Improvement Of Crystal Quality Research Articles

Characterisation of vapour phase grown CdTe and (Cd,Zn)Te for detector applications

Abstract The growth of CdTe from the vapour phase offers several improvements in crystal quality and homogeneity. CdTe and (Cd, Zn)Te were grown by the modified Markov technique. The transport properties and the detector performance are given and compared to melt grown material.

Crystallization and improvement of crystal quality for x-ray diffraction of maltooligosyl trehalose synthase by reductive methylation of lysine residues.

Maltooligosyl trehalose synthase, one of the two enzymes in the coupled trehalose biosynthesis system in Sulfolobus acidocaldarius, has been purified and crystallized. The chemical modification of this enzyme by reductive methylation of lysine residues significantly improved the crystal quality for X-ray diffraction experiments. The crystals of the modified enzyme belong to orthorhombic space group P212121, with unit-cell parameters a = 56.70, b = 140.1, c = 205.2 A measured at cryo-temperature, and are found to contain two enzyme molecules per asymmetric unit.

High quality β-SiC films obtained by low-temperature heteroepitaxy combined with a fast carbonization step

The present paper describes a new low-temperature process for the deposition of crystalline cubic silicon carbide on [100] silicon using methylsilane as precursor. A substantial improvement of crystal quality was achieved by applying a special fast carbonization step based on ethylene diluted in argon. The influence of the buffer-layer growth conditions, the SiC film thickness, and the deposition temperature on crystal quality and surface morphology was studied using SEM, PDS, and X-ray diffraction. In comparison to SiC layers grown without a carbonization step the FWHM of the [200] reflex was reduced from 0.54° to 0.21°, the surface roughness from 350nm to 38nm and the defect density by a factor of 9. The state-of-the-art quality was proved by the comparison to commercial films from Cree Research Inc., presumably grown at considerably higher temperatures.

Multiwavelength output from a Nd:YAG/Cr:LiSAF hybrid laser

A prototype solid-state, multispectral hybrid laser has been designed and tested. The laser provides simultaneous outputs at several wavelengths. The hybrid-laser concept is based on the efficient use of flash-lamp-pump energy distributed between two complementary lasing materials, Nd:YAG and Cr:LiSAF, that share the same pump cavity. The prototype Q-switched hybrid laser provides dual-fundamental-wavelength output at 850 and 1064 nm as well as frequency-doubled output at 532 nm. The laser achieved 3.6% slope efficiency (combined) in free-running operation and 2.4% when Q switched. Higher efficiencies can be obtained with improvements in laser crystal quality and pump cavity configuration.

Expression, purification, crystallization and preliminary X-ray diffraction results from Campylobacter jejuni ferritin.

The prokaryotic ferritin gene of Campylobacter jejuni was overexpressed in Escherichia coli under control of the bacteriophage T7 promoter and the protein (Cj-FTN) purified. Preliminary crystallization experiments have been performed using the hanging-drop vapour-diffusion method with ammonium sulfate as the precipitant. Diffraction studies show the crystals belong to the I432 space group (a = 151.52 A). Structure solution by molecular replacement is in progress while crystal quality improvement is carried out.

Heteroepitaxial technologies on Si for high-efficiency solar cells

The improvements of the AlGaAs crystal quality grown on Si substrate and the AlGaAs/Si tandem solar cell have been studied with varying the growth conditions. The crystal quality of the AlGaAs layer was evaluated by photoluminescence, deep level transient spectroscopy, time-resolved photoluminescence and double crystal X-ray diffraction while varying the thermal cycle annealing temperature. The optimum thermal cycle annealing temperature and the buffer layer thickness for the growth of high efficiency AlGaAs/Si tandem solar cells have been presented. The active-area conversion efficiency of 21.2% and 21.4% (AMO and 1 sun at 27°C) has been demonstrated with two-terminal and four-terminal configuration, respectively, by the perfect photocurrent matching between the top cell and the bottom cell.

MOCVD growth of high efficiency current-matched [formula omitted] tandem solar cell

The improvements of the AlGaAs crystal quality grown on Si substrate and the AlGaAs Si tandem solar cell have been studied by varying the growth conditions and solar cell structure. The crystal quality of the AlGaAs layer was evaluated by time resolved photoluminescence and double crystal X-ray diffraction while varying the thermal cycle annealing temperature. The optimum thermal cycle annealing temperature and the buffer layer thickness for the growth of high efficiency AlGaAs Si tandem solar cells have been presented. The active-area conversion efficiency of 21.2 and 21.4% (AM 0 and 1 sun at 27°C) has been demonstrated with two-terminal and four-terminal configurations, respectively, by perfect photocurrent matching between the top cell and the bottom cell.

