Temperature evolution of the band-gap in BiFeO3 traced by resonant Raman scattering

TL;DR

This study uses resonant Raman scattering across multiple wavelengths to investigate how the electronic band-gap of BiFeO3 changes with temperature, revealing the shrinking of an indirect band-gap while direct transitions stay stable.

Contribution

It provides new insights into the temperature-dependent electronic band structure of BiFeO3 using a novel multi-wavelength Raman approach.

Findings

The optical band-gap variation is linked to the shrinking of an indirect electronic band-gap.

Resonant Raman signatures identify direct and indirect electronic transitions and in-gap levels.

The energies of direct electronic transitions are nearly temperature independent.

Abstract

Knowledge of the electronic band structure of multiferroic oxides, crucial for the understanding and tuning of photo-induced effects, remains very limited even in the model and thoroughly studied BiFeO3. Here, we investigate the electronic band structure of BiFeO3 using Raman scattering with twelve different excitation wavelengths ranging from the blue to the near infrared. We show that resonant Raman signatures can be assigned to direct and indirect electronic transitions, as well as in-gap electronic levels, most likely associated to oxygen vacancies. Their temperature evolution establishes that the remarkable and intriguing variation of the optical band-gap can be related to the shrinking of an indirect electronic band-gap, while the energies for direct electronic transitions remains nearly temperature independent.