Estudo teórico da estabilidade e propriedades eletrônicas de defeitos em nanotubos de GaN

Abstract

The stability and electronic properties of antisities, vacancies and substitutional Si and C impurities in GaN nanotubes are studied using spin-polarized density functional theory within the local density approximation (LDA) to the exchange-correlation functional. We investigated these defects in both nanotubes the zigzag (10,0)and the armchair (6,6), which have 10.4 and 11.2 Å in diameter, respectively. These nanotubes are semiconductors and energetically metastable with respect to the GaN bulk phase, the zigzag have a direct band gap while the armchair an indirect one. Our results show that antisities have lower formation energies in nanotubes as compared with the bulk GaN. For both chiralities, the NGa antisities introduce an occupied level and GaN antisities an empty and two occupied levels within the band gap. For nitrogen vacancies (VN), in the equilibrium geometry, two Ga atoms have their bonds reconstructed forming a pentagon and the other Ga atom remain with a dangling bond. In the minimum energy configuration, the gallium vacancy (VGa) have a more complex reconstruction: two N atoms have their bonds reconstructed forming a N −N dimer and the other N atom bonds to a Ga atom nearest neighbor to the VGa site that moves in direction to the empty site (vacancy). Vacancies are energeticaly less favorable as compared with antisities and the defective level into the band gap present a spin splitting giving rise to a net magnectic moment of 1 μB. Substitutional C and Si impurities have the lowest formation energy between the studied defects, being negative for Si in a Ga site. For the impurities in a N site (SiN and CN) a deep acceptor level with a spin splitting around 0.8 eV is observed. The impurities in a Ga site (SiGa and CGa) exhibit donor properties, suggesting the formation of defect-induced n-type GaN nanotubes.