166 research outputs found

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

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    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

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

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    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

    Self-Assembly and Characterization of Germanium Quantum Dots on Silicon by Pulsed Laser Deposition

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    Self-assembled Ge quantum dots (QD) are grown on Si(100)-(2×1) by pulsed laser deposition (PLD). In situ reflection-high energy electron diffraction (RHEED) and post-deposition atomic force microscopy (AFM) are used to study the growth dynamics and morphology of the QDs. Several films of different thicknesses were grown at a substrate temperature of 400°C using a Q-switched Nd:YAG laser (λ = 1064 mu, 40 ns pulse width, 23 J/cm2 fluence, and 10 Hz repetition rate). At low film thicknesses, but clusters that are faceted by different planes, depending on their height, are observed after the completion of the wetting layer. With increasing film thickness, the size of the clusters grows, and they gradually lose their facetation and become more rounded. With further thickness increase, the shape of these clusters becomes dome-like with some pyramids observed among the majority of domes. The effect of the laser fluence on the morphology of the grown clusters was studied. The cluster density was found to increase dramatically while the average cluster size decreased with the increase in the laser fluence. For a laser fluence of 70 J/cm2, dome-shaped clusters that are smaller than the large huts formed at 23 J/cm2 were observed. At a substrate temperature of 150°C, misoriented three-dimensional (3D) clusters formed producing only a RHEED background. At 400 and 500°C, huts and a lower density of domes formed, respectively. Above 600°C, 3D clusters formed on top of a discontinuous textured layer. As an application, pulsed laser deposition is used to fabricate multilayered Ge quantum-dot photodetector on Si(100). Forty successive Ge quantum dot layers, each covered with a thin Si layer, were deposited. Deposition and growth are monitored by in situ reflection-high energy electron diffraction and the morphology is further studied by ex situ atomic force microscopy. The difference in the current values in dark and illumination conditions was used to measure the device sensitivity to radiation. Spectral responsivity measurements reveal a peak around 2 μm, with responsivity that increases three orders of magnitude as bias increases from 0.5 to 3.5 V. The effects of laser-induced electronic excitations on the self-assembly of Ge quantum dots on Si(100)-2×1 grown by pulsed laser deposition are also studied. Electronic excitations, due to laser irradiation of the Si substrate and the Ge film during growth, are shown to decrease the roughness of films grown at a substrate temperature of ∼120°C. At this temperature, the grown films are nonepitaxial. However, electronic excitation results in the formation of an epitaxial wetting layer and crystalline Ge quantum dots at ∼260°C, a temperature at which no crystalline quantum dots form without excitation under the same deposition conditions. Finally, the very early stages of formation of Ge but clusters on Si(100) has been studied by UHV STM. Growth starts by the formation of a very low density of asymmetric huts with high aspect ratios. Further deposition results in a higher density of clusters characterized by their narrow size and height distributions. These clusters are almost of the same lateral size as those deposited at lower thicknesses

    MBE growth and characterisation of metastable transition metal sulphides

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    Abstract unavailable please refer to PD

    Magnetic Tunnel Junctions based on spinel ZnxFe3-xO4: Magnetic Tunnel Junctions based onspinel ZnxFe3-xO4

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    Die vorliegende Arbeit befasst sich mit magnetischen Tunnelkontakten (magnetic tunnel junctions, MTJs) auf Basis des Oxids Zinkferrit (ZnxFe3-xO4). Dabei soll das Potential dieses Materials durch die Demonstration des Tunnelmagnetowiderstandes (tunnel magnetoresistance, TMR) in zinkferritbasierten Tunnelkontakten gezeigt werden. Dazu wurde ein Probendesign für MTJs auf Basis der „pseudo spin valve“-Geometrie entwickelt. Die Basis für dieseStrukturen ist ein Dünnfilmstapel aus MgO (Substrat) / TiN / ZnxFe3-xO4 / MgO / Co. Dieser ist mittels gepulster Laserabscheidung (pulsed laser deposition, PLD) hergestellt. Im Rahmen dieser Arbeit wurden die strukturellen, elektrischen und magnetischen Eigenschaften der Dünnfilme untersucht. Des weiteren wurden die fertig prozessierten MTJ-Bauelemente an einem im Rahmen dieser Arbeit entwickeltem und aufgebautem TMR-Messplatz vermessen. Dabei ist es gelungen einen TMR-Effekt von 0.5% in ZnxFe3-xO4-basierten MTJs nachzuweisen. Das erste Kapitel der Arbeit gibt eine Einführung in die spintronischen Effekte Riesenmagnetowiderstand (giant magnetoresistance, GMR) und Tunnelmagnetowiderstand (TMR). Deren technologische Anwendungen sowie die grundlegenden physikalischen Effekte und Modelle werden diskutiert. Das zweite Kapitel gibt eine Übersicht über die Materialklasse der spinellartigen Ferrite. Der Fokus liegt auf den Materialien Magnetit (Fe3O4) sowie Zinkferrit (ZnxFe3-xO4). Die physikalischen Modelle zur Beschreibung der strukturellen, magnetischen und elektrischen Eigenschaften dieser Materialien werden dargelegt sowie ein Literaturüberblick über experimentelle und theoretische Arbeiten gegeben. Im dritten Kapitel werden die im Rahmen dieser Arbeit verwendeten Probenpräparations- und Charakterisierungsmethoden vorgestellt und technische Details sowie physikalische Grundlagen erläutert. Die Entwicklung eines neuen Probendesigns zum Nachweis des TMR-Effekts in ZnxFe3-xO4-basierten MTJs ist Gegenstand des vierten Kapitels. Die Entwicklung des Probenaufbaus sowie die daraus resultierende Probenprozessierung werden beschrieben. Die beiden letzten Kapitel befassen sich mit der strukturellen, elektrischen und magnetischen Charakterisierung der mittels PLD abgeschiedenen Dünnfilme sowie der Tunnelkontaktstrukturen

