Nanowires with inhomogeneous heterostructures such as polytypes and periodic twin boundaries are interesting due to their potential use as components for
optical, electrical, and thermophysical applications. Additionally, the incorporation
of metal impurities in semiconductor nanowires could substantially alter their
electronic and optical properties. In this highlight article, we review our recent
progress and understanding in the deliberate induction of imperfections, in terms of both twin boundaries and additional impurities in germanium nanowires for new/enhanced functionalities. The role of catalysts and catalyst–nanowire interfaces for the growth of engineered nanowires via a three-phase paradigm is explored. Three-phase bottom-up growth is a feasible way to incorporate and engineer imperfections such as crystal defects and impurities in semiconductor
nanowires via catalyst and/or interfacial manipulation. “Epitaxial defect transfer”
process and catalyst–nanowire interfacial engineering are employed to induce
twin defects parallel and perpendicular to the nanowire growth axis. By inducing
and manipulating twin boundaries in the metal catalysts, twin formation and
density are controlled in Ge nanowires. The formation of Ge polytypes is also
observed in nanowires for the growth of highly dense lateral twin boundaries.
Additionally, metal impurity in the form of Sn is injected and engineered via
third-party metal catalysts resulting in above-equilibrium incorporation of
Sn adatoms in Ge nanowires. Sn impurities are precipitated into Ge bi-layers
during Ge nanowire growth, where the impurity Sn atoms become trapped with
the deposition of successive layers, thus giving an extraordinary Sn content
(>6 at.%) in Ge nanowires. A larger amount of Sn impingement (>9 at.%) is further encouraged by utilizing the eutectic solubility of Sn in Ge along with impurity
trapping.