Heavily doped by nitrogen single crystals of 6H-SiC were completely transformed into 3C-SiC ones by annealing in vacuum at presence of Si vapor for 1 hour at 2180 K or 4 hours at 2080 K. Mechanism of solid-to-solid transformation have been studied. Calculated nitrogen concentration from the Hall effect and EPR spectra for transformed crystals show its decreasing value in 3C-SiC. Data show appearance of new defects - donors and acceptors - that make nitrogen optically and electrically non-active. These defects accompany the process of transformation

Vlaskina, S.I.

Semiconductor Physics Quantum Electronics & Optoelectronics

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

Semiconductor Physics, Quantum Electronics & Optoelectronics. 2002. V. 5, N 2. P. 152-155.' 2002, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine152PACS: 77.84.BMechanism of 6H-3C transformation in SiCS.I. VlaskinaInstitute of Semiconductor Physics, NAS of Ukraine, 45  prospect Nauky, 03028 Kiev, UkrainePhone: +380(44) 269 3792; fax +380(44) 265 8342; e-mail:businkaa@mail.ruDong Seoul College, 461-714, 423,Bokjung-Dong, Sungnam- city, Kyonggi-do, KoreaPhone: 82(031)7202141; fax 82(0342)7202261; e-mail:svitlana@haksan.dsc.ac.krAbstract. Heavily doped by nitrogen single crystals of 6H-SiC were completely transformedinto 3C-SiC ones by annealing in vacuum at presence of Si vapor for 1 hour at 2180 K or 4hours at 2080 K. Mechanism of solid-to-solid transformation have been studied. Calculatednitrogen concentration from the Hall effect and EPR spectra for transformed crystals show itsdecreasing value in 3C-SiC. Data show appearance of new defects  donors and acceptors that make nitrogen optically and electrically non-active. These defects accompany the processof transformation.Keywords: silicon carbide, phase transformation.Paper received 11.02.02; revised manuscript received 28.05.02; accepted for publication 25.06.02.1. IntroductionMany attempts have been made to explain polytypism inSiC, ZnS, CdJ2. The transformation from one polytypeto another takes place in gas phase [1-2] or in solid state[3-4] and can be considered both from thermodynamicand kinetic points of view. In most experiments dealingwith polytypism, β-SiC (cubic phase) has been reportedto transform to α-SiC (hexagonal phase), in particularthe 6H phase, by high-temperature annealing. Manyauthors have studied these β→α transformations in SiC.In general, the 6H polytype is stable at high tempera-tures, while 3C-SiC at low ones.  A few reports of thereverse transformation, i.e., 6H→3C, confirmed theirrealization even at high temperatures. A number of thesetransformations have been made under high nitrogen pres-sures and at high temperatures in hard nitrogen dopedLely crystals. The authors concluded that nitrogen sta-bilized the 3C polytype at high temperatures. In generalimpurities are believed to have an important influence onthe stability of different polytypes of SiC.  Acceptor im-purities, such as B or Al, are reported to stabilize layersin a hexagonal environment, while donor impurities, suchas N or P, stabilize layers in a cubic environment. Actualmechanism of polytypic transformation in solid state asa kinetic problem has been considered both as diffusiveprocess and dislocation processes. The present paper dis-cusses some possibilities of 6H-3C transformation andtransformation mechanism.2. Experimental results and discussionHard dark green heavily nitrogen doped 6H-SiC sam-ples (concentration of donors nitrogen Nd-Na about1019cm3) with thickness about 500 µm were transformedpartly or fully at 3C-SiC by the method described in [5].Heavily nitrogen doped 6H-SiC crystals were exposed to1500-2500 K temperature at the presence of Si vapor invacuum crucible.  After the transformation process, crys-tals became yellow, thinner (100-200 µm) and not trans-formed part of initial 6H-SiC that became white or evenslightly violet. The crystals are completely transformedinto 3C-SiC when 6H-SiC is annealed for 1 hour at 2180K or 4 hours at 2080 K. The quantity of transformed6H→3C SiC was calculated using phase-contrast micro-scope for crystals annealed for various times at differenttemperatures. Fig. 1a shows the content of 3C-SiC in 6H-SiC phase  (η) versus temperature. The content of 3C-SiC(in %) was calculated as a ratio of transformed area tocommon area using photos of crystals heated at differenttemperatures for the same time (t = 1 hour).S.I. Vlaskina: Mechanism of 6H-3C transformations in SiC153SQO, 5(2), 2002According with Avraami-Kolmogorov equation:( ).exp1 nkt−−=η . (1)Usually n value shows a mechanism of transforma-tion. From the kinetic of transformation (Fig. 2b) n isclose to 3 (n=3) in our case.