Crystallization under microgravity of acidic phospholipase A 2 from venom of Agkistrodon halys Pallas

A series of space crystallization experiments of an acidic phospholipase A 2 from venom of Agkistrodon halys Pallas were performed in August 1992 and in July 1994. On both missions, compared with earth-grown crystals in control experiments, space-grown crystals were larger, had fewer nuclei, and exhibited better habit. Significant improvements in crystal quality were achieved on 1994's mission, and the space-grown crystals exhibited stronger diffraction patterns and diffracted X-rays to high resolutions. Relative Wilson plots indicated that the space-grown crystals had higher internal order than the best laboratory-grown crystal. In addition, the results of the two space experiments suggested that the optimization of the space crystallization conditions, especially when an unconventional crystal growth method is engaged, plays an important role in exploiting microgravity more efficiently.

Current state-of-the-art industrial and research applications using room-temperature CdTe and CdZnTe solid state detectors

Improvements of CdTe crystal quality and significant progress in the growth of large ingots of high resistivity CdZnTe material enable the fabrication of larger area detectors in single element form or monolithic arrays. These advances allow for the development of imaging devices of improved spatial resolution for industrial, research and medical applications. CdTe and CdZnTe detectors operate in single photon counting mode or in current mode (charge integrating mode). The paper presents advantages of CdTe and CdZnTe over common scintillator type detectors, but also presents the shortcomings of the former detectors with respect to charge collection which limit the yields of good spectrometers. The paper reviews industrial and research applications utilizing these detectors and in particular describes in detail two imaging systems for security screening and custom inspection. These systems are characterized by large dynamic range and good spatial resolution and are composed of large arrays of CdTe spectrometers and discriminator grade detectors. A wide energy range detector assembly, for astrophysical research of gamma ray bursts composed of CdTe, HgI 2 and CdZnTe spectrometers in two dimensional arrays is also presented.

Synthesis of SiGe and SiGeC alloys formed by Ge and C implantation

In this study, near surface SiGe and SiGeC alloys with germanium peak composition up to 16 at.% were formed using high-dose ion implantation and subsequent solid phase epitaxial growth. Rutherford backscattering spectroscopy (RBS) channeling spectra and cross-sectional transmission electron microscopy (XTEM) studies showed that high quality Si0.92Ge0.08 and Si0.91Ge0.08C0.01 crystals were formed at the Si surface, while Si0.84Ge0.16 and Si0.82Ge0.16C0.02 layers had extended defects. X-ray diffraction experiments demonstrated that carbon could reduce the lattice strain in SiGe alloys but without significant crystal quality improvement as detected by RBS channeling spectra or XTEM observations.

Structural properties of ion beam synthesized iron - cobalt silicide

Surface and buried layers of ternary silicide were fabricated by implantation of iron and cobalt into (100) silicon wafers. For the surface layers two sets of samples with different iron to cobalt ratios were prepared. In the first set, cobalt was implanted first followed by iron and the implant order was reversed in the second set. In all cases the total implanted dose (Fe+Co) was kept constant. For the buried ternary silicides two samples were prepared with equal doses of iron and cobalt. In the first sample, cobalt was again implanted first with the implant order being reversed for the second sample. The physical properties of the synthesized layers were investigated by Rutherford backscattering spectrometry (RBS), x-ray photoelectron spectroscopy (XPS) and cross sectional transmission electron microscopy (XTEM). The results indicate that the implantation order is critical to the subsequent development of the synthesized layer. The surface layers with iron implanted first were non-crystalline and showed no significant improvement of crystallinity with increasing anneal temperature. However, the surface layers with cobalt implanted first exhibited a large improvement of crystal quality with increasing anneal temperature. Segregation into separate iron- and cobalt-rich layers was also observed for surface layers where cobalt was implanted first for Co:Fe . The crystalline quality of the buried layers was also determined by the implant order, in a similar way to that for the surface layers.

