Formation of orthorhombic (Zr,Ta)O₂ in thin Zr-Ta-O films

The formation of orthorhombic (Zr,Ta)O₂ was found in annealed thin Zr-Ta-O films with various tantalum concentrations deposited by co-sputtering a ZrO₂ target and a mixed ZrO₂/Ta₂O₅ target. In the as-deposited state, all films were amorphous. After annealing, tetragonal (Zr,Ta)O₂ for [Ta]/([Ta] + [Zr]) ≤ 0.19 and orthorhombic (Zr,Ta)O₂ for [Ta]/([Ta] + [Zr]) > 0.19 were formed. Thin films with excess of tantalum ([Ta]/([Ta] + [Zr]) ≥ 0.5) decomposed into two orthorhombic phases upon crystallization: (Zr,Ta)O₂ and tantalum-rich (Ta,Zr)₂O₅. The Rietveld analysis of X-ray diffraction patterns revealed that the crystal structure of (Zr,Ta)O₂ can be described with the non-centrosymmetric space group Pbc2₁. The broad range of tantalum concentrations, in which orthorhombic (Zr,Ta)O₂ is formed as a single crystalline phase, is promising for the use of this compound in ferroelectric field effect transistors.