Browsing by Subject "Thin Films"

Now showing 1 - 15 of 15
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    AC Susceptibility and Anisotropic Magnetoresistance: A Study of Thin Magnetic Films
    (2019-12) Booth, Kevin
    The differential ac magnetic susceptibility of thin magnetic films was determined using the anisotropic magnetoresistance (AMR) to measure the response of the magnetization to an applied ac magnetic field. The ac susceptibility was measured as a function of an applied dc magnetic field. The frequency of the applied ac field was varied between 5Hz to 5000Hz. The ferromagnetic films investigated were permalloy, cobalt, nickel, and nickel with an antiferromagnetic nickel oxide layer on one surface. For all the samples investigated, the differential susceptibility magnitude was a function of the dc field magnitude and was frequency dependent, decreasing with increasing frequency.
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    Continuos Doping of La2CuO4+x Thin Films
    (2015-09) Kinney, Joseph
    Finding more efficient ways of exploring the doping phase diagrams of high temperature superconductors as well as probing the fundamental properties of these materials are essential ingredients for driving the discovery of new materials. We use a doping technique involving gating with ionic liquids to systematically and continuously tune the Tc of superconducting La2CuO4+x thin films. We probe both the transport properties and the penetration depth of these samples and find that Homes scaling, lambda^-2 ~ sigma*Tc, is obeyed, consistent with these materials being in the dirty limit. This result is independent of the precise mechanism for the gating process as all of the parameters of the scaling relationship are determined by direct measurements on the films.
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    Defect Engineering in Perovskite Oxide Thin Films and Heterostructures
    (2019-03) Prakash, Abhinav
    Perovskite oxide (ABO3 type compounds) is an important class of materials exhibiting a wide range of functionalities. However, in comparison to conventional semiconductors such as silicon, they possess orders of magnitude lower room-temperature electron mobilities. For example, in doped SrTiO3, the best reported room-temperature value of electron mobility has remained below 10 cm2 V−1 s−1 for over five decades. The realization of a perovskite oxide semiconductor with high room-temperature mobility would constitute a significant advancement, enabling novel physical and perhaps even a plethora of new and more realistic device concepts. Very recently a key step in this direction was taken via the growth of bulk doped BaSnO3, where room-temperature mobilities as high as 320 cm2 V−1 s−1 was reported. Thin films of BaSnO3 show much lower room-temperature mobility values ranging between 1-180 cm2 V−1 s−1 and highly dependent on the growth method, choice of substrate, and dopants. Although these findings have been encouraging for fundamental studies and potential applications in room-temperature oxide electronics, there still remains many open fundamental questions and challenges including the role of defects on the properties of BaSnO3 and the scattering mechanisms that limit the mobility in thin films from reaching values close to bulk mobility. These questions will be addressed in this thesis by studying thin films of BaSnO3 grown by molecular beam epitaxy. One of the challenges with the growth of BaSnO3 is the high electronegativity (low oxidation potential) of tin suggesting that stronger oxidizing conditions such as ozone or high-pressure oxygen plasma are required to achieve full oxidation of Sn. Such extreme oxidation conditions in an ultra-high vacuum molecular beam epitaxy system may lead to undesirable consequences such as oxidation of elemental sources leading to flux-instabilities, filament oxidation, and potential damage to vacuum pumps. As the first step in this direction, a new radical-based hybrid MBE approach for tin-based compounds is developed. For BaSnO3 growth, Ba is supplied through effusion from a cell, Sn using a chemical precursor (hexamethylditin – (CH3)6Sn2), and oxygen using a radio frequency plasma source. The unique aspect of our approach is that hexamethylditin forms highly reactive Sn• radicals, which facilitate the growth of phase-pure, stoichiometric films even in weak oxidizing environment such as molecular oxygen. Using this approach, synthesis of phase-pure and epitaxial BaSnO3 with scalable growth rates and layer-by- layer control over thicknesses is reported. Reflection high-energy electron diffraction is used to describe the strain relaxation behavior of BaSnO3. Various characterization techniques are employed for establishing the stoichiometric growth condition such as X-ray diffraction for lattice parameter measurements, Rutherford backscattering spec- trometry for quantification of cation (Sn:Ba) ratio, atomic force microscopy for imaging the surface