Characterization techniques

Before the illustration of the results obtained in this part of the project, a brief explanation about the characterization techniques used to study doped-CeO₂ film will be reported.

4.7.1 X-Ray Diffraction (XRD)

X-ray diffraction is a multi-function technique used to identify the crystalline phases of material and to analyze structural properties. In particular it has been proved to be very useful in thin film growth to verify the in-plane and out-of-plane orientation of grains, their size and phase composition. With high resolution facilities it is also possible to detect strain fields due to heteroepitaxial growth and defect presence.

Fig. 4.10 – Geometry of X-ray diffractometry

Three types of measurements were made with XRD: θ/2θ-scan, ω-scan and ϕ-scan.

Conventional θ/2θ XRD pattern provides several information about the dimension of the elementary cell, the content of elementary cells, the crystallite dimension and hints about

out-of-plane orientation of the thin film analyzed. from the diffraction pattern, the lattice relative parameter (d) can be calculated by the Bragg’s law: d = nλ/a senθ where λ is the X-ray wavelength.

The ω-scan XRD measurements (known as rocking curves) quantifies the out-of-plane orientation of the film by tilting of a very small angle ω (within the diffraction plane) once selected the diffraction peak (at a fixed 2θ angle). By measuring the full width at half maximum (FWHM) value of the profile, ∆ω, we obtain information about the degree of the out-of-plane orientation.

The pole figures reveal the in-plane texture of thin films. The measure is taken at a fixed scattering angle and consists of a series of ϕ-scans (in-plane rotation around the φ−axis passing from the centre of the sample) at different tilt angles around ψ-axis. For this technique, we can obtain information about the grain alignment on the plane parallel to the substrate surface.

The presented θ/2θ-scan and ω-scan measurements have been carried out at IMEM-CNR on a Siemens D500 while the pole figures have been performed at University of Trento (Siemens D500T).

4.7.2 Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS)

The SEM is an instrument that produces a magnified image by using electrons instead of light to form an image. A beam of electrons is produced at the top of the microscope by an electron gun. The electron beam follows a vertical path through the microscope, in vacuum. The beam travels through electromagnetic fields and lenses, which focus the beam down-ward the sample. Once the beam hits the sample electrons will scatter through the specimen within a defined area called the interaction volume. During the electron beam-sample interactions, secondary products like secondary electrons, backscattered electrons, X-rays, heat and light will be formed. Detectors collect backscattered electrons and secondary electrons and convert them into a signal that is sent to a screen where the image is formed.

81 The backscattered electrons are primary electrons coming out from the surface because of scattering events without significantly changing their original energy. The backscattering yield depends on the atomic number of the interacting atom, thus an image contrast show different phases made with different atoms. Another type of contrast can be created from topographic irregularities (i.e. the surface roughness) which deviates the primary electrons towards different directions. The spatial resolution of backscattered electrons is about 1µm.

Secondary electrons are emitted from excited atoms. Since their energy is much smaller than that of backscattered electrons, only those electrons within 100 Ǻ from the surface can be collected. The Energy Dispersed X-ray Spectroscopy (EDS) studies the X-ray radiation produced by the interaction between electron beam and atoms of the sample. When an excited core electron decays, a x-ray with a specific wavelength is emitted. The wavelength of the radiation depends on the transition level and it is a fingerprint of the emitting atom. By analyzing the X-ray emission one can semi-quantitatively analyze the film stoichiometry and the composition of secondary phases on the surface.

Fig. 4.11 – Interaction between the e-beam and the sample surface

4.7.3 Rutherford Back Scattering (RBS)

The RBS is an ion scattering technique that is used for compositional thin film analysis.

RBS allows quantification without the use of reference standards. During an RBS measurement, high-energy (0,1 – 3 MeV) He²⁺ ions are directed onto a sample and the energy distribution and yield of the backscattered He²⁺ ions at a given angle is recorded.

Since the backscattering cross section for each element is known, it is possible to obtain

qualitative and quantitative depth profiles from the RBS spectra (for thin films that are less than 1mm thick). It is based upon the elastic two-particle scattering of energetic ions with sample atoms via the repulsive Coulomb force of the positively charged atomic nuclei. The ions penetrate into the matter and lose their kinetic energy, at the beginning mainly in collisions with electrons until they come to rest in a depth of several micrometers. At the surface a very small fraction of the ions approach atomic nuclei close enough to be scattered at large angles. Backscattered ions can leave the sample and reach a particle spectrometer, where their energy is analysed.

Fig. 4.12 – schematic interaction between ions and atoms of thin film in RBS system

For scattering at the sample surface, the only energy loss is due to momentum transfer to the target atom. The ratio of the projectile energy after and before a is defined as the kinematic factor:

2

0

2 2 0 2 0

0

1 cos













+

−

= +

= m M

sen m M m

E

K E θ θ (Eq. 4.4)

where E1 is the energy of the ion after the impact, E0 is the incident ion energy, m0 is the mass of the projectile ion and M is the mass of the atom target. Knowing the geometry of the system, one can draw the mass (and hence the type) of the atom hit by the projectile.

Besides atom identification, the RBS can also detect the depth profiling of the atoms in the sample. This is due to the energy that a particle loses when it backscatters from an element at some depth in a sample. When probing particles penetrate to some depth in a dense medium, projectile energy dissipates because of interactions with electrons

83 stopping). Effectively, these particles have measurable less energy than a particle which backscatters from the same element on the sample surface. The amount of energy a projectile loses through the sample depends on the projectile type, its velocity, the chemical composition and the density of the sample material. Dedicated application program have been studied to calculate and simulate different stopping power in order to interpret the very intricate data collected from multi-layer systems.