Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO-Si photoanode.

Metal oxide semiconductor junctions are central to most electronic and optoelectronic devices, but ultrafast measurements of carrier transport have been limited to device-average measurements. Here, charge transport and recombination kinetics in each layer of a Ni-TiO-Si junction is measured using the element specificity of broadband extreme ultraviolet (XUV) ultrafast pulses. After silicon photoexcitation, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in characteristic spectral shifts in the XUV edges. Meanwhile, the electrons remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiO, shifting the Ti spectrum to a higher oxidation state, followed by electron-hole recombination at the Si-TiO interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously.