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It is a well-known fact in the materials science community that the use of low-energy atom impacts during thin film deposition is an effective tool for altering the growth behavior and for increasing the crystallinity of the films. However, the manner in which the incident atoms affect the growth kinetics and surface morphology is quite complicated and still not fully understood. This provides a strong incentive for further investigations of the interaction among incident atoms and surface atoms on the atomic scale. These impact-induced energetic events are non-equilibrium, transient processes which complete in picoseconds. The only accessible technique today which permits direct observation of these events is molecular dynamics (MD) simulations. This thesis deals with MD simulations of low-energy atom impact phenomena on metal surfaces during crystal growth. Platinum is chosen as a model system given that it has seen extended use as a model surface over the past few decades, both in experiments and simulations. In MD, the classical equations of motion are solved numerically for a set of interacting atoms. The atomic interactions are calculated using the embedded atom method (EAM). The EAM is a semi-empirical, pair-functional interatomic potential based on density functional theory. This potential provides a physical picture that includes many-atom effects while retaining computational efficiency needed for larger systems. Single adatoms residing on a surface constitute the smallest possible clusters and are the fundamental components controlling nucleation kinetics. Small two-dimensional clusters on a surface are the result of nucleation and are present during the early stages of growth. These surface structures are chosen as targets in the simulations (papers I and II) to provide further knowledge of the atomistic processes which occur during deposition, to investigate at which impact energies the different kinetic pathways open up, and how they may affect growth behavior. Some of the events observed are adatom scattering, dimer formation, cluster disruption, formation of three-dimensional clusters, and residual vacancy formation. Given the knowledge obtained, papers III and IV deal with growth of several layers with the aim to study the underlying mechanisms responsible for altering growth behavior and how the overall intra- and interlayer atomic migration can be controlled by low-energy atom impacts. / <p>On the day of the defence date the status of article II was Accepted.</p>

Ag thin film growth on ZnO substrates has been investigated theoretically using multi-timescale simulation methods. The models are based on an atomistic approach where the interactions between atoms are treated classically using a mixture of fixed and variable charge potential energy functions. After some preliminary tests it was found that existing fixed charge potential functions were unreliable for surface growth simulations. This resulted in the development of a ReaxFF variable charge potential fitted to Ag/ZnO surface interactions. Ab initio models of simple crystal structures and surface configurations were used for potential fitting and testing. The dynamic interaction of the Ag atoms with the ZnO surface was first investigated using single point depositions, via molecular dynamics, whereby the Ag impacted various points on an irreducible symmetry zone of the ZnO surface at a range of energies. This enabled the determination of the relative numbers of atoms that could penetrate, reflect or bond to the surface as a function of incident energy. The results showed that at an energy of up to 10 eV, most atoms deposited adsorbed on top of the surface layer. The second part of the dynamic interaction involved a multi-timescale technique whereby molecular dynamics (MD) was used in the initial stages followed by an adaptive kinetic Monte Carlo (AKMC) approach to model the diffusion over the surface between impacts. An impact energy of 3 eV was chosen for this investigation. Ag was grown on various ZnO surfaces including perfect polar, O-deficient and surfaces with step edges. Initial growth suggests that Ag prefers to be spread out across a perfect surface until large clusters are forced to form. After further first layer growth, subsequent Ag atoms begin to deposit on the existing Ag clusters and are unlikely to join the first layer. Ag island formation (as mentioned within the literature) can then occur via this growth mechanism. O-deficient regions of ZnO surfaces result in unfavourable Ag adsorption sites and cause cluster formation to occur away from O-vacancies. In contrast, ZnO step edges attract deposited Ag atoms and result in the migration of surface Ag atoms to under-coordinated O atoms in the step edge. Various improvements have been made to the existing methodology in which transitions are determined. A new method for determining defects within a system, by considering the coordination number of atoms, is shown to increase the number of transitions found during single ended search methods such as the relaxation and translation (RAT) algorithm. A super-basin approach based on the mean rate method is also introduced as a method of accelerating a simulation when small energy barriers dominate. This method effectively combines states connected by small energy barriers into a single large basin and calculates the mean time to escape such basin. To accelerate growth simulations further and allow larger systems to be considered, a lattice based adaptive kinetic Monte Carlo (LatAKMC) method is developed. As off-lattice AKMC and MD results suggest Ag resides in highly symmetric adsorption sites and that low energy deposition events lead to no penetrating Ag atoms or surface deformation, the on-lattice based approach is used to grow Ag on larger perfect polar ZnO surfaces. Results from the LatAKMC approach agree with off-lattice AKMC findings and predict Ag island formation. Critical island sizes of Ag on ZnO are also approximated using a mean rate approach. Single Ag atoms are placed above an existing Ag cluster and all transition states are treated as belonging to a single large super-basin . Results indicate that small Ag clusters on the perfect ZnO surface grow in the surface plane until a critical island size of around 500 atoms is reached. Once a critical island size is reached, multiple Ag ad-atoms will deposit on the island before existing Ag atoms join the cluster layer and hence islands will grow upwards. A marked difference is seen for second layer critical island sizes; second layer Ag islands are predicted to be two orders of magnitude smaller (< 7 atoms). This analysis suggests that Ag on ZnO (0001) may exhibit Stranski-Krastanov (layer plus island) growth.

