Center for Functional Nanomaterials Seminar

Semiconductor nanowires (NWs) are one of the most promising building blocks for near future nano-electronics because they provide a new route to continuing miniaturization as well as a wealth of opportunities in nanoscale science and technology. There are many NW systems in which the majority carrier type is controlled by intentional doping, but in all cases the actual dopant concentration, its spatial distribution and the fraction of active dopants are unknown. We use Kelvin Probe Force Microscopy (KPFM) to measure both the longitudinal and radial dopant distribution in Phosphorous-doped SiNWs grown by the Vapor-Liquid-Solid (VLS) method. The results show a non-uniform doping profile along the SiNWs, and this is explained due to vapor-solid surface doping during their growth. This process also induces inhomogeneous radial doping profile which was observed via successive chemical etching of a single NW and measuring its surface potential using KPFM. We find that the radial active dopant distribution within a single n-type silicon nanowire decreases by almost two orders of magnitude from the wire surface to its core. Furthermore, the dopant profile is consistent with a very large diffusion coefficient of phosphorous in the silicon nanowires of D ~1x10-19 m2s-1. We have also used KPFM to image individual deep acceptor type trapping centers in a single undoped SiNW, grown with an Au catalyst. The switching between occupied and empty trap states is reversibly controlled by the back-gate potential of such intrinsic nanowire transistors. In addition, we extract a trap energy level of EC-ET=0.65eV±0.1V and estimate a concentration of~2x1016 cm-3. These trapping centers may be related to the unintentional incorporation of Au atoms during the VLS growth.