducting. The superconducting temperature of the (La1−xCax)2CaCu2O6+y crystals grown at 11 bar oxygen is 28 K. The application of high pressure oxygen during crystal growth is an effective method to get bulk superconductivity in the crystals.

Determination of trapping center parameters of Tl 2InGaS 4-layered crystals by thermally stimulated current measurements

We have carried out thermally stimulated current measurements on as-grown Tl 2InGaS 4-layered single crystals in the low temperature range 10–60 K with different heating rates. We found experimental evidence for the presence of two shallow hole trapping centers with activation energies of 4 and 10 meV. We have determined the trap parameters using various methods of analysis, and these agree with each other. Their capture cross-sections have been found to be 6.5 × 10 −26 and 4.0 × 10 −25 cm 2, respectively, and their concentrations are 2.1 × 10 12 and 6.5 × 10 12 cm −3, respectively. It is concluded that in these centers retrapping is negligible, as confirmed by the good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping.

Relaxor ferroelectric characteristics and temperature-dependent domain structure in a (110)-cut(PbMg1∕3Nb2∕3O3)0.75(PbTiO3)0.25single crystal

Relaxor ferroelectric characteristics of (110)-cut ${(\mathrm{Pb}{\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}{\mathrm{O}}_{3})}_{0.75}{(\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3})}_{0.25}$ single crystal has been revealed by temperature-dependent hysteresis loops and dielectric permittivities measurements. Ferroelectric domain structure and evolution in the as-grown and the poled samples have also been studied by means of temperature-dependent piezoresponse force microscopy. The result from the as-grown single crystal reveals that the microdomain size distribution follows an exponential cutoff, and the crystal exhibits a transition from ferroelectric microdomain to paraelectric phase upon heating; while being cooled back to room temperature, new microdomains are rebuilt. By contrast, lamellar-shaped macrodomains have been observed in the poled sample and transitions from macro- to microdomain structures at $90\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and from microdomain structure to paraelectric phase at $115\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ have been observed upon heating, respectively.

Study of point defects in as-grown and annealed bismuth germanate single crystals

Point defects and their clusters in bismuth germanate single crystals free from grain boundaries and having low density of dislocations were studied by high-resolution diffuse X-ray scattering measurements. Differences in defects in the colourless crystals (type A) and the crystals having yellow tinge (type B), which were grown with different raw materials, were investigated. In addition, interesting differences in defect structures in specimens from different regions of the same boule were investigated. Specimens with diffracting surfaces along (111), (112) and (100) planes were studied. A multicrystal X-ray diffractometer employing a well collimated and highly monochromated Mo Kα1 beam and set in (+,−,−,+) configuration was employed. The diffraction curves of all the samples were quite narrow with half-widths in the range 7–11 arcsec, which are close to the theoretically expected values, if instrumental broadenings are taken into account. The observed distribution of diffuse X-ray scattering (DXS) intensity showed that not all the point defects are isolated but a significant fraction are agglomerated into clusters. Experimental data of DXS intensity were analysed by using a phenomenological model for a small concentration of dislocation loops wherein the point defects are loosely clustered with weak interactions among them. From this analysis, the cluster radius R cl, cluster volume A cl, the number of point defects within a cluster N cl and the relative concentration of the point-defect clusters among the samples were estimated. It was observed that cluster sizes do not vary from sample to sample. However, it was found that the concentration of clusters is approximately twice in the coloured sample compared with that of the colourless sample from the same boule. Annealing of the crystals at 1273 K produced an increase in point-defect clusters by a factor of ∼200. It was accompanied by a reduction in volume of clusters by a factor of ∼0.14.

Effects of doping, electron irradiation,H+ and He+ implantation on the thermoelectric properties ofBi2Se3 single crystals

As-grown single crystals of Bi2Se3 are doped with varying percentages of tellurium. These crystalsare irradiated and implanted with electrons of energy 8 MeV andH+ andHe+ ionsof energy 1.26 MeV for comparative studies on their properties. Effects on the thermoelectric propertiesof Bi2Se3 due to high-energy electron bombardment (8 MeV),H+ and He+ ion implantation and doping are studied at temperatures ranging from 150 to 380 K. Crystalhomogeneity and surface dislocations are determined using EDAX and SEM. Hot-probe and Halleffect measurements show that as-grown, electron irradiated and ion implanted crystals aren-type. Thermal diffusivity measurements prove the effective scattering mechanism (phonons) inBi2Se3 crystals and provide a valid reason for reduced thermo-power in doped crystals.

