Nanobeam X-ray Fluorescence Dopant Mapping Reveals Dynamics of in Situ Zn-Doping in Nanowires

The properties of semiconductors can be controlled using doping, making it essential for electronic and optoelectronic devices. However, with shrinking device sizes it becomes increasingly difficult to quantify doping with sufficient sensitivity and spatial resolution. Here, we demonstrate how X-ray fluorescence mapping with a nanofocused beam, nano-XRF, can quantify Zn doping within in situ doped III-V nanowires, by using large area detectors and high-efficiency focusing optics. The spatial resolution is defined by the focus size to 50 nm. The detection limit of 7 ppm (2.8 × 10¹⁷ cm^-3), corresponding to about 150 Zn atoms in the probed volume, is bound by a background signal. In solar cell InP nanowires with a p-i-n doping profile, we use nano-XRF to observe an unintentional Zn doping of 5 × 10¹⁷ cm^-3 in the middle segment. We investigated the dynamics of in situ Zn doping in a dedicated multisegment nanowire, revealing significantly sharper gradients after turning the Zn source off than after turning the source on. Nano-XRF could be used for quantitative mapping of a wide range of dopants in many types of nanostructures.