Il Soo Kim

Erosion characteristics of silicon nitride ceramics

Abstract Silicon nitride ceramics with three different microstructures were prepared using three different silicon nitride powders and β-silicon nitride whiskers in order to study their erosion characteristics. Samples with a coarse microstructure were prepared by using a commercially available silicon nitride powder seeded with the whiskers. Samples with fine microstructures were obtained using fine silicon nitride powders that had been milled in toluene or in distilled water. The powder milled in toluene gained both carbon and oxygen during milling. Heating the powder at 1673 K for 10 h in a flowing nitrogen atmosphere reduced carbon and oxygen contents to the level of as-received powder. The powder milled in distilled water gained a significant amount of oxygen during milling, and silicon oxynitride (Si2N2O) was detected by X-ray diffraction analysis (XRD) from the sample sintered with it. The sample sintered with the powder milled in distilled water showed the finest microstructure and the lowest erosion rate among the samples. Erosion occurred mainly by dislodgment of grains.

SiC Crystal Growth by Sublimation Method with Modification of Crucible and Insulation Felt Design

SiC crystal boules with different shapes were prepared using sublimation physical vapor transport technique (PVT) and then their crystal quality was systematically investigated. The temperature distribution in the growth system and the crystal shape were controlled by modification of crucible and insulation felt design, which was successfully simulated using “Virtual Reactor” for flat structure design and concave structure design. The SiC polytype proved to be the n-type 6H-SiC from the typical absorption spectrum of SiC crystal. The defect density of SiC crystal boules with concave structure was slightly lower than that of flat structure and the crystal quality of SiC crystal boules with both flat structure and concave structure was significantly improved as the SiC crystal grows during the PVT methods.

High Quality SiC Crystals Grown by the Physical Vapor Transport Method with a New Crucible Design

SiC single crystal ingots grown by sublimation physical vapor transport (PVT) technique were prepared and then the SiC crystal quality with varying crucible design employing a guide tube and tantalum foil was systematically investigated. The growth rate of 2-inch SiC crystal grown by these crucible designs was about 0.3 mm/hr. The n-type and p-type 2”-SiC single crystals exhibiting the polytype of 6H-SiC were successfully fabricated. The doping concentration level of below ~1017/cm3 was extracted from the absorption spectrum and Hall measurement. The densities of micropipes and inclusions in SiC crystal boules grown using the graphite/Ta foil double layer guide tube were significantly decreased. Finally we improved crystal quality through the introduction of new crucible design.

Initial Stage Modification for 6H-SiC Single Crystal Grown by the Physical Vapor Transport (PVT) Method

Two SiC single crystal ingots were prepared using sublimation PVT techniques through the different process procedure and then their crystal quality was systematically compared, because the present research was focused to improve the quality of SiC crystal by modifying the initial stage of the PVT growth. Before the main growth step for growing SiC bulk crystal, initial stage period where growth rate was kept to relatively low rate of <10μm/h was introduced to conventional process procedure. N-type 2”-SiC single crystals exhibiting the polytype of 6H-SiC was successfully fabricated. As compared to the characteristics of SiC crystal grown using the conventional schedule, the quality of SiC crystal grown with modifying the initial stage was significantly improved, exhibiting decrease of defect formation such as micropipe and polytype formation.

Hydrogen Effect on SiC Single Crystal Prepared by the Physical Vapor Transport Method

We investigated the effects of hydrogen addition to the growth process of SiC single crystal using sublimation physical vapor transport (PVT) techniques. Hydrogen was periodically added to an inert gas for the growth ambient during the SiC bulk growth. Grown 2”-SiC single crystals were proven to be the polytype of 6H-SiC and carrier concentration levels of about 1017/cm3 was determined from Hall measurements. As compared to the characteristics of SiC crystal grown without using hydrogen addition, the SiC crystal grown with periodically modulated hydrogen addition definitely exhibited lower carrier concentration and lower micropipe density as well as reduced growth rate.

An Inserted Epitaxial Layer for SiC Single Crystal Growth by the Physical Vapor Transport Method

SiC single crystal ingots were prepared onto different seed material using sublimation PVT techniques and then their crystal quality was systematically compared. In this study, the conventional SiC seed material and the new SiC seed material with an inserted SiC epitaxial layer on a seed surface were used as a seed for SiC bulk growth. The inserted epitaxial layer was grown by a sublimation epitaxy method called the CST with a low growth rate of 2μm/h. N-type 2”-SiC single crystals exhibiting the polytype of 6H-SiC were successfully fabricated and carrier concentration levels of below 1017/cm3 were determined from the absorption spectrum and Hall measurements. The slightly higher growth rate and carrier concentration were obtained in SiC single crystal ingot grown on new SiC seed materials with the inserted epitaxial layer on the seed surface, maintaining the high quality.

Process and Crucible Modification for Growth of High Doped 4H-SiC Crystal with Larger Diameter

The present research was focused to investigate various process parameters influenced on the large 4H-SiC crystal growth on a 6H-SiC seed by PVT method. The crucible diameter along horizontal axial direction and inserted graphite ring was modified to change the growth parameter like the temperature gradient. In the initial stage of growth, foreign polytypes such as 6H/4H were observed on 6H-SiC seed, indicating the growth temperature to be unstable on crystal surface. However, from the middle of growth step, 4H-SiC was successfully formed in the ingot with the modification of growth pressure and a SIMS analysis confirmed the high doping concentration in grown 4H-SiC crystal.

Effect of the Seed Polarity for High Quality 4H-SiC Crystal Grown on 6H-SiC Seed by PVT Method

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.

The Effect of Process Parameters on 4H-SiC Single Crystal Grown by a PVT Method

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.

Non-Polar SiC Crystal Growth with m-Plane(1-100) and a-Plane(11-20) by PVT Method

The present research was focused to produce 2 inch wafers from small rectangular seeds and to investigate the quality of non-polar SiC substrates grown by a conventional PVT method. The non-polar SiC seeds were prepared by cutting along <0001> direction of 6H-SiC crystal grown on (0001) basal plane. As SiC ingot grows, many defects in connected region were gradually diminished. While the full width at half maximum (FWHM) values of m-plane SiC substrate measured along a-direction and c-direction were 60 arcsec and 70 arcsec, respectively, and the FWHM values of a-plane SiC substrate measured along m-direction and c-direction were 27 arcsec and 31 arcsec respectively. The stacking faults lying in the basal plane can be detected by molten KOH etching as linear etch pits extending along <0001> on the (11-20) surface and the carrier concentration was observed by Raman spectrum.