Interesting Evidence for Template‐Induced Ferroelectric Behavior in Ultra‐Thin Titanium Dioxide Films Grown on (110) Neodymium Gallium Oxide Substrates

Abstract

This paper reports the first-ever presentation of evidence for room-temperature ferroelectric behavior in anatase-phase titanium dioxide (a-TiO2). It is shown that behaviour strongly indicative of ferroelectric behavior is induced in ultra-thin (20nm to 80nm) biaxially-strained epitaxial films of a-TiO2 deposited by liquid injection chemical vapour deposition onto (110) Submitted to neodymium gallium oxide (NGO) substrates. The structural properties of the films were analyzed by x-ray diffraction and high-resolution transmission electron microscopy, which showed significant orthorhombic strain in films. Possible ferroelectric behavior was probed by piezoresponse force microscopy (PFM). The films on NGO showed a switchable dielectric spontaneous polarization, the ability to retain polarization information written into the film using the PFM tip for extended periods (several hours) and at elevated temperatures (up to 100°C) without significant loss, and the disappearance of the polarization at a temperature between 180 and 200°C, indicative of a Curie temperature within this range. This combination of effects is believed to constitute strong experimental evidence for ferroelectric behavior, which has not hitherto been reported in a-TiO2 and opens up the possibility for a range of new devices and materials applications. A model is presented for the effects of large in-plane strains on the crystal structure of anatase which provides a possible explanation for the experimental observations.

Ferroelectric or non-ferroelectric: Why so many materials exhibit “ferroelectricity” on the nanoscale

“…335 However, charge injection can result in longlived states with relaxation times that are extremely long (e.g. hours or even days, and even persisting at high temperature 344,345 ), and therefore, these also cannot be used as unambiguous proof of ferroelectric switching.…”

Comment on “Interesting Evidence for Template‐Induced Ferroelectric Behavior in Ultra‐Thin Titanium Dioxide Films Grown on (110) Neodymium Gallium Oxide Substrates”

“…In rutile, the A 2u mode exhibits much lower frequency (172 cm −1 at 300 K) than in anatase, therefore its tendency to softening on cooling is remarkable . For that reason rutile exhibits incipient ferroelectric behavior at low temperatures and theoretical calculations predicting its softening under strain are plausible. Unfortunately, IR and dielectric studies of strained rutile film are still missing.…”

“…Among the main TiO 2 structures—the two tetragonal, rutile and anatase, the orthorhombic brookite and the monoclinic akaogiite—rutile is the most studied. It is characterized by incipient ferroelectricity, with a dramatic increase of the dielectric constant with decreasing temperature and absence of the ferroelectric phase transition even near 0 K. Interestingly enough for the ferroelectrics community, a series of theoretical studies have predicted the appearance of strain‐induced spontaneous polarization in bulk rutile structures and thin films . Nevertheless, no experimental evidence of ferroelectricity has been reported as‐yet for the rutile phase of TiO 2 .…”

Solution Processable Metal Oxide Thin Film Deposition and Material Growth for Electronic and Photonic Devices

“…The primary drawback is the cost per layer for expensive precursors, and advancements in ALD rely on alternative precursor development. Further improvements on this technique can be obtained through the adoption of a vapour based technique and using atomic vapour deposition (AVD), which is commonly known as liquid injection vapour chemical vapour deposition (Li‐CVD) . Overall, the transition to solution based methods is primarily driven by the need for lower temperatures and the coverage of non‐transitional substrates.…”