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 precursor. The growth mode has previously been described as CrO3 vaporizing in the 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 two completely different methods, namely, molecular-beam epitaxy (MBE) and CVD. We focus on the CVD process itself, and show the importance of an intermediate 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 ex situ, and use it directly for the growth of high-quality CrO2 epitaxial layers, avoiding any contamination caused by the decomposition of CrO3 to Cr8O21. We discuss in parallel our failed attempts to deposit CrO2 from either CrO3 or Cr and oxygen plasma by MBE and our experiments with the CVD process, 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 CVD 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 Institute of Physics.