Bonding structure and morphology of chromium oxide films grown by pulsed-DC reactive magnetron sputter deposition

Chromium oxide (CrO) thin films were grown by pulsed-DC reactive magnetron sputter deposition in an Ar/O discharge as a function of the O fraction in the gas mixture (f) and for substrate temperatures, T, up to 450 °C. The samples were analysed by Rutherford backscattering spectrometry (RBS), spectroscopic ellipsometry (SE), atomic force microscopy (AFM), scanning (SEM) and transmission (TEM) electron microscopy, X-ray diffraction (XRD), and X-ray absorption near-edge structure (XANES). On unheated substrates, by increasing f the growth rate is higher and the O/Cr ratio (x) rises from ∼2 up to ∼2.5. Inversely, by increasing T the atomic incorporation rate drops and x falls to ∼1.8. XRD shows that samples grown on unheated substrates are amorphous and that nanocrystalline CrO (x = 1.5) is formed by increasing T. In amorphous CrO, XANES reveals the presence of multiple Cr environments that indicate the growth of mixed-valence oxides, with progressive promotion of hexavalent states with f. XANES data also confirms the formation of single-phase nanocrystalline CrO at elevated T. These structural changes also reflect on the optical and morphological properties of the films.Financial support from grants FIS2012-38866-C05-05 (Ministerio de Economía y Competitividad, Spain) and P2013/MIT-2775 (Comunidad Autónoma de Madrid, Spain) is greatly acknowledged. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement n.° 226716. Finally, ARC acknowledges funding from “Juan de la Cierva” program by MINECO under contract number JCI-2012-14509