Structural analysis of perovskite thin films and multilayers

T. Číž, M. Kiaba, O. Caha, A. Dubroka

Department of Condensed Matter Physics, Faculty of Science, Masaryk University, Kotlářská 2, 
61137 Brno, Czech Republic

444338@mail.muni.cz, caha@physics.muni.cz

Perovskite oxides with chemical formula ABO₃ have many unique applications in modern matter science such as superconductivity, magnetoresistance or ferroelectricity. Compounds studied in this research are LaFeO₃/SrTiO₃ (LFO/STO). LFO is an insulator and in bulk form has no macroscopic magnetoelectric coupling or control of magnetism by electric field. In combination with semiconducting STO is created polar-nonpolar interface which makes it possible to achieve the above mentioned properties [1, 2].        The aim of this research is measuring the thickness of thin layers, roughness of interfaces and density of these compounds with method X-ray structural analysis, namely x-ray reflectivity and coplanar diffraction were used. The samples were prepared using pulsed laser deposition (PLD) technique in oxygen atmosphere at high temperature of approximately 700 ^oC and using compound polycrystalline targets.

The layer thickness t can be determined from the angular position of the daughter’s maximum on the reflection curve. These maxima are caused by the interference of waves reflected from the upper and lower layers. The roughness of the interface σ is largely influenced by the slope of the reflection curve. The density ρ is determined by the critical angle Θ_c [3].

The goal of the experiment is to measure the reflected intensity of the X-ray beam in dependence on the reflection angle Θ and its comparison with numerical simulation. This simulation allows to obtain all mentioned parameters of sample. In Fig. 1 is reflection curve, where is easy to recognise equidistant thickness oscillations. We have determined the thickness of the film as 11 nm and roughness of 0.6 nm.

Figure 1. Example of measured reflectivity curve fitted by the simulation. The thickness can be determined from the frequency of the oscillations. This sample is a single layer of LaFeO₃/SrTiO₃ with thickness of 11 nm

            Another part of this research is observation of reflected X-ray radiation from multilayers STO/LFO/STO. Here are discussed two different approaches. First of them is considering of each single atomic plane in stack. These samples were prepared using pulsed laser deposition (PLD) method by first depositing several atomic planes of the same material on top of each other, and subsequently applying the atomic planes of the other compound (complex model in Fig. 2). This process was subsequently repeated. The second approach neglect this manufacturing process and consider only one layer of each material with appropriate repetition. In Fig. 2 is superlattice with (3+3) x 4 monolayers. The data were fitted within the second approach with a model of 4 periods, each consisting of STO layer with thickness of 1.2 nm and LFO layer with thickness of 0.37 nm. The roughness of interface comes out as 0.46 nm for STO layer and 0.03 for LFO layer. If we sum the layer thickness of the individual layers in the complex model, we get similar results.                                                                                                                                                     

A total of about 12 samples with different thicknesses and number of periods were measured. All parameters were also determined for all. Finally, we have used method called reciprocal space mapping (RSM). This measurement has confirmed that all these samples are pseudomorphic.

Figure 2. Reflection curve of superlattice STO/LFO/STO with (3+3) x 4 monolayers with two different simulations. In this case occurs extinction of maxima.

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