Vacancy-mediated climbing motion of dislocations in Ge/Si films: atomic-scale insights via molecular dynamics

Silicon-germanium and pure germanium are appealing for the microelectronic industry because of their full compatibility with silicon technology combined with several attractive properties, including high saturation velocity, high carrier mobility, and adjustable bandgap. Epitaxial deposition of Ge (or SiGe) on Si is unavoidably accompanied, at least for thick enough films, by nucleation of dislocations. Reducing the density of linear defects threading through the film and reaching the free surface, i.e. the typical active device region, is one of the main present goals of the wide industrial and academic community devoted to the integration of different materials on Si. Threading dislocations, indeed, can be detrimental in terms of device performances. A profound understanding of the microscopic mechanisms determining the typical arrays of dislocations can surely help in devising strategies for controlling the evolution of the defects. Molecular Dynamics (MD) is a valid tool that can be used to shed light on dislocation dynamics. Indeed, several works appeared in the Literature, analyzing dislocation gliding in semiconductor thin films [1, 2]. In non-standard conditions, at high enough temperatures and/or in the presence of a sufficient density of point defects, however, dislocations can move also via climbing. At variance with gliding, climbing does not involve the movement of the defect along the typical glide planes. Here we present a set of classical MD simulations, made using the LAMMPS code [3], based on the Tersoff potential [4], aimed at understanding the atomic-scale mechanisms leading to climbing towards the Si/Ge interface of typical dislocations in a Ge/Si(001). To observe climbing we have devised the following original procedure. Straight dislocation are conveniently inserted in a simulation cell including a Ge film over a Si substrate. At a certain rate, individual vacancies are created in the cell and evolved for a certain time at a high-temperature T= 1100K, sufficient to allow for vacancy diffusion to the core of the dislocation. We observe vacancies progressively decorating the dislocation core until a shift of the full core is achieved. Based on the trajectories observed in our MD simulations we can directly observe the influence of the dislocation on the random walk of the vacancies, which are irreversibly attracted towards the core of the defect. Clustering of vacancies both at the core or in its proximity is however observed in some cases, hindering full climbing. The mechanism leading to disgregation and reformation of the core is very clearly observed at the atomic scale, providing new interesting details on a mechanism poorly investigated so far in the literature. [1] A. Marzegalli, Phys Rev B, 88, 165418 (2013). [2] A. Marzegalli, Appl Phys Lett 86, 041912 (2005). [3] S. Plimpton, J Comp Phys, 117, 1-19 (1995). [4] J. Tersoff, Phys Rev B, 39, 5566 (1989). [5] A. Stukowski, Modelling Simul. Mater. Sci. Eng. 18, 015012 (2010).