Effects of Nonequilibrium Growth, Nonstoichiometry, and Film Orientation on the Metal-to-Insulator Transition in NdNiO₃ Thin Films

Abstract

Next-generation devices will rely on exotic functional properties not found in traditional systems. One class of materials of particular interest for applications are those possessing metal-to-insulator transitions (MITs). In this work, we probe the relationship between variations in the growth process, subsequent variations in cation stoichiometry, and the MIT in NdNiO3 thin films. Slight variations in the growth conditions, in particular the laser fluence, during pulsed-laser deposition growth of NdNiO3 produces films that are both single-phase and coherently strained to a range of substrates despite possessing as much as 15% Nd-excess. Subsequent study of the temperature-dependence of the electronic transport reveals dramatic changes in both the onset and magnitude of the resistivity change at the MIT with increasing cation nonstoichiometry giving rise to a decrease (and ultimately a suppression) of the transition and the magnitude of the resistivity change. From there, the electronic transport of nearly ideal NdNiO3 thin films are studied as a function of epitaxial strain, thickness, and orientation. Overall, transitioning from tensile to compressive strain results in a systematic reduction of the onset and magnitude of the resistivity change across the MIT, thinner films are found to possess sharper MITs with larger changes in the resistivity at the transition, and (001)-oriented films exhibit sharper and larger MITs as compared to (110)- and (111)-oriented films as a result of highly anisotropic in-plane transport in the latter.

Reproducibility and off-stoichiometry issues in nickelate thin films grown by pulsed laser deposition

“…Figure 5a) and volume than the more stoichiometric ones. This is compatible with the increase of the (out-of-plane) Ni-O-Ni bond angles towards 180 • suggested from the XPS analysis and, consistent with the results of Breckenfeld et al 16 Beyond the influence of the experimentally observed Nd excess on the electronic properties of the NNO films grown from a mixed-phase target, it proved difficult to provide a satisfactory explanation for the deviation of the films composition from stoichiometry. PLD is a non-equilibrium process, and using mixed phase targets may result in preferential ablation of one component.…”

“…This result is consistent with previous reports of cationic off-stoichiometry in NNO films using Rutherford backscattering spectrometry. 16 We note that even sample C shows some content of Ni 2+ . In view of the excellent transport properties of this sample, this small deviation from pure 3+ valence probably Ni-2p and Nd-3d higher resolved scans for A and C films are depicted in (b) and (c), respectively.…”

“…The difference in the out-of-plane lattice parameters inferred from the positions of the (00l) peaks is thus likely to reflect a change in the unit-cell volume resulting from variations in compositions. 16 In Figure 3 we plot the resistivity curves as a function of the temperature obtained in a coolingto-warming cycle for all samples. Surprisingly, very different behaviors are observed.…”

“…In particular, the MIT appears to be very sensitive to chemical doping, 9,10 electricfield effect 6,11 and epitaxial strain. 6,[12][13][14] 16 On the other hand, Hauser et al reported that the off-stoichiometry of the NNO films is effectively not affecting the temperature at which the MIT occurs but that, only the strain state needs to be considered. 17 These contradictory results suggest that there is a problem of reproducibility of the physical properties of NNO thin films that are generally grown via physical vapor deposition techniques onto similar templates.…”

Aspects of the synthesis of thin film superconducting infinite-layer nickelates

“…Both of these features have been previously observed in undoped nickelate films. 17,34 The cross-sectional HAADF STEM image of this film reveals that it is densely populated with vertical RP-type faults ( Figure 1(c)). These defects are formed when an AO rocksalt layer (where A corresponds to the A-site cation) stabilizes in between the perovskite layers.…”