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Title: Engineering of artificially layered lanthanum-based perovskites using pulsed laser deposition
Author: Saniee, Nessa Fereshteh
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2013
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With the advances made over the past two decades in microelectronics, the necessities to the fabrication of high quality materials, from semiconductors to oxides, has become increasingly stringent. To adapt to the increasing demand of new device structures, often a single material will not exhibit all the desired properties, and combining two or more materials to form artificial structures with tailored electronic and magnetic properties become essential. A deeper understanding of the relationship between physical properties of these materials and their nanostructure is therefore required. This can be achieved through atomic engineering, whereby new materials are fabricated specifically to have the desired properties, leading to new devices and applications. In this thesis the ability to fabricate high quality interfaces and superlattice structures between different lanthanum-based perovskites (LaMO3 M=Ni and Mn) is demonstrated. Using a pulsed laser deposition equipped with in-situ reflection high-energy electron diffraction (RHEED), single unit-cell control of the deposition process could be achieved. Since the single layer thickness of the unit-cell can significantly influence the macroscopic properties of such superlattices, it is important to distinguish between the effects arising due to short periodicity and those properties from the growth of each constituent layer. Therefore detailed analysis of the layers and superlattice structures is essential to completely understand the nature of these new materials properties. Unusual magnetic properties, such as spin glass-like behaviour and an exchange bias, were observed for the LaMnO3 films grown at high temperatures in lower oxygen pressure. This indicates an intrinsically inhomogeneous magnetic state in the locality of the transition. It has been shown that the crystal structure of LaNiO3 is much more sensitive to the growth temperature and prone to decomposition into a secondary phase of La2NiO4, that is preferentially grown in the plane of the film detected by grazing-incidence angle XRD and scanning TEM. RHEED data, including strong intensity oscillations and streaky patterns, suggest that the growth of both these materials can be optimised with purely 2D growth mode, and hence making it possible to grow single unit cell layers of these materials. The resulting strong RHEED intensity oscillations and sharp XRD features indicate high quality material growth. However small variations in the growth process lead to the formation of a Ruddlesden-Popper phase. In this thesis the ability to identify the octahedral connectivity, enabled by significantly improved epitaxial growth of short-period superlattices has been shown for the first time using precise surface analysis and atomic resolution transmission electron microscope. A Jahn-Teller type distortion of the MnO6 octahedra evolves during the deposition to reduce the symmetry by increasing the degeneracy of the energy levels leading to a (√2 × √2)R45◦) surface reconstruction. This method can be used as an emerging route to detection of the deterministic octahedral distortions by engineering potential new functionalities by identifying the phase of the terminating layer.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics