With recent trends in miniaturization in the electronics sector, ferroelectrics have gained popularity due to their applications in non-volatile RAM. Taking one step further researchers are now exploring multiferroic devices that overcome the drawbacks of ferroelectric (FE) and ferromagnetic (FM) RAM’s while retaining the advantages of both. The work presented in this dissertation focuses on the growth of FE and FM thin film structures. The primary goals of this work include, (1) optimization of the parameters in the pulsed laser deposition (PLD) of FE and FM films and their heterostructures, (2) development of a structure-property relation that leads to enhancements in electric and magnetic polarizations of these structures, (3) investigation of doping on further enhancement of polarizations and coupling between the FE and FM layers. The materials of choice are La0.7Sr0.3MnO3 (LSMO) as the ferromagnetic and PbZr0.52Ti0.48O3 (PZT) as the ferroelectric component. Epitaxial thin film capacitors were grown using PLD. The work starts with the establishment of the optimum deposition conditions for PZT and goes on to describe results of attempts at performance enhancement and tuning using two methods. It is demonstrated that ferroelectric and ferromagnetic properties can be tuned by inserting a ferromagnetic buffer layer of CoFe2O4 (CFO) between PZT and LSMO. One of the key findings of this work was the anomalously high ferroelectric polarizations produced by lanthanum (La) doped PZT films. This work attempts to shine light on a possible mechanism that leads to such high enhancements in polarization.

Titanium is one of the earth-abundant elements, and its oxides including titanium dioxide (TiO2) and strontium titanium oxide (SrTiO3) are widely used in technologies of electronics, energy conversion, catalysis, sensing, and so on. Generally, the Ti ions in these compounds have a valence of 4+ with the outer shell electron configuration of 3d0. In this thesis, we explore interface and titanite containing Ti3+ ions with 3d1 itinerate electrons, which we believe open new doors towards some new titanite-based technologies. In the first part of this thesis (Chapter 3), we will discuss the nanoscale chemical and valence evolution at a metal/oxide interface: Ti/SrTiO3. In many devices, metal-oxide interfaces are ubiquitous and play important roles in the performance of a wide range of electronic and optoelectronic devices. This motivated us to examine the microscopic structure of the interfaces between strontium titanium oxide and metals. In this work, one unit cell of cubic perovskite Ti2O3 was observed at the Ti/SrTiO3 interface, and oxygen diffusion depth of ~3.2 nm was observed in the sample fabricated at room temperature. Meanwhile, oxygen vacancy domains in the SrTiO3 substrates was observed and characterized by low angle annular dark field (LAADF) imaging and electron energy loss spectra (EELS). In the main part of this thesis, we will focus on the structure and physical properties of Ti2O3, a titanite which has received less attention so far in the research community. Different from TiO2 and SrTiO3, Ti2O3 has a much narrower band-gap (~0.1 eV), and we will discuss some preliminary results of its physical properties and potential applications. In Chapter 4, we will discuss the photothermal application and mid-infrared photodetectors using Ti2O3 nanoparticles based on its ultra-narrow bandgap. Photo-thermal effect via a Ti2O3/membrane structure is further applied to seawater desalination. A high temperature of 70 °C was achieved when this Ti2O3/membrane double layer structure floating on top surface of water subjected to the white light illumination of 7 kW/cm2. Furthermore, room temperature mid-infrared (10 μm) photodetectors based on Ti2O3/graphene hybrid structure was fabricated and studied. The photoresponsivity of this hybrid device, operated from 4.5 to 10 μm, is above ~ 100 A/W, which, to our knowledge, is the highest value for the mid-infrared photodetectors operating in the photocurrent (PC) mode. In chapter 5, structure, optical, transport properties of Ti2O3 epitaxial thin films on sapphire fabricated by pulsed laser deposition (PLD) will be discussed. By tailoring growth conditions, two different: trigonal and orthorhombic, of Ti2O3 were stabilized on Al2O3 substrates. More interestingly, the orthorhombic Ti2O3 has never been reported, and, moreover, superconductivity (~8 K) and high temperature ferromagnetism (up to 700 K) was discovered in this new stabilized phase. More details of the physical properties of Ti2O3 will be discussed in the following chapters of this dissertation.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19187号 / 工博第4064号 / 新制||工||1627(附属図書館) / 32179 / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 陰山 洋, 教授 阿部 竜, 教授 田中 勝久 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

