Rare earth scandate thin films by atomic layer deposition: effect of the rare earth cation size

Abstract

A series of amorphous REScO₃ films was deposited by atomic layer deposition (ALD) using rare earth (RE) β-diketonate precursors RE(thd)₃ (thd = 2,2,6,6-tetramethyl-3,5-heptanedionato) together with ozone in an attempt to study the effect of the RE³⁺ cation size on various film properties. A clear correlation between the deposition rate and the RE³⁺ cation radius was established. REScO₃ films with a metal ratio RE : Sc close to the stoichiometric one, viz. RE : Sc = 1, and a small excess of oxygen were realized by adjusting the metal precursor pulsing ratio. Small amount of carbon was found as impurity in all the films, concentrations varying from 1–3 at% (RE = La, Gd or Dy) to 0.4–0.5 at% (RE = Er or Lu). Also the crystallization temperature and the resulting phase were affected by the RE³⁺ cation size. The REScO₃ films were found to crystallize either as an orthorhombic perovskite phase or as a solid solution of the cubic C-type oxides. High crystallization temperatures of 800–900 °C were observed for LaScO₃, GdScO₃ and DyScO₃. All the films gave smooth C–V curves with very small hysteresis (typically <35 mV). The highest dielectric constant (κ ≈ 24) was found for DyScO3. Also the leakage current densities were small, typically in a range of 10⁻⁶–10⁻⁹ A cm⁻² at 1 V. The results confirm that REScO₃ films deposited by ALD are potential candidates for new generation high-κ gate dielectrics.