Estudo de primeiros princípios de defeitos nativos em monóxido de berílio

Abstract

The present thesis shows a study of native defects (vacancies and antisites) in Beryllium Monoxide (BeO) by means of first principle calculations within Density Functional Theory (DFT). We made use of Local Density Aproximation (LDA) in order to approach correlation and shift term, and ultrasoft pseudopotentials in order to describe the electron-core interaction. The Kohn-Sham orbitals were described by plane waves expansion method. BeO was studied in two cristaline structures: wurtzite and zinc blend. Our results confirm the greater stability of wurtzite, as well as that both wurtzite and zinc blend are semiconductors with a broad gap. We noticed that all defects we have studied have shorter formation energy in a BeO wurtzite structure than in a zinc blend one. We showed that native defects in BeO, either in wurtzite or zinc blend, introduce levels in the forbidden band of the crystal causing considerable changes in eletronic properties, and, for some defects, we identified metal features. Charge Density and Projected State Density (PDOS) calculations allowed us to determine the origin of such levels. For vacancies, we noticed that defects levels have always originated from neighboring vacancy sites, and that. Be vacancy makes BeO to have a metal feature. In antisites, defects levels originated from the substitutional atoms themselves as well as from their neighboring atoms. When in a BeO wurtzite structure, both Be that is O substitutional in (BeO) and O that is Be substitutional in (OBe) keep the semiconductor feature of the material with a significant energy gap decrease. However, in zinc blend structure, the same defects change the semiconductor feature to metallic.