morphology, electronic transport for measuring the carrier concentrations, resistivity, and electron mobility in lanthanum-doped BaSnO3 films, and time-domain thermoreflectance for determining the thermal conductivity. With the combination of these techniques, existence of a self-regulating “growth-window” is demonstrated. Through controlled La-doping in BaSnO3 films, a highest room-temperature electron mobility of 120 cm2 V−1 s−1 is achieved on a -5.12 % lattice-mismatched SrTiO3 substrate. The optimal doping range for the highest mobility is found to be 5.0 × 1019 cm−3 to 5.0 × 1020 cm−3. Mobility decreases at higher or lower doping concentrations. Temperature-dependent measurements of mobility provide insights into the scattering mechanisms limiting the mobility at different doping concentrations and temperatures. While dislocation scattering is found to be dominant at low doping regime, ionized impurity scattering plays a major role at high doping levels. At intermediate doping concentrations, both scattering mechanisms control the transport behavior. Phonon scattering accounts for the decreasing trend in mobility with increasing temperature. Building upon these findings which revealed mobility-limiting mechanisms in uniformly doped BaSnO3, the final step involves the development of modulation doping approach n BaSnO3-based heterostructures. The basic idea behind modulation doping is to sep- arate electrons from their ionized donors. Favorable band offsets in BaSnO3–SrTiO3 and BaSnO3–SrSnO3 systems are established. Taking BaSnO3–SrSnO3 as the model heterostructure, electron transfer from La-doped SrSnO3 to BaSnO3 is demonstrated, resulting in dramatic changes in the transport behavior. Results are encouraging and clearly suggest that electrons in BaSnO3 can be separated from ionized dopants. The transport, however, is still limited by dislocations and defects at the interface which should be the focus of future studies.
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    Design and fabrication of state of the art uncooled thermopile infrared detectors with cavity coupled absorption
    (2013-06) Shea, Ryan Patrick
    We present the design, fabrication, and characterization of uncooled thermopile infrared detectors with cavity coupled absorption in the long wave infrared with performance exceeding all published works. These detectors consist of a two die optical cavity which enhances absorption in the desired spectral range while rejecting unwanted noise off resonance. The electrical transduction mechanism is a thermopile consisting of four thermoelectric junctions of co-sputtered Bi2Te3 and Sb2<\sub>Te3<\sub> having a room temperature unitless thermoelectric figure of merit of .43. Processing steps are described in detail for the fabrication of extremely thermally isolated structures necessary for highly sensitive detectors. Optical characterization of the devices reveals a responsivity of 4700 V/W, thermal time constant of 58 ms, and specific detectivity of at least 3.0x109<\super> cmHz1/2/W. Also presented are a theoretical proposal for a midwave infrared detector using semiconductor selective absorption to enhance detectivity beyond the blackbody radiation limit and a new method for the analysis of radiation thermal conduction in highly thermally isolated structures.
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    Development and Characterization of Magnetostrictive GaFe and Plasmonic Gold Thin Films
    (2015-04) Estrine, Eliot
    As device sizes continue to shrink into the nano-scale, material development becomes increasingly important. This presents new deposition and characterization challenges which must be overcome to produce the next generation of devices. Magnetostrictive GaFe (galfenol) is one such material in which development of deposition and characterization techniques is necessary to enable new MEMS devices. In addition, plasmonic gold Near Field Transducers (NFTs) used in Heat Assisted Magnetic Recording (HAMR) require new characterization options to understand device failure modes as well as new gold deposition processes to improve device reliability. While these applications are very different, the underlying material deposition and characterization challenges involving thin film crystallinity are very similar. Magnetostriction measurements of electrodeposited galfenol show that it is possible to achieve thin films of this material over a wide range of compositions using electrodeposition. In addition, grain refinement in gold was achieved through alloying which shows the potential to create more robust thin films while maintaining gold's desirable plasmonic properties. Finally, advanced characterization processes using Electron Back Scatter Diffraction (EBSD) were also developed to analyze thin film crystal structure and its role in NFT stability. These results will further progress in the fields of MEMS and HAMR as well as provide the basis for identifying and solving materials challenges in the future.
