Measurement of active dopant distribution and diffusion in individual silicon nanowires

We have measured the radial distribution and diffusion of active dopant atoms in individual silicon nanowires grown by the vapor?liquid?solid (VLS) method. Our method is based on successive surface etching of a portion of a contacted nanowire, followed by measurement of the potential difference between the etched and unetched areas using Kelvin probe force microscopy (KPFM). The radial dopant distribution is obtained by fitting the measured potentials with a three-dimensional solution of Poisson equation. We find that the radial active dopant distribution decreases by almost 2 orders of magnitude from the wire surface to its core even when there is no indication for tapering. In addition, the dopant profile is consistent with a very large diffusion coefficient of D ∼ 1 - 10¹⁹ m² s^?1. This implies that phosphorus (P) diffusion during the VLS growth is remarkably high and subsequent thermal annealing must be used when a homogeneous dopant distribution is required.