Heterojunction Internal Photoemission of $\lambda\sim\textsf{2.8}~\mu\text{m}$ by Ge/Si Core/Shell Nanowires

The photodetector of Ge/Si core/shell nano-wires can detect λ ~ 2.8 μm -long infrared, far beyond the absorption edge of both semiconductors, at room temperature. The device of single nanowire grown on heavily doped Si (111) shows typical rectifying behavior despite p-p isotype of nanowire and substrate. Under illumination, the present devices show large responsivity of 35 A/W at -0.5 V. The analysis of current-voltage characteristics shows that Ge/Si nanowire on p-type silicon device follows the model of semiconductor heterojunction rather than Schottky junction. This result implies that the interface between nanowire and substrate is the main barrier of charge transport in the present nanowire infrared detector. In this paper, the parameter values of the heterojunction of nanowire and substrate are quantitatively investigated with the thermionic transport model. The analysis of the energy band structure shows that even longer wavelength infrared can be detected through the photoemission of the holes over the reduced heterojunction barrier of 0.37 eV.