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    Graphene Oxide - Towards A Comprehensive Characterization Scheme
    (2015-09) Ismail, Issam
    Graphene oxide (GO) is a near-2D material derived via oxidation of graphite and exploited in nanocomposites and optoelectronics. Following a literature review, the modified Tour-Dimiev (MTD) method was singled out for making GO, with the introduction of modifications tailored towards producing large sheets by starting with a large graphite size, tuning the oxidation conditions, employing temperature control and a modified wash routine. The product was characterized using wide-angle x-ray diffraction, x-ray photoelectron and Raman spectroscopy, revealing near completeness of graphite conversion, high oxygen content of GO and a comparable degree of defects to literature reports on the same. We imaged MTD-GO via fluorescence quenching microscopy (FQM) and atomic force microscopy (AFM). We compared the analytical capabilities of the image analysis software ImageJ with MATLAB, introducing several MATLAB subroutines to mitigate image analysis issues. We image-analyzed MTD-GO, concluding that GO size and thickness are statistically uncorrelated and described by lognormal and normal distributions respectively. We demonstrated that AFM captures small particles better than FQM, and that these two techniques can be combined to obtain a complete picture of polydisperse sample size distributions. Next, we modeled polydisperse dilute dispersions of oblate spheroids and discs in shear, uniaxial and biaxial extension using microhydrodynamic models found in the literature. We used the shear model to fit experimental shear data on a number of serially diluted sheet dispersions to obtain the dimensions and distributions thereof. The systems analyzed were MTD-GO, commercial GO before and after sonication, and a literature dataset on aqueous layered double hydroxides. Finally, we conducted novel Langmuir trough experiments with MTD-GO to understand the mechanisms surrounding the air-water interfacial assembly of GO. We were able to successfully transfer our films from the air-water interface onto a simple and versatile substrate such as surface-treated glass. We correlated film morphology in situ using Brewster Angle Microscopy and ex situ through FQM imaging of Langmuir-Blodgett-coated glass slides, to the pressure-area isotherm. We established that film packing occurs at low surface pressures. Finally, we showed that GO shows weak, pH-dependent intrinsic surface activity.
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    Magnetic Anisotropies and Damping in Epitaxial Iron Thin Films
    (2021-08) Etheridge, James
    In the research presented in the following thesis, the magnetic properties of a setof Fe/InAs(001) heterostructures with thicknesses ranging from 1.4 nm to 39.0 nm are investigated through the use of ferromagnetic resonance, x-ray diffraction, and magnetometry. The magnetic anisotropy results are heavily dependent on Fe thickness. The ferromagnetic resonance data point to an anisotropic relaxation of the Fe film that induces a shear strain in the Fe lattice. The shear strain produces an extra term of magnetoelastic origins in the free energy density resulting in several interesting results including a rotation of the uniaxial easy axis. Several x-ray diffraction experiments were performed to confirm the anisotropic relaxation of the Fe film. The magnetic damping of the samples were also investigated, yielding results that were anisotropic. The cause of the damping results can most likely be attributed to two-magnon scattering.
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    Solution Synthesis of Metal Sulfide Nanoparticles and Thin Films for Solar Photovoltaics
    (2019-12) Trejo, Nancy
    Metal sulfides, such as copper zinc tin sulfide (CZTS), zinc sulfide (ZnS), and tin sulfide (SnS), are sustainable materials suitable for energy production, energy storage, and microelectronics. In thin film solar cells, environmentally benign and earth abundant elements can provide safe alternatives to toxic and scarce materials. CZTS and SnS can replace CdTe and copper indium gallium diselenide (CIGS) as light absorbing materials, while ZnS can replace CdS as the n-type material. SnS also has applications in thermoelectrics, piezoelectrics, lithium ion batteries and valleytronics, many of which take advantage of its layered 2D structure. This thesis focuses on the solution synthesis of metal sulfide nanoparticles and films. CZTS nanoparticles and SnS nanoplates are made using an organic hot-injection synthesis method, while ZnS films are made with chemical bath deposition (CBD). Solution syntheses are potentially more economical than commonly used vapor deposition methods such as evaporation and sputter deposition. Solution based methods are also versatile, scalable, and offer control over nanocrystal size and shape. Furthermore, nanocrystal dispersions can be used to create large area semiconductor films using roll-to-roll coating and processing. We studied CZTS grain growth in coatings comprised of CZTS nanocrystals (NCs). We synthesized CZTS thin films from colloidal nanocrystal dispersions dropcast onto a Mo-coated soda lime glass and annealed in a sulfur environment. Mo is a common electrical contact in thin film solar cells and soda