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

Relatore: Cecilia I. Ventura, Centro Atomico Bariloche (AR)

Aula Newton
04 Marzo 2008 ore 15.30

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 tuning the electronic properties of narrow band semiconductors. We propose the appearance of another kind of single-site defect( beta-Sn) that may account for these facts. Accordingly, we have examined 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-Sn defect relaxes towards a cubic octahedral configuration, facilitating the nucleation of metallic white tin and, thus, segregation as found in amorphous samples. Using the information stemming from these local defect calculations, we also built a simple statistical model to investigate at which concentration these beta-Sn defects can be formed in thermal equilibrium. These results coincide with experimental findings, concerning the critical concentration above which the homogeneous alloys cannot be formed at room temperature. Our model also predicts that at lower temperature the critical concentration increases, as observed. Single site effective-field calculations we performed, provide further support to our hypothesis. ** Collaboration work with: Rafael Barrio (Inst. de Fisica, UNAM, Mexico); Javier Fuhr (C.A.B., Argentina) and Ruben Weht (Centro Atomico Constituyentes, Buenos Aires, Argentina).