Influence of strain relaxation on the structural stabilization of SmNiO3 films epitaxially grown on (0 0 1) SrTiO3 substrates

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Abstract

The structural stabilization of SmNiO3 (SNO) films epitaxially grown by an injection MO-CVD process on (0 0 1) SrTiO3 (STO) substrates is investigated. Using high-resolution X-ray diffraction (XRD), we show that SNO can be stabilized on STO with a minor amount of secondary phases and with a layer thickness reaching several hundreds of nanometers. The film quality is discussed by means of the simulation of X-ray reflectivity and XRD profiles that evidence smooth surfaces and interfaces. The actual lattice parameter of bulk (i.e. strain free) SNO is calculated as a function of the deposited thickness. It turns out firstly that the stabilization of SNO is achieved because the lattice mismatch between STO and SNO is not as high as expected (1.6% instead of 2.8%) and secondly the layer chemical composition varies with the film thickness. Finally, the well-known dissociation of the SNO phase into NiO and Sm2O3 is clearly correlated to the relaxation of epitaxial strain.

Introduction

The perovskite materials of the family RNiO3 (R = rare-earth) are of interest for their sharp metal to insulator (MI) transition which temperature can be modulated by changing the nature of the rare-earth [1], [2]. On one hand, thin films with tunable MI transition can pave the way for a variety of applications such as sensors, modulated switches, or thermochromic coatings [3]; on the other hand, they offer the opportunity to study the relationship between structural and electronic properties. SmNiO3 (SNO) belongs to the orthorhombically distorded perovskites RNiO3 with GdFeO3 structure (space group Pbnm) and can be described in the pseudo-cubic symmetry with a lattice parameter equal to 3.795 Å. Most R-nickelates exhibit a complex anti-ferromagnetic ordering in the insulating phase at low temperature allowing the study of the interplay between magnetic and electric instabilities. Unfortunately, the two transitions (magnetic and metal–insulator) coincide for a number of R-nickelates, and then obscure the straightforward interpretation of experimental results. Among all these R-nickelates, SmNiO3 has the two-fold advantage of having a well-separated MI and magnetic transition [4], thus offering a promising suitability for applications. This separation allows independent studies of these two transitions. Nevertheless, since the nickel must adopt the less stable Ni3+ oxidation state to form RNiO3, these materials are prepared in the bulk either at low temperatures and atmospheric oxygen pressures or at high temperatures and high oxygen pressures. In this way, Demazeau et al. [5] were the first to describe successful nickelates synthesis. Another way to stabilize RNiO3 phases is to use epitaxial strain [6], [7]. Novojilov et al. [6] showed that the single-phase SmNiO3 is only obtained on the perovskite-like substrate with the minimum lattice mismatch (m = −0.13%): LaAlO3. On perovskite-like substrates with higher mismatch a phase mixture of the perovskite phase with NiO and Sm2O3 is observed as the film thickness reaches a critical thickness [8] and the larger the mismatch is the lower the critical thickness is. In the latter case, films deposited on SrTiO3 (STO) present a low crystalline quality and the secondary phases Sm2O3 and NiO are reported to be in large amount. This was attributed to the high in-plane tensile strain due to the 2.8% lattice mismatch (aSTO = 3.905 Å).

Because of their thermodynamic instability and their tunable MI transition, RNiO3 compounds allow us to study the relationship between structural and electronic properties. Nevertheless, one of the long-standing questions associated with RNiO3 is the origin of the structural stabilization of the RNiO3 phase. In the present study, we will focus on the influence of the microstructure on the epitaxial stabilization of SmNiO3 films deposited on SrTiO3 substrates. We will prove that the SNO phase is stabilized because the pseudo-cubic lattice parameter of the bulk SNO phase is higher than its usually assumed value. Consequently, the lattice mismatch between these two perovskites is not as high as expected. Besides, using a careful inspection of XRD data we will investigate the strain state of five layers as a function of the deposited thickness (from 22 to 500 nm) and evidence a correlation between the strain relaxation of the SNO layers and their chemical composition.

Section snippets

Film growth

SNO films were grown by injection MO-CVD “band flash” [9] on single-crystalline (0 0 1) STO substrates. The liquid started solution is prepared with tris(tetramethylheptanedionato) samarium, Sm(tmhd)3, and bis(tetramethylheptanedionato) nickel, Ni(tmhd)2, dissolved in 1,2-dimethoxyethane. Droplets of samarium and nickel precursors of a few microliters are injected in the reactor. A porous band assisted by a gas flow carries the solid reactives to the evaporation zone. Precursors evaporate at 230 

Film thickness

The thickness of the SNO layers has been evaluated by means of two techniques depending on the thickness range of the layers. For thin layers, the thickness has been obtained by the simulation of X-ray reflectivity profiles, whereas for thicker layers (t  100 nm) the thickness has been extracted from the simulation of (0 0 2) scans recorded perpendicular to the sample surface.

For samples 1 and 2 RSMs have been recorded in the vicinity of the origin of the reciprocal space (Fig. 1a and c). In Fig. 1

Discussions

Since the nickel must adopt the less stable Ni3+ oxidation state to form SNO, this phase is unstable when prepared under normal temperature and pressure conditions [20] and dissociates into Sm2O3 and NiO. To circumvent this, epitaxial strain can be used to form metastable SmNiO3. As R increases the layer tends to recover its bulk lattice parameter. During this relaxation process, we showed that SNO partially dissociates to form NiO. Complementary measurements on highly dissociated films showed

Conclusions

In summary, we have succeeded in the stabilization of epitaxial films of SmNiO3 on (0 0 1) SrTiO3 substrates by an MO-CVD process. These films present a high crystalline quality principally attested by a very low dissociation rate of this metastable phase and by smooth surfaces and interfaces. This stabilization process has been achieved because the actual lattice mismatch between SmNiO3 and SrTiO3 is not as high as usually expected (1.6 instead of 2.8%). Using reciprocal space mapping the actual

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