Reacted amorphous layers: Tantalum and niobium oxides

The main rule governing the formation of amorphous reacted phases is that one of the components should have a mobility much lower than that of the others. This is based on the simple observation that the crystallization of a multicomponent system requires that all of the components should be mobile. The rule, which is general and independent of the initial state (solid or gas) of the reactants, is verified by comparing the mobilities of oxygen and of the metal atoms during the low-temperature oxidation and subsequent high-temperature heat treatment of bilayers initially composed of niobium and tantalum metal thin films. The high mobility of oxygen allows the oxides to be formed at low temperature in an amorphous condition, while the low mobility of the metal ions prevents the mixing of these ions except at high temperatures.