First-principles Study On Magnetism In C-doped BN

In recent years, diluted magnetic semiconductors (DMSs), in whichsemiconductors are doped by a few percent of non-magnetic ions, have gainedincreasing interest for their potential applications in spintronic devices. However,some experimental works reported that the magnetic TM dopants in TM dopedsemiconductors form magnetic precipitates in the forms of clusters or secondaryphases. These extrinsic magnetic behaviors are undesirable for practical application ofDMS. Recently, there have been reports of high-temperature ferromagnetism in someoxide and nitride semiconductors by doping nonmagnetic elements. At the same time,first-principles calculations showed that appropriate nonmagnetic anion or cationsubstitutions in several oxide, nitride or sulfide semiconductors can induce intrinsicferromagnetism. Bulk BN is a compound that occurs in four structures, that is,hexagonal BN, rhombohedra BN, cubic BN and wurtzite BN. As an importantwide-gap semiconductor, the magnetic properties of BN and effect of nonmagneticdopants on the magnetic properties of doped BN materials have gained extensiveinterests. In this thesis, we investigate the effects of nonmagnetic C dopants on thecrystal structure, electronic structures and magnetic properties of C-doped BN by firstprinciples calculations based on density functional theory. The calculations wereperformed using Vienna ab-initio simulation package (VASP) and the C doped BNsystem was modeled with a supercell, in which B atoms substitute B and N atoms,respectively. The calculated results are as follows:For C-doped hexagonal and rhombohedral BN, the nearest-neighbor N atomsaround the substitutional C at B site move inward, while the nearest-neighbor B atomsaround the substitutional C at N site move outward. The2p electons of thesubstitutional C are localized and the2p states of C are spin polarized. Asubstitutional C atom introduces magnetic moment of about1.0μBand the introducedmagnetic moment mainly comes from the partially occupied2p orbitals ofsubstitutional C. The2p orbitals of the second neighbor B or N atoms around C alsoprovide a contribution for moment. The coupling between the magnetic momentsinduced by C doping is antiferromagnetic. Moreover, the calculations for C dopedcubic and wurtzite BN show that the ground state of the systems are non-magneticand the2p electons of the substitutional C are more delocalized, thus2p states of Care not polarized.