Structural change induced on an atomic scale by equilibrium sulphur segregation in tilt germanium grain boundaries

In the present study, structural modifications induced by equilibrium sulph ur segregation in pure tilt germanium {710}< 001 >, Sigma = 25 (theta = 16. 26 degrees) and {551}< 011 >, Sigma = 51 (theta = 16.10 degrees) grain boun daries (GBs) were investigated using high-resolution electron microscopy co upled to electron-energy-loss spectroscopy and supported by structural mode lling and image simulations. Our results showed that the as-grown Sigma = 2 5 GB is composed of two parts: a stable structural region and a variable pe rturbed core. On the basis of our simulations, it is shown that this bounda ry can only be formed by a multiplicity of configurations which are energet ically close to each other but differently configured along the boundary pl ane. When sulphurized, drastic changes in the structure of the GB were obse rved. Energy-filtered electron microscopy imaging revealed a sulphur enrich ment at the perturbed part of the boundary. Although sulphur segregation at the boundary is detected, no information can at the present stage be extra cted on segregation sites and bonding configurations because of the complex ity of the boundary structure. To simplify this aspect, a simpler GB, that is germanium Sigma = 51, was studied. The structure of such a GB is a well- known configuration, that is a Lomer dislocation, which is basically a five fold ring adjacent to a sevenfold ring. After sulphur treatment, high-resol ution electron microscopy imaging also shows significant contrast modificat ions apparently concentrated on the dislocation core. Chemical imaging indi cates again the presence of sulphur enrichment along the boundary plane str ongly sustaining that equilibrium sulphur segregation in the Ge(S) system o ccurs into the GB and therefore confirms our previous results on the Sigma = 25 GB. One can therefore argue that it is the presence of those odd-membe red rings at the boundary, which should possess a specific crystallographic and electronic nature, coupled to the electronic properties of sulphur, th at are responsible for the preferential segregation into the boundary.