lime glass is used because it contains impurities (Na and K) known to improve grain growth in CIGS and CZTS. Unfortunately, sulfur easily diffuses through the NCs and reacts with Mo to create MoS2. Higher annealing times increases CZTS grain growth but also increase MoS2 growth. To increase CZTS grain growth and restrict MoS2 growth, we incorporated sodium impurities from the vapor phase. CZTS grain sizes improved with increasing sodium concentration, while MoS2 growth was limited. We investigated the synthesis of Zn(S,O) thin films via chemical bath deposition. Zn(S,O) films were deposited on Mo-coated silicon, from aqueous solutions of ZnSO4, SC(NH2)2, and NH4OH. Compositional depth profiles revealed that oxygen incorporation depends on reaction temperature, SC(NH2)2 concentration, and NH4OH concentration. Oxygen percentage increased with increasing reaction temperature and SC(NH2)2 concentration, and it remained constant with increasing ZnSO4 concentration. The NH4OH concentration controls the solubility of ZnS and Zn(OH)2 and, as a result, controls the oxygen percentage in the films. The films with the lowest oxygen contained ~13% oxygen. To reduce oxygen concentration below this level, we used an alternative synthesis based on the thermal decomposition of zinc diethyldithiocarbamate in organic solvents. This resulted in ZnS films with ~7% oxygen, an improvement on the CBD synthesized films, but carbon contamination emerged as a new problem. Finally, we present a facile chemical synthesis of SnS nanoplates with thickness ranging from only a few bilayers (3 - 10 nm) to ~200 nm and lateral sizes of several microns, via thermal decomposition of a single precursor, tin(IV) diethyldithiocarbamate (Sn(dedtc)4) dissolved in oleic acid (OA) and injected into hot (300 - 340 °C) oleylamine (OLA). Using a battery of characterization methods that include tip-enhanced Raman spectroscopy (TERS) and FTIR, we delineate the roles of oleylamine and oleic acid in the synthesis, and rationalize the factors that determine the thickness and lateral sizes of the nanoplates. Initially 3 - 7 nm thick and 10s of nm wide SnS2 nanoplates nucleate and grow but these are subsequently reduced by oleylamine to form SnS nanoplates. The SnS nanoplate morphology depends on the OA/OLA ratio with a narrow window near ~1 yielding few-layer (3 - 10 nm) thick and several micron wide SnS plates. The FTIR and TERS data suggest that few layer SnS nanoplates form near OA/OLA ~1 because deprotonated oleic acid and an oleylamide that forms upon reaction of OA and OLA adsorb and block island nucleation and growth on SnS nanoplates. We demonstrate an intriguing new property of SnS nanoplates whereby nanoplate dispersions respond to an electric field by forming dendritic patterns.
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    Structure and Transport in Epitaxial BaSnO3: Doping, Mobility and the Insulator-Metal Transition
    (2018-08) Ganguly, Koustav
    The recent discovery of high room temperature electron mobility in wide band gap BaSnO3 (BSO) has generated exceptional interest in this perovskite oxide for electronic devices. Outstanding issues with regards to epitaxial films include understanding transport mechanisms, determining the optimal dopant, and understanding the role of structural defects (like dislocations) in limiting mobility. Here, we discuss detailed temperature and field-dependent electronic transport in both oxygen vacancy and La-doped BSO films grown via high pressure oxygen sputter deposition. High-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM) confirm phase-pure, close to stoichiometric, smooth, epitaxial BSO(001). Film thickness, growth rate, deposition temperature, and substrate (i.e., lattice mismatch) have all been systematically varied and related to mobility. Detailed transport accompanied with STEM has been used to understand the structure-electronic property relationships and reveal the correlation between misfit and threading dislocations in BSO thin films. As-grown undoped, insulating films can be made conductive with controllable n-type doping by vacuum reduction, resulting in 300 K Hall mobilities up to 35 cm2V-1s-1 (on LaAlO3(001)) at 5×1019 cm-3. The mobility-electron density relation has been probed in this manner, down to 2×1017 cm-3, the lowest electron density probed in BSO till date. 2% La-doped BSO films, on the other hand, demonstrate 300 K electron mobilities up to 70 cm2V-1s-1 at ~2 ×1020 electrons per cm3. With increasing film thickness a clear insulator-metal transition is observed with both dopants, likely related to defect density near the substrate. The low temperature upturn in resistivity observed in metallic-like BSO has been analyzed using out-of-plane and in-plane magnetoresistance (MR) measurements. Two-dimensional weak localization (WL) has been identified as the underlying mechanism behind this low temperature quantum correction. Overall, the results not only validate the technique of high-pressure oxygen sputtering as a viable approach to produce high quality BSO films, but also provide insight into the mobility-electron density relation, and mobility-limiting factors in these films. The mobility values reported in this thesis are record values for sputtered films and are comparable to that obtained via pulsed laser deposition (PLD) in previous studies.
