Molecular Beam Epitaxy of KTaO$_3$

TL;DR

This paper reports the successful growth of high-quality KTaO3 thin films via molecular-beam epitaxy, demonstrating smooth, defect-free interfaces and strain coherence with various substrates, advancing the material's potential for electronic applications.

Contribution

It introduces a novel MBE growth method for KTaO3 films with detailed characterization of film quality and interface properties.

Findings

Films are smooth with abrupt interfaces.

No extended defects observed in electron microscopy.

Films are coherently strained to substrates.

Abstract

Strain-engineering is a powerful means to tune the polar, structural, and electronic instabilities of incipient ferroelectrics. KTaO3 is near a polar instability and shows anisotropic superconductivity in electron-doped samples. Here, we demonstrate growth of high quality KTaO3 thin films by molecular-beam epitaxy. Tantalum was provided by both a suboxide source emanating a TaO2 flux from Ta2O5 contained in a conventional effusion cell as well as an electron-beam-heated tantalum source. Excess potassium and a combination of ozone and oxygen (10 \% O3 + 90 \% O2) were simultaneously supplied with the TaO$_2$ (or tantalum) molecular beams to grow the KTaO$_3$ films. Laue fringes suggest that the films are smooth with an abrupt film/substrate interface. Cross-sectional scanning transmission electron microscopy does not show any extended defects and confirms that the films have an atomically abrupt interface with the substrate. Atomic force microscopy reveals atomic steps at the surface of the grown films. Reciprocal space mapping demonstrates that the films, when sufficiently thin, are coherently strained to the SrTiO$_3$ (001) and GdScO$_3$ (110) substrates.