First-principles study of planar Humble defects in Ge and GeSi alloys

Planar defects are widespread in group IV elements. The Humble defect is a special type of {001} planar defect that has gained much attention in recent years since its experimental observation in Ge and Ge_0.8Si_0.2. A detailed study of the electronic properties of Humble defects is still lacking in the literature. In this work, we perform first-principles density functional theory (DFT) calculations to study Humble defects in both Ge and Ge_0.8Si_0.2. We further compare our theoretical results with Si L3 edge electron energy loss spectra (EELS) measured at room temperature, finding excellent agreement, but only when core-hole effects are accounted for. Moreover, a comparison of the EELS observed in the vicinity of the Humble defect versus in the bulk provides a fingerprint for different types of local atomic-bonding environments in Ge_0.8Si_0.2. Our DFT calculations reveal that Humble defects can enlarge the electronic band gap, and hence may potentially be used in band engineering. DFT studies using a hybrid functional, which provides an improved description of band gaps, are also presented.