Defeitos extensos em semicondutores: discordância parcial de 90°

Abstract

In the present work we investigate electronic and structural properties of a 90° partial dislocation in Si [111]. The calculations were made with first-principles methods based on Density Functional Theory (DFT) with the local density approximation (LDA) as stated by Ceperley and Alder and parametrized by Perdew and Zunger and generalized gradients (GGA). The electron-ion in-teraction were described using norm-conserving pseudopotentials by Bachelet-Hamman-Schlíiter (BHS) in the Kleinman-Bylander form and ultrtasoft pseu­dopotentials based on Vanderbilt development. In Silicon, as well as in other zinc-blende semiconductors, dislocations generally belong to the [111] glide plane, lying along the [110] directions, that is the most closely-packed plane for Silicon, leading to reconstructions. How-ever, for a structure wíthout reconstruction, our calculation indicate one energy o o formation of 0.57 (eV/A), which reconstructs with an energy gain of 0.13 (eV/ A) and is identified by single period (SP) reconstruction. Inserting alternating kinks in the core of it, we have a double period (DP) reconstruction that lower o the energy of the system by 0.1 (eV/A) related to SP, being the most stable reconstruction. Acting as sources and sinks for point defects, the dislocation structural component is related primarily with the strain and stress field inside and outside the core that interacts with stress field caused by impurities. These interactions have important effects on transport properties of carries. In this way, we study Ge and As substitutional and vacancy in the presence of a Silicon 90° partial dislocation. For As substitutional and a vacancy in a SP reconstruction we observe an energy difference, for one site far from the core and one inside the core, of 0.26 eV and 0.98 eV respectively in favor of a site in the core. Although for Ge substitutional no preference were observed. Concluding, we did a comparative study between LDA (GGA) approxi­mation with norm-conserving (ultrasoft) pseudopotential for SP and DP recons­tructions in a 90° partial dislocation. Now, using an ultrasoft pseudopotential and GGA, we did a systematic study of a vacancy in both reconstructions. This shows that for DP, the most energetically site is not in the core, but above it with an energy difference of the 0.31 eV related to the most energetically favorable site in the core.