Morphology and crystallization of ThO2 thin films on polycrystalline Ir

Abstract As part of a study of low work function coatings for enhanced electron thermionic and field emission, very thin films of ThO 2 (~ 40 nm thick) grown by physical vapor deposition on polycrystalline Ir have been characterized by Auger electron spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy. Following resistive heating to > 1750 K strong morphological heterogeneity was observed in the ThO 2 film suggesting significant sensitivity to local surface variations in the polycrystalline Ir substrate. Observed growth paradigms include: step-flow, quasi-laminar and triangular hillocks; pyramidal structures on large terrace regions; highly anisotropic growth and coalescence of pyramids; and overlapping regions where multiple growth modes exist. Despite the variety of morphological structures, XRD confirmed a preferred (111) orientation for the ThO 2 fluorite crystallites, which is in good agreement with theoretical predictions and previous work on ThO 2 films. Further, analogous behavior of calcium fluorite thin films, where similar morphological features were observed, has been reported. Among the morphologies in the ThO 2 /Ir system, quasi-laminar growth suggests the possibility of improving the efficacy of very thin films of ThO 2 for enhanced electron thermionic and field emission.