Structure and Transport Properties of Epitaxial Oxide Thin Films: From Synthesis to Characterization

Epitaxial thin films and heterostructures based on perovskite oxides have attracted significant attention in physics since perovskites exhibit an enormous range of electrical, magnetic, and optical properties, making them exciting systems for studies of the fundamental physical mechanisms of interactions between electron, lattice, and spin degrees of freedom. This dissertation has been focused on ferroelectricity in lowdimensional ferroelectric materials using ultra-thin ferroelectric epitaxial films (BaTiO3) with a metallic electrode (SrRuO3) by studying polarized ordering of the crystal structure and electronic transport through the films. High quality and highly oxidized epitaxial films are a prerequisite for the clear observation of physical properties such as ferroelectricity which depends on a sensitive balance of lattice structure, dynamics, and charge distribution. Measurements in low dimensional, ultra-thin films require a controlled surface status through in-situ characterization. As is demonstrated here, fundamental physical phenomena on surfaces and in ultra-thin films are easily modified due to reactivity in ambient air, even for oxide materials generally considered inert. This study is centered on in-situ low energy electron diffraction and scanning tunneling spectroscopy of BaTiO3 films grown on SrRuO3 electrodes on a SrTiO3 substrate. Results show out-of-plane polarized structure and polarization switching, which provide evidence of ferroelectricity in films down to 4 ML. Surface reconstruction in 1-2 ML thick BaTiO3 films is seriously affected by the interface between BaTiO3 films and SrRuO3 bottom electrode. Our observation in epitaxial BaTiO3 films indicates the existence of ferroelectricity with a lower limit (4 ML) for the minimum thickness than theoretical expectation (6 ML), which results from the difference of film stress, termination on films, and depolarizing screening

Reconstructions at the Interface in Complex Oxide Heterostructures with Strongly Correlated Electrons

Strongly correlated oxides exhibit a rich spectrum of closely competing orders near the localized-itinerant Mott insulator transition leaving their ground states ripe with instabilities susceptible to small perturbations such as lattice distortions, variation in stoichiometry, magnetic and electric fields, etc. As the field of interfacial engineering has matured, these underlying instabilities in the electronic structure of correlated oxides continue to be leveraged to manipulate existing phases or search for emergent ones. The central theme is matching materials across the interface with disparate physical, chemical, electronic, or magnetic structure to harness interfacial reconstructions in the strongly coupled charge, spin, orbital, and lattice degrees of freedom. In this dissertation, we apply the above paradigm to cuprate-manganite and cuprate-titanate interfaces. We examine ultrathin YBa2Cu3O7/La2/3Ca1/3MnO3 multilayers, where interfacial charge reconstruction modulates the distribution of charge carriers within the superconducting planes and thereby act as dials to tune through the cuprate doping phase diagram. The ultrathin nature of the cuprate layers allows the reconstructed states to be resolved free of a bulk admixture. The depleted carriers are observed to directly enter the CuO2 planes. With increasing manganite thickness, magnetic correlations are introduced, and coupling between interfacial Cu and Mn develops. The reconstructions in spin and electronic degrees of freedom found in cuprate-manganite heterostructures are expected to completely mask all other competing interactions. To this end, SrTiO,3 is incorporated as a spacer material in cuprate-titanate multilayers to reveal the role of dimensionality, interlayer coupling, and broken translational symmetry. At the unit cell limit, a decrease in carrier concentration is found that directly correlates with underdoping from lost charge reservoir layers at the interface, while increased titanate layer thickness is found to augment the carrier concentration with the charge reservoir layers but has no effect on the doping within the superconducting planes. Also spectroscopic evidence for charge transfer across the interface between Cu and Ti is shown to support a recent theoretical prediction of pre-doping at the cuprate-titanate interface in response to a polar discontinuity at the interface