⋅⋅−−=3331exp1 grcr VVtpiη           (2)Vcr  velocity of nuclear creation,Vgr  velocity of nuclear growth.204060801001800 1900a )h, %T, OC  2,0 3,1 3,5 3,9 4,2-2,1-1,6-1,1-0,6-0,1lg  tlglg(1-)h-1t =  1910t =  81 10t =  8 81 0t =  8 61 0b )10 /T, K4 -17894,5 4,6 4,7 4,8æ)ln[/ln]tT  •(1- h)1/31/3Fig. 1. a  Temperature dependence of 3C-SiC content in 6H-SiC phase, b  calculation of n, c - activation energy.Creation of crystallization centers is necessary for acrystallization process.The velocity of this creation is as follows:.1 kTWeccrV−⋅=(3)where:W  activation energy for new nucleus creation,k  the Boltzmann constant,T  temperature.The velocity of new phase creation is constant.The velocity of a new phase growth is:,2 kTUecgrV−⋅⋅= ν (4)U  average activation energy of self-diffusion of newand old phases.Characteristic frequency:,3331exp1exp1 ⋅⋅−−=−−= grVcrVtnkt piη (5)⋅⋅⋅⋅−−=−− 33exp1 teeTC kTUkTWη . (6)⋅⋅⋅⋅−=−−−kTUkTWeetTC33exp1 η . (7)( ) ,3expexp1ln 3 −⋅−⋅⋅⋅−=−kTUkTWtTCη               (8)( )33expexp1lntTCkTUkTW=⋅⋅⋅−− η. (9)Transformation activation energy was calculated ac-cording:( )[ ]kTUWTBt+⋅⋅−−⋅=− 3exp1ln 3131η , (10)( ) −−=+311lnln3ηBtkTWU(11)From Fig. 1c:./201403molKcalWU ±=+ (12)This calculated activation energy is very close to theenergy of sublimation.154 SQO, 5(2), 2002S.I. Vlaskina: Mechanism of 6H-3C transformations in SiCThis calculation was based on the assumption of crea-tion of new centers providing both crystallization andgrowing processes of these centers. It means that it isbased on solid-state transformation going on with newphase-disordered 6H-SiC structure and through forma-tion of an intermediate multilayer structure. Fact of clos-ing this energy to the gas transformation energy showsthe similar nature of both processes.A possible model of this transformation is a site chang-ing according to A→B, B→C or C→A and changingstacking sequence ABCACB (6H-SiC) to ACB (3C-SiC)followed by final 6H→3C conversion through formationof an intermediate multilayer structure. It can be both adiffusion process and the dislocation one.As seen from the Hall effect measurements, free car-rier concentration at T = 300 K (noncompensated im-purities  NdNa) became 1015 cm3, that is decreasedby three orders of magnitude. The mobility was in-creased. In some very small transformed 3C crystals,mobility was 790 cm2/V•s, free  carrier concentration6.1•1016 cm3, conductivity 7.75 ohm1•cm1. Some sam-ples had mobility 960 cm2/V•s.Several samples had very high resistivity (conductivityabout 6•103 ohm1•cm1 and less), and we could not makemeasurements.In general, it is necessary to do the Hall measure-ments only at one plane of originally grown 3C-SiC crys-tals because the concentration and all parameters aredifferent at available planes due to differing velocities ofthe crystal growth in various directions and at differentnitrogen doping. But we could not make such measure-ments using transformed crystals. In Fig. 2, shown arethe results of Hall measurements in transformed crystals(temperature dependences of the free carrier concentra-tion).Temperature dependence of mobility is shown in Fig.3. Some deep donor levels at 0.35–0.1 eV and 0.74–0.04eV appear in the crystal. Usual nitrogen levels are about0.1 eV. In Fig. 2 black points are experimental, and an-other ones theoretically calculated for a two-level sys-tem. Lines 1 and 2 are indicative of this fact. Deep level0.35 eV appears at low temperatures only. At tempera-tures close to 400 K this level is exhausted, and free car-rier concentration is 2•1017 cm3. Compensation of thislevel is very small (about 1% as it was calculated accord-ingly to equation for doped and compensated semicon-ductors). New deep levels (0.74 eV) appear at 400K. Theconcentration of these levels is very high. The tempera-ture dependence of mobility (Fig. 3) shows some mate-rial property improvement and argues in favour of ab-sence of new scattering centers. But, in general, the mo-bility is increased.The nitrogen concentration calculated from EPR spec-tra for transformed crystals shows a decreasing nitrogenconcentration in 3C-SiC.All these data show appearance of new defects - do-nors and acceptors that make nitrogen optically and elec-trically non-active. These can be vacancies (Si-vacancy -as an acceptor, C-vacancy -as a donor or more compli-13141516171819201.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5lg(Carrier æoncentration,  cm-3 )10 / T(K )3 -10.901.101.301.502.40 2.48 2.56 2.64lg(Mobility, cm / V•s)-3lg T(K)cated complexes, namely: nitrogen-vacancy, vacancy-vacancy, or some impurities play more important role inthese processes). Nature of these defects has to be under-stood later  now we dont know it exactly. But thesedefects accompany processes of transformation.3. ConclusionSo, 6H→3C transformation in SiC was investigated as asolid-state transformation. Tight proximity of meaningsvalid for the transformation and sublimation energies aswell as for the gas phase transformation one enabled usto consider this 6H→3C conversion as accompanied byFig. 2. Temperature dependence of the free electron concen-tration.Fig. 3. Temperature dependence of the free electron mobility.S.I. Vlaskina: Mechanism of 6H-3C transformations in SiC155SQO, 5(2), 2002some reconstruction processes. At the same time, it is clearthat some new defects participate in these processes. Prob-ably, these defects are complexes of doping elements withsilicon and carbon vacancies. And vacancies play an im-portant role in the transformation process.  Some electri-cal properties were investigated, too.References1. A.J. Verma, P.Krishna, Polymorphism and Polytypism in Crys-tals, Wiley, New York, (1966).2. P.Krishna., R.C. Marshall, Direct transformation from the2H to the 6H-structure in single crystal SiC// J.Cryst.Growth,9, pp.310-327 (1971).3.    H.Yagodzinsky, Polytypism in SiC crystals// Acta crystals, 7,pp.300-307 (1954).4. J.W. Yang, P.Pirous, The aÞb polytypic transformation inhigh-temperature indented SiC// J.Mater.Res, 8 (11),pp.2902-2906 (1993).5. Vlaskina S., Shin D.H. 6H to 3C Polytype transformation inSilicon Carbide// Jpn.J.AppI.Phys. 38(1A), pp.L27-L29(1999).

Mechanism of 6H-3C transformation in SiC

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