Electronic structure and morphology of chromium oxide thin films grown by reactive pulsed magnetron sputtering

Chromium oxide appears in different phases such as CrO (II), Cr₂Or₃ (III) CrO₂ (IV), and CrO₃ (VI); the number in brackets denoting the valence of the Cr atoms. This structural variety results in a wide range of applications. For example, Cr₂O₃ is the most stable oxide under normal conditions and exhibits high hardness and low friction, being used as protective coating in magnetic recording devices or solar absorber materials.¹ CrO₂ is a half-metallic ferromagnet and, therefore, good candidate for Spintronics.² Finally, CrO₃ is highly toxic, corrosive, and carcinogenic; but widely used in electroplating.³ Clearly, practical applications demand processing with a precise control of the phase formation. In this work, CrO_x films were grown by pulsed magnetron sputtering with different Ar/O₂ mixtures and substrate temperatures up to 500ºC. The samples were analyzed by Rutherford backscattering, ellipsometry, atomic force microscopy, transmission and scanning electron microscopies, X-ray diffraction and X-ray absorption spectroscopy. On unheated substrates, films exhibit X-ray amorphous character and a direct correlation between the O₂ content in the gas, [O₂], and the growth rate and stoichiometry. Remarkably, a range of mixed-valence oxides is observed, with higher valence Cr states promoted with [O₂]. On the contrary, substrate heating favors Cr₂O₃ formation and, accordingly, single-phase nanocrystalline (nc) films are identified. The amorphous films are compact, displaying a surface morphological transition from granular-like to smooth with [O₂]. Upon heating, the morphology develops into a porous fibrillar-like structure, as evidenced by empty voids between nc-Cr₂O₃ grains. These results show a clear correlation between the bonding structure and the microstructure of the different CrO_x structures. REFs: ¹ Hones et al. Surf. Coat. Technol. 120-121 (1999) 277; ² Stewart et al. Phys. Rev. B 79 (2009) 144414; ³ Sarto et al. Carcinogenesis 3 (1982) 1011.