GROWTH AND CHARACTERIZATION OF THIN MANGANITE FILMS AND IN- SITU ANALYSIS OF THE LASER INDUCED PLASMA

It has been shown that Pulsed Laser Ablation is a suitable technique for the deposition of thin films with a complex stoichiometry. This thesis comprises investigations of the ablation phenomena which are concerned with the plume expansion/dynamics, as well as studies about the film growth. In particular, Pulsed Reactive Crossed Beam Laser Ablation (PRCLA) has been selected for film deposition due to several advantages, such as an overall deposition pressure lower compared to classical PLD and the possibility to grow films without oxygen deficiencies. The perovskite-type compounds, e.g. La0.6Ca0. 4MnO3, were selected for these studies, because they exhibit a spectacular increase of the bulk resistivity by several orders of magnitude in the presence of a magnetic field. This effect has been named colossal magnetoresistance. A possible integration of those materials in device applications based on the sensitivity to magnetic fields represents one of the major tasks for industrial applications, such as magnetic recording. The main goal for future applications is to obtain high magnetoresistance (MR) values at temperatures as close as possible to room temperature. The purpose of this study is to establish the optimum deposition conditions, i.e. substrate materials, target to substrate distance and post annealing steps, for the growth of the manganite materials. A good epitaxial quality of the films allows us to study the influence of the crystallinity on the transport properties of the films compared to the bulk material. The film microstructure and the strain induced at the substrate–film interface are important factors that influence the crystallinity of the manganite compounds, film composition, and the electrical properties. The presence of oxygen vacancies in the manganites affects dramatically the transport properties of the materials, e.g. the magnetoresistance ratio. Heavy annealing of the manganite films leads to an increase of the oxygen content and of the Mn species in the perovskite structure. As a consequence, the bulk resistivity decreases and the magnetoresistance ratio increases. Different annealing treatments have shown that desired electrical properties, i.e. conductivity and magnetoresistance, are obtained after heavy annealing in O2 . A more complex perovskite compound, i.e. La0.6Ca0. 4Mn0.8Fe0. 2O3, has also been investigated. In this material some of the Mn ions (JahnTeller ions) are substituted by Fe ions (non JahnTeller ions), which can be used to probe the importance of the double exchange interaction (electron hopping in the MnO2−Mn network) for magnetoresistance. The structural analysis reveal that Fe substitution in the Mn site does not change the structure significantly, but the exchange of the Mn by Fe results in a loss of the magnetoresistance properties, which confirms the importance of the DE mechanism. The composition of the plume generated by laser ablation from a La0.6Ca0. 4MnO3 manganite target was studied for different irradiation wavelengths, i.e. 193 nm, 266 nm and 308 nm. A correlation of the plume dynamics and composition with the resulting properties of the grown films would allow to tune the film properties for an envisioned application. A quadrupole mass spectrometer was employed as main analytical tool to study the ablation plume. The kinetic energy of the positive/negative ions and neutral species in the plume was measured using an electrostatic deflection energy analyzer, while the mass distribution of the species was analyzed with a quadrupole mass filter. The data indicate that the ionic yield increases with increasing photon energy. In vacuum, the oxygen species possess higher kinetic energies compared to the other plume species, suggesting that the oxygen deficiencies in these films would be due to re-sputtering of the growing films by the high kinetic energy species. A comparison between vacuum and PRCLA conditions (8 ×10−2 Pa O2 and 200 kPa N2O (τ= 400 μs)) reveals that the ablation plume and composition change drastically in the presence of the gas pulse. The N2O gas pulse, synchronized with the laser, leads to the creation of a large amount of reactive atomic oxygen species, diatomic species, and to a decrease of the kinetic energy of the ablated species. The increase in the plume reactivity, i.e. concentration of atomic oxygen, is desired when manganite thin films are grown, in order to incorporate the necessary amount of oxygen into the film. The kinetic energy of the particles from the plume, which can also be controlled by the irradiation wavelength and background pressure, is important for the epitaxial growth of the film (together with the substrate temperature). A large amount of negative ions is also created by the ablation process during PRCLA. They may play an important role in the plume dynamics and composition, and may also influence considerably the properties of the films. The formation mechanism of negative species and their role in the growth of the oxides remains an open question at the moment. Complementary spectroscopic methods, such as plume imaging and emission spectroscopy have been applied to analyze whether the observed Ca deficiencies in La0.6Ca0. 4MnO3 can be associated with a large angular spread of the light species within the plume. A similar compound containing an even lighter element, (LiMn2O4), was chosen to study possible different temporal and spatial dynamics of the different species. The data reveal that the lithium species have a very large angular distribution. This would imply that all films containing Li as light element grown by PLD are deficient in the light element. The analysis reveal therefore that the large angular distribution of the light elements in the plume is indeed reflected in the film composition. Using the same alternative spectroscopic methods, e.g. plume imaging and emission spectroscopy, a study regarding the dynamics of the plume induced by fs and ns ablation of a La0.6Ca0. 4CoO3 target was also performed because in films grown by fs irradiation Ca but also Co deficiencies were observed. The comparison of the plume for ablation with the two different pulse lengths reveals a different expansion in vacuum for fs and ns ablation. The Co I species reveal higher kinetic energies, even faster than the ions, in the case of fs irradiation, while for nsirradiation all species exhibit similar velocities. Additionally, a more ellipsoidal shape of the plume was observed for fs ablation, while ns ablation results in a more spherical shape. In the presence of a background gas pronounced changes were detected for fs and ns irradiation. All species revealed lower kinetic energies, and the plume appears less ellipsoidal in shape for fs irradiation. Additionally, an even stronger deceleration of the plume created by fs irradiation is detected (even slower than the species for ns irradiation), and the Co species reveal in background gas the same velocities and distribution as the other species.