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 sulphur segregation in pure tilt germanium {710}〈001〉, Σ = 25 (θ = 16.26°) and {551}〈011〉, Σ = 51 (θ = 16.10°) grain boundaries (GBs) were investigated using high-resolution electron microscopy coupled to electron-energy-loss spectroscopy and supported by structural modelling and image simulations. Our results showed that the as-grown Σ = 25 GB is composed of two parts: a stable structural region and a variable perturbed core. On the basis of our simulations, it is shown that this boundary can only be formed by a multiplicity of configurations which are energetically close to each other but differently configured along the boundary plane. When sulphurized, drastic changes in the structure of the GB were observed. Energy-filtered electron microscopy imaging revealed a sulphur enrichment at the perturbed part of the boundary. Although sulphur segregation at the boundary is detected, no information can at the present stage be extracted on segregation sites and bonding configurations because of the complexity of the boundary structure. To simplify this aspect, a simpler GB, that is germanium Σ = 51, was studied. The structure of such a GB is a well-known configuration, that is a Lomer dislocation, which is basically a fivefold ring adjacent to a sevenfold ring. After sulphur treatment, high-resolution electron microscopy imaging also shows significant contrast modifications apparently concentrated on the dislocation core. Chemical imaging indicates again the presence of sulphur enrichment along the boundary plane strongly sustaining that equilibrium sulphur segregation in the Ge(S) system occurs into the GB and therefore confirms our previous results on the Σ = 25 GB. One can therefore argue that it is the presence of those odd-membered rings at the boundary, which should possess a specific crystallographic and electronic nature, coupled to the electronic properties of sulphur, that are responsible for the preferential segregation into the boundary.