Domain structure and evolution in (PbMg1∕3Nb2∕3O3)0.75(PbTiO3)0.25 single crystal studied by temperature-dependent piezoresponse force microscopy

Ferroelectric domain structure and evolution in (110)-cut relaxor ferroelectric (PbMg1∕3Nb2∕3O3)0.75(PbTiO3)0.25 (PMN-25PT) single crystal have been studied by means of temperature-dependent piezoresponse force microscopy. It revealed that, during heating, the as-grown PMN-25PT single crystal exhibits a transition from a ferroelectric microdomain structure to a paraelectric phase; while after being cooled back to room temperature the microdomain structure is rebuilt. Domains with size of tens of nanometers were also observed embedded in the microdomains. By contrast, the poled sample exhibits transitions from macro- to microdomain structures at 90 °C and from microdomain structure to paraelectric phase at 115 °C. These direct observations are consistent with temperature-dependent relative permittivity measurements. In situ polarization switching of the domains was also demonstrated.

Growth of ZnSe single crystals from Zn–Se–Zn(NH 4) 3Cl 5 system

A modified chemical vapor transport method in a closed-tube system was developed to grow ZnSe single crystals from zinc–selenium –Zn(NH 4) 3Cl 5 system, where Zn(NH 4) 3Cl 5 served as the chemical transport agent to avoid the kinetics limitations of the congruent sublimation conditions to ZnSe crystals growth. The growth temperature of 1001–1005 °C was employed. The ZnSe crystal with a dominated (1 1 1) singular face was obtained. The structure characteristic and crystalline quality of as-grown ZnSe single crystal were investigated by several methods such as XRD, rotating orientation X-ray diffraction, double-crystal diffraction, photoluminescence spectrum and optical microscope. The infrared transmission spectra and ultraviolet absorption measurement of as-grown crystal were also reported. The microscopic observation on the surface of as-grown ZnSe crystal revealed that two-dimension dendrite crystal growth governs the crystal growth in the system. Comparing this result with the growth of ZnSe–Zn(NH 4) 3Cl 5 system suggested that ZnSe single-crystal grown directly from two elements in one step has lower purity and quality than ZnSe crystal grown from ZnSe polycrystal source system.

Flux Growth and Dielectric Properties of Relaxor-Based Pb(In<sub>1/2</sub>Nb<sub>1/2</sub>)O<sub>3</sub>-PbTiO<sub>3</sub> Single Crystals

Relaxor-based piezoelectric single crystals of lead indium niobate-lead titanate (0.63Pb(In1/2Nb1/2)-0.37PbTiO3, abbreviated as PIN-PT), in the vicinity of the morphotropic phase boundary, were prepared by a flux method. The compositions of the flux, Pb3O4 or PbF2, play different role in perovskite stability and phase development. Thus, pure perovskite PIN-PT single crystals with the size of 2~5 mm were obtained by using Pb3O4 flux as well as a small amount of B2O3 additive. The microstructure and the phase development of the as-grown single crystals were investigated by scanning electron microscopy and transmission electron microscopy and X-ray diffraction. Furthermore, the dielectric properties of the <100>-oriented PIN-PT single crystals were measured in the temperature range between 20°C and 400°C.

Infrared radiation of zinc selenide single crystals

Long-wave photoluminescence (PL) spectra of both as-grown and Au-doped n-ZnSe single crystals are studied in the temperature range from 81 to 300 K. A narrow band of infrared (IR) radiation centered at 878 nm (1.411 eV) manifests itself in the low-temperature PL spectrum. It is established that this band intensity first increases and then decreases with increasing concentration of doping impurity. With increasing excitation radiation intensity, spectral position of the IR PL band is unchanged and its intensity increases under the linear law. With increasing excitation radiation wavelength, the IR PL band intensity increases, it becomes narrower and shifts towards long wavelengths. It is shown that the observed IR radiation is caused by recombination of free electrons with holes localized on associative acceptors ( V Zn - Au i + ) - in the ZnSe:Zn:Au crystals or ( V Zn - D + ) - in the undoped crystals.

