  Metal-oxide-semiconductor junctions are central to most electronic and
optoelectronic devices. Here, the element-specificity of broadband extreme
ultraviolet (XUV) ultrafast pulses is used to measure the charge transport and
recombination kinetics in each layer of a Ni-TiO2-Si junction. After
photoexcitation of silicon, holes are inferred to transport from Si to Ni
ballistically in ~100 fs, resulting in spectral shifts in the Ni M2,3 XUV edge
that are characteristic of holes and the absence of holes initially in TiO2.
Meanwhile, the electrons are observed to remain on Si. After picoseconds, the
transient hole population on Ni is observed to back-diffuse through the TiO2,
shifting the Ti spectrum to 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.
