Strain Effects in SrHfO$_{3}$ Films Grown by Hybrid Molecular Beam Epitaxy

TL;DR

This study investigates how strain influences the structural phases of SrHfO₃ films grown by hybrid molecular beam epitaxy, revealing a phase transition to an I4/mcm structure under compressive strain, contrary to previous reports.

Contribution

It provides experimental evidence of strain-induced phase changes in SrHfO₃ films, challenging earlier assumptions and combining advanced characterization with DFT models.

Findings

Strain induces octahedral tilt distortions in SrHfO₃.

Compressed SrHfO₃ adopts an I4/mcm phase with specific tilt patterns.

Results contrast with previous reports of phase stability under strain.

Abstract

Perovskite oxides hetero-structures are host to a large number of interesting phenomena such as ferroelectricity and 2D-superconductivity. Ferroelectric perovskite oxides have been of significant interest due to their possible use in MOSFETs and FRAM. SrHfO$_3$ (SHO) is a perovskite oxide with pseudo-cubic lattice parameter of 4.1 $\mathring{A}$ that previous DFT calculations suggest can be stabilized in a ferroelectric P4mm phase, similar to STO, when stabilized with sufficient compressive strain. Additionally, it is insulating, possesses a large band gap, and a high dielectric constant, making it an ideal candidate for oxide electronic devices. In this work, SHO films were grown by hybrid molecular beam epitaxy with a tetrakis(ethylmethylamino)hafnium(IV) source on GdScO$_3$ and TbScO$_3$ substrates. Equilibrium and strained SHO phases were characterized using X-ray diffraction, X-ray absorption spectroscopy, and scanning transmission electron microscopy to determine the perovskite phase of the strained films, with the results compared to density functional theory models of phase stability versus strain. Contrary to past reports, we find that compressively-strained SrHfO$_3$ undergoes octahedral tilt distortions and most likely takes on the I4/mcm phase with the a$^0$a$^0$c$^-$ tilt pattern.