Surface Dimer Engineering and Properties of GaAs(N)(Bi) Alloys

Due to the significant bandgap narrowing induced by dilute fractions of N and Bi in III-V semiconductors, emerging dilute nitride-bismide semiconductor alloys are of significant interest for long-wavelength applications ranging from temperature-insensitive laser diodes to ultra-high efficiency multijunction photovoltaic cells. However, both dilute nitride and dilute bismide devices have exhibited significant sensitivity to the local atomic environments of N or Bi solute atoms, while their incorporation mechanisms are not well understood. In this work, we investigate the role of the surface reconstruction on doping, alloy formation, and electronic and optical properties of GaAs(N)(Bi) alloys. For GaAs(Bi), we examine the influence of surface reconstruction on silicon dopant incorporation and electronic properties. Si incorporation into GaAs(Bi) with an (nx3) surface reconstruction leads to n-type conductivity, while growth with a (2x1) reconstruction leads to p-type conductivity. We hypothesize that the presence or absence of surface arsenic dimers prevents or enables dopant incorporation into arsenic lattice sites. We consider the influence of bismuth anions on arsenic-dimer mediated dopant incorporation and the resulting electronic transport properties, demonstrating the applicability of this mechanism to mixed anion semiconductor alloys. For GaAsNBi alloys, we examine the influence of Bi and N fluxes on N and Bi incorporation. The incorporation of Bi is found to be independent of N flux, while the total N incorporation and the fraction of N atoms occupying non-substitutional lattice sites increase with increasing Bi flux. A comparison of channeling nuclear reaction analysis with Monte Carlo – molecular dynamics simulations indicates that the non-substitutional N primarily incorporate as (N-As) interstitial complexes. We discuss the influence of Bi adatoms on the formation of arsenic-terminated [110]-oriented step edges with a (1x3) surface reconstruction and the resulting enhancement in total N incorporation via the formation of additional (N-As). We also consider the influence of Bi as an incorporating surfactant on chemical ordering in GaAsN:Bi alloys. While epitaxy with a (2x1) reconstruction leads to random GaAsN formation, the introduction of a Bi flux induces long-range chemical ordering of the {111} planes. We propose a mechanism in which Bi enhances the formation of dimer rows aligned along the [110] direction in the (2x1) surface reconstruction, facilitating N incorporation beneath surface dimers and Bi incorporation between dimer rows to form alternating N-rich and Bi-rich {111} planes. These findings suggest a route to tailoring the local atomic environment of N and Bi atoms in a wide range of emerging dilute nitride-bismide alloys. Finally, we have examined the alloy composition dependence of the energy bandgap and electronic states in GaAsNBi alloys. Using direct measurements of N and Bi mole fractions, via ion beam analysis, in conjunction with direct measurements of the out-of-plane misfit via x-ray rocking curves, we determine a new "magic ratio" for lattice-matching of GaAsNBi alloys with GaAs substrates. In addition, using a combination of photoreflectance and photoluminescence spectroscopy, we determine a new map of the composition- and misfit-dependence of the energy bandgaps, along with revealing the energetic position of Bi-related states at approximately 0.18 eV above the valence band maximum.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147570/1/joccena_1.pd

Reflection High-Energy Electron Diffraction Studies of Indium Phosphide (100) and Growth on Indium and Indium Nitride on Silicon (100)

Study of the effects of atomic hydrogen exposure on structure and morphology of semiconductor surfaces is important for fundamental properties and applications. In this dissertation, the electron yield of a hydrogen-cleaned indium phosphide (InP) surface was measured and correlated to the development of the surface morphology, which was monitored by in situ reflection high-energy electron diffraction (RHEED). Atomic hydrogen treatment produced a clean, well-ordered, and (2x4)-reconstructed InP(100) surface. The quantum efficiency, after activation to negative electron affinity, and the secondary electron emission were shown to increase with hydrogen cleaning time. RHEED patterns of low-index InP(100) surface were modified by the step structure and resulted in splitting of the specular beam at the out-of-phase diffraction condition. Quantitative RHEED showed reduction in the average terrace width and a decrease of the adatom-vacancy density with hydrogen exposure. This suggests that atomic hydrogen etching occurs preferentially at terrace edges, and thermal diffusion on the surface causes changes in the terrace edge morphology, which result in the observed decrease in the average terrace width. The results show that the decrease in the surface disorder, measured from the RHEED intensity-to-background ratio, correlated with the increased quantum efficiency. The growth of group-III metals on Si surfaces has become an attractive area of research because of its scientific importance and great potential in technological applications. In this work, the growth dynamics, structure, and morphology of indium (In) on a vicinal Si(100)-(2×1) surface by femtosecond pulsed laser deposition (fsPLD) were studied using in situ RHEED and ex situ atomic force microscopy. Indium was found to grow on Si(100) by the Stranski-Krastanove mode. At room temperature, the initial growth formed strained two-dimensional (2D) layers in the In(2×1) structure followed by growth of three-dimensional islands. During the 2D growth, the surface diffusion coefficient of deposited In on the In(2×1) layer was estimated to be in the order of 10−14 cm2/s, from recovery of the RHEED intensity. This was attributed to surface diffusion of In clusters by step flow mode. The results suggest that fsPLD of In removed the reconstruction of the Si(100)-(2×1) surface in the early growth and resulted in the initial In(2x1) structure. Next, growth of In on Si(100)-(2×1) was studied at temperature of 350–420°C and showed formation of In(4×3) structure. The growth stages, probed by RHEED intensity relaxation, proceed in a two-step process, formation of small In clusters and surface diffusion to the terrace step edges with activation energy of 1.4±0.2 eV and diffusion rate constant of 1.0±0.1x1011 s −1. The terrace width dynamics and the related surface processes were studied during growth of the In(4×3) phase with increase in film coverage. Finally, the fsPLD was used to grow nitride films of InN on Si(100) substrates. A buffer layer of In was grown on Si(100) by fsPLD prior to growth of InN and different nitridation procedures were used

