Dec 25, 2019

Reducing Iron in Single Crystal Silicon Grown Using CZ Process

The metal iron, that is present in graphite hot zone components used in CZ silicon process, contaminates the crystal. To eliminate and reduce contamination, graphite components were coated with two protective layers including a first protective layer of silicon carbide and a second protective layer of silicon. The effectiveness of these coatings to prevent iron contamination was tested. At test temperature of 1100{degree sign}C and 900{degree sign}C, the Fe concentrations in the monitor wafer that was exposed to graphite sample with layers of silicon and silicon carbide coating was ten times lower. Several hot zone structural components were coated using this method and were used to grow single crystal silicon. The wafers from these crystals were tested for iron concentrations using standard SPV method and are found to be much lower when compared to wafers from crystals grown using only silicon carbide coated graphite components.

Source:IOPscience

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Dec 18, 2019

Characterization of Crystal Quality by Crystal Originated Particle Delineation and the Impact on the Silicon Wafer Surface

Characterization of Si wafers by delineation of crystal originated particles (COP) provides insight into size and radial distribution of crystal related defects. A good correlation of the COP densities with gate oxide integrity and flow pattern defect densities is observed. The density and size distribution of COP in Czochralski Si ingots can be modified by the pulling rate and the cooling conditions of the crystal and is further influenced by high doping concentrations. The COP densities are comparable on wafers with (100) Si and (111) Si orientation as well as on p‐ and n‐type wafers with moderate doping level. No COP are found on float zone (FZ) and on epitaxially grown wafers. Crystal defects are also delineated by chemomechanical polishing and can be detected on the wafer surface as light point defects (LPD). LPD densities, however, do not necessarily correlate with the corresponding COP densities after SC1 treatment and do not reflect the quality of the crystals because polishing delineates only part of the larger crystal defects to a size which is above the detection limits of commercially available scanning surface inspection systems. High temperature annealing results in reduction of defect sizes and partial dissolution of COP. Investigations of FZ and oxygen doped float zone indicate that oxygen is participating in the formation of COP.

Source:IOPscience

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Dec 12, 2019

Relationship between Basal Plane Dislocation and Local Basal Plane Bending in PVT-Grown 4H-SiC Crystals

Synchrotron monochromatic beam x-ray topography (SMBXT) shows black and white contrast of BPDs for Burgers vectors of opposite signs based on the principle of ray tracing. We have evaluated the ratio of black and white contrast of BPDs along both [1120] and [1100] directions across multiple 4-inch and 6-inch diameter 4H-SiC substrates. Results show the predominance of white contrast BPDs along both radial directions indicating that the basal planes on Si face are bent in a convex manner. Line scans of 0004 reflection using HRXRD was carried out which further confirmed the nature of basal plane bending in these wafers. Detailed analysis on the subsequent wafers across the crystal boule reveals the inheritance of basal plane bending behavior in these wafers. The radius of curvature in 6-inch wafers was found to be larger than the 4-inch wafers. Additional studies on the effect of low angle grain boundaries were also discussed.

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Dec 4, 2019

Characterization of Crystal-Originated Particles in Silicon Nitride Doped, CZ-Grown Silicon Wafers

Grown-in crystal-originated particles (COPs) on the surface of silicon nitride-doped Czochralski (CZ)-grown silicon wafers were characterized using atomic force microscopy and scanning electron microscopy. These nanometer-scale COPs are categorized into kite-shaped, parallelepiped-plate and needle-shaped COPs, respectively, with unique features distinctively different from the octahedral voids commonly found in conventional CZ-grown silicon wafers. Based on the experimental data obtained, it is postulated that nitrogen dopants in the silicon crystals could influence the formation of these COPs with different morphologies and sizes. This may be supported by a simple analysis of the mapping distribution of COPs on the nitride-doped CZ-grown silicon wafer, which reveals that the densities of the smaller-size parallelepiped-plate and needle-shaped COPs are negligible at the center of the silicon wafer but increase to a significant proportion comparable to that of the kite-shaped COPs at the outer edges of the silicon wafer along the radial directions. These observations are thought to correlate well with the presence of nitrogen dopants and the radial concentrations of the excess free vacancy.

Source:IOPscience

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Nov 27, 2019

Macrodefect-free, large, and thick GaN bulk crystals for high-quality 2–6 in. GaN substrates by hydride vapor phase epitaxy with hardness control

On the basis of a novel crystal hardness control, we successfully realized macrodefect-free, large (2–6 in.) and thick +c-oriented GaN bulk crystals by hydride vapor phase epitaxy. Without the hardness control, the introduction of macrodefects including inversion domains and/or basal-plane dislocations seemed to be indispensable to avoid crystal fracture in GaN growth with millimeter thickness. However, the presence of these macrodefects tended to limit the applicability of the GaN substrate to practical devices. The present technology markedly increased the GaN crystal hardness from below 20 to 22 GPa, thus increasing the available growth thickness from below 1 mm to over 6 mm even without macrodefect introduction. The 2 and 4 in. GaN wafers fabricated from these crystals had extremely low dislocation densities in the low- to mid-105 cm−2 range and low off-angle variations (2 in.: <0.1°; 4 in.: ~0.2°). The realization of such high-quality 6 in. wafers is also expected.