Self-organized growth of [formula omitted] heterostructure nanocylinders by organometallic vapor phase epitaxy

Free-standing GaAs InAs heterostructure wires as thin as 20 nm and as long as 1 μm have been formed by vapor-liquid-solid (VLS) growth during organometallic vapor phase epitaxy. The grown wires were analyzed by transmission electron microscopy, which revealed that the crystal structure of the GaAs portion coincides with that of zincblende, and the InAs portion coincides with that of wurtzite. The atomic composition along the heterojunction was also measured by energy dispersive X-ray analysis. The composition changes within a width of 5 nm at the heterojunction interface. The InAs GaAs wires show a photoluminescence peak around 1.5 eV at 14 K, which indicates significant improvement in crystal quality over conventional GaAs InAs layer structures.

A clarification of optical transition of β-FeSi 2 film

Highly oriented β-FeSi 2 film on Si(111) was prepared by reactive deposition solid phase epitaxy. Photoreflectance measurements have been carried out near the absorption edge at room temperature. The exact direct transition has been determined at E g = 0.871 eV. Photocurrent spectra of the sample indicated that the surface recombination velocity becomes very small due to the improvement of crystal quality.

A new route to study fluorination of high-T c superconductors: Crystal growth with fluorine ion as additive

After the discovery of the high temperature superconductor YBa2Cu307_y (YBCO), much research effort was devoted to the effect of cationic [1] and anionic [2] substitutions on the superconductivity. One of the most important areas of study was substituting oxygen by fluorine by (i) solid state reaction [3], (ii) ionic implantation [4] and (iii) solid-gas reaction [5]. After extensive investigation, most researchers agreed that the fluorination did not make any important improvement to the superconductivity under such solid state conditions and relatively low temperature. In this letter, we report the influence of fluorine ion as additive on the crystal growth from high temperature solution. A small amount of BaF2 greatly improved the crystal growth and the superconducting transition temperature (Te) by reducing aluminium contamination of Cu-O chains from the alumina crucible. The conductance spectra for Cu-O chains by scanning tunnelling microscopy (STM) showed that such contamination made the chain layer more insulating. No Fwas found in the crystal. A growth model with a monomolecular layer spiral dislocation mechanism is proposed, which enables to understand the function of F-. High quality crystals with sharp superconducting transition at 94 K have been obtained in a yttria crucible by using BaF2 as additive. The starting materials for the crystal growth were YBCO, BaCO3, CuO and BaF2. The crystal growth was carried out by spontaneous nucleation from a self-flux system with a small amount of BaF2 as additive. In order to select an appropriate system for the crystal growth, phase diagrams [6] of the flux system and the growth system were explored by improved thermogravimetric analysis (ITGA) and differential thermal analysis (DTA), from which the eutectic composition of 0.28 BaO + 0.72 CuO was chosen as solvent and the solute concentration and growth temperature were introduced from the liquidus range of the growth system. Alumina or yttria crucibles were used as containers for the crystal growth. In fact, most compounds, including BaF2, are not suitable as flux for the crystal growth of YBCO [7] because of its low stability. The crystal could be grown mainly from the self-flux system. However, we could reduce the concentration of BaF2 in the self-flux to such a low level as an additive that YBCO remains the major crystalline phase. Sometimes small amounts of a particular additive can dramatically affect the nucleation and crystal growth, resulting in improvements of crystal size or quality a n d modification of the habit. We have used B203, BaF2, Bi203 and BaC12 as additives for the crystal growth. Only BaF2 achieved improvements of both crystal growth and superconductivity [8]. Table I shows the effects of various concentrations of BaF2 on the cyrstal growth under the same solute concentration. The optimum amount of BaF2 appears

Influence of divalent cations in protein crystallization.

We have tested the effect of several cations in attempts to crystallize the ligand-bound forms of the leucine/isoleucine/valine-binding protein (LIVBP) (M(r) = 36,700) and leucine-specific binding protein (LBP) (M(r) = 37,000), which act as initial periplasmic receptors for the high-affinity osmotic-shock-sensitive active transport system in bacterial cells. Success was achieved with Cd2+ promoting the most dramatic improvement in crystal size, morphology, and diffraction quality. This comes about 15 years after the ligand-free proteins were crystallized. Nine other different divalent cations were tried as additives in the crystallization of LIVBP with polyethylene glycol 8000 as precipitant, and each showed different effects on the crystal quality and morphology. Cd2+ produced large hexagonal prism crystals of LIVBP, whereas a majority of the cations resulted in less desirable needle-shaped crystals. Zn2+ gave crystals that are long rods with hexagonal cross sections, a shape intermediate between the hexagonal prism and needle forms. The concentration of Cd2+ is critical. The best crystals of the LIVBP were obtained in the presence of 1 mM CdCl2, whereas those of LBP, with trigonal prism morphology, were obtained at a much higher concentration of 100 mM. Both crystals diffract to at least 1.7 A resolution using a conventional X-ray source.