    Growth and Behaviors of InN/GaN Multiple Quantum Wells by Plasma-Assisted Molecular Beam Epitaxy

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    Fully realizing the potential of InGaN semiconductors requires high quality materials with arbitrary In-content. To this date the growth of In-rich InGaN films is still challenging since it suffers from the low growth temperatures and many detrimental alloying problems. InN/GaN multiple quantum wells (MQWs) and super lattices (SLs) are expected to be promising alternatives to random InGaN alloys since in principle they can achieve the equivalent band gap of InGaN random alloys with arbitrarily high In-content and at the same time bypass many growth difficulties. This dissertation focuses on studying the growth mechanisms, structural properties and energy structures of InN/GaN MQWs. Molecular beam epitaxy (MBE) growth of InN/GaN MQWs were carried out at 550 â—‹C and 680 â—‹C, which are close to the low and high ends of the allowed growth temperature window. Reflection high energy electron diffraction (RHEED) was demonstrated to be a valuable tool for understanding the MQW growth. By associating the RHEED intensity transient features with surface atomic processes such as the adsorption/desorption of metal species, the growth process was successfully monitored in situ. Also, at the high growth temperature, RHEED was successfully used to study the adsorption/desorption kinetics of indium surface coverage to gain knowledge of how to control InN deposition. The MQW growth at 680 â—‹C show that indium surface coverage over 2 MLs before GaN capping is a key factor for consistent quantum well formation. The consistent PL emissions at ~375 nm were attributed to the insertion of 1-ML thick QWs. At 550 â—‹C, both PL emission and QW thickness showed a self-regulating behavior. The redshift of PL emissions with the InN deposition saturated at ~423 nm while the QW apparent thickness were no more than 2 MLs. The residual indium accumulation identified by RHEED suggests that QWs are generally InGaN layers instead of coherent InN layers, which is supported by k.p calculations. Finally, a growth mechanism was proposed to explain the preservation, structural and optical properties of the quantum wells

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

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    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

    Melting and Solidification Study of Indium and Bismuth Nanocrystals Using Reflection High-Energy Electron Diffraction

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    As technology begins to utilize nanocrystals for many chemical, biological, medical, electrical, and optoelectrical applications, there is a growing need for an understanding of their fundamental properties. The study of melting and solidification of nanocrystals is of interest to fundamental understanding of the effect of reduced size and crystal shape on the solid-liquid phase transition. Melting and solidification of as-deposited and recrystallized indium and bismuth nanocrystals were studied using reflection high-energy electron diffraction (RHEED). The nanocrystals were thermally deposited on highly oriented 002-graphite substrate at different deposition temperatures. The growth dynamics of the nanocrystals was studied using in situ RHEED while the morphology and size distributions were studied using ex situ real image technique (atomic force microscopy (AFM) or scanning electron microscopy (SEM)). RHEED observation during deposition showed that 3D nanocrystals of indium are directly formed from the vapor phase within the investigated temperature range, 300 K up to 25 K below the bulk melting point of indium. On the other hand, bismuth condensed in the form of supercooled liquid droplets at temperatures above its maximum supercooling point, 125 K below the bulk melting point of bismuth. Below the maximum supercooling point, bismuth condensed in the solid phase. Post deposition real images showed that the formed nanocrystals have morphologies and size distributions that depend on the deposition temperature, heat treatment, and the amount of the deposited material. As-deposited nanocrystals are found to have different shapes and sizes, while those recrystallized from melt were formed in similar shapes but different sizes. The change in the RHEED pattern with temperature was used to probe the melting and solidification of the nanocrystals. Melting started early before the bulk melting point and extended over a temperature range that depends on the size distribution of the nanocrystals. Nanocrystals at the lower part of the distribution melt early at lower temperatures. With the increase in temperature, more nanocrystals completely melt with the thickness of the liquid shell on the remaining crystals continuing to grow. Due to size increase after melting, recrystallized bismuth nanocrystals showed a melting range at temperatures higher than that of as-deposited. However, recrystallized indium nanocrystals showed an end melting point nearly equal to that of-the recrystallized ones except for the 1.5-ML film which showed an end melting point ∼10 K higher than that of as-deposited
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