First-principles Study On Magnetic Properties Of Defects In BN

Traditionally, Diluted magnetic semiconductors (DMS) are produced by doping semiconductors with transition-metal (TM) elements. It was found that the magnetic TM dopants in TMs-doped DMS segregate to form ferromagnetic clusters or precipitates, which are obstacles for the practical applications of DMS. Recently, experiments demonstrate that room temperature ferromagnetism can be obtained in semiconductors in the absence of magnetic ion dopants, which is refereed as d0 magnetism. These works are significant because they demonstrate alternastive ways to produce DMS. As an importantⅢ-Ⅴsemiconductor, BN has received much attention for their potential applications in the field of spintronics. The experimental researches demonstrated that high temperature ferromagnetism can be obtained in BN with defects.In this thesis, first-principles calculations based on the density functions theory are used to study the crystal structure and magnetic properties of BN with vacancies. The strong correlation effects for anion-2p states are introduced by means of GGA+U scheme as implemented in VASP.For the cubic, wurtzite, hexagonal and rhombohedra BN system, which are modeled by supercell with a vacancy, the results are as follows:①The system has a nitrogen vacancy of the c-BN, the concentration of the vacancy is 1.56%. The ground state of the supercell is half–metallic, The spin-split impurity band emerged in the bottom of the conduction band,the impurity band is from the 2p orbital of B atoms. The calculated magnetic moment of the B32N31 supercell by GGA and GGA+U is 1.0μB, the magnetic moment is from the N vacancy the nearest neighboring 2p orbital of B atoms. The calculated by GGA+U,the band gap has increased, the spin-split degree of the B 2p orbital has reduced.②The system has a nitrogen vacancy of the w-BN, the concentration of the vacancy is 1.39%. The system is insulation. The addition of the N vacancy, The spin-split impurity band emerged in the bottom of the conduction band, the impurity band is from the 2p and 2s orbitals of B atoms, The calculated by GGA , the magnetic moment of the supercell is1.0μB. The calculated spin-split degree of the B 2p orbital has reduced and energy level of impurity has increased by GGA+U.③The system has a nitrogen vacancy of the h-BN, the concentration of the vacancy is 1.39%. The calculated by GGA, the magnetic moment of the supercell is 1.0μB, the magnetic moment is from the N vacancy the nearest neighboring 2p orbital of B atoms. The calculated by GGA and GGA+U, the result are in the same.④The system has a nitrogen vacancy of the r-BN, the concentration of the vacancy is 1.85%. The calculated of the result by GGA and GGA+U are in the same. The spin-split impurity band emerged in the bottom of conduction band. The calculated the magnetic moment of the supercell by GGA is 1.0μB, the magnetic moment is from the neighbor 2p orbital of B atoms.⑤The system has one boron vacancy of the c-BN, the concentration of the vacancy is 1.56%. The calculated by GGA, the spin-split impurity band emerged in the top of the valence band. The impurity band is formed from the 2p orbital of the N atoms, and the magnetic moment is from the B vacancy the nearest neighboring 2p orbital of the N atoms. The ground state of supercell is metallic. The calculated magnetic moment of the supercell is 3.0μB. The calculated by GGA+U, The system is insulation.⑥The system has a boron vacancy of the w-BN, the concentration of the vacancy is 1.39%. The system is metallic. The calculated by GGA+U, The system is insulation. The magnetic moment of the supercell from 1.2μB to 3.0μB.⑦The system has a boron vacancy of the h-BN, the concentration of the vacancy is 1.39%. The calculated by GGA, the spin-split impurity band emerged in the top of the valence band, the Fermi level is traversing impurity band, the system is metallic. The magnetic moment of the supercell is 0.2μB. The addition of U effective the spin-split degree of impurity band has increased. The ground state of the BN from metallic to half-metallic. The magnetic moment of the supercell is3.0μB, ⑧The system has a boron vacancy of the r-BN, the concentration of vacancy is 1.85%. The calculated by GGA shows that the ground state of the BN is metallic. The magnetic moment of the supercell is 0.13μB. The calculated by GGA+U, the impurity band in the band gap has increased, the ground state of theBN is half-metallic. For the cubic BN system, in which are modeled by supercell with two vacancies, the magnetic coupling between the magnetic moments are investigated and the results are as follows:①The system has two nitrogen vacancies, we named N0-N1,N0-N2,N0-N3 and N0-N4 configurations, and the distance of two N vacancies are 2.556 ?,3.615?,4.427? and 5.112?. For the arrangement of ferromagnetic: In addition to the N0-N4 configuration, the N0-N1,the N0-N2 and the N0-N3 configurations, the magnetic moment of the three configurations are 2.0μB and the magnetic moment of the N0-N4 configuration is 0μB. The calculated by GGA+U, the magnetic moment of the four configurations are 2.0μB. For the arrangement of anti-ferromagnetic: The magnetic moment of the four configurations are 0μB. The calculated by GGA show that the ground states of the N0-N2 configurations is anti-ferromagnetism. The calculated four configurations by GGA+U are anti-ferromagnetism.②The system has two boron vacancies, we named B0-B1,B0-B2,B0-B3 and B0-B4 configurations. For the arrangement of ferromagnetic: The magnetic moment of four configurations are 6.0μB. For the arrangement of anti-ferromagnetic: The magnetic moment of four configurations are 0μB. The calculated by GGA shows that the ground state of the B0-B3 configuration is ferromagnetism. The ground states of the B0-B1,the B0-B2 and the B0-B4 configurations are anti-ferromagnetism.The addition of U make the ground state of the B0-B2 configuration form anti-ferromagnetism to ferromagnetism.