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Electronic Structure of Cubic Silicon Carbide with Substitutional 3D Impurities at Si and C Sites N

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Electronic Structure of Cubic Silicon Carbide with Substitutional 3D Impurities at Si and C Sites N Semiconductors, Vol. 37, No. 11, 2003, pp. 1243–1246. Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 11, 2003, pp. 1281–1284. Original Russian Text Copyright © 2003 by Medvedeva, Yur’eva, IvanovskiÏ. ELECTRONIC AND OPTICAL PROPERTIES OF SEMICONDUCTORS Electronic Structure of Cubic Silicon Carbide with Substitutional 3d Impurities at Si and C Sites N. I. Medvedeva, E. I. Yur’eva, and A. L. IvanovskiÏ Institute of Solid State Chemistry, Ural Branch of Russian Academy of Sciences, Yekaterinburg, 620219 Russia e-mail: [email protected] Submitted January 20, 2003; accepted for publication January 25, 2003 Abstract—The full-potential linear muffin-tin orbital method is used to calculate the electronic structure and cohesive energies for cubic silicon carbide doped with transition 3d metal impurities (Me = Ti, V, Cr, Mn, Fe, Co, Ni), substituting Si or C at the corresponding sublattice of the atomic matrix. It is established that all 3d impurities mainly occupy silicon sites. For the Ti Si substitution, dopant impurity levels are located in the conduction band of SiC, whereas doping silicon carbide with other 3d impurities gives rise to additional donor or acceptor levels in the band gap. For 3C-SiC the effect of impurities on the lattice parameter (with the substitutions Me Si) and on the impurity local magnetic moments (with the substitutions Me Si, C) is studied. © 2003 MAIK “Nauka/Interperiodica”. Silicon carbide is a promising material for extremal culations of spin polarization by the full-potential aug- electronics. Using intentional doping with atoms of mented plane wave method (APW) showed that, for the 3d metals (Me), one can appreciably vary the electrical, states of an isolated Ti atom in 3C-SiC, the relaxation magnetic, and mechanical properties of SiC [1–6]. For phenomena play an important role in formation of the example, the introduction of vanadium atoms compen- parameters of hyperfine interactions; moreover, the sates for the uncontrollable presence of oxygen and substitutional defect (Ti Si) is stabler than the nitrogen atoms, resulting in improvement in the dielec- interstitial defect [9]. The LMTO and tight binding cal- tric performance of the material [1]. The introduction of culations were performed for SiC:Me systems, where titanium and nickel atoms improves the SiC strength Me=Ti, V [10] and Me = Cr, Mn, Fe, Co [11]. Accord- characteristics [2, 3], whereas the iron and cobalt atoms ing to [11], Cr, Fe, and Co predominantly replace car- are the catalysts for the process of thin wire fabrication bon, whereas Mn atoms mainly occupy silicon sites. from crystals of cubic silicon carbide [4]. The chrome For doping of SiC with Ti, V, and Ni, the effect of relax- and manganese impurities are promising for producing ation of carbon atoms was considered in [12]. It was magnetic materials on the basis of doped silicon car- found that the substitution Me Si is accompanied bide [5, 6]. by radial displacement of carbon atoms away from the impurity center and that the substitution energy is pos- Information on the mechanism of the effect of itive; it is lowest for titanium and increases with the 3d elements on the properties of a SiC:Me system can atomic number of the impurity. The possibility of dou- be obtained by studying how the electronic spectrum of ble defects forming (Me and a lattice vacancy) in SiC the atomic matrix is modified by impurity states and was also considered using theoretical calculations in then how the impurity type (donor or acceptor) depends the context of the cluster model [13]. on the possible impurity sites in the lattice. These stud- ies are closely related to the problems of forecasting The aim of this study is to analyze the tendencies in changes in lattice and cohesive properties of the matrix: the variation of structural, electronic, and energy prop- the effects of structural relaxation, doping-induced erties of cubic silicon carbide as a result of doping with variation in the stability of the system, etc. 3d metals and to establish how these properties depend on the type and position of the substitutional impurity. Recently, the solution of these problems for impu- The metals Me = Ti, V, Cr, Mn, Fe, Co, Ni were chosen rity systems on the basis of cubic silicon carbide as impurities; they could be introduced to the sites in (3C-SiC) doped with 3d metals has attracted consider- both of Si and C sublattices. able attention [7–13]. For example, the energy states of