Monocrystalline SiC ingots were grown by a modified Lely method using 6H-SiC seed crystals with (0001) base plane. The crystal growth was carried out in the temperature range 2200-2500 ⁰C at Ar pressure from 2 to 40 mbar. The rate of growth varied between 0.3 and 1.5 mm/hour in the C-axis direction. At growth time of about 15 hours we obtained the ingots with 35 mm useful diameter. To determine the polytype composition of SiC ingots the Raman scattering technique was used. The structural defects were investigated by means of reflection and transmission light microscopy and by selective etching. In the best ingots the dislocation density did not exceed 102 cm⁻², the micropipe density - 10-20 cm⁻², and blocks were absent

Avramenko, S.F.

Kiselev, V.S.

Valakh, M.Ya.

Visotski, V.G.

Semiconductor Physics Quantum Electronics & Optoelectronics

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

7 6 ' 1999,  Institute of  Semiconductor Physics, National Academy of  Sciences of UkraineSemiconductor  Physics, Quantum  Electronics & Optoelectronics. 1999. V. 2, N 1. P. 76-79.1. IntroductionSilicon carbide has a unique combination of electronicand physical properties for application in various fieldsof modern engineering such as high power devices whichoperate at high temperature, and devices for microwaveelectronics. The modified sublimation Lely method is theonly growth technique providing large bulk SiC crystals[1-3]. The most serious drawback of this method is lowstructural perfection of crystals. Commercially available6H- and 4H-SiC substrates contain different types ofdefects, namely: dislocations, dislocation loops, misori-ented domains (a mosaic structure), inclusions, blocksand, especially, micropipes [4]. The crystal defects do notallow to realize completely advantages of silicon carbide.This is currently important for progress in the develop-ment of new semiconductor devices of the next genera-tion. The concentration of these defects depends mainlyon the growth conditions. Therefore, the aim of thepresent paper is to study the influence of the initial stagegrowth on the structure of SiC crystals.PACS 81.05.  C, D, E, G, HInvestigation of structural perfection of SiC ingots grownby a sublimation methodS. F. Avramenko, V. S. KiselevSpecial Design Office of the Institute of Semiconductor Physics, National Academy of  Sciences of Ukraine, Kyiv, 252028, Ukraine,fax: 380(44) 265-19-57, e-mail: kisvs@usa.netM. Ya. Valakh, V. G. VisotskiInstitute of Semiconductor Physics, National Academy of  Sciences of Ukraine, Kyiv, 252028, Ukraine,fax: 380(44) 265-83-42, e-mail: valakh@isp-div6.kiev.uaAbstract. Monocrystalline SiC ingots were grown by a modified Lely method using 6H-SiC seedcrystals with (0001) base plane. The crystal growth was carried out in the temperature range 2200-2500 oC at Ar pressure from 2 to 40 mbar. The rate of growth varied between 0.3 and 1.5 mm/hourin the C-axis direction. At growth time of about 15 hours we obtained the ingots with 35 mm usefuldiameter. To determine the polytype composition of SiC ingots the Raman scattering techniquewas used. The structural defects were investigated by means of reflection and transmission lightmicroscopy and by selective etching. In the best ingots the dislocation density did not exceed102 cm-2, the micropipe density  10-20 cm-2, and blocks were absent.Keywords: silicon carbide, modified Lely method, Raman scattering, defects.Paper received 19.03.99; revised manuscript received 30.03.99; accepted for publication 31.03.99.2. Equipment and method of growthWe have used the resistively heated industrial furnaceRedmet-30. The sublimation vapor transport systemwe designed for SiC ingot growth is shown in Fig. 1. Thegrowth system consists of concentric graphite heater,crucible made from vacuum-tight graphite with 180 mmin diameter and 200-mm long, and two heat screens forthermal insulation made of graphite, rigid carbon foamand carbon foil. We used a double wall or coaxial ar-ranged cylinder type crucible [5]. A porous carbon mem-brane separated the growth cavity in the crucible. At hightemperature the source material diffused through themembrane to the seed crystal which has lower tempera-ture in comparison with the source material. As a seedwe used 6H-SiC Lely [5] crystals with (0001) base plane.The diameters of the seeds lay between 8 and 10 mm.The seeds were etched in KOH melt (T = 600 oC, t == 3-5 min). The orientation of (0001) Si and (0001) Ctype plane was determined by this process. The sourcematerial used was the polycrystalline SiC powder of p-S. F. Avramenko et al.: Investigation of structural perfection of SiC ingots grown ...77SQO, 2(1), 1999or n-type conductivity, which was synthesized by us fromSi and C powders. The crystal growth was carried out inthe temperature range 2200-2500 oC and at the Ar pres-sure