Unleashing Strain Induced Ferroelectricity in Complex Oxide Thin Films via Precise Stoichiometry Control

Abstract

Strain tuning has emerged as a powerful means to enhance properties and to induce otherwise unattainable phenomena in complex oxide fi lms. However, by employing strain alone, the predicted properties sometimes fail to emerge. In this work, the critical role of precise stoichiometry control for realizing strain-induced ferroelectricity in CaTiO 3 fi lms is demonstrated. An adsorption controlled growth window is discovered for CaTiO 3 fi lms grown by hybrid molecular beam epitaxy, which ensures an excellent control over the Ti:Ca atomic percent ratio of <0.8% in the fi lms. Superior ferroelectric and dielectric properties are found for fi lms grown inside the stoichiometric growth window, yielding maximum polarization, dielectric constant, and paraelectric-toferroelectric transition temperatures. Outside this growth window, properties are severely deteriorated and ultimately suppressed by defects in the fi lms. This study exemplifi es the important role of precise compositional control for achieving strain-induced properties. Untangling the effects of strain and stoichiometry on functional properties will accelerate both fundamental discoveries yet to be made in the vast materials design space of strained complex oxide fi lms, as well as utilization of strain-stabilized phenomena in future devices.

Stabilizing hidden room-temperature ferroelectricity via a metastable atomic distortion pattern

“…On the other hand, when it is too high, rough surfaces and extra peaks in RHEED patterns have been observed. This feature is consistent with the off-stoichiometric CaTiO 3 thin film, reported previously 37 , 38 . In addition, the CaTiO 3 thin film grown on a (LaAlO 3 ) 0.3 –(Sr 2 AlTaO 6 ) 0.7 (LSAT) (001) substrate with optimal laser energy density has the same c -axis lattice parameter as films grown by molecular beam epitaxy (Fig.…”

Three-dimensional atomic scale electron density reconstruction of octahedral tilt epitaxy in functional perovskites

“…CaTiO 3 on NdGaO 3 and DyScO 3 , which are significantly higher than that of 70 and 170 K, reported in thick films (10 nm) in literatures 47 , 49 . Most strikingly, CaTiO 3 on LSAT exhibits significant SHG signal from 4 up to 900 K. However, literature reports a T C of 140–190 K for thick CaTiO 3 films (>10 nm) on LSAT 48 , 49 . Room temperature electron densities reconstructed by COBRA confirm a paraelectric state for CaTiO 3 on NdGaO 3 , a weak polarization of the film on DyScO 3 and a large polarization of the film on LSAT (see Supplementary Figure 6 ).…”

“…Green squares show measurements on 10 nm CaTiO 3 at 10 K 49 . Blue triangle shows measurement on 20 nm CaTiO 3 at 77 K 48 . The in-plane polarizations obtained by COBRA method at 30 K for 8 u.c.…”

Hybrid reflections from multiple x-ray scattering in epitaxial oxide films

“…Many high-quality, single-crystal, heteroepitaxial oxide films are highly oriented relative to their substrates in-plane, as were these two films, as shown through reciprocal-space maps or reflection high energy electron diffraction of both the film and the substrate. [18][19][20][21][22][23][24][25][26] We conclude that hybrid reflections may be expected from heteroepitaxial oxide thin films that: (1) are grown close to cube-on-cube relative to their substrates; (2) have an in-plane lattice constant close to that of the underlying substrate, though which may or may not be perfectly coherent;…”

“…111, 131903-1 quality. [16][17][18][19][20][21][22][23][24][25][26] (We describe specific structural quality criteria below.) For such high-quality films, we present a simple relationship between film and substrate lattice constants and the angles 2h at which hybrid reflections can be expected to occur.…”