Molecular beam epitaxial growth and charcterization of mismatched InGaAs and InAlAs layers on InP

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1993.Includes bibliographical references (leaves 147-158).by Brian R. Bennett.Ph.D

Valence changes at interfaces and surfaces investigated by X-ray spectroscopy

In this thesis valence changes at interfaces and surfaces of 3d and 4f systems are investigated by X-ray spectroscopy, in particular X-ray photoemission (XPS), X-ray absorption (XAS) and resonant photoemission spectroscopy (ResPES). The first part addresses the electronic properties of the oxides LaAlO3, LaGaO3 and NdGaO3 grown by pulsed laser deposition on TiO2-terminated SrTiO3 single crystals along (001)-direction. These polar/non-polar oxide interfaces share an insulator to metal phase transition as a function of overlayer thickness including the formation of an interfacial two dimensional electron gas. The nature of the charge carriers, their concentration and spatial distribution, and the band alignment near the interface are studied in a comparative manner and evaluated quantitatively. Irrespective of the different overlayer lattice constants and bandgaps, all the heterostructures behave similarly. Rising Ti3+ concentration is monitored by Ti 2p XPS, Ti L-edge XAS and by resonantly enhanced Ti 3d excitations in the vicinity of EF (ResPES) when the layer number n increases. This indicates that the active material is in all cases a near interface SrTiO3 layer of 4nm thickness. Band bending in SrTiO3 occurs but no electric field is detected inside the polar overlayers. Essential aspects of the findings are captured by scenarios where the polar forces are alleviated by surface defect creation or the separation of photon generated electron-hole pairs in addition to the electronic reconstruction at n = 4 unit cells layer thickness. Furthermore, deviations from an abrupt interface are found by soft X-ray photoemission spectroscopy which may affect the interface properties. The surface sensitivity of the measurements has been tuned by varying photon energy and emission angle. In contrast to the core levels of the other elements, the Sr 3d line shows an unexpected splitting for higher surface sensitivity, signaling the presence of a second strontium component. From a quantitative analysis it is concluded that during the growth process a small amount of Sr atoms diffuse away from the substrate and segregate at the surface of the heterostructure, possibly forming strontium oxide. In the second part of this thesis the heavy fermion superconductors CeMIn5 (M = Co, Rh, Ir) are investigated by temperature- and angle-dependent XPS. In this material class the subtle interplay between localized Ce 4f and itinerant valence electrons dominate the electronic properties. The Ce 3d core level has a very similar shape for all three materials and is indicative of weak f-hybridization. The spectra are analyzed using a simplified version of the Anderson impurity model, which yields a Ce 4f occupancy that is larger than 0.9. The temperature dependence shows a continuous, irreversible and exclusive broadening of the Ce 3d peaks, due to oxidation of Ce at the surface.In der vorliegenden Dissertation werden Valenzänderungen an Grenzflächen und Oberflächen mittels Verfahren der Röntgenspektroskopie untersucht, zu denen die Röntgenphotoelektronen- (XPS), die Röntgenabsorptions- (XAS) und die resonante Photoelektronenspektroskopie (ResPES) gehören. Kapitel 3 behandelt die elektronischen Eigenschaften der Oxide LaAlO3, LaGaO3 und NdGaO3, welche mittels Laserdeposition (PLD) auf TiO2-terminierte SrTiO3 Einkristalle entlang (001)-Richtung gewachsen wurden. Diese polaren/nicht-polaren Oxidgrenzflächen weisen einen Isolator-Metall Phasenübergang als Funktion der Schichtdicke auf, bei dem sich ein zwei dimensionales Elektronengas an der Grenzfläche bildet. Die Eigenschaften dieser Ladungsträger, deren Konzentration und räumliche Ausdehnung, sowie der Verlauf der Energiebänder an der Grenzfläche werden vergleichend untersucht und quantitativ bestimmt. Es wird gezeigt, dass sich die drei untersuchten Grenzflächen, trotz unterschiedlicher Gitterkonstanten und Energiebandlücken, ähnlich verhalten. Das mit der Schichtdicke ansteigende Ti3+ Signal wird im Ti 2p XPS, Ti L-Kante XAS und durch die resonant verstärkten Ti 3d Anregungen nahe EF (ResPES) nachgewiesen. Daraus lässt sich schlussfolgern, dass in allen Fällen eine SrTiO3 Schicht mit einer Dicke von 4nm der eigentlich aktive Bereich ist. Im SrTiO3 tritt eine Bandverbiegung auf, ein elektrisches Feld in der polaren Deckschicht kann jedoch nicht nachgewiesen werden. Grundlegende Aspekte dieser Ergebnisse sind in einem Szenario vereinbar, bei dem die polaren Kräfte durch die Entstehung von Oberflächendefekten, durch die Trennung von photoneninduzierten Elektronen-Lochpaaren und durch eine elektronische Umordnung bei 4 uc Schichtdicke eliminiert werden. Des Weiteren werden Abweichungen von einer abrupten Grenzfläche mittels weich-Röntgenphotoelektronenspektroskopie festgestellt, die die Grenzflächeneigenschaften beeinflussen können. Für oberflächenempfindlichere Messbedingungen zeigt die Sr 3d Anregung, im Gegensatz zu Rumpfniveaus anderer Elemente, eine unerwartete Aufspaltung, was nur durch das Vorhandensein einer zweiten chemischen Strontiumkomponente zu erklären ist. Aus quantitativen Betrachtungen lässt sich schließen, dass einige Strontiumatome während des Wachstums an die Oberfläche diffundieren und möglicherweise Strontiumoxid gebildet wird. Der zweite Schwerpunkt der vorliegenden Arbeit ist die Untersuchung von Schwer-Fermionen Supraleitern CeMIn5 (M = Co, Rh, Ir) mittels temperatur- und winkelabhängiger XPS. Bei dieser Materialklasse dominiert das feine Zusammenspiel zwischen lokalisierten Ce 4f und frei beweglichen Leitungselektronen die elektronischen Eigenschaften. Das Ce 3d Rumpfniveauspektrum besitzt für die drei Materialien eine sehr ähnliche Form, die auf eine schwache f-Hybridisierung schließen lässt. Die Spektren werden mittels einer vereinfachten Version des Anderson-Impurity Modells analysiert, wobei sich eine Ce 4f Besetzung von mehr als 0,9 ergibt. Die Temperaturabhängigkeit zeigt eine kontinuierliche und irreversible Verbreiterung ausschließlich für die Ce 3d Anregung, dieser Umstand kann einer Oxidation der reaktiven Ceratome an der Oberfläche zugeordnet werden

