Layer-Resolved Ultrafast XUV Measurement of Hole Transport in a Ni-TiO2-Si Photoanode

TL;DR

This study uses ultrafast XUV pulses to measure layer-specific charge transport in a Ni-TiO2-Si junction, revealing ultrafast hole dynamics and recombination processes crucial for electronic device performance.

Contribution

It introduces a broadband XUV technique for element-specific, layer-resolved measurement of ultrafast charge transport in metal-oxide-semiconductor junctions, providing new insights into hole dynamics.

Findings

Holes transport from Si to Ni in ~100 fs

Electrons remain on Si initially

Hole back-diffusion and recombination observed

Abstract

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.