Development of a machine learning interatomic potential for exploring pressure-dependent kinetics of phase transitions in germanium.

We introduce a data-driven potential aimed at the investigation of pressure-dependent phase transitions in bulk germanium, including the estimate of kinetic barriers. This is achieved by suitably building a database including several configurations along minimum energy paths, as computed using the solid-state nudged elastic band method. After training the model based on density functional theory (DFT)-computed energies, forces, and stresses, we provide validation and rigorously test the potential on unexplored paths. The resulting agreement with the DFT calculations is remarkable in a wide range of pressures. The potential is exploited in large-scale isothermal-isobaric simulations, displaying local nucleation in the R8 to ß-Sn pressure-induced phase transformation, taken here as an illustrative example.

Nanostructured 3C-SiC on Si by a network of (111) platelets: a fully textured film generated by intrinsic growth anisotropy.

In this paper, we address the unique nature of fully textured, high surface-to-volume 3C-SiC films, as produced by intrinsic growth anisotropy, in turn generated by the high velocity of the stacking fault growth front in two-dimensional (111) platelets. Structural interpretation of high resolution scanning electron microscopy and transmission electron microscopy data is carried out for samples grown in a hot-wall low-pressure chemical vapour deposition reactor with trichlorosilane and ethylene precursors, under suitable deposition conditions. By correlating the morphology and the X-ray diffraction analysis we also point out that twinning along (111) planes is very frequent in such materials, which changes the free-platelet configuration.

Anisotropic extended misfit dislocations in overcritical SiGe films by local substrate patterning.

In this work we will show how local substrate patterning leads to a long range controlled propagation of dislocations in SiGe films grown on Si(001) substrates. Dislocations preferentially nucleate in the inhomogeneous strain field associated with the patterned pits, and then partialize on the local (111) surfaces which form the pit sidewalls. The resulting V-shaped defects extend for several microns and effectively block the propagation of randomly nucleated dislocations which propagate in the perpendicular direction. The surface morphology and strain fields associated with the extended defects have been characterized by atomic force microscopy and µRaman spectroscopy, and the defects have been directly observed with high resolution transmission electron microscopy.

Critical shape and size for dislocation nucleation in Si1-xGex islands on Si(001).

The critical volume for the onset of plastic strain relaxation in SiGe islands on Si(001) is computed for different Ge contents and realistic shapes by using a three-dimensional model, with position-dependent dislocation energy. It turns out that the critical bases for dome- and barnlike islands are different for any composition. By comparison to extensive atomic force microscopy measurements of the footprints left on the Si substrates by islands grown at different temperatures (and compositions), we conclude that, in contrast with planar films, dislocation nucleation in 3D islands is fully thermodynamic.