Papers by Keyword: Growth Rate

Abstract: To prepare bulk single-crystal REBCO superconductors by the new single-direction growth method (SDMG: Single-Direction Melt-Growth), it is necessary to produce a large-area seed of high quality, for example, based on EuBCO. Since the samples prepared by the SDMG method copy the structure of the seed, for the production of large-area seeds it is necessary to optimize the time-temperature regime in order to grow seeds with a suitable structure and composition and minimize structural defects (limiting the amount of subgrains and others). A higher growth rate was used in comparison with the standard growth rates used to produce EuBCO seeds of larger dimensions. The increased growth rate in the crystal growth window reduces the outflow of the melt from the sample, and thus it is possible to achieve a single-crystal sample in the entire volume of the precursor. The samples were produced at different growth rates: 1; 2; 3 and 5 °C/h. The microstructure of the samples was studied by polarized light microscopy and scanning electron microscopy. The size and distribution of Eu211 particles in the sample volume and the subgrain structure were studied on the fabricated samples.

Abstract: The thermoelastic stress, mechanical properties and defect content of bulk 4H n-type SiC crystals were investigated following adjustments to the PVT growth cell configuration that led to a 40% increase in growth rate. The resulting 150 mm wafers were compared with wafers produced from a control process in terms of wafer bow and warp, and dislocation density. Wafer shape was found to be comparable among the processes, indicating minimal impact on internal stress. Threading edge and threading screw dislocation densities increased and decreased, respectively, while basal plane dislocation densities were unaffected by the increase in growth rate. Loss of wafer planar stability was observed in certain cases. The elastic modulus was measured to be in the range of approximately 420-450 GPa for selected stable and unstable wafers, and was found to correspond to resistivity.

Abstract: Stress-controlled fatigue tests were conducted on round-bar specimens to understand the fatigue behavior of precipitate-strengthened Cu–6Ni–1.5Si alloy. The cracks were initiated at the grain boundaries, followed by growth along the crystallographic slip planes in the adjacent grains. The crack growth data of plain specimens exhibited a large scatter, resulting in a difficulty of the measurement of crack growth rate. To evaluate the small-crack growth rate of the alloy, the plain specimens with a small blind hole as the crack starter were fatigued. The crack growth rate of small cracks from the hole was uniquely determined by a term σ_a ⁿl and the material constant, n, was 5.3. The term σ_a ⁿl with n = 5.3 was applied to the plain specimen, showing good applicability of the term to small cracks in the plain specimen.

Abstract: The effect of varying growth rate on the formation of defects in homo-epitaxially grown cubic silicon carbide (3C-SiC) is studied. Three growth rates are considered (30, 60 and 90 μm/hr) demonstrating that as the growth rate increases the density of point defects, as demonstrated by photo- luminescence, and stacking faults (SFs), as measured by a KOH etching procedure, increase. Scanning transmission electron microscopy images demonstrate generation, annihilation and closure of SFs as a function film thickness. High resolution X-ray diffraction is used to uncover the higher quality of homo-epitaxial with respect hetero-epitaxial films through the examination of the sample mosaicity and SF density.

Abstract: Tantalum carbide (TaC) coating, produced in an ultrahigh temperature chemical vapor deposition (CVD) process, exhibited high thermal and chemical stabilities, low emissivity, and high purity. The present research investigated through modeling the TaC emissivity and reactivity impacts on the temperature, C/Si ratio and growth rate in SiC physical vapor transport (PVT) process. The TaC coating effect was further validated by SiC crystal shape and quality through a series of growth experiments. As our study revealed, TaC coating can act as a protection and isolation layer to extend graphite component life, improve radial temperature uniformity, maintain SiC sublimation stoichiometry, suppress impurity migration, and reduce energy consumption. Ultimately a TaC-coated graphite crucible set is expected to improve SiC PVT process control and product quality.

Abstract: In this work many steps concerning the epitaxial layer growth on 4H-SiC are studied, evaluated and optimized to obtain high quality 4H-SiC epitaxy. The processes evaluated have been studied on a Hot Wall CVD reactor. The first step related to the substrate surface etching has been tuned by choosing the H2 flow, temperature and process time at which most of defects (mainly stacking faults) are not propagated. Then, the buffer layer step has been optimized by increasing the thickness at which an effective reduction of defect density and an improved electrical performance of power devices have been detected. Also, during the buffer layer growth a strong dependence between basal plane dislocations propagation and the growth rate has been observed. A crucial step carefully studied has been the drift layer growth. It was optimized by increasing the growth rate (13<GR<15µm/h) that results in a lower defectiveness, good thickness and doping uniformity. Final stage concerning the cooling of the process has been strongly revisited. A significant decreasing of the morphological defects (carrots, pits) and stacking faults has been observed by slowing the cool down process (~ 25 °C/min).

Abstract: The modified crucible in the top seeded solution growth (TSSG) method was proposed to get the stable growth condition and the enhanced growth rate. The temperature gradient and the distribution of dissolved carbon in the solvent were obtained by numerical approaches. Compared simulation results and experiment, we investigated the region where is the most of dissolved carbon and it is supplied to the seed through what path. The cross-sectional samples of crucibles and grown SiC crystals were systematically analyzed to investigate the effect of the crucible modification on SiC crystal growth.

Abstract: Stacking Faults (SFs) are the main defect of 3C-SiC material and in this work a detailed study of this typology of defect is presented. We studied the behavior of SFs with High Resolution XRD and STEM analysis. The homo-epitaxial growth was proposed as a solution for the reduction of SFs density in 3C-SiC material and the influence of the growth condition on the SFs density was studied. The knowledge of the mechanism of SFs reduction is crucial for the development of a high quality material for devices fabrication.

Abstract: Si_0.56Cr_0.4M_0.04 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rh, and Pd) solvents were investigated to identify new multi-component materials in which carbon is highly soluble, because solubility is a key parameter in solution growth of SiC. The solubility of carbon in Si_0.56Cr_0.4Co_0.04 was 8.37 at%, the highest value among the tested multi-component materials. This is about 2.5 times the solubility of carbon in Si_0.6Cr_0.4. These results show that addition of a small amount of a transition metal enhanced the solubility of carbon in Si_0.6Cr_0.4. This technique for determining carbon solubility is effective for investigating crystal growth using solvents with several components, for which complex thermodynamic calculations are necessary.

Abstract: A series of potassium dihydrogen phosphate (KDP) crystals are grown from various saturation temperature solutions by “point seed” technique. The prismatic faces growth rate of KDP crystals are measured by laser polarization system. The micromorphology of the steps on (100) face are comparatively researched by atomic force microscope (AFM) systematically. The results show that step bunching decreases with the supersaturation rise from 0.01 to 0.03 when the saturation temperature is around 35°C, the bunching increases in the case around 45°C, 55°C and 65°C, which is more significant around 65°C. The terrace width has the similar trend, it increases with the rise of supersaturation at different temperatures. The step slope has various trend with the change of supersaturation at various temperature. The normal growth rate and tangential velocity both increase with the rise of the supersaturation. In addition, at the same supersaturation, the higher the temperature, the faster the growth rate.