Source:IOPscience

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Nov 20, 2019

Growth of bulk GaN crystals by the Na-flux point seed technique

In this paper, progress in the Na-flux point seed technique (SPST) will be reviewed. Bulk GaN crystals with a diameter of 2.1 cm, a height of 1.2 cm, and large dislocation-free areas were successfully produced by SPST. Panchromatic cathodoluminescence images of a wafer sliced parallel to the c-face from the crystal showed the lack of dark spots due to dislocations over a large area of the wafer. Structural properties were evaluated using synchrotron X-ray diffraction analysis at SPring-8. The full width at half maximum of the 006 rocking curve was found to be 2.1 arcsec, close to the calculated value of 2.0 arcsec for a perfect GaN crystal, indicating that crystals grown by SPST have an almost perfect structure. In addition, we have extended the use of SPST to the coalescence growth of GaN crystals to increase the wafer diameter and obtained a 2 in. GaN wafer with a low dislocation density and a low curvature by this technique.

Source:IOPscience
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Nov 13, 2019

Defect characterization of β-Ga2O3 single crystals grown by vertical Bridgman method

The characteristics of structural defects observed on (100) wafers in β-Ga2O3 single crystals grown by directional solidification in a vertical Bridgman furnace were studied in terms of crystal growth conditions. No high-dislocation-density regions near the wafer periphery were observed owing to the lack of adhesion between the as-grown crystal ingot surface and the crucible inner wall, and directional solidification growth in a crucible with a very low temperature gradient resulted in β-Ga2O3 single crystals with a low mean dislocation density of 2.3 × 103 cm−2. Line-shaped defects up to 150 µm long in the [010] direction were detected at a mean density of 0.5 × 102 cm−2, which decreased with decreasing growth rate. The line-shaped defect structure and formation mechanism were discussed.

Source:IOPscience

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Nov 6, 2019

Effect of Quartz Crystal Form on the Measurement Performance of Multi-dimensional Force Sensor

Quartz wafer is an important part of the force sensor of piezoelectric multi-dimensional force sensor. Its structural form affects the overall measurement performance of force-sensitive components and sensors. In this paper, the shape of the quartz crystal group was first analysed, and then two different types of quartz crystal group models were established using finite element software, and their simulation analysis was performed. The corresponding relationship between the deformation of the two different shapes of quartz crystal set under the same external load and the limitation of external space size was studied. The same axial load and tangential load were applied to two different types of quartz crystals respectively, and the corresponding maximum equivalent stress and stress distribution regions were obtained. This paper provides an important reference for the fabrication of quartz crystal arrays and the selection of quartz wafer shape for piezoelectric multi-dimensional force sensors.

Source:IOPscience

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Oct 29, 2019

Thermomechanical Study and Fracture Properties of Silicon Wafer under Effect of Crystal Orientation

Designing and optimizing the performance of a piezoresistive pressure microsensor involves not only studying the piezoresistive properties of polysilicon but also analyse the thermomechanical behaviour of its most sensitive internal part. This work focuses on an analytical, numerical and experimental study of a silicon wafer used in a pressure sensor. A thermomechanical study was conducted to assess the effects of temperature and material orientation on the deflexion and failure mode of Silicon membrane. The chosen analytical approach is based on Kirchhoff theory in which a square Silicon membrane is subjected to a distributed pressure over all its surface. Thermomechanical simulations and fracture analysis are carried out using Abaqus software. An experimental thermomechanical technique to determine maximum wafer deflexion and its effect on crack initiation and fracture mode was implemented. Deflexions tests were carried out on a bending machine using specimens of p-Si doped wafers. Deflexion values are measured for each applied temperature. Some preliminary tests were performed to determine the impact of temperature and Silicon wafers orientation on maximum deflexion when fracture occurs. The test's results confirmed that the failure mode is highly brittle and follow each time crystal orientations 0° and 45°. Furthermore, at room temperature, fracture occur at 389 μm of displacement, and grows by 5.2 % when



Source:IOPscience


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Aug 22, 2019

Point defects in ZnO crystals grown by various techniques

In the present work point defects in ZnO crystals were characterized by positron lifetime spectroscopy combined with back-diffusion measurement of slow positrons. Defects in ZnO crystals grown by various techniques were compared. Hydrothermally grown ZnO crystals contain defects characterized by lifetime of 181 ps. These defects were attributed to Zn vacancies associated with hydrogen. ZnO crystals prepared by other techniques (Bridgman, pressurized melt growth, and seeded chemical vapour transport) exhibit shorter lifetime of 165 ps. Positron back-diffusion studies revealed that hydrothermally grown ZnO crystals contain higher density of defects than the crystals grown by other techniques. The lowest concentration of defects was detected in the crystal grown by seeded chemical vapor transport.


