Study On PVT Method 6-8 Inch Low Defect 4H-SiC Crystal Growth Based On Numerical Simulation

Silicon carbide（SiC）has outstanding characteristics such as high thermal conductivity,high breakdown field strength,high saturation electron drift rate,high bonding energy,and radiation resistance,and has broad application prospects in emerging fields such as electric vehicles and photovoltaics.The Physical Vapor Transfer（PVT）method is the mainstream process for SiC crystal growth,with the advantages of simple principles and relatively mature technology.However,the inability to directly observe high temperature conditions limits the depth of research.The evolution of temperature and flow fields in crystal growth cavities and their mechanisms of action on the growth process are not yet clear.Crystal growth is prone to spontaneous nucleation,forming defects such as polycrystalline,polytype,inclusions,micropipes,and dislocations,resulting in high defect density and low yield of crystals 6 inches and larger.Numerical simulation of physical chemical process is an efficient research method.This paper simulates the growth of 4H-SiC crystals,analyzes the evolution of raw materials and the synergistic effects of temperature and flow fields on defect formation.By using raw material annealing,thermal field design,and Ta C coating structure,radial gradient growth conditions were obtained.The preparation of 6-inch 4H-SiC single crystals with zero micropipes,low dislocation density,and flat micro convex morphology was achieved.By constructing a temperature field with radial uniformity and small edge axial temperature gradient,low defect 8-inch single crystals were successfully grown.The Virtual Reactor（VR）simulation tool was used to simulate the growth process of SiC crystals using PVT method using multi-physical field coupling.The deviation between the model and the experimental temperature measurement point was within 10 K.By slicing and characterizing crystals under different conditions,it was found that the graphitization process of powder leads to an increase in the temperature of the growth interface and a decrease in the temperature gradient.In the early and late stages of crystal growth,the gas phase is rich in Si and C,respectively.Rich Si induces the formation of polytype defects and micropipe defects,while rich C causes the formation of C inclusion defects and the proliferation of dislocation defects.By analyzing the stress and dislocation defects of crystals with different thicknesses,it was found that areas with protruding crystal edges and larger radial temperature gradients have a higher density of stress and dislocation defects.Research on the temperature and flow fields in the early stage of crystal growth shows that the weak synergistic effect between the radial flow field distribution,which is first strong,then weak,and then strong,and the gradual temperature field is an important reason for the distribution of wave shaped growth rate and the"M"type growth morphology,increasing edge stress and defect density.The effect of the above conditions on defect formation was verified through polarized light images and KOH corrosion detection.Research on the factors affecting the growth of low defect crystals was conducted.Purification annealing of SiC powder under high temperature conditions achieved varying degrees of reduction in free Si and impurities,eliminated the formation of crystal polymorphism,and significantly reduced the density of micropipe defects.Using a high-temperature and low-pressure recrystallization process to increase the purity of the powder to 5N.Based on the field synergy effect,a radial gradient crystal growth condition was constructed by regulating the temperature and flow fields,maintaining a stable step-flow growth mode.It was found that crystals with nearly flat and slightly convex morphology have extremely low defect density.Microscopic and corrosion defect detection was conducted on crystals corroded by graphite on the side,and it was found that corrosion will promote the formation of C inclusions and dislocation defects.However,the application of Ta C coating can adsorb C components and effectively isolate the graphite structure,resulting in the growth of 6-inch 4H-SiC single crystals with low dislocation defects,and micropipes<0.05 cm^-2.By maintaining the driving force of crystal growth through relative movement of the coil,the crystal growth rate and convexity can be improved.Higher temperatures and lower pressures can significantly increase the growth rate,but will expand the radial differences at the growth interface,leading to the formation of defects in the edge and center regions.The temperature field structure of 8-inch crystal growth was studied.Adopting the expansion technology route and combining with temperature field simulation design,a matching expanded graphite structure was used to obtain 8-inch SiC seed crystals through three rounds of expansion growth.Expanding the 6-inch thermal field to 8-inch,it was found that the radial temperature uniformity deteriorates rapidly with size expansion.By introducing a seed crystal cavity structure,a temperature field structure with radial uniformity and small edge axial temperature gradient is constructed.Using graphite plates,bonded wafers,and spliced seed crystals to verify crystal growth conditions,it was found that the growth rates of polycrystals and single crystals were similar under experimental conditions,and polycrystals with slightly convex contours were successfully grown.A study was conducted on the influencing factors of surface temperature uniformity of 8-inch seed crystals.By changing the heating power,heating frequency,induction coil diameter,induction coil height,insulation layer thickness,and spacing between raw materials and seed crystals of the intermediate frequency power supply,the effects of these factors on the surface temperature and temperature gradient of the seed crystal were simulated.It was found that the coupling effects of various parameters were found,and the parameter range of each influencing factor needs to be determined based on specific thermal field requirements to obtain the best solution.By utilizing the optimized temperature field structure for crystal growth,8-inch 4H-SiC crystals with single crystal form,low microtubules,and defect density were successfully grown,with microtubule density<0.5 cm^-2 and dislocation density<3900 cm^-2.