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published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Intensive research interests in condensed matter physics have been focused at the strongly correlated electron systems. Most of the efforts were devoted in hole-doped manganites with a double exchange interaction between Mn3+/Mn4+. Recently, tetravalent ions substitution has also stimuli much attention as a supplement for the hole-doping. Such electron-doped manganites may be of great potential for the development of all-manganites devices. Manganites are extremely sensitive to external disturbances, such as magnetic fields, electric fields, currents, mechanical strain, and photo illumination, etc. These extraordinary properties make manganites promising for practical applications. In this thesis, the field modulation on physical properties in electron/hole-doped manganites films and heterojunctions were investigated. The effects of tetravalent hafnium doping on the structural, transport, and magnetic properties of polycrystalline La1-xHfxMnO3(LHMO) (0.05 ≤x ≤0.3) were studied systematically. A phase diagram was obtained for the first time through magnetization and resistance measurements in a broad temperature range. An abnormal enhancement of magnetization was observed at about 42 K. It was further confirmed that the second magnetic phase MnO2in LHMO gives rise to such a phenomenon. The dynamic magnetic properties of LHMO, such as relaxation and aging processes, were studied, demonstrating a spin-glass state at low temperature accompanied by a ferromagnetic phase. Heterojunctions composed of n-type SrTiO3-δand p-type GaAs exhibited excellent rectifying behavior from 40K to room temperature. The photocarrier injection effect and a colossal photo-resistance were observed. Strong dependences on both temperature and bias voltage were found as well, which might be under stood by considering the band structure of the formed p-njunction. By employing an ultrathin SrTiO3buffer layer,La0.8Ca0.2MnO3films could be epitaxially grown on GaAs substrates. The heterostructures exhibit good rectifying behavior with a paramagnetic-ferromagnetic transition at ~200K. The variation of diffusion voltage with temperature in these heterostructures could be explained by the effects of the Hund’s rule coupling between the La0.8Ca0.2MnO3and the buffer layer. The effects of the strain induced by ferroelectric poling on the magnetic and electric properties have been investigated by using 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3(PMN-PT) substrates. The polarization of the PMN-PT crystal reduces the biaxial tensile strain in the formed La0.9Hf0.1MnO3layer.It results in a significant decrease in resistance and an enhancement of the phase transition temperature as well as the magnetization. The impact of the lattice strain on the charge/orbital ordering state was also been studied. The modification of charge/orbital ordering phase by the electric fields and ferroelectric polarization suggested that the unstable states in the manganites are sensitive to the strain. Heterojunctions of La1-xHfxMnO3/Nb:SrTiO3 were fabricated and investigated under different fields (electric, magnetic and optic). These heterojunctions exhibited excellent rectifying behavior in a wide temperature range. Their properties could be significantly modulated by magnetic fields. Prominent photovoltaic effect was also observed in the formed junctions. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

In this thesis, molecular-beam epitaxy (MBE) of three-dimensional (3D) topological insulator (TI) Bi_2 Se_3 thin films on different substrates is presented. The substrates experimented include InP(111)A, GaAs(111)A, InP(001) and GaAs(001). Multiple characterization techniques are employed to investigate the film’s structural, morphological and electrical properties. To facilitate growth of high quality epitaxial Bi_2 Se_3, thermal treatment of the substrate surfaceturnsout to be crucial for both InP(001) and InP(111). On the other hand, for high-index epitaxial Bi_2 Se_3 growth on GaAs(001), the In_2 Se_3 buffer layer has to be employed. Twin defects in epitaxial Bi_2 Se_3 (111) thin films on hexagonal substrates have been found inevitable in the past. In this study, however, such defects are successfully suppressed on InP(111)A and GaAs(111)Asubstrates, as evidenced in electron diffraction and morphological measurements. The prerequisite for the twin-free Bi_2 Se_3 (111) epitaxy appears to be the step-flow growth mode on the purposely treated stepped substrate surfaces, where deposits incorporate in film at step edges. The lattice of InP or GaAs substrate then plays a guiding role for epitaxial Bi_2 Se_3. Twin suppression is also seen to occur for growth on vicinal and islanded InP(111)A substrate, where a high density of steps inherently exists on surface. Transport studies on such single-domain Bi2Se3epifilms show superior electronic characteristics when compared to those of twinned films grown on, e.g., Si(111). The Shubnikov–de Haas (SdH)oscillations due to bulk state Landau quantization are observed in the magnetoresistance (MR) measurements of Bi_2 Se_3films grown on InP(111)A. So far, a majority of experimental work of 3D TIs is exclusively on the (111) surfaces, primarily due to the ease to obtain such a surface by cleavage or by growth. On the other hand, for strong topological insulator, nontrivial surface states are expected to exist on other surfaces as well, which remain to be experimentally confirmed. In this study, a high-index epitaxial Bi_2 Se_3is achieved by epitaxial growth on facetted InP(001) substrate. The latter is obtained by a cautious thermal treatment of the substrate wafer under Se flux, where the rhombohedral In_2 Se_3buffer layer forms, facilitating the growth of Bi_2 Se_3 (221) film.Such a high index Bi_2 Se_3 film is evidenced by low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED) and x-ray diffraction (XRD) measurements. The unique strapped morphology on Bi_2 Se_3 (221) surface is revealed by scanning tunneling microscopy (STM). Angle-resolved photoemission spectroscopy (ARPES) measurements unambiguously show the Dirac surface states elucidating the 3D topological nature ofBi_2 Se_3. Significantly, constant energy plot shows an anisotropic Fermi surface, being ofellipticalshape, which qualitatively agrees with the theoretical calculation. Transport studies of such Bi_2 Se_3(221) films reveal the ratio of conductivities along directions parallel and transverse the van der Waals (vdW) gaps to be as high as 4.4. