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Closing the gap between experiments and simulations in the investigation of high-pressure silicon phase transitions calls for advanced, new-generation modeling approaches. By exploiting massive parallelization, we here provide molecular dynamics (MD) simulations of Si nanoindentation based on the Gaussian Approximation Potential (GAP). Results are analyzed by exploiting a customized Neural Network Phase Recognition (NN-PR) approach, helping to shed light on the phase transitions occurring during the simulations. Our results show that GAP provides a realistic description of silicon phase transitions. With the support of NN-PR method, the formation mechanism and stability of high-pressure phases are comprehensively studied. Additionally, we also show how simulations based on the less demanding and widely-used Tersoff potential are still useful to investigate the role played by the indenter tip modeling. However, high-pressure phases obtained with GAP are more consistent with observations made in nanoindentation experiments, removing a spurious phase that is shown by Tersoff simulations. This behavior is explained on the base of relative phase stability with comparison with Density Functional Theory (DFT) calculations. This work provides insight into the application of state-of-the-art Machine Learning (ML) methods on nanoindentation simulations, enabling further understanding of the phase transition mechanisms in silicon.

The indirect electronic bandgap is the Achille’s heel of Silicon, hindering monolithic integration of lasers for Silicon photonics. Metastable hexagonal polytypes of SiGe are very promising to achieve the direct gap within the Si technology. The main approaches to get these hexagonal polytypes and particularly the hexagonal diamond (2H) phase are discussed. The best quality of 2H Si and Ge has been obtained by exploiting core/shell nanowires [1]: a wurtzite GaAs/P core provides the crystallographic template for the Si/Ge hexagonal shell and together with the lower surface energy of the hexagonal phases allows his epitaxial growth [2]. We will discuss the main problems of this approach from a theoretical perspective, particularly focusing on crystalline defects affecting the hexagonal Si/Ge nanowires (Fig. 1). Then, pressure induced phase transitions in Si and Ge will be presented, focusing on the multiscale modelling of nanoindentation process. In this contest, results obtained with unique methods and novel tools, such as solid-state nudge elastic band (NEB) and machine learning Interatomic potentials, will be also presented [3]. Finally, 2D -hexagonal inclusions in Si and Ge will be discussed. Classically, these would be extended crystalline defects in the cubic phase of Si and Ge, but we will show how these defects could be exploited to form quantum wells with direct gap, potentially very interesting for quantum and optoelectronic applications. [1] E. Fadaly et al., Nano Lett. 2021, 21, 8, 3619–3625. [2] E. Scalise et al., Appl. Surf. Science 2021, 545, 148948. [3] G. Ge et al., ACTA MATERIALIA, 263(15 January 2024) [10.1016/j.actamat.2023.119465]

Heteroepitaxial films of Ge on Si(001) are receiving wide attention due to several possible applications in micro- and opto-electronics. Understanding the dynamic behavior of linear defects, such as dislocations, is key. They are unavoidably present in such systems due to the lattice mismatch between the two materials, and can directly influence devices performances. It has been experimentally demonstrated more than fifteen years ago that a suitable choice of the growth parameters allows for the formation of a nicely ordered net of 90 ∘ dislocations at the Ge/Si interface, improving the overall film quality and strain relaxation uniformity. Atomic-scale details on the set of mechanisms leading to such an outcome are however still missing. Here we present a set of classical molecular dynamics simulations shedding light on the full set of microscopic processes driving to the experimentally observed array of linear defects. This includes simple gliding of 60 ∘ dislocations and vacancy-promoted climbing and gliding. The importance of the particular experimental conditions, involving a low-temperature stage followed by an increase in temperature, is highlighted.

The crossing of stacking faults (SFs) (namely dislocation forests) in 3C-SiC is experimentally probed to cause detrimental effects on the electronic properties of the layer. By means of classical molecular dynamics simulations, for the first time the evolution of two crossing partial dislocation loops is shown, revealing the mechanism that leads to the formation of a complex defect at their intersection. The obtained atomistic structure is then further refined by ab initio calculations, revealing that this defect does not cause defect states within the gap of the pristine material. The case of the intersection of two double-dislocation loops, that is, involving double SFs, in the hypothesis that the evolution follows the same mechanism found for single-dislocation loops is also investigated. Contrary to the single-plane intersection, the simulations reveal that the junction formed by double-dislocation loops does cause intragap states, thus suggesting detrimental effects on the electronic properties of the 3C-SiC layers.

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.

Inversion domain boundaries are extended defects affecting cubic silicon carbide (3C-SiC) layers grown on the on-axis (001) Si wafers. Their impact on the device performance is still debated and a clear connection to their electronic properties at the atomistic scale is still missing. By comparing the first-principles atomistic models and scanning transmission electron microscopy imaging, the most relevant structures of inversion domain boundaries laying both in the {110} and {111} planes are identified. Their calculated electronic structures shed light on the relevance that these extended defects may have in the degradation of the performances of the 3C-SiC devices.