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    Tailoring the Microstructure of 2D Molecular Sieve Materials for Thin Film Applications
    (2018-05) Shete, Meera
    Zeolites and metal organic frameworks (MOFs) are microporous materials, with pores of molecular dimensions, that are of interest in a variety of applications including catalysis, adsorption, ion-exchange, separation membranes etc. With a global need of developing clean energy resources and reducing the carbon footprint of existing processes, they are being increasingly sought after as catalysts for the conversion of biomass to chemicals and fuels, in separation membranes to replace the existing energy intensive industrial separations with clean energy-efficient processes and for capture and storage of carbon dioxide. Their performance in these applications depends mainly on their pore size but also on our ability to tune their microstructure (crystal morphology and size, orientation, phase purity, defect densities etc.) as desired for an optimum performance. Recent advances in synthesis of molecular sieve materials have resulted in the development of advanced morphologies such as hierarchical materials, core-shell catalysts, two-dimensional nanosheets and thin films. However, a lot of the reports in the literature focus on conventional crystals and studies focusing on nanoscale crystal growth control are still in their infancy. This dissertation focuses on developing synthetic methods that will enable us to tailor the microstructure of 2D molecular sieve materials at a nanoscale approaching single-unit-cell dimensions with a goal of optimizing their performance in thin film applications. A novel coating technique was applied to isolate 2D MFI zeolite nanosheets and form monolayer coatings on versatile supports such as Si wafers. Using this as a prototype, growth conditions were developed that lead to unprecedented control of zeolite MFI growth at a scale approaching single-unit-cell dimensions. It was demonstrated that these growth conditions are robust enough and can be used to grow zeolite MFI crystals of varied sizes and morphology. It also enabled us to precisely control the microstructure of MFI thin films leading to the development of a material that had one of the lowest reported dielectric constant. Furthermore, the nanoscale growth control also allowed us to tailor the design of hierarchical catalysts by controllably thickening the zeolite domains in them and open opportunities to design multifunctional catalysts. A scalable and direct synthesis of Cu(BDC) MOF nanosheets was developed. Hybrid nanocomposites incorporating the MOF nanosheets in polymer matrices were fabricated which demonstrated significantly improved performance for CO2/CH4 separation.
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    Thermal heat transport characterization for macroscale, microscale, and nanoscale heat conduction
    (2008-12) Anderson, Christianne Vanessa Duim Riberiro
    Several theoretical and experimental methods for predicting the thermal conductivity of thin dielectric ¯lms and carbon nanotubes are presented based on two schools of thought: (1) the physics of the Boltzmann Transport Equation (BTE), and (2) Molec- ular Dynamics (MD) simulations. First, in relation to models based on the BTE, this thesis highlights temporal and spatial scale issues by looking at a uni¯ed theory that bridges physical aspects presented in the Fourier and Cattaneo models. This newly developed uni¯ed model is the so called C- and F-Processes heat conduction model. The model introduces the dimensionless heat conduction model number which is the ratio of thermal conductivity of the fast heat carrier F-Processes to the total thermal conductivity comprised of both fast F-processes and slow heat carrier C-processes. Prior work has claimed that macroscopic heat transfer models cannot explain mi- croscale heat transfer. First, this dissertation provides arguments by showing how the C-F model is able to extend the use of \macroscopic" constitutive relations for the prediction of thermal conductivity at the \microscopic" level for thin ¯lms, mul- tilayer structures, and includes both dielectrics and metals. Second, the in°uence of external mechanical strain on the thermal conductivity of single-wall carbon nan- otubes is studied using direct molecular dynamics simulations with Terso®-Brenner potential for C-C interactions. Three types of external mechanical strain, namely, axial compression, tension, and torsion are studied. In all three cases, the thermal conductivity does not degrade much, i.e., it remains within 10% of the pristine nan- otube values for lower applied strain below the values required for structural collapse. At higher applied strain, structural collapse occurs, and major reductions in the ob- served thermal conductivity for axially compressed and torsionally twisted tubes are observed.