isolated 3d impurities in the silicon sublattice in the The SiC:Me systems (of formal stoichiometry 3C-SiC:Me (Me = Ti, V,…, Ni, Cu) system were calcu- Si0.875Me0.125C and SiC0.875Me0.125 with substitutions lated by using the method of linear muffin-tin orbitals Me Si and Me C, respectively) were simulated (LMTOs) and Green functions [7, 8]. It was concluded by the 16-atom supercells MeSi7C8E8 and MSi8C7E8; that the system is minimally destabilized by the atoms eight “empty” spheres (E) were introduced into each at the beginning and at the end of the 3d series [7]. Cal- cell to accomplish close packing. Such a choice of the 1063-7826/03/3711-1243 $24.00 © 2003 MAIK “Nauka/Interperiodica” 1244 MEDVEDEVA et al. Lattice parameter, Å Ionic radius, Å unbounded scalar-relativistic full-potential method [14], with an exchange–correlation potential of the form sug- gested in [15]. 4.36 1.5 We calculated the total and partial densities of elec- tronic states, atomic magnetic moments (MM), total energies (Et), and cohesion energies (Ec). The effects of structural relaxation were evaluated using the equilib- 4.32 1.4 rium values of lattice parameters a; the latter were obtained from the minimality condition on Et for the optimized SiC:Me system, where the coordinates of the atoms nearest to the impurity were varied without 4.28 1.3 breaking the total symmetry of the crystal (we described the results on local relaxation in [12]). We drew our con- clusions concerning the preferential position of the sub- stitutional atom (Me Si or Me C) by comparing 0 Ti V CrMnFeCoNi Si C the corresponding energies Ec(Ec and Ec ) defined as the difference of the total energy E of SiC:Me and the Fig. 1. Impurity-type dependence of the calculated lattice t parameters and ionic radii for the 3C-SiC:Me system [16]. sum of the energies of the corresponding free atoms. The dashed line corresponds to the theoretical lattice The results of the calculation are shown in Figs. 1 parameter for the 3C-SiC lattice. and 2 and in the table. The parameters of the electronic spectrum, namely, the total density of states at the cell is optimal, since it allows one to include the effects Fermi level N(EF), the band gap Eg, Ec, and the MMs of of structural relaxation of the nearest atoms without 3d atoms in SiC:Me (see table), are found to be quite substantial increase in computing time. The use of the different for different substitutions. supercell model implies the ordering of impurities in Since Ec (Me Si) > Ec (Me C) for all impu- the calculation, whereas in doped materials disorder rity atoms (see table), the impurities considered must can affect the matrix properties. However, for low predominantly replace silicon atoms. This is the basic impurity concentrations, impurity–impurity interaction difference from [11], where it was concluded that, for is weak, and the effect of disorder in impurity positions Cr, Fe, and Co, the most probable sites are those of car- on the electronic spectrum is negligible. The calcula- bon. For substitution of silicon atoms, the largest values tions were performed by the self-consistent spin- Si of Ec correspond to Ti and V impurities, for which the C energy (Ec ) is lowest. For these impurities, the prefer- Cohesion energy E , density of states at the Fermi level c ∆ Si N(EF), (energy band gap Eg), and 3d impurity magnetic ence energy E, defined as the difference between Ec moment (MM) C ∆ and Ec , is highest. In contrast, E is minimal for Cr, –1 3C-SiC : Me Ec, eV/atom N(EF), eV MM Mn, and Fe. Therefore, under certain conditions, one 3C-SiC 7.39 (E = 2.4 eV) 0 can expect the impurities Cr, Mn, and Fe to be located g at both Si and C sites; this, however, is improbable for Ti Si 7.30 (Eg = 2.4 eV) 0 Ti and V atoms. Ti C 5.88 9.07 0 Analysis of the calculated effects of lattice relax- V Si 7.19 4.17 0.9 ation [dependence of lattice parameter a on impurity V C 6.07 4.84 0 type at the Si site for SiC:Me systems (see Fig. 1)] shows that the replacement of silicon by titanium, vana- Cr Si 6.90 0.11 2.0 dium, and chrome must result in an increase in the lat- Cr C 6.61 10.78 1.3 tice parameter compared to undoped 3C-SiC and that Mn Si 6.95 4.05 1.1 the replacement of Si by Fe, Co, and Ni should lead to Mn C 6.66 7.87 1.0 a slight decrease in the lattice parameter; this behavior Fe Si 7.02 0.41 0 indicates a correlation with the ionic radii of Me. The general tendency also manifests itself in the decrease in Fe C 6.75 (Eg = 0.08 eV) 0 a with increasing atomic number N of the impurity: at Co Si 7.03 8.16 0 the beginning of the series (Ti, V, and Cr atoms), the Co C 6.63 3.23 0 slope of curve a/N is greater than at the end of the series Ni Si 7.04 4.40 0 (Mn, Fe, Co, and Ni atoms).
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