Growth of large dimension BaWO4 crystal by the Czochralski method

In this paper, polycrystalline materials of BaWO4 were synthesized by solid-state phase method, and a single crystal of BaWO4 was successfully grown along a and c-axis direction by using the Czochralski method. Up to 20×22×80mm3 BaWO4 crystal was obtained, and X-ray powder diffraction results show that the as-grown BaWO4 crystal belongs to the scheelite structure. The effective segregation coefficients of Ba and W of the BaWO4 crystal were measured by the X-ray fluorescence method, and the effective segregation coefficients of Ba and W were near 1. The rocking curve from (200) diffraction plane of as-grown BaWO4 single crystal was measured on the High-resolution X-ray diffractometer D5005, and the full-width at half-maximum value was found to be 26.64′′ The density and hardness of the BaWO4 crystal was measured, the measured density was in agreement with the calculated result, and the Mohs hardness was about 4.

The influence of crystal imperfections on the shape of exciton emission spectrum in ZnO single crystals

A distortion of exciton emission spectrum, namely, relative decrease of free exciton band intensity with respect to that of its phonon replicas in undoped as-grown ZnO single crystals has been shown to result from enhanced reabsorption of shorter wavelength emission in the crystal due to formation of optical absorption tail. Some crystal imperfections, in all probability, dislocations decorated with shallow donors are supposed to be responsible for the effect.

Determination of the ionization energy of nitrogen acceptors in zinc oxide using photoluminescence spectroscopy

Photoluminescence spectroscopy of nitrogen-related emissions in ZnO is used to establish the ionization energy of the substitutional nitrogen acceptor. The temperature dependence of the nitrogen-related electron-acceptor (e,A0) emission band has been monitored in as-grown single crystals of ZnO. Line shape analysis of this band is used to determine the acceptor ionization energy. The temperature dependence of the band gap for ZnO was included in our analysis and the low-temperature acceptor ionization energy for substitutional nitrogen at an oxygen site in ZnO was found to be EA=209±3 meV. Our line shape analysis indicates a small temperature-dependent decrease in EA for T>5 K.

Investigation of transition metal oxides by ultrasonic microscopy

Abstract Ultrasonic microscopy is now well established in non-destructive testing and has become a versatile method for investigating solid materials. It may serve both as imaging technique for large scale inspection of sample homogeneity and also as measuring device for a local determination of elastic properties. Our investigations of single crystalline vanadium dioxide nicely demonstrate this capability of flexible use and are exemplary for the beneficial effect of ultrasonic microscopy on materials research, in general. Here we focus on three different subjects: the detection of odd phases in as-grown single crystals, the inspection of structural transformations accompanying the metal–insulator transition and the investigation of elastic properties performed on small crystallites.

Floating zone growth and characterization of Ca 2Fe 2O 5 single crystals

We report the growth of inclusion and sub-grain free Ca 2Fe 2O 5 (CFO) single crystals by the floating zone method. Single crystals were successfully obtained with volumes of up to 1.0 cm 3. The X-ray rocking curve of the CFO single crystal has the full-width at half-maximum (FWHM) of 0.065°. The oxygen content in the as-grown CFO single crystal was measured by the thermo-gravimetric (TG) method in Ar/H 2 flow. Two magnetic transitions at T 1≈140 K and T 2≈60 K were observed in the CFO crystal.

Cd-vacancy-related excitonic emission in CdTe

The dominant luminescence emission of excitons bound to a neutral acceptor defect (A 0, X) in as-grown high purity CdTe single crystals has been studied by Cd-dip treatment, doping with Cu and Ag, and irradiation with γ-rays from 60Co. It has been demonstrated that, by Cd-dip treatment, the dominant (A 0, X) emission can be annihilated to be a double-structure emission with their energy at 1.5896 and 1.5885 eV, respectively. Our doping experiments have unambiguously shown that (A 0, X) at 1.5896 eV is associated with residual impurities of Cu and (A 0, X) at 1.5885 eV with Ag. Further experiment with γ-ray irradiation has revealed a new (A 0, X) emission at 1.5892 eV in the double structure. We suggest that this emission is associated with Cd-vacancies introduced by γ-rays. To our knowledge, this is the first report on the isolated Cd-vacancies showing (A 0, X) emission with the energy position different from 1.5896 eV.