Oxygen plasma-assisted epitaxial growth of tetragonal CuO(001) and CoxCu1-xO(001) thin films

The growth of ultrathin tetragonal CuO films by pulsed laser deposition was reported several years ago. In this work, we demonstrate that the same phase can be synthesized using oxygen-plasma–assisted molecular beam epitaxy. We show that it remains stable up to a thickness of 6 nm, significantly exceeding the thickness range previously achieved by PLD. Using x-ray diffraction, we resolve its crystal structure and confirm that it consists of a tetragonally distorted rock-salt phase in coherent epitaxy on SrTiO 3 (001), as initially hypothesized. We precisely determine the tetragonal distortion, obtaining a c/a ratio of 1.342(2) for a 3 nm thick film. We also show, using resonant x-ray diffraction, that Co can partially substitute for Cu without altering the crystal structure. The structural stability of this novel CuO phase upon doping is a key asset for potential future applications in both magnetism and superconductivity.