title: Bias dependence of sub-bandgap light detection for core-shell silicon nanowires.
abstract: We experimentally demonstrate a vertically arrayed silicon nanowire-based device that exhibits voltage dependence of photoresponse to infrared sub-bandgap optical radiation. The device is fabricated using a proximity solid-state phosphorus diffusion method to convert the surface areas of highly boron-doped silicon nanowires into n-type, thus forming a radial core-shell p-n junction structure. Prominent photoresponse from such core-shell Si nanowires is observed under sub-bandgap illumination at 1310 nm. The strong bias dependence of the photoresponse and other device characteristics indicates that the sub-bandgap absorption is attributed to the intrinsic properties of core-shell Si nanowires rather than the surface states. The attractive characteristics are based on three physical mechanisms: the Franz-Keldysh effect, quasi-quantum confinement effect, and the impurity-state assisted photon absorption. The first two effects enhance carrier tunneling probability, rendering a stronger wave function overlap to facilitate sub-bandgap absorption. The last effect relaxes the k-selection rule by involving the localized impurity states, thus removing the limit imposed by the indirect bandgap nature of Si. The presented device uses single-crystal silicon and holds promise of fabricating nanophotonic systems in a fully complementary metal-oxide-semiconductor (CMOS) compatible process. The concept and approach can be applied to silicon and other materials to significantly extend the operable wavelength regime beyond the constraint of energy bandgap.
authors.affiliation: Department of Electrical and Computer Engineering, Jacobs School of Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0409, USA. y5zhou@ucsd.edu
authors.forename: Yuchun
authors.initials: Y
authors.lastname: Zhou
authors.forename: Yu-hsin
authors.initials: YH
authors.lastname: Liu
authors.forename: James
authors.initials: J
authors.lastname: Cheng
authors.forename: Yu-Hwa
authors.initials: YH
authors.lastname: Lo