Effects of V/III supply ratio on improvement of crystal quality of zincblende GaN grown by gas source molecular beam epitaxy using RF-radical nitrogen source

The effects of the V/III supply ratio on the crystal quality of zincblende GaN grown on (001) GaAs substrates by gas source molecular beam epitaxy, using a 13.56 MHz RF-radical nitrogen source, was investigated. The surface morphology was drastically improved by optimizing the V/III ratio, resulting in a featureless smooth surface. The measured electron concentration increased with increasing the V/III supply ratio. This phenomenon is explained by the electrons originating from nitrogen vacancies being compensated by defects due to the poor crystal quality of GaN grown at lower V/III supply ratios.

Ternary iron-cobalt silicide fabricated by ion beam synthesis

Ternary silicide layers were fabricated by the implantation of iron and cobalt into (100) silicon wafers. Two sets of samples with different iron to cobalt ratios were prepared, with cobalt being implanted first followed by iron in the first set, and the implantation order being reversed for the second set. In all cases a total Fe + Co dose of 5 × 10 17 cm −2 was used. The structural properties of the synthesised layers were assessed by Rutherford backscattering spectrometry (RBS) and cross-sectional transmission electron microscopy (XTEM). Our results indicate that for the samples where cobalt was implanted first epitaxial layers are formed after implantation. However, when iron was implanted first the layers are non-crystalline. For the samples with iron implanted frrst no significant improvement in crystal quality was observed with increasing anneal temperature. However, for those samples where cobalt was implanted first we observed a strong dependence of the crystal quality on the annealing temperature. For the samples with cobalt doses ≥ 2.5 × 10 17 cm −2, where cobalt was implanted first, segregation of iron and cobalt within the synthesised layer is observed.

Quantum confinement effects of CdS nanocrystals in a sodium borosilicate glass prepared by the sol-gel process

Experimental evidences of both weak and strong confinement regimes are reported on CdS nanocrystals embedded in a sodium borosilicate glass matrix. A method, based on the sol-gel technique, is used for the preparation of CdS-activated glass. This route is capable of providing nanocrystals covering a wide range of radii with small size dispersion. Low-temperature linear-absorption spectra have been analyzed in terms of excitons and electron-hole confinements by fitting the results of a numerical calculation to experimental findings. The model used, in the envelope-function formalism, involves both a Lorentzian broadening of the exciton energy states inside each nanocrystal and a Gaussian size distribution. The improvement of crystal quality and the sharpening of the size distribution by thermal annealing is also studied versus both time and temperature of treatment. It is shown that we can keep a tight control on the crystallinity, average size, and size distribution of the nanocrystals by rather simple adjustments and short treatments.

Purification and single crystal growth of niobium by combined zone refining and electrotransport

High quality, high purity Nb single crystals were produced by combining DC electrotransport with float-zone refining. A significant improvement in residual resistivity ratio, RRR ( ϱ 300 K/ ϱ 4.2 K), and crystal quality is observed when the directions of the applied field and the zone motion are parallel. A reduction in RRR is observed when the directions are opposed. The best crystals produced had RRRs up to 2600, which is nearly a factor of 2 higher than that achieved by zone refining alone.

Improvement of crystal quality and laser characteristics by zero net strain structure

Zero net strain structure is very useful for strained multiple quantum well (MQW) lasers for which many numbers of quantum wells are required. We demonstrate the effectiveness of the zero net strain structure to prevent the degradation of the crystal quality up to 15 wells, which cannot be realized by the conventionally strained MQW (well: 1.5% compressive strain, 30 Å thick). We also show that the zero net strain structure has finite critical thickness and dislocations other than 60° misfit dislocations are introduced into the zero net strain structure beyond the critical thickness. Narrower distribution of threshold current than that of the conventionally strained lasers, high relaxation oscillation frequency of 5.5 GHz/ √mW and high differential gain of 6.3 X 10 -16 cm 2 have been realized for the 1.55 μm laser with zero net strain MQW with 15 wells by the suppression of degradation of crystal quality.