A precise understanding of the fundamental correlation between synthesis, microstructure and physical properties is of vital importance towards rational design of improved functional epitaxial thin films. With the presence of heterogeneous interface and associated inhomogeneous lattice strain, film microstructure becomes sensitive to subtle interfacial perturbations and hence may exhibit intriguing physical properties. Control of the epitaxial film functionality requires accurate knowledge of the actual film chemistry, interfacial defects and associated strain field. This dissertation reports in-depth microstructural characterization of the intrinsic chemical inhomogeneity in selected epitaxial thin films including superconducting Fe1+yTe1-xSex/SrTiO3(STO) heterogeneous systems, the flux-pinning defects at both of conversional YBa2Cu3O7-δ (YBCO)/substrate lateral interfaces and vertical interfaces of YBCO&BaSnO3(BSO) nanocomposite films, and the misfit dislocation core configurations of STO/MgO and MgO/STO heterostructures pair, using the state-of-the-art aberration-corrected scanning transmission electron microscopy (CS-corrected STEM) in combination with geometric phase analysis (GPA). For the first time, the local atomic arrangement of Te and Se as well as interstitial Fe(2) has been clearly revealed in superconducting Fe1+yTe1-xSex/STO epitaxial films. We found that the film growth atmosphere can greatly affect the film stoichiometry, the homogeneity of Se/Te ordering and thus the overall film superconductivity. YBCO/substrate interface mismatch and YBCO&BSO vertical interface contact have been explored through substrate selection and doping-concentration variation. We observed a diverse nature of intrinsic defects in different YBCO/substrate heterosystems; thermal stable defects capable of maintaining individual strain field have been found effective in flux-pinning. Along the vertical heterointerface of YBCO/BSO, misfit dislocations were found throughout the film thickness. It adds another dimension to the flux-pinning landscape design. Four basic misfit dislocation core configurations of a STO/MgO heterosystem have been identified, and found strongly dependent on the actual interface disordering such as substrate atomic-height steps and interdiffussion. To precisely quantify the heterointerface lattice strain, we first conducted systematic investigations on the accuracy of STEM-based GPA. Follow our protocol, 1 pm accuracy has been proven in the STEM fast-scan direction with a spatial resolution less than 1 nm. The effectiveness and reliability of this optimized GPA strain profile were demonstrated in both applications of a relaxed STO/MgO and a partially strained LaAlO3/STO heterointerfaces, respectively.

The recent discoveries of topological insulators (TI) has opened a new realm for study¬ing topological systems and exploring the exotic properties they offer. The in-built topological protection against direct backscattering and absence of localization makes two-dimensional (2D) surface states of bismuth chalcogenide-based strong TI a promising platform for studying interesting phenomena in condensed matter physics like dissipation-less transport, quantum anomalous hall effect, topological magnetoelectric effect, majo¬rana fermions etc. and also makes this system very suitable for applications in the fields of electronics and spintronics. However, realization of these novel states can be difficult because of scattering of surface states from different types of disorders (intrinsic or ex¬trinsic) or the presence of parallel channels in the bulk of the sample which can dominate over surface transport. The main goal of this thesis is to evaluate the performance of TI as an electronic element and look into elastic and inelastic scattering processes and kinetics of these scatterers. In most part of this work we concentrate on the magnitude and origin of low-frequency flicker noise or the 1/f-noise, a key performance marker in electronics, to characterize the electrical transport in TI. In this work we have studied 1/f-noise in both mechanically exfoliated TI-flakes and epitaxially grown TI films by varying chemical potential and temperature. Our study of exfoliated TI-flakes with a wide range of thickness (10 nm to 80 μm) suggests that whereas at thinner (<100 nm) samples and at low temperature (<70 K), the electrical transport happens entirely at the surface, resistance fluctuations in the surface states are mainly caused by potential fluctuations caused by generation-recombination processes in the bulk of TI. Study of 1/f-noise in MBE-grown magnetically doped TI reveals signature of hopping transport through localized bulk mid gap states. These states can either be Cr-impurity band or disorder-induced mobility edge states of bulk valence band. Our study of quantum transport in exfoliated TI-devices indicate presence of a de-coherence mechanism which saturates phase-coherence length and temperature below T< 3 K and results from a unique scattering mechanism caused by localized magnetic moments in these systems

