First-principles Study On Ferromagnetism In C-doped AIN

Traditionally, Diluted magnetic semiconductors(DMS) are produced by doping semiconductors with magnetic transition-metal (TM) elements.It was found that the magnetic TM-doped DMS sergregate to form ferromagnetic cluster,precipitates or secondary phases,which are obstacles for applications of DMS. Recent years, many researchers have found that non-magnetic ion doping in oxides and nitrides semiconductors obtain high temperature ferromagnetism. Researches found that semiconductors (oxides, sulfides and nitrides) with non-magnetic anion C, N doped obtian room temperature ferromagnetism. Due to the frequent usage of C as dopant ofⅢ-Ⅴnitrides, many works have been made to study the structural and electronic properties and the formation energies of substitutional C impurities in AlN. So far, there has not been a report on the magnetic properties of nonmagnetic anion C-doped AlN. In this thesis, first-principles calculations based on the density functions theory are used to study the electronic structure and magnetic properties of the C doped AlN system. The strong correlation effects for anion-2p states are introduced by means of GGA+U scheme as implemented in VASP. The main work and the results are as follows1.The pure AlN and the C-doped system of Al36N35C were first investigated. GGA result shows that there is no magnetic moment in pure AlN, but the magnetic moment of Al36N35C is 1μB, which comes from the partially occupied doped C and its first neighboring Al and first neighboring N atoms. The DOS of Al36N35C shows that C substitution induces the spin splitting impurity states above the top of the valence band. The impurity states are mostly formed by 2p state of C and its first neighboring N atoms. The U correction enlarges splitting between the occupied and unoccupied, change the distributions of spin density. It transforms the ground state of C-doped AlN to an insulating state from half-metallic.2. Next the system of Al36N34C2 is investigated to study the magnetic coupling between the moments produced by C doping. Six different C-C positions were chosen in the supercell. The results show that all the ground state of configurations are ferromagnetic for GGA. Configuration (0,4) and (0,5) are the most stable for GGA and GGA+U. The ground state of configuration (0,3) is anti-ferromagnetic for GGA+U.Conclusions:A substitutional C dopant in AIN introduces magnetic moment of 1.0μB, which comes from the partially occupied 2p orbitals of the C,its first neighboring Al and first neighboring N atoms(GGA) or out-of-plane first and fifth neighboring N atoms (GGA+U),among which the atomic moment of the C is the biggest. The U correction for the anion-2p states obviously changes the magnetic moment distribution of Al and N atoms and transforms the ground state of C-doped AIN to insulating from half-metallic. The C atoms can induce ferromagnetic ground state with long-range couplings between the moments in C-doped AIN. The ferromagnetic coupling can be explained in terms of the two band coupling model.