Source:IOPscience

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Aug 9, 2019

Influence of Mn doping on CuGaS2 single crystals grown by CVT method and their characterization

1 and 2 mole% of Mn doped CuGaS2 (CGS) single crystals were grown by the chemical vapour transport (CVT) technique using iodine as the transporting agent. The analysis of the single crystal x-ray diffraction data suggests that the doping of 1 and 2 mole% Mn in the CGS single crystal does not affect the tetragonal (chalcopyrite) crystal structure. The optical absorption spectrum shows that the Mn ion induces a very strong absorption band in the UV–visible–near IR regions. The values of the crystal parameter (Dq) and the Racah parameter (B) calculated from the absorption spectra show d electron delocalization in the host crystal CGS. Room temperature photoluminescence spectra of undoped CGS only exhibited a band–band emission. But 1 and 2 mole% Mn doped CGS single crystals show two distinct CGS and Mn2+ related emissions, both of which are excited via the CGS host lattice. Raman spectra of 1 and 2 mole% Mn doped CGS single crystals exhibit a high intensity peak of the A1 mode at 310 cm−1 and 300 cm−1, respectively. EDAX, optical absorption and Raman spectrum studies reveal that Mn2+ ions are substituted in the Ga3+ ions and incorporated into the CGS lattice. The magnetization of Mn doped CGS single crystals was measured as a function of the magnetic field and temperature. Paramagnetic behaviour typical of spin S = 5/2 expected for Mn2+ (d5) magnetic centres was observed in the temperature range 2 K < T < 300 K. In Mn doping, the increase in bulk conductivity of the Mn doped CGS single crystals at room temperature indicates an increase in the hole concentration.


Source:IOPscience

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Aug 1, 2019

Growth of square Si single bulk crystals with large side-face widths using noncontact crucible method

The noncontact crucible method was used to prepare square Si single bulk crystals. The size of the square part of the ingots was determined by the side-face width of the four-cornered pattern that appeared on the top surface. We obtained square Si single crystals with sizes of 9.4 × 9.7 and 10.9 × 11.0 cm2 that had no fan-shaped {110} faces and had diagonal lengths of up to 91% of the crucible diameter. To obtain large square Si single bulk crystals with a large side-face width using the present method, the importance of establishing a larger low-temperature region in the Si melt while maintaining a smaller initial temperature reduction was considered.



Source:IOPscience

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Jul 23, 2019

Near-infrared photoluminescence in germanium oxide enclosed germanium nano- and micro-crystals

We have studied the near-infrared photoluminescence properties of free-standing germanium nano-crystals (20 nm on average) and micro-crystals (60 µm on average) at 80–300 K. Two peaks were observed at ~1.0 and ~1.4 eV from both the nano- and micro-crystals. The integrated PL (IPL) intensity of the nano-crystals is about an order of magnitude stronger than that of the micro-crystals and the IPL is also enhanced by ageing in air for both crystals. The ~1.0 eV peak position does not change with either the crystal size or temperature. We suggest that the deep traps located at the interfacial region between the surface GeO2 layer and the bulk crystal Ge is responsible for the near-infrared PL.



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Jul 17, 2019

Crystal growth and optical characterization of an organic single crystal for frequency conversion applications

Organic nonlinear optical 2-methylquinolinium L-malate single crystals have been grown by slow evaporation solution growth technique from a mixture of 2-methylqiunoline and L-malic acid in ethanol solution at ambient temperature. Single crystal X-ray diffraction analysis reveals that grown crystal in monoclinic system with non-centrosymmetric space group P21 and the lattice parameters are a = 7.35 Ǻ, b = 26.51 Ǻ, c = 10.83 Á, α = γ = 90° β = 102.95º and V = 2057.4 Ǻ3. UV-vis spectrum indicates that the crystal is transparent (75%) in the entire visible region with a cut-off wavelength of 437 nm and optical energy band gap Eg is found to be 2.71 eV. Microhardness measurement reveals the mechanical strength of the grown crystal. The photoluminescence spectrum shows the blue emission of the crystal. Laser damage threshold studies was carried out to ascertain the suitability of grown crystal for laser applications.The relative second harmonic generation efficiency of 2-methlquinolinium L-malate crystal was found to be two times greater than that of KDP.



Source:IOPscience

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Jul 9, 2019

On the optical properties of undoped and rare-earth-doped yttrium aluminium garnet single crystals

Optical absorption and photoluminescence (PL) measurements were performed on single crystals of undoped Y3Al5O12 (YAG) and a number of rare-earth-doped YAG to study the effect of dopant type and concentration, growth atmosphere, post-growth annealing and UV irradiation on the optical properties of YAG crystals. The presence of hydrogen in the growth atmosphere was found to be essential for enhancing the incorporation of Ce ions in the Ce3+ state in Ce-doped YAG (Ce : YAG). Annealing in air was shown to have no effect on the PL emission of Ce : YAG crystals. An absorption peak around 256 nm was observed in the undoped YAG and Ce : YAG crystals after air anneal at 1200 °C. Optical absorption and annealing experiments support the association of the 256 nm peak with Fe impurities and oxygen ions. UV irradiation modifies the valency of impurities and generates electronic defects leading to an increase in the optical density of YAG crystals. Optimizing the growth and annealing conditions is critical in order to develop Ce : YAG single crystals as efficient scintillators.



