Is carrier mobility a limiting factor for charge transfer in TiO₂/Si devices? A study by transient reflectance spectroscopy

Abstract

<p>TiO₂ coatings are often deposited over silicon-based devices for surface passivation and corrosion protection. However, the charge transfer (CT) across the TiO₂/Si interface is critical as it may instigate potential losses and recombination of charge carriers in optoelectronic devices. Therefore, to investigate the CT across the TiO₂/Si interface, transient reflectance (TR) spectroscopy was employed as a contact-free method to evaluate the impact of interfacial SiO_x, heat-treatments, and other phenomena on the CT. Thin-film interference model was adapted to separate signals for Si and TiO₂ and to estimate the number of transferred carriers. Charge transfer velocity was found to be 5.2 × 10⁴ cm s⁻¹ for TiO₂ heat-treated at 300 °C, and even faster for amorphous TiO₂ if the interfacial SiO_x layer was removed using HF before TiO₂ deposition. However, the interface is easily oversaturated because of slow carrier diffusion in TiO₂ away from the TiO₂/Si interface. This inhibits CT, which could become an issue for heavily concentrated solar devices. Also, increasing the heat-treatment temperature from 300 °C to 550 °C has only little impact on the CT time but leads to reduced carrier lifetime of ¡3 ns in TiO₂ due to back recombination via the interfacial SiO_x, which is detrimental to TiO₂/Si device performance.</p>