UC Berkeley

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-TiO₂-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 TiO₂, shifting the Ti spectrum to a higher oxidation state, followed by electron-hole recombination at the Si-TiO₂ interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO₂ and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously.