Free Growth of 4H-SiC by Sublimation Method

Abstract

4H-SiC crystals of cylindrical shape 25 mm and 45 mm in diameter have been grown by the Modified Lely method in a graphite crucible with a guide. The crystals had not mechanical contacts with the walls of the crucible and no formation of polycrystalline SiC during the growth to decrease the stresses. This was confirmed by the facets observed at the edges of the crystals. Numerical simulations of the growth process have been performed. Introduction Silicon carbide is a wide band gap semiconductor with unique properties that make it suitable for application in high frequency, high temperature and high power devices. These applications prompted increased interest in growth of high quality, large diameter single crystals with defect reduction. The objective of this paper is the study of 4H-SiC growth by the Modified Lely method without polycrystalline rim and mechanical contact with graphite walls of the crucible in order to decrease the stresses. Besides, the aim was to obtain crystals with a cylindrical shape and flat top surface to minimize the number of low grain boundaries. Numerical modeling has been used to optimize the design of the crucible. Experimental details 4H-SiC crystals , 25 and 45 mm in diameter, have been grown by the Modified Lely method over the 2000-2050°C temperature range (measured by the top pyrometer : 1 in Fig. 2) in argon [1] (Fig.1). An experimental set-up with RF heating and graphite crucible was used. The crucible (Fig.2) was wrapped in a graphite felt for thermal insulation, and the whole assembly was placed inside a water cooled quartz reactor. Under these experimental conditions we obtained 4H single crystals with a thickness of 5-10 mm. The growth rate was 200-300 μm/h. An increase of the growth rate resulted in 6H-SiC growth. The SiC source powder was loaded both between a dense graphite crucible and a thin walled inner graphite cylinder and inside of the inner cylinder. With this configuration it is possible to reduce the influence of the leakage of Si and the reaction of Si from the main central source with the walls in order to maintain an excess of Si over the seed. To eliminate the influence of the periphery we cut out the seed after gluing to the graphite holder and avoided 6H polycrystalline SiC growth at the periphery. 4H-SiC wafers of 25 and 45 mm in diameter were used as seeds. The ingots were grown on the C-face of seeds which are 8° off-axis. The distance between the powder source and the seed was 15-20 mm. The growth process consisted of : (a) annealing in vacuum at a temperature lower than 1000°C, (b) heating of the crucible up to growth temperature at a high argon pressure, (c) decrease of the pressure (3-5 torr) and growth at low argon pressure. Fig. 2 : Schem right-half par Numerical d Intense effor profiles in th especially rad without a gr minimize the in growth ca Knudsen equ total mass flu Here γi is th equilibrium p ) 25 mm (thickness 5mm Fig.1. Photographs of “as-grown” 4H-SiC atic representation of the Fig. 3 : Compar t of the reactor with guide in the growth ca etails t has been focused on the crucible design to optim e growth area [1-7]. Electromagnetodynamics must iative heat transfer within the growth cavity [1]. The aphite guide (3 on Fig. 2) was optimized by heat radial temperature gradient (Fig. 3). In order to consi vity, we have used a model based on the heterogeneo ations (Eq. 1) to relate the partial pressures of the spec x i R : ( ) RT 2 M P P R i eq i i i i S i π α − γ = (i=Si,SiC2,Si2C) e sticking coefficient and αi is the evaporation c ressures eq i P obey to mass action law. 2605 K without guide radial gradient : axial gradient : 2.Graphite crucible 4. Graphite foam 5. SiC seed 6. SiC powder 7. Induction coils 3. Graphite guide 1.Top pyrometer measure 45 mm (thickness 10mm)