(Invited) Beyond LaNiO3 - Direct Epitaxy of More Exotic Rare Earth Nickelates By ALD

Abstract

The report of direct low temperature ALD epitaxy of metallic LaNiO3 in 2020 spurred a lot of interest in the ALD- and electronics communities [1]. Thin films deposited at temperatures as low as 225 °C by thermal ALD exhibit MBE/PLD-like quality, and may be a door-opener for direct integration of functional oxides in a range of applications. Not only are the films metallic with low resistivity (≈100 μΩ cm), they also serve as templates for integration of other compounds adopting the perovskite structure. In other words, they are perovskite-templating conductors that can be used as electrodes in oxide-electronics.LaNiO3 is, however, considered the most boring of the rare-earth nickelates. Being metallic at all temperatures and exhibiting close to ohmic behavior, it offers few surprises even if it is versatile in terms of applicability. Other rare-earth nickelates are more interesting from a fundamental perspective, exhibiting current driven resistivy variation and metal-insulator transitions (MITs) at temperatures determined by the size of the rare earth cation (indirectly the Ni-O-Ni bond angle).ALD is proving to be a brilliant tool to study the rare earth nickelates. Low temperature epitaxy gives ample opportunity to study temperature dependent behavior of pristine films. The intrinsic thickness control provides the possibility to study thicknes dependency on both resistivity variation and MIT.In this talk, I will highlight how we have used the learnings from direct epitaxy of LaNiO3 to deposit NdNiO3, SmNiO3 and EuNiO3. I will note similarities and differences in the processes, underpinning why careful process and precursor control is of the essence for these ternary oxides. I will go on to show how these materials may be interesting in electronics applications, discussing their functional electronic properties. Composition, thickness, thermal history and strain all prove to be important parameters when tuning in on wanted parameters. These compounds exhibit a remarkable versatility in electronic properties such as MIT and resistivity modulation.I will furthermore talk about some of the more challenging rare-earth nickelates, such as PrNiO3 and TbNiO3. Why are these challenging, and can we overcome these challenges? Their electronic properties may be of wide interest, and so processes for preparing the by ALD would carry significant impact.I will also spend some time on the underlying mechanisms allowing direct epitaxy of the rare-earth nickelates. Are other materials than nickelates attainable? Why are the processes for rare-earth nickelates so special, and how can we use the learnings from these processes to open for other B-site cations?This is a very good example of why novel process development by ALD is very important, showing how small changes in processing carry large implications for deposited films.[1] H. H. Sønsteby et al., Nat. Commun. 11, 2872 (2020)