Source:IOPscience

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Jun 20, 2019

Development of a dual-ended readout detector with segmented crystal bars made using a subsurface laser engraving technique

Depth of interaction (DOI) information is indispensable to improving the sensitivity and spatial resolution of positron emission tomography (PET) systems, especially for small field-of-view PET such as small animal PET and human brain PET. We have already developed a series of X'tal cube detectors for isotropic spatial resolution and we obtained the best isotropic resolution of 0.77 mm for detectors with six-sided readout. However, it is still challenging to apply the detector for PET systems due to the high cost of six-sided readout electronics and carrying out segmentation of a monolithic cubic scintillator in three dimensions using the subsurface laser engraving (SSLE) technique. In this work, we propose a more practical X'tal cube with a two-sided readout detector, which is made of crystal bars segmented in the height direction only by using the SSLE technique. We developed two types of prototype detectors with a 3 mm cubic segment and a 1.5 mm cubic segment by using 3  ×  3  ×  20 mm3 and 1.5  ×  1.5  ×  20 mm3 crystal bars segmented into 7 and 13 DOI segments, respectively, using the SSLE technique. First, the performance of the detector, composed of one crystal bar with different DOI segments and two thorough silicon via (TSV) multi-pixel photon counters (MPPCs) as readout at both ends of the crystal bar, were evaluated in order to demonstrate the capability of the segmented crystal bars as a DOI detector. Then, performance evaluation was carried out for a 4  ×  4 crystal array of 3  ×  3  ×  20 mm3 with 7 DOI segments and an 8  ×  8 crystal array of 1.5  ×  1.5  ×  20 mm3 with 13 DOI segments. Each readout included a 4  ×  4 channel of the 3  ×  3 mm2 active area of the TSV MPPCs. The three-dimensional position maps of the detectors were obtained by the Anger-type calculation. All the segments in the 4  ×  4 array were identified very clearly when there was air between the crystal bars, as each crystal bar was coupled to one channel of the MPPCs; however, it was necessary to optimize optical conditions between crystal bars for the 8  ×  8 array because of light sharing between crystal bars coupled to one channel of the MPPCs. The optimization was performed for the 8  ×  8 array by inserting reflectors fully or partially between the crystal bars and the best crystal identification performance was obtained with the partial reflectors between the crystal bars. The mean energy resolutions at the 511 keV photo peak for the 4  ×  4 array with air between the crystal bars and for the 8  ×  8 array with partial reflectors between the crystal bars were 10.1%  ±  0.3% and 10.8%  ±  0.8%, respectively. Timing resolutions of 783  ±  36 ps and 1.14  ±  0.22 ns were obtained for the detectors composed of the 4  ×  4 array and the 8  ×  8 array with partial reflectors, respectively. These values correspond to single photon timing resolutions. Practical X'tal cubes with 3 mm and 1.5 mm DOI resolutions and two-sided readout were developed.

Source:IOPscience

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Jun 14, 2019

Fabrication of p-type lithium niobate crystals by molybdenum doping and polarization

The lack of p-type lithium niobate limits it serving as an active material. A series of Mo-doped and pure congruent lithium niobate crystals were grown by Czochralski method under different polarization conditions. Their dominant carrier species were characterized by holographic experiment. The results showed dominant charge carrier species may be changed from electrons to holes when lithium niobate crystal was doped with Mo ions and polarized under the current of 70mA for 30 minutes. It indicated that p-type lithium niobate crystal could be fabricated by Mo-doping and suitably controlling the polarization condition. Mo-doped lithium niobate crystals can be a promising candidate for active components.



Source:IOPscience

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Jun 5, 2019

Melt growth and properties of bulk BaSnO3 single crystals

We present the first-time growth of bulk BaSnO3 single crystals from the melt by direct solidification, their basic electrical and optical properties as well as their structural quality. Our measurement of the melting point (MP) of BaSnO3 amounts to 1855 °C  ±  25 K. At this temperature an intensive decomposition and non-stoichiometric evaporation takes place as the partial pressure of SnO(g) is about 90 times higher than that of BaO(g). X ray powder diffraction identified only the BaSnO3 perovskite phase, while narrow rocking curves having a full width at half maximum of 26 arcsec and etch pit densities below 106 cm−2 confirm a high degree of structural perfection of the single crystals. In this respect they surpass the structural properties of those single crystals that were reported in the literature. The electrical conductivity of nominally undoped crystals depends on the growth conditions and ranges from insulating to medium n-type conductivity. After post-growth annealing in an oxidizing atmosphere undoped crystals are generally insulating. Doping the crystals with lanthanum during growth results in a high n-type conductivity. For a La doping concentration of 0.123 wt.% we measured an electron concentration of 3.3  ×  1019 cm−3 and an electron mobility of 219 cm2 V−1 s−1. Based on optical absorption measurements we determined an energy of 3.17  ±  0.04 eV at 5 K and of 2.99  ±  0.04 eV at 297 K for the indirect band gap of BaSnO3.



