Bias Dependence of Sub-Bandgap Light Detection for Core―Shell Silicon Nanowires

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.