Non-substitutional single-atom defects in the Ge_{1-x}Sn_{x} alloy

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

PHYSICAL REVIEW B - CONDENSED MATTER AND MATERIALS PHYSICS

AMER PHYSICAL SOC

Año: 2009 vol. 79 p. 155202 - 155202

0163-1829

Ge-{1−x}Sn_x alloys have proved difficult to form at large x, contrary to what happens with other group IV semiconductor combinations. However, at low x they are typical examples of well-behaved substitutional >compounds, which is desirable for harnessing the electronic properties of narrow-band semiconductors. In this paper, we propose the appearance of another kind of single-site defect beta-Sn, consisting of a single Sn atom in the center of a Ge divacancy, that may account for these facts. Accordingly, we examine the electronic and structural properties of these alloys by performing extensive numerical ab initio calculations around local defects. The results show that the environment of the beta defect relaxes toward a cubic octahedral configuration, facilitating the nucleation of metallic white tin and its segregation, as found in amorphous samples. Using the information stemming from these local defect calculations, we built a simple statistical model to investigate at which concentration these beta defects can be formed in thermal equilibrium. These results agree remarkably well with experimental findings, concerning the critical concentration above which the homogeneous alloys cannot be formed at room temperature. Our model also predicts the observed fact that at lower temperature the critical concentration increases. We also performed single-site effective-field calculations of the electronic structure.