Pulsed Laser Deposition Growth of Delafossite (CuFeO$_{2})$ thin films and multilayers*

Owing to its narrow band gap (\textless 2 eV) and p-type conductivity delafossite CuFeO$_{2}$ is attractive for applications in the field of solar energy conversion. Obtaining pure phase CuFeO$_{2}$ thin films, however, is relatively difficult. It is necessary to maintain the lowest possible Cu valency ($+$1) in order to avoid forming the comparably stable spinel compound CuFe$_{2}$O$_{4}$. We present a systematic study of the pulsed laser deposition (PLD) growth conditions for epitaxial (00.1) oriented CuFeO$_{2}$ thin films on Al$_{2}$O$_{3}$ (00.1) substrates. The secondary impurity phase, CuFe$_{2}$O$_{4}$, was removed completely by optimizing the growth conditions. RHEED, XRD and TEM showed that the pure phase delafossite films are highly epitaxial to the substrate. The chemical purity was verified by Raman and XPS. The indirect bandgap of 1.15 eV was measured using infrared reflectivity, and is in agreement with the CuFeO$_{2}$ bulk value. Finally, we discuss the growth and structural characterization of delafossite multilayers, CuFeO$_{2}$/CuGaO$_{2}$.