Epitaxial RuO$_2$ and IrO$_2$ films by pulsed laser deposition on TiO$_2$(110)

TL;DR

This study systematically investigates the epitaxial growth of RuO₂(110) and IrO₂(110) films on TiO₂(110) substrates using pulsed laser deposition, addressing growth challenges and optimizing conditions for high-quality films.

Contribution

It provides detailed growth conditions, growth modes, and structural characterization of epitaxial RuO₂ and IrO₂ films, highlighting differences in growth behavior and film quality.

Findings

RuO₂ exhibits layer-by-layer growth at high deposition rates.

Low deposition rates lead to island formation due to lattice mismatch.

Films up to 30 nm (RuO₂) and 13 nm (IrO₂) are highly crystalline with well-defined defects.

Abstract

We present a systematic growth study of epitaxial RuO$_2$(110) and IrO$_2$(110) on TiO$_2$(110) substrates by pulsed laser deposition. We describe the main challenges encountered in the growth process, such as a deteriorating material flux due to laser induced target metallization or the delicate balance of under- vs over-oxidation of the 'stubborn' Ru and Ir metals. We identify growth temperatures and oxygen partial pressures of 700 K, $1\times 10^{-3}$ mbar for RuO$_2$ and 770 K, $5\times 10^{-4}$ mbar for IrO$_2$ to optimally balance between metal oxidation and particle mobility during nucleation. In contrast to IrO$_2$, RuO$_2$ exhibits layer-by-layer growth up to 5 unit cells if grown at high deposition rates. At low deposition rates, the large lattice mismatch between film and substrate fosters initial 3D island growth and cluster formation. In analogy to reports for RuO$_2$ based on physical vapor deposition, we find these islands to eventually merge and growth to continue in a step flow mode, resulting in highly crystalline, flat, stoichiometric films of RuO$_2$(110) (up to 30 nm thickness) and IrO$_2$(110) (up to 13 nm thickness) with well defined line defects.