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May 28, 2019

High-dimensional generalizations of the kagomé and diamond crystals and the decorrelation principle for periodic sphere packings

In this paper, we introduce constructions of the high-dimensional generalizations of the kagomé and diamond crystals. The two-dimensional kagomé crystal and its three-dimensional counterpart, the pyrochlore crystal, have been extensively studied in the context of geometric frustration in antiferromagnetic materials. Similarly, the polymorphs of elemental carbon include the diamond crystal and the corresponding two-dimensional honeycomb structure, adopted by graphene. The kagomé crystal in d Euclidean dimensions consists of vertex-sharing d-dimensional simplices in which all of the points are topologically equivalent. The d-dimensional generalization of the diamond crystal can then be obtained from the centroids of each of the simplices, and we show that this natural construction of the diamond crystal is distinct from the Dd + family of crystals for all dimensions . We analyze the structural properties of these high-dimensional crystals, including the packing densities, coordination numbers, void exclusion probability functions, covering radii and quantizer errors. Our results demonstrate that the so-called decorrelation principle, which formally states that unconstrained correlations vanish in asymptotically high dimensions, remarkably applies to the case of periodic point patterns with inherent long-range order. We argue that the decorrelation principle is already exhibited in periodic crystals in low dimensions via a 'smoothed' pair correlation function obtained by convolution with a Gaussian kernel. These observations support the universality of the decorrelation principle for any point pattern in high dimensions, whether disordered or not. This universal property in turn suggests that the best conjectural lower bound on the maximal sphere-packing density in high Euclidean dimensions derived by Torquato and Stillinger (2006 Expt. Math. 15 307) is, in fact, optimal.


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May 23, 2019

High purity germanium crystal growth at the University of South Dakota

High-purity germanium crystal growth is challenging work, requiring the control of individual crystal properties such as the impurity distribution, the dislocation density, and the crystalline structure. Currently, we grow high-purity germanium crystals by the Czochralski method in our laboratory in order to understand the details of the growing process, especially for large diameter crystals. In this paper, we report the progress of detector-grade germanium crystal growth at the University of South Dakota.


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May 9, 2019

Synthesis of novel liquid crystal compounds and their blood compatibility as anticoagulative materials

The objective of this study was to synthesize new types of cholesteric liquid crystal compounds and study the anticoagulative properties of their composite membranes. Three kinds of cholesteric liquid crystal compounds were synthesized and characterized by infrared spectroscopy, differential scanning calorimetry and optical polarizing microscope. The polysiloxane, as a substrate, was blended with three liquid crystal compounds and was then used as membranes. The anticoagulative property of different polysiloxane liquid crystal composite membranes was identified by the blood compatibility tests. Three cholesteryl liquid crystals synthesized in this work contained hydrophilic soft chains and presented iridescent texture owned by cholesteric liquid crystals in the range of their liquid crystal state temperature, but only cholesteryl acryloyl oxytetraethylene glycol carbonate was in the liquid crystal state at body temperature. When liquid crystals were blended with polysiloxane to form polysiloxane/liquid crystal composite membranes, the haemocompatibility of these membranes could be improved to some extent. The blood compatibility of composite membranes whose hydrophilic property was the best was more excellent than that of other composite membranes, fewer platelets adhered and spread, and showed little distortion on the surface of materials.


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Apr 30, 2019

Growth process of pentacene crystals obtained by physical vapor transport technique

The growth process of pentacene crystals obtained by the physical vapor transport technique is discussed on the basis of the results of optical microscopy and X-ray topography. At the initial stage of physical vapor growth, many pentacene whisker crystals were grown. After that, the morphology of the whiskers developed to dendrites. Then, the whisker crystals themselves became the main branches of the dendritic crystals formed at the initial stage, and many side branches were created from the lateral surfaces of the whisker crystals. Some of the dendritic crystals continued to develop to form plateletlike crystals with a dendritic configuration. Regarding the plateletlike crystals with a parallelogram configuration, traces of the dendrites are thought to sometimes be left inside these plateletlike crystals on the basis of the results of optical microscopy and X-ray topography. Plateletlike crystals with a parallelogram configuration are considered to sometimes grow via the formation of dendritic crystals.


