Non-substitutional Sn defects in Ge-(1-x)Sn_x alloys for opto-and nanoelectronics

R. A. BARRIO; J. D. QUERALES FLORES; J. D. FUHR; C. I. VENTURA

JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM

SPRINGER

Lugar: Berlin; Año: 2012 vol. 26 p. 2213 - 2217

1557-1939

Important technological applications are envisaged for Ge1−x Snx alloys. They provide a route to obtain direct-gap group IV materials, tuneable by concentration. Therefore, these alloys are ideal candidates for optoelec-tronic devices, highly compatible with Si integrated circuits. Contrary to other binary alloys with group IV elements, homogeneous Ge1−x Snx alloys, as required for device applications, have proven difficult to form above a certain temperature-dependent critical Sn concentration. Through a detailed ab-initio local defect study, and the proposal of astatistical model for the formation of these alloys, we predicted that a new type of Sn defect (β-Sn), consisting of a single Sn atom in the centre of a Ge divacancy, might be formed. The environment of this defect relaxes towards a cubic octahedral configuration, facilitating the nucleation of white tin and its segregation, as found in amorphoussamples. We confirmed that Sn would enter substitutionally in the Ge lattice, but above a temperature-dependent critical concentration, non-substitutional β-Sn defects should be formed, consistent with experimental observations. In this paper we introduce a two-site substitutional equiv alent for the non-substitutional β-Sn defect in Ge, as needed in order to be able to include β-Sn in electronic structure calculations with effective-field electronic models for disorder, like the Virtual Crystal Approximation (VCA). The equivalent substitutional model is derived in order to takeinto account the different symmetries in the immediate environment of the substitutional α and non-substitutional β defect sites and their effect on the electronic structure.