  Sn-containing Si and Ge alloys belong to an emerging family of semiconductors
with the potential to impact group IV semiconductor devices. Indeed, the
ability to independently engineer both lattice parameter and band gap holds the
premise to develop enhanced or novel photonic, optoelectronic, and electronic
devices. With this perspective, we present detailed investigations of the
influence of Ge1-y-xSixSny layers on the optical properties of Si- and Ge-based
heterostructures and nanowires. We found that adding a thin Ge1-x-ySixSny
capping layer on Si or Ge greatly enhances light absorption especially in the
near IR range leading to an increase in short-circuit current density. For the
Ge1-y-xSixSny structure at thicknesses below 30 nm, a 14-fold increase in the
short-circuit current is predicted with respect to bare Si. This enhancement
decreases by reducing the capping layer thickness. Conversely, decreasing the
shell thickness was found to improve the short-circuit current in
Si/Ge1-y-xSixSny and Ge/Ge1-y-xSixSny core/shell nanowires. The optical
absorption becomes very important when increasing the Sn content. Moreover, by
exploiting optical antenna effect, these nanowires show an extreme light
absorption reaching an enhancement factor, with respect to Si or Ge nanowires,
on the order of ~104 in Si/Ge0.84Si0.04Sn0.12 and ~12 in Ge/Ge0.84Si0.04Sn0.12
core/shell nanowires. Furthermore, we analyzed the optical response of the
addition of a dielectric capping layer consisting of Si3N4 to the
Si/Ge1-y-xSixSny core-shell nanowire and found about 50% increase in
short-circuit current density for a dielectric layer thickness of 45 nm and a
core radius and shell thickness superior to 40 nm. The core/shell optical
antenna benefits from a multiplication of enhancements contributed by leaky
mode resonances in the semiconductor part and antireflection effects in the
dielectric part.