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Apr 25, 2019

Angular dependences of the luminescence and density of photon states in a chiral liquid crystal

Luminescence spectra of a laser dye-doped chiral liquid crystal have been studied in a wide range of angles (up to 60°) to the axis of its helical structure using a semicylindrical quartz prism, which made it possible to observe the shift and evolution of the photonic band gap in response to changes in angle. Using measured spectra and numerical simulation, we calculated the spectral distributions of the density of photon states in such a cholesteric crystal for polarised and unpolarised light, which characterise its structure as that of a chiral one-dimensional photonic crystal.


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Apr 18, 2019

Depth-of-interaction measurement in a single-layer crystal array with a single-ended readout using digital silicon photomultiplier

We present the first experimental evaluation of a depth-of-interaction (DOI) positron emission tomography (PET) detector using a digital silicon photomultiplier (dSiPM). To measure DOI information from a mono-layer array of scintillation crystals with a single-ended readout, our group has previously proposed and developed a new method based on light spread using triangular reflectors. Since this method relies on measurement of the light distribution, dSiPM, which has a fully digital interface, has several merits for our DOI measurement. The DOI PET detector comprised of a dSiPM sensor (DPC-3200-22-44) coupled with a 14   ×   14 array of 2 mm  ×  2 mm  ×  20 mm unpolished LGSO crystals. All crystals were covered with triangular reflectors. To obtain a good performance of the DOI PET detector, several parameters of detector were selected as a preliminary experiment. Detector performance was evaluated with the selected parameters and the optimal experimental setup, and a DOI measurement was conducted by irradiating the crystal block at five DOI positions spaced at intervals of 4 mm. Maximum-likelihood estimation was employed for DOI positioning and the optimal DOI estimation scheme was also investigated in this study. As a result, the DOI PET detector showed clear crystal identification. The energy resolution (full-width at half-maximum (FWHM)) averaged over all depths was 10.21%  ±  0.15% at 511 keV, and time resolution averaged over all depths was 1198.61   ±   39.70 ps FWHM. The average DOI positioning accuracy for all depths was 74.22%  ±  6.77%, which equates to DOI resolution of 4.67 mm. Energy and DOI resolutions were uniform over all crystal positions except for the back parts of the array. Furthermore, additional simulation studies were conducted to verify the results of our DOI measurement method that is combined with dSiPM technology. In conclusion, our continuous DOI PET detector coupled with dSiPM is a promising PET/MRI detector with DOI-encoding capability.

single crystal.


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Apr 9, 2019

Modeling Czochralski growth of oxide crystals for piezoelectric and optical applications

Numerical modeling is applied to investigate the impact of crystal and crucible rotation on the flow pattern and crystal-melt interface shape in Czochralski growth of oxide semi-transparent crystals used for piezoelectric and optical applications. Two cases are simulated in the present work: the growth of piezoelectric langatate (LGT) crystals of 3 cm in diameter in an inductive furnace, and the growth of sapphire crystals of 10 cm in diameter in a resistive configuration. The numerical results indicate that the interface shape depends essentially on the internal radiative heat exchanges in the semi-transparent crystals. Computations performed by applying crystal/crucible rotation show that the interface can be flattened during LGT growth, while flat-interface growth of large diameter sapphire crystals may not be possible.


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Apr 3, 2019

Pyroelectric generation of 2D spatial soliton sets in a bulk of lithium niobate crystal

The generation of two-dimensional bright spatial soliton sets in lithium niobate sample has been experimentally demonstrated at light wavelength of 532 nm, contribution of pyroelectric effect into nonlinear optical response of the crystal, and spatial modulation of one-dimensional beam along direction normal to the crystal optical axis. Diameters of soliton beams and channel waveguides formed within the crystal bulk by these solitons are near to 20 μm at light polarization corresponding to extraordinary wave of the crystal.


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Mar 25, 2019

Orientation-dependent crystal instability of gamma-TiAl in nanoindentation investigated by a multiscale interatomic potential finite-element model

The anisotropic mechanical behavior of γ-TiAl alloys has been observed and repeatedly reported, but the effect of crystallographic orientations on the crystal instability of γ-TiAl is still unclear. In this paper, the orientation-dependent crystal instability of γ-TiAl single crystals was investigated by performing nanoindentation on different crystal surfaces. All the nanoindentations are simulated using an interatomic potential finite-element model (IPFEM). Simulation results show that the load–displacement curves, critical indentation depth and critical load for crystal instability as well as indentation modulus, are all associated with surface orientations. The active slip systems and the location of crystal instability in five typical nanoindentations are analyzed in detail, i.e. the (0 0 1), (1 0 0), (1 0 1), (1 1 0) and (1 1 1) crystal surfaces. The predicted crystal instability sites and the activated slipping systems in the IPFEM simulations are in good agreement with the dislocation nucleation in molecular dynamics simulations.