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Atomic layer deposition (ALD) has emerged as an important technique for depositing thin films in both scientific research and industrial applications. The goal of this work is to integrate functional materials using ALD including high-κ dielectric, LaAlO₃, ferroelectric BaTiO₃, photocatalytic CoO, and room temperature ferromagnetic thin films of Co metal for spin-transfer torque random-access memory applications. The work is also to demonstrate the formation of a quasi-two-dimensional electron gas (2-DEG) at the γ-Al₂O₃/SrTiO₃ heterointerface enabling a method for all-oxide device manufacturing using ALD. High permittivity oxide thin films are needed to replace SiO₂ in complementary metal oxide semiconductor (CMOS) transistors. The replacement of SiO₂ by hafnium oxide-based high-κ materials in CMOS devices in 2007 was a revolutionary development in semiconductor front end of line. The continued device feature shrinking requires higher-κ dielectrics, compared to HfO₂-based materials. Crystalline perovskite oxides, such as SrTiO₃, LaAlO₃, and BaTiO₃, etc. have from high to very high dielectric constant and being proposed to replace HfO₂-based materials in CMOS devices if the leakage problem is resolved. The work explores the monolithic integration of crystalline perovskite oxide films with Si(001) using combined molecular beam epitaxy (MBE) and ALD techniques. Four unit cells of SrTiO₃ were grown directly on Si(001) by MBE and transferred in-situ into the ALD chamber for further depositions. The integration of oxide thin films on Si(001) using the MBE-ALD technique allows us to maintain clean oxide/Si(001) interfaces since low temperatures (180–250 °C) were maintained during the ALD deposition. The goal of my work is also to explore processes to enable area selective deposition of cobalt (II) oxide, CoO. The effectiveness of poly(trimethylsilylstyrene) in selectively inhibiting surface nucleation of CoO on SiO₂ and MgO substrates is demonstrated. Carbon-free cobalt thin films are formed by reducing CoO using Al and Sr metals to scavenge oxygen from CoO. The work explores the ability to control the structure and morphology of the resultant cobalt film by tuning the reduction conditions, allowing us to tune magnetic properties of the cobalt thin film. My work also focuses on the growth of γ-Al₂O₃ on the TiO₂-terminated SrTiO₃ substrate at temperatures higher than 300 °C. The formation of a quasi-2-DEG is found at the γ-Al₂O₃/TiO₂-terminated SrTiO₃ interface. In-situ x-ray photoelectron spectroscopy reveals the presence of Ti³⁺ feature at the heterointerface. Conductivity at the interface was found to be proportional to the amount of Ti³⁺ species. Oxide quasi-2-DEG might provide opportunities for new generations of all-oxide electronic devices using ALD.

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Great interest has been directed toward the study of polymer thin films recently due to their emerging applications, and appreciable deviated properties and phenomena as compared with bulk polymers. Carbon dioxide (CO₂) has received attention as an environmentally benign alternative to hazardous industrial solvents. Unlike conventional liquid solvents, the density and hence the solvent strength of supercritical CO₂ can be tuned by small variations in pressure, temperature or both. The objective of this work is to study the interaction between high pressure CO₂ and polymer systems. We introduced the methodology used in this dissertation. The combination of gradient theory of inhomogeneous systems and Sanchez-Lacombe Equation of State is used to calculate the interfacial properties, such as interfacial density profile, interfacial tension and interfacial thickness. We first investigated the adsorption of supercritical fluid on polymer surfaces. We showed analytically that surface adsorption of high pressure fluid on an attractive surface is proportional to the compressibility of the fluid. We have also investigated numerically the sorption of supercritical CO₂ on poly(dimethylsiloxane) (PDMS) and polyisobutylene (PIB), and supercritical 1,1-difluorethane on PS. By calculating the Gibbs adsorption and adsorption layer thickness of the supercritical fluids, we found in all cases that maximum adsorption occurred when the supercritical fluid was near its compressibility maximum. We then examined the compatibilization effect of supercritical fluid on two incompatible polymers. We calculated the interfacial density profile, interfacial thickness and interfacial tension between the two polymers with and without the supercritical fluid. We found that the interfacial tension was decreased and the interfacial thickness was increased with high pressure super-critical fluid for the ternary systems we have investigated. No enhancement or deleterious effects on compatibilization were observed as the critical point was approached and the compressibility became large. We also examined the morphological structures of asymmetric poly(ethylene oxide)-b-poly(1,1'-dihydroperflurooctyl methacrylate) (PEO-b-PFOMA) thin films upon annealing in supercritical CO₂. The strong affinity between PFOMA and CO₂ was found to induce phase segregation when annealing PEO-b-PFOMA films as compared with vacuum at the same temperature.