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    Thin-Film Synthesis of Metal Halide Perovskites for Optoelectronics
    (2020-08) Clark, Catherine
    Metal halide perovskites (MHPs), like the archetypal methylammonium lead iodide (MAPbI3), have emerged in the last decade as promising materials for efficient, low-cost optoelectronics. MHP solar cells have already reached efficiencies >25%, rivaling established technologies like single-crystal Si. Yet several challenges prevent the widespread commercialization of MHPs, including their instability in ambient conditions, their toxicity, and the need for scaleable fabrication techniques. Fundamentally, the origins of important material properties relating to carrier transport and recombination are still not well understood. Thin film deposition techniques that enable detailed study of process-structure-property relationships and are commercially relevant are consequently becoming increasingly essential. This thesis seeks to address these challenges through the design, implementation, and utilization of a carrier-gas assisted vapor deposition (CGAVD) method that can grow MHP films with highly tunable stoichiometries and morphologies. Alongside the design of a CGAVD system with six independently controllable experimental parameters, an analytical model is developed and experimentally validated that allows the determination of robust and repeatable growth regimes and the prediction of material deposition rates. Harnessing this technique, we demonstrate the ability to deposit MASnI3 and MASnBr3 films and to systematically vary their compositions across a wide range, and realize corresponding changes in film microstructures (grain size, coverage) and electronic properties (resistivity, carrier concentration, mobility). Control of grain size and film texturing is also achieved independent of stoichiometry via modulation of chamber pressure and substrate temperature. The benefits of CGAVD are further highlighted by the successful growth of novel all-MHP heterojunctions. Two stable pairings are identified: MAPbBr3/MASnBr3 and CsPbBr3/MASnBr3. Design rules to control the mixing of heterojunctions are developed by exploring the dependence of mixing rate on MHP layer composition and grain size. Finally, through a collaboration with Physical Electronics, we optimize the use of XPS depth-profiling for MHPs and investigate which ions are diffusing in a layered structure that exhibits mixing. Moving forward, the incorporation of CGAVD-grown heterojunctions and Pb MHPs into optoelectronic devices will harness the tunability of this system towards a deeper understanding of process-structure-property relationships in MHP thin films and novel layered structures.
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    Time-Resolved Magneto-Optical Kerr Effect for the Study of Ultrafast Magnetization Dynamics in Magnetic Thin Films
    (2020-05) Lattery, Dustin
    As traditional complementary metal oxide semiconductors (CMOS) struggle to extend previous industrial trends, new technologies must be researched and delivered. One of the most important aspects that must be considered is the transport of heat within the material. By advancing the design of materials and interfaces, heat transfer within electronic devices can be improved. At the same time, novel technologies that rely on the magnetism of thin films also need to have their transient magnetic behavior optimized. By measuring the magnetic response of the materials, engineers can select the best-matched materials to design and fabricate devices with lower power consumption and higher processing speed, and thus improved performance. Such material transport studies require new methods and metrology development that can provide highly sensitive and accurate characterization of the materials. The time-resolved magneto-optical Kerr effect (TR-MOKE) technique is capable of probing both thermophysical and magnetic properties of a variety of materials, and it offers superb spatial (micrometer) and temporal (sub-picosecond) resolutions. In this thesis, information about this technique will be discussed including thorough examples of its applications in the study of magnetization dynamics.