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Mar 18, 2019

Laser recovery of grinding-induced subsurface damage in the edge and notch of a single-crystal silicon wafer

The edges and notches of silicon wafers are usually machined by diamond grinding, and the grinding-induced subsurface damage causes wafer breakage and particle contamination problems. However, the edge and notch surfaces have large curvature and sharp corners, thus it is difficult to be finished by chemo-mechanical polishing. In this study, a nanosecond pulsed Nd:YAG laser was used to irradiate the edge and notch of a boron-doped single-crystal silicon wafer to recover the grinding-induced subsurface damage. The reflection loss and the change of laser fluence when irradiating a curved surface were considered, and the damage recovery behavior was investigated. The surface roughness, crystallinity, and hardness of the laser recovered region were measured by using white light interferometry, laser micro-Raman spectroscopy, and nanoindentation, respectively. The results showed that after laser irradiation the damaged region was recovered to a single-crystal structure with nanometric surface roughness, and the surface hardness was also improved. This study demonstrates that laser recovery is a promising post-grinding process for improving the surface integrity of the edge and notch of silicon wafers.



Source:IOPscience

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Mar 12, 2019

Liquid crystal director fields in micropores of photonic crystals

Photonic crystals made of macroporous silicon and liquid crystals are optical materials with a tunable dispersion relation in the infrared frequency range. Three-dimensional structures can be realized by manufacturing a two-dimensional array of pores that show a spatially periodic variation of the pore diameter. The director fields of cholesteric liquid crystals confined to such modulated pores show different topologies that depend essentially on the ratio between the helix pitch of the liquid crystal and the pore size.



Source:IOPscience

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Mar 5, 2019

Study on the Properties of High Purity Germanium Crystals

In the crystal growth lab of South Dakota University, we are growing high purity germanium (HPGe) crystals and using the grown crystals to make radiation detectors. As the detector grade HPGe crystals, they have to meet two critical requirements: an impurity level of ~109 to 10 atoms /cm3 and a dislocation density in the range of ~102 to 104 / cm3. In the present work, we have used the following four characterization techniques to investigate the properties of the grown crystals. First of all, an x-ray diffraction method was used to determine crystal orientation. Secondly, the van der Pauw Hall effect measurement was used to measure the electrical properties. Thirdly, a photo-thermal ionization spectroscopy (PTIS) was used to identify what the impurity atoms are in the crystal. Lastly, an optical microscope observation was used to measure dislocation density in the crystal. All of these characterization techniques have provided great helps to our crystal activities



Source:IOPscience

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Feb 18, 2019

Simulation study of PET detector limitations using continuous crystals

Continuous crystals can potentially obtain better intrinsic detector spatial resolution compared to pixelated crystals, additionally providing depth of interaction (DoI) information from the light distribution. To achieve high performance sophisticated interaction position estimation algorithms are required. There are a number of algorithms in the literature applied to different crystal dimensions and different photodetectors. However, the different crystal properties and photodetector array geometries have an impact on the algorithm performance. In this work we analysed, through Monte Carlo simulations, different combinations of realistic crystals and photodetector parameters to better understand their influence on the interaction position estimation accuracy, with special emphasis on the DoI. We used an interaction position estimation based on an analytical model for the present work. Different photodetector granulation schemes were investigated. The impact of the number of crystal faces readout by photodetectors was studied by simulating scenarios with one and two photodetectors. In addition, crystals with different levels of reflection and aspect ratios (AR) were analysed. Results showed that the impact of photodetector granularity is mainly shown near the edges and specially in the corners of the crystal. The resulting intrinsic spatial resolution near the centre with a 12 × 12 × 10 mm3 LYSO crystal was 0.7–0.9 mm, while the average spatial resolution calculated on the entire crystal was 0.77 ± 0.18 mm for all the simulated geometries with one and two photodetectors. Having front and back photodetectors reduced the DoI bias (Euclidean distance between estimated DoI and real DoI) and improved the transversal resolution near the corners. In scenarios with one photodetector, small AR resulted in DoI inaccuracies for absorbed events at the entrance of the crystal. These inaccuracies were slightly reduced either by increasing the AR or reducing the amount of reflected light, and highly mitigated using two photodetectors. Using one photodetector, we obtained a piecewise DoI error model with a DoI resolution of 0.4–0.9 mm for a 1.2 AR crystal, and we observed that including a second photodetector or reducing the amount of reflections reduced the DoI bias but did not significantly improve the DoI resolution. Translating the piecewise DoI error model obtained in this study to image reconstruction we obtained a spatial resolution variability of 0.39 mm using 85% of the FoV, compared to 2.59 mm and 1.87 mm without DoI correction or with a dual layer system, respectively.