ranging from 2 to 40 mbar. The average rate growthvaried from 0.3 to 1.5 mm/hour in the C-axis direction.When the growth time was as long as 15 hours we ob-tained the ingots with 35 mm useful diameter.3. Results and discussionTo determine SiC ingots polytype composition we usedthe Raman scattering technique [6-9]. Structural defectswere investigated by means of reflection and transmis-sion light microscopy. In order to study crystal parame-ters, ingots were cut into plates in two directions: paral-lel and perpendicular to the C axis. The thickness of theplates varied from 0.5 to 1.0 mm. The facets of these plateswere the crystallographic planes {0001}, {1120}, {1100}.It enabled to study not only the basis plane {0001}, par-allel to the surface of the seed, but also facets {1120}and {1100}. The plates were polished with diamondpastes and etched in KOH melt. Thermoelectric powermeasurements were used to determine the conductivitytype. The etching in KOH melt is polishing for a plane{0001} C and selective for a plane {0001} Si. In the lat-ter case, the etch pits are seen on the surface. Their shapedepends on the tilt of dislocations and on the type ofdefects. The shape of dislocation etch pits is the invertedhexagonal pyramid, and the etch pits for tilted disloca-tions are the deformed hexagonal pyramid. The defor-mation is dependent on the angle between the disloca-tion and {0001} Si plane. The etch pits of closed disloca-tion loops have the same shape but are smaller and  of-ten situated in pairs. Other microdefects and the inclu-sions of other phases have etch pits with flat bottom (in-verted truncated pyramid). Dislocations or their partslocated in a subsurface layer, microtwin lamellas andblock boundaries are seen as grooves. Dislocations, dis-location loops, blocks and pipes were found to be themain defects in ingots. The concentration of these de-fects depends mainly on the structural quality of seed,its preparation and growth conditions. Our results haveshown that the main reasons of these defects formationare:1) Conditions of growing. Defects of crystalline structurecan appear at different stages of crystal growth. Butthe majority of defects are generated at the initialgrowth stage due to the formation of 3C-SiC. Thismodification of SiC is of low stability; it transformsto a-SiC at high temperature. This transformationleads to the emergence of pores, inclusions and pipesin growing crystals. Correct selection of Ar pressureat the initial growth stage allows one to avoid the for-mation of 3C-SiC. Moreover, the stability of growthconditions (temperature of growth, pressure and va-por equilibrium) also essentially influences the struc-tural perfection of crystals.2) Fixation of the seed. Pipes can be caused by defects inthe graphite seed-holder and incorrect fixation of aseed.3) Quality of the seed. The largest part of defects canoriginate from the defects of the seed crystal and itssurface. Therefore, to obtain high-quality crystals oneshould carefully choose seed crystals.Shown in Fig. 2 is the appearance of the typical SiCcrystals obtained by the Lely method and used as a seedsFig. 1. Design of the equipment: 1  crucible of vacuum-tightgraphite, 2  heater, 3  first screen, 4  second screen, 5 rotated rod.Fig. 2. The appearance of the typical SiC crystals obtained byLely method (1) and ingots (2,3) grown in this work. Time ofgrowing: 2  5 hours, 3  10 hours.S. F. Avramenko et al.: Investigation of structural perfection of SiC ingots grown ...7 8 SQO, 2(1), 1999(1) and ingots (2,3) grown in this work. Fig. 3 shows theschematic cross-section of the ingot along the C axis. Itcan be conventionally divided into three parts taking intoaccount a content of defects: the seed (1), the monocrys-talline part (2), the outer part (3). The blocks in the area(3) frequently consist of various polytypes (15R, 6H, or4H) differing from the polytypes of the seed (6H-SiC)and of the growing crystal (6H- or 4H). Significant elas-tic deformations arise at the part (3), these sometimesextend into the area (2).Fig. 4 represents the Raman spectra for various partsof the ingot. As the absorption coefficient for Ar laserradiation is small, the Raman spectra give informationabout a polytype structure averaged by the entire lightpath inside the ingot. The spectra show that the periph-eral, polycrystalline part and the bulk of the ingot con-sist of 15R and 6H polytypes, respectively, while the seed-ingot interface contains the inclusions of the 4H poly-type.The shape of the ingots is determined by the configu-ration of a thermal field used. The main structural de-fects (dislocations and micropipes) originating from theseed surface propagate along the vector grad T; they arenormal to the ingot surface.The polarity of SiC seed (Si-type or C-type plane) in-fluences the polytype structure of growing