Electronic Properties of Extended Defects in Germanium: A First-Principles Study

Germanium, with its superior carrier mobility and potential for direct band gap engineering, is an attractive material for advanced electronic, optoelectronic, and quantum applications. However, its epitaxial growth on silicon frequently incorporates extended defects such as dislocations and stacking faults, an issue well-documented in the literature for decades and considered inevitable. These defects significantly alter germanium's electronic properties. Recent advancements in computational technologies enable us to explore the impact of these defects through ab initio calculations assisted by machine learning-based interatomic potentials. Dislocations, modelled using dipoles within periodic boundary conditions, create electronic trap states within the band gap, which may affect device functionality but could also be leveraged for engineering optical and spin properties. In contrast, stacking faults induce hexagonal inclusions that form Type-I quantum wells with a tuneable direct band gap. Recent experiments on epitaxially grown germanium nanostructures have confirmed the presence of hexagonal inclusions, exhibiting strong photoluminescence qualitatively comparable with our theoretical predictions. Our results suggest that these extended defects, traditionally seen as detrimental, can be harnessed to tailor the optoelectronic properties of germanium, opening new pathways for quantum technologies and photonic devices.