Papers by Keyword: Sublimation

Abstract: To avoid unpredictable time to float due to variable pH of the gastric fluid in each subject, sublimation technique is the new interesting approach to prepare non-effervescent floating drug delivery system (FDDS). In this study, a floating matrix tablet using borneol as sublimable agent was developed. The system consists of drug, matrix-forming polymer and sublimable agent. The highly porous and low-density floating matrix tablet was obtained when borneol was sublimated. The floating matrix tablets immediately floated and maintained buoyancy over 8 h. The sublimation of borneol increased with increasing sublimation temperature and incubation time. Increasing amount of sublimable agent exhibited lower density of the floating tablet because of higher porous structure. The results demonstrated that boneol was one of good candidates to be used as sublimable agent for non-effervescent floating tablets. However, physical properties of the matrix floating tablets were needed to be concerned.

Abstract: We have developed RAF (Repeated a-face) growth method which is high quality bulk crystal growth technology [1, 2]. A block crystal more than 150 mm square size was produced by the RAF growth method. Since c-face growth crystal was produced on the seed obtained from the block crystal, high quality 150mm 4H-SiC wafer was achieved. This paper reports the results of the quality evaluation.

Abstract: In this paper we present a concept on the defect generation and annihilation during the homoepitaxial growth step of cubic silicon carbide by sublimation epitaxy on templates grown by chemical vapor deposition on silicon substrates. Several structural defects like stacking faults, twins and star defects show opposite evolution from the template layer into the sublimation grown material. While single planar defects tend to annihilate with increasing layer thickness, the defect clusters assigned to the star defects are enlarging. These issues contribute to a balance of how to achieve the best possible quality on thick layers.

Abstract: Transport properties of multigraphene layers on 6H-SiC substrates was studied. It was found that the curves of magnetoresistance and Shubnikov- de Haas oscillations shown the features, typical for single-layred graphene. The low temperature resistance demonstrated an increase with temperature increase, which also corresponds to a behavior typical for single-layered graphene (antilocalization). However at higher temperatures the resistance decreased with an increase of temperature, which corresponds to a weak localization. We believe that the observed behavior can be explained by a parallel combination of contributions to the conductivity of single-layered graphene and of multigraphene.

Abstract: Large-diameter SiC single crystals are grown at II-VI by the sublimation technique. 100mm substrates of semi-insulating 6H SiC and n-type 4H SiC are produced as commercial products; in development, diameter expansion to 125mm has been achieved. Over the last two years, significant improvements have been made in crystal quality. The values of FWHM of x-ray rocking curves are typically 20-40 arc-seconds for 6H SI wafers and 12-30 arc-seconds for 4H n+ wafers. Micropipe density is less than 3 cm-2, and less than 0.1 cm-2 in best substrates. Electrical resistivity of SI substrates is, typically, of 10¹¹ Ω•cm or above. For 4H n+ substrates, the typical dislocation density is about 9×10³ cm^-2 and the typical BPD density is less than 1×10³ cm^-2.

Abstract: The wide size distribution of the hexagonal etch pit of screw dislocations (SD) in 4H-SiC wafer was found in spite of the narrow size distribution of the SD pit in epitaxial film. Calculation on the basis of the strain energy equation indicated that etch pit size depends on the Burgers vector and dislocation tilt. Size variation of SD etch pits in 4H-SiC wafer fabricated by sublimation method is explained to be caused by the dislocation tilt by observing the sizes and the positions of etch pits from the surface of the epitaxial film to the inside of 4H-SiC wafer. The SDs in 4H-SiC wafer fabricated by sublimation method propagate to c-axis direction in macroscopic but changing tilt in microscopic.

Abstract: The single crystal ingots by using a sublimation technique were grown on 6H-SiC dual-seed crystals with opposite face polarities and then SiC crystal wafers sliced from the SiC ingot were systematically investigated to find out the polarity dependence of the crystal quality. The growth rate of the SiC crystal grown in this study was about 0.2mm/hr. N-type 2’’ SiC crystals exhibiting the 4H- and 6H-SiC polytype were successfully fabricated on C-face and Si-face, respectively. The incorporation of nitrogen donors in the SiC crystals grown on the C-face seed crystal was exhibited to be higher than in SiC crystals grown on a Si-face crystal. When the SiC crystal ingot proceeded to grow, the SiC crystal region grown on the C-face seed crystal was enlarged compared to the SiC crystal region on the Si-face seed crystal.

Abstract: Extensive study of various process parameters to influence on the growth of 4H-SiC crystal has been carried out using the transformation of the 6H-SiC seed by a PVT method. The axial temperature gradients were increased throughout increasing the crucible length along growth direction in order to enhance the growth rate and transformed crystal yield. The N2/Ar gas ratio used during the crystal growth related with carrier concentration/mobility of grown crystal. In the initial stage of growth, foreign polytypes such as 6H/15R were observed on 6H-SiC seed crystal but 4H crystals were entirely grown after the process optimization. While the typical absorption spectrum of SiC seed crystal indicated that the SiC polytype was the 6H-SiC with fundamental absorption energy of about 3.02eV, absorption spectrum of grown SiC crystal exhibited 4H-SiC with fundamental absorption energy of about 3.26eV. The entirely transformed SiC region exhibited lower micropipe density than 6H/4H transition region.

Abstract: The initial stage of heteroepitaxial growth of 3C-SiC and homoepitaxial growth of 6H-SiC on nominal 6H-SiC on-axis substrates has been studied. Before 3C-SiC starts to nucleate, 6H-SiC grows in a step-flow growth mode due to a slight off-orientation of the substrate surface already at about 1500oC. In the 1650-1700oC temperature interval 3C-SiC nucleates as 2D islands. A distance away from the 3C-SiC island 6H-SiC grows in step-flow mechanism. In the vicinity of the 3C-SiC islands the 6H-SiC growth steps start to change direction and even split into two steps with the equal height of 0.5 nm, which is approaching the unit cell size of cubic SiC. When the supersaturation is lower in comparison with the conditions for 3C-SiC growth, there is only formation of 6H-SiC, i.e. homoepitaxial growth. The growth mode of 6H-SiC is dependent on temperature. At the lowest temperature there is spiral growth while at higher temperature 2D nucleation is preferred.

Abstract: The co-doping of nitrogen and aluminum has been studied in the sublimation epitaxy growth process. It is shown that the doping may be tuned from n-type to p-type by effect of substrate doping, growth face and volume of the growth crucible. The co-doped layers show a nearly ideal I V characteristic and luminescence at room temperature.