crystals. Thedifference of the binding energy values for the (0001) Cand (0001) Si surfaces of SiC allows one to control a poly-type structure of growing crystals [10,11]. Using thecharge caused by doping with boron on the surface of6H-SiC seed (0001) C, we obtained the 4H-SiC ingots,while on the (0001) Si surface we always got the 6H poly-type. This result was obtained in several dozens of exper-iments at the growth temperature T < 2400 oC. Howev-er, rising the temperature up to 2500 oC, we obtained only6H-SiC polytype ingots on the (0001) C surface. Thus,one can control the polytype structure using (0001) Csurface by varying the technological conditions of growth(temperature, impurity and so on) [11]. In Fig. 5 the Ra-man spectra of plates cut from two different ingots arerepresented. In the first case, the growth was carried outon a surface (0001) Si of the seed and, in the second one,on a surface (0001) C of 6H-SiC seed.140 160 180 2000,00,51,01,52,02,53,0 Intensity, arb.un.Raman shift, cm-13214H6H15R140 160 180 2000,00,51,01,52,0Intensity, arb.un.Raman shift, cm-121  Fig. 3. The shape of the crystal: 1  seed, 2  monocrystallinepart, 3  peripheral part.Fig. 4. Raman spectra for various sites of the same ingot: 1 interface seed/ingot, 2  mono-crystalline part, 3  peripheralpart.Fig. 5. Raman spectra of ingots grown on 6H-SiC seed: 1 6H-SiC grown on a surface (0001) Si, 2  4H-SiC grown on asurface (0001) C.S. F. Avramenko et al.: Investigation of structural perfection of SiC ingots grown ...79SQO, 2(1), 19993. ConclusionsThe construction of the crucible designed by us, with athermal field of a hemispherical configuration, allows toconduct effective growth of SiC crystals of large sizes.Using seeds of 8 to 10 mm in diameter, we received theplates with the diameter up to 35 mm.The investigation of structural defects in silicon car-bide ingots obtained by the modified Lely method hasbeen carried out. It has been shown that there are threecharacteristic parts in the grown crystals that differ bythe defect composition. The main defects are dislocations,screw dislocations, dislocation loops, blocks and pipes.The seed/ingot junction area plays the main role of a dis-location source. The pipes mainly originate from theboundaries of the seed in peripheral parts of ingots. Theseparts of ingots consists of misoriented blocks. The caus-es of structural defect formation are analyzed. It is shownthat the modified Lely method developed allows to pro-duce SiC substrates of fairly high quality. In the bestingots the concentration of dislocations did not exceed102-103 cm-2, micropipes  10-20 cm-2, and blocks wereabsent.References1. Yu. M.Tairov, V. F. Tsvetkov, General principles of growinglarge-size single crystals of various silicon carbide polytypes //J.Cryst.Growth, 52, pp.146-150 (1981).2. Yu. M. Tairov, V. F. Tsvetkov, Crystal growth and characteriza-tion of polytype structures, p.111, Pergamon Press, London (1983).3. G. Ziegler, P. Lang, D. Theis, C. Weyrich, Single crystal growthof SiC substrate material for blue light emitting diodes // IEEETransactions on ED, ED-30(4), pp.277-281 (1983).4. S. F. Avramenko, M. Ya. Valakh, V. S. Kiselev, O. T. Sergeevand M. Ya. Skorokhod, Study of structural perfection of a-Sili-con Carbide crystals //Met.Phys.Adv.Tech., 15, pp.729-735 (1996).5. S. Nishino, Bulk growth of SiC, Chap.8 in Properties of SiliconCarbide, Ed. G.L. Harris, pp.163-169, INSPEC, Washington DC,USA (1995).6. D. F. Feldman, J. H. Parker Jr, W. Choyke, L. Patrick Ramanscattering in 6H-SiC //Phys.Rev., 170(3), pp.698-704 (1968);D.F.Feldman, J.H.Parker Jr,W.Choyke, L.Patrick Phonon dis-persion curves by Raman scattering in SiC. Polytypes 3C, 4H,6H, 15R and 21R. //Phys.Rev. 173(3) pp.787-793 (1968).7. V.V. Artamonov, M.Ya.Valakh, M.P.Lisitsa, V.Ye.Nizkova,Determination of a polytycal composition of SiC joints (in Rus-sian) //Ukr.fiz.zhurn., 32(6), ð.848-851 (1987).8. V. V. Artamonov, A. V. Pogorelov, M. Renucci, M. Ya. Valakh,Micro-Raman characterization of SiC polytype natural hetero-structures //Proc. SPIE, 2543, pp.46-51 (1995).9. S. Nakashima and H. Harima, Raman investigation of SiC poly-types //Phys.Stat.Sol., 162(1), pp.39-66 (1997).10. R. A. Stein, P. lanig, S. Leibenzeder, Influence of surface energyon the growth of 6H- and 4H-SiC //Materials Science and Engi-neering, B11, pp.69-71 (1992).11. Yu. A. Vodakov, E. N. Mokhov, A. D. Roenkov, D. T. Saidbek-ov, Effect of crystallographic orientation on the polytype stabili-zation and transformation of silicon carbide //Phys.Stat.Sol., 51(1),pp.209-215 (1979).

Investigation of structural perfection of SiC ingots grown by a sublimation method

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Investigation of structural perfection of SiC ingots grown by a sublimation method

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)