Source:IOPscience

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Feb 12, 2019

Simulation of quantum magnetism in mixed-spin systems with impurity-doped ion crystals

We propose the realization of linear crystals of cold ions that contain different atomic species for investigating quantum phase transitions and frustration effects in spin systems beyond the commonly considered case of $s=\frac {1}{2}$ . Mutual spin–spin interactions between ions can be tailored via the Zeeman effect by applying oscillating magnetic fields with strong gradients. Further, collective vibrational modes in the mixed ion crystal can be used to enhance and to vary the strength of spin–spin interactions and even to switch the nature of the interacting forces from a ferro- to an antiferromagnetic character. We consider the behavior of the effective spin–spin couplings in an ion crystal of spin-1/2 ions doped with high-magnetic-moment ions with spin S = 3. We analyze the ground state phase diagram and find regions with different spin orders including ferrimagnetic states. In the most simple nontrivial example, we deal with a linear {Ca+,Mn+,Ca+} crystal with spins of $\{\frac {1}{2},3,\frac {1}{2}\}$ . To demonstrate feasibility with current state-of-the-art experiments, we discuss how quantum phases might be detected using a collective Stern–Gerlach effect of the ion crystal and high-resolution spectroscopy. Here, the state-dependent laser-induced fluorescence of the indicator spin-1/2 ion, of species 40Ca+, is used to reveal also the spin state of the simulator spin-3 ions, 50Mn+, which does not possess suitable levels for optical excitation and detection.


Source:IOPscience

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Feb 1, 2019

Enhanced mobility in organic field-effect transistors due to semiconductor/dielectric interface control and very thin single crystal

A perfect organic crystal while keeping high quality semiconductor/dielectric interface with minimal defects and disorder is crucial for the realization of high performance organic single crystal field-effect transistors (OSCFETs). However, in most reported OSCFET devices, the crystal transfer processes is extensively used. Therefore, the semiconductor/dielectric interface is inevitably damaged. Carrier traps and scattering centers are brought into the conduction channel, so that the intrinsic high mobility of OSCFET devices is entirely disguised. Here, very thin pentacene single crystal is grown directly on bare SiO2 by developing a 'seed-controlled' pentacene single crystal method. The interface quality is controlled by an in situ fabrication of OSCFETs. The interface is kept intact without any transfer process. Furthermore, we quantitatively analyze the influence of crystal thickness on device performance. With a pristine interface and very thin crystal, we have achieved the highest mobility: 5.7 cm2 V−1 s−1—more than twice the highest ever reported pentacene OSCFET mobility on bare SiO2. This study may provide a universal route for the use of small organic molecules to achieve high performance in lamellar single crystal field-effect devices.



Source:IOPscience

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Jan 21, 2019

The density and compositional analysis of titanium doped sapphire single crystal grown by the Czocharlski method


Titanium doped sapphire (Ti:Al2O3) crystal has attracted attention not only as beautiful gemstones, but also due to their applications as high power laser action. It is very important crystal for tunable solid state laser. Ti:Al2O3 crystals have been success grown using the Czocharlski method with automatic diameter control (ADC) system. The crystals were grown with different pull rates. The structure of the crystal was characterized with X-Ray Diffraction (XRD). The density of the crystal was measurement based on the Archimedes principle and the chemical composition of the crystal was confirmed by the Energy Dispersive X-ray (EDX) Spectroscopy. The XRD patterns of crystals are showed single main peak with a high intensity. Its shows that the samples are single crystal. The Ti:Al2O3 grown with different pull rate will affect the distribution of the concentration of dopant Ti3+ and densities on the sapphire crystals boules as well on the crystal growth process. The increment of the pull rate will increase the percentage distribution of Ti3+ and on the densities of the Ti:Al2O3 crystal boules. This may be attributed to the speed factor of the pull rate of the crystal that then caused changes in the heat flow in the furnace and then causes the homogeneities is changed of species distribution of atoms along crystal.


Source:IOPscience

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Jan 14, 2019

High-Temperature Hardness of Bulk Single-Crystal AlN


The hardness of single-crystal aluminum nitride (AlN) 0.5-mm-thick wafers was measured at elevated temperatures and compared with that of other semiconductors. A Vickers indentation method was used to determine the hardness under an applied load of 0.5–5 N in the temperature range 20–1400°C. The average hardness was measured as 17.7 GPa at room temperature. AlN exhibits a hardness higher than that of GaN in the entire temperature range investigated. A high mechanical stability for AlN at high temperatures is deduced.


Source:IOPscience

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Jan 9, 2019

The impact of subsurface damage on the fracture strength of diamond-wire-sawn monocrystalline silicon wafers

We describe a multi-diamond-wire saw for cutting monocrystalline silicon bricks into thin (120 µm) and thick (200 µm) wafers and label as fresh- and worn-wire sides. While almost no difference was found in the fracture stress of the thick wafers(200 µm) cut from either side, the thin (120 µm) wafers showed a lower fracture stress in those from the fresh-wire side compared to the worn-wire side. This is a remarkable result when wafers are sawn with conventional diamond wire. On the contrary, wafers sawn with improved diamond wire (100d-M6/12) showed a higher fracture stress compared to those cut with conventional diamond wire (100d-M8/16), for both the fresh- and worn-wire sides. Observing the subsurface areas of wafers by micro-Raman spectroscopy, we succeeded in quantifying the defective silicon fraction as the Raman crystallinity factor (Φc). We found that wafers having a higher fracture strength had a larger Φc.



Source:IOPscience

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