Measurements of Bandgap of Epitaxial BiFeO$_{3}$ Films by UV-VIS Absorption and Cathodoluminescence Spectroscopies

We report measurements of the bandgap of pure-phase epitaxial BiFeO$_{3}$ thin films on (001)-oriented SrTiO$_{3}$ substrates, via UV-VIS absorption and cathodoluminescence (CL) spectroscopies. 70 nm thick BiFeO$_{3}$ films were grown using ultrahigh vacuum RF magnetron sputtering at substrate temperatures between 500 \r{ }C and 600 \r{ }C. X-ray diffractometry shows that samples grown in this temperature range are epitaxial and pure-phase. UV-VIS absorption spectra show a consistent bandgap of 2.5 $\pm $ 0.03 eV for all growth temperatures. A small tail in the UV-VIS absorption spectra just below the band gap extends down to 2.2 eV, indicating some electronic states within the bandgap. The bandgap was confirmed via CL measurements, where a bandgap of 2.46 $\pm $ 0.01 eV was obtained for samples at growth temperatures of 550 \r{ }C and 600 \r{ }C. To our knowledge, this report is the first sdetailed measurement of electronic band and defect structure for epitaxial BiFeO$_{3}$ films and confirms theoretical predictions.