Dreams of Molecular Beams: Indium Gallium Arsenide Tensile-Strained Quantum Dots and Advances Towards Dynamic Quantum Dots (Moleculare Radiorum Somnia: Indii Gallii Arsenicus Tensa Quanta Puncta et ad Dinamicae Quantae Puntae Progressus)

Through the operation of a molecular beam epitaxy (MBE) machine, I worked on developing the homoepitaxy of high quality InAs with a (111)A crystallographic orientation. By tuning substrate temperature, we obtained a transition from a 2D island growth mode to step- ow growth. Optimized MBE parameters (substrate temperature = 500 °C, growth rate = 0.12 ML/s and V/III ratio ⩾ 40) lead to growth of extremely smooth InAs(111)A films, free from hillocks and other 3D surface imperfections. We see a correlation between InAs surface smoothness and optical quality, as measured by photoluminescence spectroscopy. This work establishes InAs(111)A as a platform for future research into other materials from the 6.1 Å family of semiconductors grown with a (111) orientation. Continuing this work, we also have determined a reproducible set of growth conditions for the self-assembly of tensile-strained In1-xGaxAs quantum dot nanostructures on InAs(111)A surfaces. During molecular beam epitaxy, In1-xGaxAs islands form spontaneously on InAs(111)A when the Ga content x ≥ 50 %. We analyze the structure and composition of InGaAs/InAs(111) samples using atomic force microscopy, transmission electron microscopy, electron energy loss spectroscopy, and photoluminescence spectroscopy. We demonstrate control over the size and areal density of the islands as a function of In1-xGaxAs coverage, In1-xGaxAs composition, and substrate temperature. Furthermore, we also present a study aimed to determining the growth conditions of In1-xGaxAs self-assembled tensile-strained QDs on GaSb(111)A surfaces. From previous work we determined that a larger band gap barrier was necessary to ensure the confinement of charge carriers in the InGaAs nanostructures. Through a series of temperature, V/III ratio, and growth rate we determined the best parameters for GaSb(111) homoepitaxy. We then studied the nucleation of optimal-morphology In1-xGaxAsQDs by locking the compositions at In0.5Ga0.5As, studying the critical pause for group V element transition and V/III ratio prior and post QD growth. Several photoluminescence techniques are employed to determine the light emission properties of these structures. Finally, we did preliminary studies on how to achieve the dynamic lateral confinement of charge carriers in 2D and 3D using near-THz surface acoustic phonon pulses in polar semiconductors. Using the acousto-electrical effect, we measure the degree to which surface acoustic waves (SAWs) confine electrons and holes limiting the number of recombination processes. Applications for this technological development include the external modulation of lateral confinement size in the SAWs and subsequent photon emission wavelength, as well as potential quantum logical gate design using acoustic pulses to drive electrons in a circuit

