Substrate selection and preparation

In the past, substrates were considered a passive element supplying only mechan- ical support. However, they have been shown to act as an active template during the epitaxial growth of materials. In particular, interfacial effects at the nanoscale have been demonstrated to play a key role in the development of new and exotic functionalities[130,131], and consequently, great efforts have been done to control the quality and the sharpness of both surface and interfaces [132,133].

Nucleation and growth of oxide heterostructures are determined by the presence of the substrate below, and therefore, their crystalline structure and surface character- istics are essential to determine the final properties and morphology of the system. During the development of this thesis, two different single-crystalline oxide sub- strates with perovskite structure have been used in the growth of the epitaxial thin films, namely SrTiO3(STO) and LaAlO3(LAO).

SrTiO3 Strontium titanate (STO) is a paradigmatic example of cubic perovskite

P m3m. STO single crystal substrates have been widely used for its compati- bility due to the low lattice mismatch with the active layers in different appli- cations, such as high temperature superconductors [134,135], ferroelectricity [136] and ionic conductivity [131]. Furthermore, STO itself present very in- teresting properties as a functional material. For instance, it can be employed as gate dielectric material due to its high-k and can be integrated on silicon without outgrowth of silicon dioxide [137], and even promoting the integra- tion of other perovskite oxides on silicon [138]. In the resistive switching community, STO has been widely studied [25, 26, 139, 140]. According to our supplier (Crystec), STO single crystal substrates have a lattice parameter aST O = 3.905Å, in good agreement with the values reported in the literature

[141] and confirmed by XRD measurements performed in our facilities.

LaAlO3 Lanthanum aluminate (LAO) exhibits an ideal cubic perovskite structure

at high temperatures, but goes through a second order phase transition to the rhombohedral R3c structure at around 800K, owing lattice parameters of a = b = c = 5.3547Åand α = β = γ = 60.113◦. This rhombohedral distortion can be described as a pseudocubic cell with aLAO = 3.79Åand α = 90.096◦.

This phase transition is accompanied by the formation of twin planes to re- lieve the stress produced by the lattice distortion. An extensive description of twinning in LAO is given by Bueble and co-workers [142]. The supplier (Crystec) provides a lattice parameter of aLAO = 3.82Å, which is close to

the literature values (aLAO = 3.789)Å) [142] and to our XRD measurements

(aLAO= 3.79Å).

In this thesis (001)-oriented LAO and STO substrates of 5 x 5 x 0.5 mm in size and one side polished were employed. The growth of the metallic perovskite oxides studied in this thesis on STO and LAO substrates leads to the growth of epitaxial films due to the the small lattice mismatch between them. Table 2.1 summarizes the strain of the films. This mismatch was calculated using the formula  = asubstrate−afilm

asubstrate :

Compound a (b) bulk (Å) ST O(%) LAO(%)

LSMO 3.873 +0.9 −2.3

YBCO* 3.886(3.821) +1.2 −1.7

LNO 3.840 +1.7 −1.3

NNO 3.806 +2.6 −0.6

TABLE 2.1: Mismatch between substrates and the materials em- ployed in this thesis. *An average cell parameter has been taken

to calculate the mismatch to the substrates. 3.853

Surface conditioning to assure a single-terminated and atomically flat morphology is a essential step prior to the deposition and growth of heteroepitaxial functional thin films. Therefore, processing of the as-received substrates is required to achieve a clean, smooth surface, single-terminated and free of impurities surfaces. In our case, the following protocol was applied:

Cleaning procedure The general cleaning procedure for the as-received substrates

includes ultrasonication in acetone and methanol during 5 minutes in each solvent. This procedure allows the removal of non-polar and polar adsor- bate impurities respectively, while maintaining the stoichiometry and with- out surface degradation. (This will be the general cleaning procedure applied for all the substrates and samples unless something else is mentioned.)

Chemical selectivity of substrate termination In the case of STO, we have fol- lowed a methodology based on the work of Koster et al. [143]. There, an etch- ing NH4F-HF solution of controlled pH is used to obtain single-terminated

surface. The process implies the chemical reaction of the Sr-O terminated planes with CO2and water by cleaning the substrates in Mili-Q purified wa-

ter. This produces SrCO3and Sr(OH)2. The latter is dissolved in the NH4F-HF

diluted solution (5:1, from Sigma-Aldrich) for 60-90 s. Afterwards, the acid is removed with a gently bath in Mili-Q water. This process leads to Ti-O2

single terminated surface.

Surface reconstruction Finally, a thermal treatment is employed to promote the

formation of the steps morphology due to the miscut. STO and LAO sub- strates are placed on an alumina crucible and are introduced in a quartz tube inside a high-temperature tubular furnace. Then, they are heated up to 900◦C at 15◦C · min−1 and remain at that temperature for 5 hours under a constant oxygen flow of 0.5L · min−1. Cooling to room temperature is performed at a rate of10◦C ·min−1. In the case of LAO, treatments at higher temperatures are reported in the literature [144] to promote surface reconstruction. However, this thermal treatent has been tested in several thesis in our research group, showing high reproducibility and successful results [145,146].

At the end of the process, atomically flat surfaces with stair-like morphology are obtained. Under this procedure, the substrates are conditioned and it leads to a reproducible growth and excellent performances of the metallic perovskite oxide films.