Thickness-Dependent Band Gap Modification in BaBiO$_{3}$

TL;DR

This study investigates how the band gap of BaBiO₃ thin films varies with thickness, revealing a transition from wide-gap to small-gap semiconductor without metallicity, linked to the oxygen breathing mode.

Contribution

It demonstrates the thickness-dependent band gap modification in BaBiO₃ and correlates it with the oxygen breathing mode, providing insights into its electronic phase behavior.

Findings

Band gap decreases with decreasing film thickness.

No metallic state observed even in ultra-thin films.

Band gap size correlates with Raman breathing mode intensity.

Abstract

The material BaBiO$_{3}$ is known for its insulating character. However, for thin films, in the ultra-thin limit, metallicity is expected because BaBiO$_{3}$ is suggested to return to its undistorted cubic phase where the oxygen octahedra breathing mode will be suppresse as reported recently. Here, we confirm the influence of the oxygen breathing mode on the size of the band gap. The electronic properties of a BaBiO$_{3}$ thickness series are studied using \textit{in-situ} scanning tunneling microscopy. We observe a wide-gap ($E_\textrm{G}$~$>$ 1.2 V) to small-gap~($E_\textrm{G}$~$\approx$ 0.07 eV) semiconductor transition as a function of a decreasing BaBiO$_{3}$ film thickness. However, even for an ultra-thin BaBiO$_{3}$ film, no metallic state is present. The dependence of the band gap size is found to be coinciding with the intensity of the Raman response of the breathing phonon mode as a function of thickness.