The development of components for ultrafast all-optical communication networks

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2000.Includes bibliographical references (p. 149-158).The need for ultrafast (greater than 100 Gbps) all-optical communication networks is amplified as the amount of data-containing communication traffic continues to grow at an exorbitant rate. Multiplexing schemes are attractive, as they enable increased transmission over silica fiber already buried and in use. One of the key components enabling Wavelength Division Multiplexed (WDM) propagation is the distributed feedback (DFB) laser. A periodic index modulation, typically buried within the device structure (hence requiring an overgrowth step), is used to select the specific operating wavelength of the laser. As the WDM wavelength spacing continues to decrease, the increasing demands placed on the fabrication of the DFB lasers bring the fabrication issues to the forefront. Similar patterned surface overgrowth issues need to be addressed for realization of wavelength-selective filters, e.g. Bragg-resonant filters, for all-optical routing. Issues pertaining to the formation of buried index contrasts, for application to future WDM network components, are addressed. Of importance to Time Division Multiplexed (TDM)-based propagation schemes is the multiplexing and demultiplexing of the ultrafast data streams. All-optical switching as well as optical (de)multiplexing are very attractive alternatives to optoelectronic conversion. Many optical switching schemes require the use of a semiconductor optical amplifier (SOA) as the nonlinear medium, as well as optical clock sources with repetition rates on the order of GHz (for integration with electronic components). For TDM network propagation rates on the order of Tbps, lasers must be capable of producing pulses shorter than a picosecond. Mode-locking of laser cavities is an attractive method of ultrashort pulse generation. A semiconductor saturable absorber mirror is a monolithically integrated device utilized for passive mode-locking of a wide variety of lasers. The development of SOAs as well as semiconductor saturable absorber mirrors is discussed.Elisabeth Marley Koontz.Ph.D

Atomic structure of thin films and heterostructure of Bi2Te3 and Bi2Se3 topological insulators

The atomic structure of transition metals doped three dimensional (3D) topological insulators (TIs) and the bonding nature of Bi2Te3 with FeSe layers and Ge(111) substrate were studied. Motivation behind transition metal doping of 3D TIs is driven by achieving long range ferromagnetism of Bi2Se3 and Bi2Te3, which is expected to give rise to different spintronic effects that can be utilise in device applications. The nature of this magnetisation depends on the location of the dopants in the Bi chalcogenide matrix. Dopants in Bi based TIs can substitute for Bi, Te, or incorporate between the quintuple layers in the van der Waals gap. Long range ferromagnetism is observed in both Cr doped Bi2Se3 and Mn doped Bi2Te3; however, the main goal of achieving room-temperature ferromagnetism in homogeneously doped TIs has proven to be difficult. In this thesis it is shown that 4.6 at-% of Cr is incorporated substitutionally on Bi sites with no phase segregation. The presences of grain boundaries can cause Cr segregation; hence by controlling the defect density a homogeneous Cr distribution could in principle be achieved even at higher concentrations. In case of Mn as a dopant, we show that the local environment of Mn in Bi2Te3 is heterogeneous. The first principal calculations revealed that the Mn dopants ferromagnetically couple in Bi2Te3 lattice. In addition, we have shown that doping of Bi2Te3 with Mn should be limited to low concentrations (< 6 at-%), higher dopants concentrations results in the formation of secondary phases. Next we have demonstrated that epitaxial growth of FexCu1-xSe on Bi2Te3 is possible regardless of their different lattice symmetries and large lattice mismatch of 19%. First-principles energy calculations revealed that this is realised through van der Waals-like bonding between the Se and Te atomic planes at the interface. Finally, we have shown that the weak van der Waals bonding between the Bi2Te3 and Ge(111) substrate can be strengthen by formation of a Te monolayer at the interface. The electronic band structure calculations revealed that this is due to the stronger atomic p-type orbital hybridization at the interface