Fabrication and Properties of Topological and Two-dimensional Thin Film Heterostructures

Spintronics explores phenomenon that interlink the spin and charge degrees of freedom. It is a field where traditional solid-state physics and materials research have created their strongest bond, with each taking alternate leading roles in a unique, fast paced, technological tango. Such a broad definition implies that the range of subjects that fall under the umbrella of spintronics is inevitably very wide. In order to develop spintronics technology, it is first necessary to fully explore potential materials and their properties; by obtaining a thorough understanding of spintronic phenomena we can effective utilize them to create spin-engineered materials and working devices. However, the spin of the carriers tend to go out of phase (both in time and space) in most of the materials thereby losing the spin coherent current, which is required in order to manipulate and utilize the spin of the moving electrons. Thus, it is of great interest now to identify and engineer the materials so that they retain not only spin coherency, but also are capable of supporting maximum spin polarization. The potential materials systems which are believed to be possessing such characteristics are Room Temperature Ferromagnetic Semiconductors (RTFM) and Topological insulators (TIs).;The primary aim of this research is to (1) integrate emerging defect-induced RTFM material, Sr3SnO, with technically important substrate Si (100) using pulsed laser deposition (PLD) technique. The films were grown under different deposition conditions in order to understand the effect of processing parameters on the film properties. The characteristics of the thin films have been investigated in detail using X-ray diffraction, TEM, X-ray photoelectron spectroscopy (XPS), UV- photoelectron spectroscopy (UPS), Physical Property Measurement System (PPMS) and superconducting quantum interference device (SQUID) in order to establish processing-structure-property correlation. The mechanisms of electrical transport and ferromagnetic properties of Sr3SnO is discussed, and we show the ability to simultaneously control both properties by manipulating the intrinsic defects, presumably oxygen vacancies. The transport mechanism follows the variable-range-hopping (VRH) model, consisting of Efros and Shklovskii (ES) and Mott VRH laws in specific temperature regions; and the ferromagnetism results are explained through the oxygen vacancy constituted bound magnetic polarons (BMP) model. An attempt has been made to elucidate the role of point defects, in controlling the carrier concentration transport and ferromagnetic characteristics of SSO films.;TIs are the other candidates to open up a novel route in spin based electronics where the gapless surface states on TIs are protected from elastic scattering on non-magnetic impurities that makes them promising candidates for low-power electronic applications. On contrary to traditional ferromagnetic materials, where the carrier spin polarization and magnetization are based on the exchange interaction, the spin properties in TIs are based on the coupling of spin- and orbit interaction connected to its momentum.;The secondary aim of this research is to (2) integrate the epitaxial Bi2Se3 thin films on c-sapphire substrates by PLD and demonstrate the existence of topologically protected 2D surface states. Detailed characterization using X-ray diffraction, Raman spectroscopy, XPS, angle-resolved UPS for valence band structure analysis, PPMS for magnetotransport measurements and SQUID for magnetic properties analysis were performed. The different Se% of the samples were prepared to investigate the characteristics and probe the role of defects as a function of processing conditions to establish the correlations between microstructure, strain and physical properties. Furthermore, highly functional hybrid structures consisting of Bi2Se3 and ferromagnetic insulating materials (FMI) were made to demonstrate the proximity-induced ferromagnetism in Bi2Se3. Two hybrid structures are studied: Bi2Se3/Cr2O3/c-sapphire and Bi2Se3/MgO/c-sapphire. The insulating behavior is more pronounced due to the additional scattering of the surface states of the Bi 2Se3 layer by interfacing with MgO and Cr2O 3. The weak antilocalization effect from the surface states is clearly suppressed, accounting for the presence of magnetic bottom layers. It provides an effective way to study the emergence of a ferromagnetic phase in TIs by the magnetic proximity effect in Bi2Se3, a step toward unveiling their exotic properties.