Le but de ce travail de thèse est de réaliser des vannes de spin « naturelles » constituées d’un matériau unique. Le matériau en question doit contenir une alternance de feuillets magnétiques et non magnétiques et présenter différents états magnétiques. Ce système modèle ne présenterait alors ni d’inter diffusion ni de rugosité aux interfaces magnétiques/non-magnétiques et pourrait constituer un système modèle pour les études des phénomènes de transport dépendant de spin. Le CoV2O6 est un oxyde polymorphe de basse dimensionnalité. Les deux phases (α et γ) présentent chacune plusieurs plateaux d’aimantation induits par un champ magnétique. Le premier objectif est de comprendre la structure cristalline et magnétique de ce composé, ce qui a été réalisé par des mesures d’aimantation, de diffraction des rayons X et de neutrons sur des poudres et monocristaux. Les résultats de la phase α sont appuyés par des calculs ab initio. Le second objectif est de déposer ce matériau en couche mince afin d’analyser ses propriétés de transport. Des films épitaxiés de γ CoV2O6 ont été obtenus sur TiO2(100) et TiO2/Pt(111) par ablation laser. Le dépôt sur l’électrode de Pt montre la présence de six variants entrainant l’observation de plateaux d’aimantation supplémentaires. / The purpose of this work is to synthesize “natural” spin valves within one unique material. The material needs a crystalline structure formed by stacking magnetic and non-magnetic sheets as well as different magnetic states. Such model system could be used for the study of spin dependent transport properties as no-roughness or diffusion at the magnetic/non-magnetic interfaces is allowed. The polymorph low-dimensional oxide CoV2O6 is such a material. Both phases (α and γ) exhibit field induced magnetization plateaus. This study’s first aim is to understand crystalline and magnetic structures. Investigations were performed on powders and single crystals using magnetization measurements as well as X-ray and neutron diffraction measurements. The magnetic properties of α CoV2O6 were supported by ab initio calculations. The second aim is to grow CoV2O6 thin films in order to analyze further the transport properties. Epitaxial γ CoV2O6 thin films were obtained by pulsed laser ablation on both TiO2(100) and TiO2/Pt(111) substrates. Films grown on Pt electrode exhibit six variants which allows observing additional magnetization plateaus.

No description available.

Group III-nitride semiconductors have received much research attention and witnessed a significant development due to their ample applications in solid-state lighting and high-power/high-frequency electronics. Numerous growth methods were explored to achieve device quality epitaxial III-nitride semiconductors. Among the growth methods for III-nitride semiconductors, molecular beam epitaxy provides advantages such as formation of abrupt interfaces and in-situ monitoring of growth. The present research work focuses on the growth and characterizations of III-nitride based epitaxial films, nanostructures and heterostructures on c-sapphire substrate using plasma-assisted molecular beam epitaxy system. The correlation between structural, optical and electrical properties of III-nitride semiconductors would be extremely useful. The interfaces of the metal/semiconductor and semiconductor heterostructures are very important in the performance of semiconductor devices. In this regard, the electrical transport studies of metal/semiconductor and semiconductor heterostructures have been carried out. Besides, studies involved with the defect induced room temperature ferromagnetism of GaN films and InN nano-structures have also been carried out. The thesis is organized in eight different chapters and a brief overview of each chapter is given below. Chapter 1 provides a brief introduction on physical properties of group III-nitride semiconductors. It also describes the importance of III-nitride heterostructures in the operation of optoelectronic devices. In addition, it also includes the current strategy of the emergence of room temperature ferromagnetism in III-nitride semiconductors. Chapter 2 deals with the basic working principles of molecular beam epitaxy system and different characterization tools employed in the present work. Chapter 3 describes the growth of GaN films on c-sapphire by plasma-assisted molecular beam epitaxy. The effects of N/Ga flux ratio on structural, morphological and optical properties have been studied. The flux ratio