A Study Of Preparation Of Silicon Carbide Crystal By Physical Vapor Transport Method

The breakdown field strength of silicon carbide(SiC)is so high that it makes SiC have broad applications in high power and high temperature devices.Studies of SiC single crystal preparation are a hot spot in the field of semiconductor materials research.SiC crystal growth technology has made some progress.However,there still exists obstacles to achieve large-scale productions.Currently,physical vapor transport(PVT)method is the primary technique to produce SiC single crystal.The prepared SiC single crystal has many defects,such as micropipe,inclusion,dislocation,etc..Purity and particle size of raw materials used for SiC single crystal growth have huge impact on the final products.In this dissertation,growth process of SiC single crystal was studied and defects in the prepared SiC single crystal were analyzed.SiC was synthesized with 99.99% high purity Si powder and C powder at 1800? and purified later.Experiment results indicated that 800? was the best temperature to remove C impurities in the prepared SiC powder and didn't produce substantial Si O2 impurities.When oxidation temperature was higher than 800?,a large number of Si O2 impurities will appear.The Si O2 impurities was removed with hydrofluoric acid to obtain high purity SiC raw material.The XRD analysis of the purified powder showed no other phases.EDS elemental analysis didn't appear other elements except Si and C,and the Si/C atomic ratio was about 1:1.The best graphite crucible position H=60 mm was obtained by adjusting the relative position of the graphite crucible.Also the maximum temperature gradient and growth rate can be reached in this position.The optimum process parameters were determined as follows: growth temperature 2300 ?,temperature gradient 1.3?·mm-1,growth pressure 20 Torr,gas flow 30 sccm.Growth rate under this process parameters was 43.91g·h-1 and average grain size was 5mm,the maximum grain size was 8mm.SiC crystal growth was carried out by seeding method with above optimum process parameters and SiC ingot with size of 10 cm was obtained.The central part of SiC ingot was single crystal,its size was about 7cm.The edge of ingot was polycrystalline SiC.SiC single crystal growth rate is 38.71g·h-1,SiC crystal will form 4H-SiC.Then,defect analysis and quality characterization of the grown crystal were investigated.There existed microtubules,inclusions,mosaics,voids and dislocations in the prepared SiC crystal.The FWHM of the 4H-SiC(0004)plane single crystal rocking curve was 165.96 arcsec.The Raman characterization indicated the presence of tensile stress in the crystal.Photoluminescence peaks were attributed to carbon vacancies VC and nitrogen impurities.In this study,SiC powder which was suitable for SiC crystal growth was prepared.The self-nucleation growth process of SiC crystal was studied and optimum growth process was obtained.The 7cm diameter 4H-SiC single crystal was epitaxially grown by 3-inch SiC single crystal seed under optimum growth process.Defects in prepared SiC single crystal were analyzed and crystal quality of SiC crystal was characterized.