We report an influence of disorder on structural and magnetic properties of ZnFe2O4 thin films grown at temperatures ranging from 400°C to 600°C by pulsed laser deposition in O2 atmosphere on SrTiO3 (100) substrates evidenced by properties of electronic transitions observed in the dielectric function. Inversion of the normal spinel structure was found to be one of the main mechanisms responsible for the increase in the magnetic response for the lowest growth temperature. The enhanced feature in the dielectric function located at ~3.5 eV, related to the transition involving tetrahedrally coordinated Fe3+ cations, corresponds to the dominating magnetic coupling between the octahedral and tetrahedral lattice sites, responsible for the overall ferrimagnetic behaviour of the film grown at the lowest temperature.

The work in this licentiate is devoted to investigating the epitaxial growth of thin Fe layers on MgAl2O4 (001) and MgO (001) substrates using dc magnetron sputtering. The aim is to qualitatively and quantitatively determine the crystal quality of the grown Fe layers depending on their thickness, substrate material, and selected deposition parameters. The effect of the crystal quality on the magnetic and electronic transport properties is discussed. The structural characterization of the epitaxial Fe thin films is carried out by x-ray reflectometry and diffraction as well as transmission electron microscopy. X-ray scattering measurements and analysis with related models allow for a quantitative determination of layering, crystal quality, and strain profiles in the growing Fe. Magnetic properties are determined using a combination of longitudinal magneto-optical Kerr effect measurements, Kerr microscopy, and scanning electron microscopy with polarization analyser. Electronic transport properties are characterized by four-point probe measurements of the thin films. The epitaxial growth of Fe is found to be highly substrate dependent: Fe layers grown on MgAl2O4 have a significantly higher crystal quality, as compared to Fe grown on MgO. The difference in crystal quality is attributed to different strain states in Fe, which is supported by theoretical calculations of the critical thickness on both substrates. Moreover, an anomalous elastic response in Fe at the thin film limit is found. The magnetic properties of Fe are weakly reflecting the differences in crystal quality of the Fe layers. However, the difference in crystal quality affects the electronic transport properties. The results of this study on epitaxial Fe layers can provide insights into strain and defect engineering in Fe thin films, which can additionally serve as model systems for finite size effects.

Piezoelectric materials are well known for their applications in surface (SAW) and bulk acoustic wave (BAW) devices such as oscillators, resonators and sensors. Quartz has been the main material used in such applications. Ternary calcium gallium germanate (CGG) structure-type materials, so-called langasites, recently emerged as very promising because of their piezoelectric properties superior to quartz. This thesis discusses the growth of langasite-type La3Ga5.5Ta0.5O14 (LGT) films by liquid phase epitaxy (LPE) technique and their chemical and structural characterization. In addition, the different techniques suitable for the growth of LGT are discussed and compared. To adjust the materials properties for given applications, doping by selected ions can be used. However, the dopants must be homogeneously distributed. In the current study, Al, Ti, Cr and Ca were investigated as dopants. In an earlier study, Al and Ti had been chosen because of their ability to substitute the octahedral site of LGT, normally occupied by Ga (CN=VI) with a segregation coefficient near unity in Czochralski growth. Doping with Ca and Cr has never been reported before, and therefore, the segregation behavior was unknown. In this study, Al, Ti and co-doping with Cr and Ca has been investigated for both X and Y-oriented films. The dopant distribution in the films was quantitatively evaluated by Secondary Ion Mass Spectroscopy (SIMS), using ion-implanted LGT substrates as standards. The drop of dopant concentration, in the SIMS profile, allows for the identification of the film-substrate interface and to accurately measure the thickness of the films. The film thickness is found to be typically of the order 0.5 to 2µm, depending on growth conditions. The solvent was found a reliable choice, as solvent ions were not incorporated in the films above the detection limits of the characterization techniques. A lead oxide solvent system is used as a solvent for the growth of LGT LPE films with different orientations. Extensive structural characterization was performed. The crystallinity of substrates and films grown with different orientations was compared by X-ray diffraction (XRD). The films show a very high structural perfection, with typically FWHM values of 0.035 for the (004) reflection of the XRD rocking curve. The films were also characterized by TEM. The optical transmittance of the films was characterized by Varian optical spectrophotometer, and the value obtained of approximately 80% is comparable with the transmittance value of the Czochralski grown polished substrate.