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    Transport and Magnetism in Bulk and Thin Film Strontium Titanate
    (2015-10) Ambwani, Palak
    SrTiO3 is a wide band-gap perovskite oxide semiconductor that is widely investigated in the bulk form, due to its remarkable electronic properties. These properties arise from its quantum paraelectric nature which enables unique features, such as, a high-mobility low-density metallic state, quantum transport in an unusual limit, and the most dilute superconducting state thus reported. Recent advances in deposition of oxide thin films and heterostructures have further led to some remarkable observations, such as, the strain-enhancement of mobility in doped thin films of SrTiO3, and the presence of 2D electron gases at interfaces and in delta-doped layers. The presence of magnetic moments and their possible ordering, and the simultaneous observation of quantum oscillations and superconductivity, have been reported in these 2D electron gases. While magnetism has been observed in heterostructures of SrTiO3, there have been limited reports on magnetism in bulk SrTiO3. The first part of this thesis (Chapter 3) discusses how circularly polarized light can induce an extremely long-lived magnetic moment in slightly oxygen-deficient but otherwise nominally pure SrTiO3-δ bulk crystals. These magnetic signals, which are induced at zero applied magnetic field and at low temperatures below ~ 18 K, can be controlled in both magnitude and sign by means of the circular polarization and wavelength of sub-bandgap illumination (400-500 nm), and point to the existence of optically polarizable "V" _"O" -related complexes in the forbidden gap of SrTiO3-δ, rather than collective or long-range magnetic order. The methods used to detect optically induced magnetization are also discussed (Appendix A). The phenomenal progress reported in thin films and heterostructures of SrTiO3 has been possible only by precise control of stoichiometry and defect density in SrTiO3 using techniques such as oxide/LASER MBE or high-temperature PLD. The next part of the thesis (Chapter 4) demonstrates that high pressure oxygen RF sputtering from a ceramic target is similarly capable of growth of high-quality, precisely stoichiometric thin films of SrTiO3. By employing homoepitaxy on SrTiO3(001) substrates, it is shown that optimization of deposition temperature (above 750 C), oxygen pressure (above 2.5 mBar) and deposition rate (below 1.5 Å/min) leads to films that are indistinguishable from the substrate via grazing incidence and wide-angle X-ray scattering. The importance of pre-annealing of substrates in oxygen above 900 C and polishing the target prior to deposition, to obtain bulk-like lattice parameters and eliminate interfacial scattering contrast, is reiterated. Detailed transport measurements were also performed on reduced films grown on LaAlO3(001) and LSAT(001) substrates. The films were found to be semiconducting with mobilities at least an order lower than bulk. Detection and quantification of trace impurities was carried out using PIXE, and the possible causes of low mobility and semiconducting transport characteristics are discussed. Despite the rapid recent progress in thin film deposition techniques, controlled dopant incorporation and attainment of high mobility in thin films of SrTiO3 remain problematic. The last part of the thesis (Chapter 5) discusses the use of analytical scanning transmission electron microscopy to study the local atomic and electronic structure of Nb-doped SrTiO3 both in ideally substitutionally-doped bulk single crystals, and epitaxial thin films. The films are deposited under conditions that would yield highly stoichiometric undoped SrTiO3, as discussed in the previous chapter, but are nevertheless insulating. The Nb incorporation in such films was found to be highly inhomogeneous on nanoscopic length-scales, with large quantities of what is deduced to be interstitial Nb. Electron energy loss spectroscopy reveals changes in the electronic density of states in Nb-doped SrTiO3 films compared to undoped SrTiO3, but without a clear shift in the Fermi edge, that is seen in bulk single crystal Nb-doped SrTiO3. Analysis of atomic-resolution annular dark-field images leads to the conclusion that the interstitial Nb is in the Nb0 state, confirming that it is electrically inactive. It is argued that this approach will enable future work establishing the vitally needed relationships between synthesis/processing conditions and electronic properties of Nb-doped SrTiO3 thin films.
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    Transverse shear microscopy: a novel microstructural probe for organic semiconductor thin films.
    (2010-08) Kalihari, Vivek
    The microstructure of ultrathin organic semiconductor films (1-2nm) on gate dielectrics plays a pivotal role in the electrical transport performance of these films in organic field effect transistors. Similarly, organic/organic interfaces play a crucial role in organic solar cells and organic light emitting diodes. Therefore, it is important to study these critical organic interfaces in order to correlate thin film microstructure and electrical performance. Conventional characterization techniques such as SEM and TEM cannot be used to probe these interfaces because of the requirement of conducting substrates and the issue of beam damage. Here, we introduce a novel contact mode variant of atomic force microscopy, termed transverse shear microscopy (TSM), which can be used to probe organic interfaces. TSM produces striking, high contrast images of grain size, shape, and orientation in ultrathin films of polycrystalline organic materials, which are hard to visualize by any other method. It can probe epitaxial relationships between organic semiconductor thin film layers, and can be used in conjunction with other techniques to investigate the dependence of thin film properties on film microstructure. In order to explain the TSM signal, we used the theory of linear elasticity and developed a model that agrees well with the experimental findings and can predict the signal based on the components of the in-plane elastic tensor of the sample. TSM, with its ability to image elastic anisotropy at high resolution, can be very useful for microstructural characterization of soft materials, and for understanding bonding anisotropy that impacts a variety of physical properties in molecular systems.