plays a major role in controlling crystal quality, morphology and emission properties of GaN films. The dislocation density is found to increase with increase in N/Ga flux ratio. The surface morphologies of the films as seen by scanning electron microscopy show pits on the surface and found that the pit density on the surface increases with flux ratio. The room temperature photoluminescence study reveals the shift in band-edge emission towards the lower energy with increase in N/Ga flux ratio. This is believed to arise from the reduction in compressive stress in the GaN films as it is evidenced by room temperature Raman study. The transport studies on the Pt/GaN Schottky diodes showed a significant increase in leakage current with an increase in N/Ga ratio and is found to be caused by the increase in dislocation density in the GaN films. Chapter 4 deals with the fabrication and characterization of Au/GaN Schottky diodes. The temperature dependent current–voltage measurements have been used to determine the current transport mechanism in Schottky diodes. The barrier height (φb) and the ideality factor (η) are estimated from the thermionic emission model and are found to be temperature dependent in nature, indicating the existence of barrier height inhomogeneities at the Au/GaN interface. The conventional Richardson plot of ln(Is/T2) versus 1/kT gives Richardson constant value of 3.23×10-5 Acm-2 K-2, which is much lower than the known value of 26.4 Acm-2 K-2 for GaN. Such discrepancy of Richardson constant value was attributed to the existence of barrier height inhomogeneities at the Au/GaN interface. The modified Richardson plot of ln(Is/T2)-q2σs2/2k2T2 versus q/kT, by assuming a Gaussian distribution of barrier heights at the Au/GaN interface, provides the Schottky barrier height of 1.47 eV and Richardson constant value of 38.8 Acm-2 K-2 which is very close to the theatrical value of Richardson constant. The temperature dependence of barrier height is interpreted on the basis of existence of the Gaussian distribution of the barrier heights due to the barrier height inhomogeneities at the Au/GaN interface. Chapter 5 addresses on the influence of GaN underlayer thickness on structural, electrical and optical properties of InN thin films grown using plasma-assisted molecular beam epitaxy. The high resolution X-ray diffraction study reveals superior crystalline quality for the InN film grown on thicker GaN film. The electronic and optical properties seem to be greatly influenced by the structural quality of the films, as can be evidenced from Hall measurement and optical absorption spectroscopy. Also, we present the studies involving the dependence of structural, electrical and optical properties of InN films, grown on thicker GaN films, on growth temperature. The optical absorption edge of InN film is found to be strongly dependent on carrier concentration. Kane’s k.p model is used to describe the dependence of optical absorption edge on carrier concentration by considering the non-parabolic dispersion relation for carrier in the conduction band. Chapter 6 deals with the analysis of the temperature dependent current transport mechanisms in InN/GaN heterostructure based Schottky junctions. The barrier height (φb) and the ideality factor (η) of the InN/GaN Schottky junctions are found to be temperature dependent. The temperature dependence of the barrier height indicates that the Schottky barrier height is inhomogeneous in nature at the heterostructure interface. The higher value of the ideality factor and its temperature dependence suggest that the current transport is primarily dominated by thermionic field emission (TFE) other than thermionic emission (TE). The room temperature barrier height and the ideality factor obtained by TFE model are 1.43 eV and 1.21, respectively. Chapter 7 focuses on the defect induced room temperature ferromagnetism in Ga deficient GaN epitaxial films and InN nano-structures grown on c-sapphire substrate by using plasma-assisted molecular beam epitaxy. The observed yellow emission peak in room temperature photoluminescence spectra and the peak positioning at 300 cm-1 in Raman spectra confirms the existence of Ga vacancies in GaN films. The ferromagnetism in Ga deficient GaN films is believed to originate from the polarization of the unpaired 2p electrons of nitrogen surrounding the Ga vacancy. The InN nano-structures of different size are grown on sapphire substrate, the structural and magnetic properties are studied. The room temperature magnetization measurement of InN nano-structures exhibits the ferromagnetic behavior. The saturation magnetization is found to be strongly dependent on the size of the nano-structures. Finally, Chapter 8 gives the summary of the present work and the scope for future work in this area of research.