Size effects in epitaxial oxide thin films

Abstract

Cover Three dimensional impression of the formation of epitaxial nanowires on an ordered mixed terminated crystal surface. The light gray blocks at the bottom represent the different areas of surface mixed termination, e.g., DyO and ScO 2 in the case of DyScO 3 . The dark blocks represent complete perovskite blocks of deposited film material, e.g., SrRuO 3 . This type of nanowire formation is described in chapters three, four and five of this thesis. The picture is generated using POVRay software and is based on actual Monte Carlo simulation results.The research described in this thesis was carried out within the Inorganic Materials Science group, Department of Science and Technology and the MESA+ institute for Nanotechnology at the University of Twente. This work is financially supported by The Netherlands Organization for Scientific Research (NWO). Nanopatterned epitaxial oxide thin films IntroductionFabricating ever smaller devices which show ever more functionality is at the heart of modern day technological development in the field of electronics. This quest for smaller and more advanced electronics, requires the fabrication of components and structures with length-scales now reaching only several nanometers (a billionth of a meter). Material properties at the nanometer scale can be very different compared to their bulk counterparts. Volume to surface area ratios change and classical laws of physics cannot always be applied; quantum effects start to play a role. A famous quote from world-renowned physicist Feynman is often referred to in this regard:"There is plenty of room at the bottom."Richard Feynman, December 1959 [1] Although popular magazines discourage the use of this quote as introduction in scientific publications [2] along with Moore's law, it perfectly summarizes the motivation for much of the research done in the field of nanotechnology in the past years and nicely fits the work done in this thesis. While intrinsic (bulk) material properties may be lost or altered by size or quantum effects, new phenomena and properties can emerge, e.g., giant magnetoresistance, which is now commonly used in hard disk drives and sensor applications. [3,4] The fabrication process of these small structures or thin films is usually different compared to conventional patterning techniques. New fabrication processes must be developed in order to create functional patterns at the nanometer scale and characterization techniques, like electron microscopy, should be improved to allow for analyzing the resulting structures and properties. In this respect, the fundamental material properties, the fabrication process and characterization methods are coupled and all require a great amount of study.An interesting group of materials for both device fabrication and fundamental materials studies is the family of metal oxides. Within this group, the per-1 2 Nanopatterned epitaxial oxide thin films ovskite family contains a range of materials which all share a common oxygen backbone, with a multitude of different properties, e.g....

Stoichiometry control in oxide thin films by pulsed laser deposition

“…Image (a) is adapted from Huijben, [40] (b) from Kuiper. [41] the sample with respect to the Laue circle is aligned in such a way that a circular pattern is observed perpendicular to the sample surface, i.e., perpendicular to a certain crystal plane orientation. These crystal orientations are labeled with their corresponding in-plane [hk] values.…”

Oxide materials as building blocks for artificial topological insulators

“…Originally, XPD is used to study the orientation of small molecules on a surface [233,234,235]. In more recent work, the structural transition from a planar-to chain-type SrCuO 2 crystal structure was observed upon decreasing film thickness by using XPD among other techniques [236] as well as the presence of the tetragonal structure in CuO thin films [237]. Furthermore, for ultra-thin PbTiO 3 (PTO) films, the ferroelectric configuration was determined by combining experimental and simulated XPD results [238,239].…”

“…The angle between the x-ray source and electron analyzer is fixed. The technique remains element specific, since the photoelectric effect forms the basis and FIGURE 6.3: Forward focussing is schematically depicted, adapted from [236]. The curvy yellow line shows the incoming photon beam and the dark green dot represent the emitting atom.…”

“…In ordered systems, variations in the photoelectron intensity are observed due to coherent scattering [236,251]. In crystalline material systems, an increased intensity is observed along the nuclear axes because the emitted photoelectrons are attracted by the Coulomb potential of the neighboring atoms [241] and experience constructive interference [242].…”

“…The size of the neighboring atoms also has an influence, since a lower atomic number implies a lower scattering power [254]. The width of the forward focussing peak depends on the energy of the photoelectron: the higher the energy, the narrower the peak [236,241,251]. Furthermore, higher kinetic energies of the photoelectrons lead to improved focussing over larger nuclear distances [251].…”