Ab Initio Study of Magnetic Properties of SiC-Based Diluted Magnetic Semiconductors

In order to obtain quantum-mechanical understanding of electronic and spin properties of SiC:TM (TM=V, Cr, Mn, Fe, Co, Ni), we have performed ab initio total energy calculations based on the density-functional theory within the generalized gradient approximation. The ferromagnetic phase was found to be energetically favorable for the case of SiC:TMSi (TM=V, Cr, Mn). These diluted ferromagnetic semiconductors exhibit a half-metallic property with metallic majority-spin electrons and insulating minority-spin electrons. Introduction Diluted magnetic semiconductors (DMS), in which magnetic elements are substituted for host elements, have been intensively studied due to their potential applicability to spin-related microelectronics [1]. Such intensive efforts have been focused to the design of a new functional material that allows utilizing the carrier control techniques developed in semiconductor industry [2]. For instance, the idea of using a ferromagnetic semiconductor for the spin injection [3] is now being pursued broadly [4]. As to the mechanism of the ferromagnetism observed in some DMS materials, the double-exchange mechanism has been proposed to explain the ferromagnetic ordering [5,6]. However, no consensus has been reached among the researchers yet. In search for the potential ferromagnetic DMSs, Katayama-Yoshida et al. employed ab initio calculations using the coherent potential approximation and proposed that the ferromagnetism can be realized in ZnS-, ZnSe-, and ZnTe-based DMSs doped with V or Cr and ZnO-based DMSs doped with V, Cr, Fe, Co, or Ni. The DMSs based on GaAs and GaN are also showing ferromagnetic order when doped with V, Cr, or Mn [7]. In this paper, we report the ab initio ultrasoft pseudopotential (PP) and projectoraugmented-wave (PAW) [8] calculations for electronic and spin properties of cubic β-SiC by varying the substituted transition metals. The calculated total energies and magnetic moments show that the ferromagnetic phase is energetically favorable compared to the paramagnetic phase for the cases of V, Cr, and Mn substituted for Si and Cr and Mn substituted for C. The analysis of the density of states reveals that SiC with V, Cr, and Mn substituted for Si should be a half-metal. Calculation method By using the Vienna Ab initio Simulation Package (VASP) [9] based on spin-polarized density functional theory (DFT) [10,11] we have performed ab initio total energy calculations for β-SiC:TM (TM=V, Cr, Mn, Fe, Co, Ni). The DFT calculations were done with the generalized gradient approximation (GGA) [12] for the exchange and correlation functional. The electron-ion interaction is treated by the PP and PAW potential. For bulk cubic β-SiC, all calculated lattice constants of the PP, Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 1237-1240 doi:10.4028/www.scientific. net/KEM.264-268.1237 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-13/03/20,18:14:22) PAW, and full potential linearized augmented plane wave (FLAPW) are found to be in good agreement with the experimental value (see Table 1). In order to simulate β-SiC:TM, we adopted the 8-atom supercell as shown in the Fig. 1. This 8-atom supercell model corresponds to the impurity concentration of 25%. For instance, in the case of β-SiC:TMSi, one Si atom out of the four Si atoms in the supercell was substituted by a TM impurity atom. With the use of this structural model, we have performed both the spin-unpolarized and the spin-polarized calculations by varying the species of the substitutional TM atoms. The momentum-space integrations are performed using the linear tetrahedron method with Blöchl correction on an (8 x 8 x 8) Monkhorst-Pack k-point grid containing the Γ point. The electron wave functions are expanded in terms of plane-wave basis set with an energy cutoff of 280 eV. To obtain the atomic structure of minimum-energy configuration [13], we applied the conjugate gradient method to relax all atoms until the force [14] acting on each atom becomes less than 0.02eV/Å in magnitude. Fig. 1. Crystal structures of cubic β-SiC with transition metal atoms substituted at (a) Si and (b) C sites. Table 1 summarizes the test calculations of the PP and PAW for the magnetic moments and magnetization energy of β-SiC:TMSi, where all atomic positions were fixed at their unrelaxed ideal sites. The results were also compared with those of all-electron calculations of the FLAPW method as implemented in WIEN2k [15]. The magnetization energy is defined as the energy difference between the ferromagnetic phase and the paramagnetic phase and a positive value indicates the stable ferromagnetic phase. As shown in Table 1, the overall agreement is very good for both types of potentials although the PP shows somewhat better agreement. Table 1. Calculated results for the equilibrium lattice constant (a0), the total magnetic moment (M) and magnetization energy (∆E). All calculations were performed within the generalized gradient approximation. Results Cubic β-SiC has the zinc-blende structure in which both the Si and the C sublattices form a face centered cubic structure. Magnetic transition metal atoms can be incorporated into Si or C sites as shown in Fig.1. First, we have investigated the ground-state properties of β-SiC:TM. Figure 2 shows the total magnetic moment of the ferromagnetic phase and the magnetization energy. The present calculations showed that in cases of the Fe, Co, and Ni substitution, β-SiC:TM, except for β-SiC:FeSi(PP), has no magnetism at all and that the paramagnetic phase is energetically favored SiC SiC:VSi SiC:CrSi SiC:MnSi a0[Å] M[μB] ∆E[eV] M[μB] ∆E[eV] M[μB] ∆E[eV] PP 4.36 1.00 0.12 2.00 0.56 3.00 0.53 PAW 4.38 1.00 0.09 2.00 0.42 2.91 0.29 FLAPW 4.37 1.00 0.02 2.00 0.53 3.00 0.44 Expt. (Ref. [16]) 4.36 1238 Euro Ceramics VIII