Epitaxial growth of CrO2 thin films by chemical-vapor deposition from a Cr8O21 precursor

Presently, the best epitaxial thin films of CrO2 are made by chemical-vapor deposition (CVD) in a two-zone furnace with oxygen flow from a CrO3 precur sor. The growth mode has previously been described as CrO3 vaporizing in th e first zone, and thermally decomposing at higher temperature in the second zone onto a substrate. In the more recent works, the focus has been on the properties of the obtained layers rather than on deposition mechanisms. In the present experimental work, we attack the epitaxial growth of CrO2 by t wo completely different methods, namely, molecular-beam epitaxy (MBE) and C VD. We focus on the CVD process itself, and show the importance of an inter mediate compound, Cr8O21, for the growth of CrO2 films. We show that it is not necessary to start the CVD from CrO3; instead, one can prepare Cr8O21 e x situ, and use it directly for the growth of high-quality CrO2 epitaxial l ayers, avoiding any contamination caused by the decomposition of CrO3 to Cr 8O21. We discuss in parallel our failed attempts to deposit CrO2 from eithe r CrO3 or Cr and oxygen plasma by MBE and our experiments with the CVD proc ess, and conclude that CrO3 does not decompose directly to CrO2 and oxygen, as was expected. We propose a hypothesis that the role of Cr8O21 in the CV D process is to exude unstable molecules of CrO4, and that the reaction on the substrate is the decomposition CrO4--> CrO2+O-2. (C) 2001 American Inst itute of Physics.