Comparative study of the structural and optical properties of epitaxial CuFeO2 and CuFe1 - xGaxO2 delafossite thin films grown by pulsed laser deposition methods

Three samples of epitaxial delafossite CuFeO2 and CuFe1 − xGaxO2 films were grown using Pulsed Laser Deposition techniques in high vacuum. The sample thicknesses were estimated to be 21 nm, 75 nm for the CuFeO2 films and ~ 37 nm for the composite sample containing gallium. The estimated gallium fraction of substituted ferric atoms was x = 0.25 for the composite sample. We present the study of the fundamental band gap(s) for each sample via observation of their respective optical absorption properties in the NIR-VIS region using transmittance and diffuse reflection spectroscopy. Predominant absorption edges measured at 1.1 eV and 2.1 eV from transmittance spectra were observed for the CuFeO2 samples. The sample of CuFe1 − xGaxO2 showed a measurable shift to 1.5 eV of the lower band-gap and a strong absorption edge located at 2.3 eV attributed to direct band to band transitions. This study also found evidence of changes between apparent absorption edges between transmittance and diffuse reflectance spectroscopies of each sample and it may be resultant from absorption channels via surface states.