Matching Items (19)
Description
Nanocrystalline (nc) thin films exhibit a wide range of enhanced mechanical properties compared to their coarse-grained counterparts. Furthermore, the mechanical behavior and microstructure of nc films is intimately related. Thus, precise control of the size, aspect ratio and spatial distribution of grains can enable the synthesis of thin films with exceptional mechanical properties. However, conventional bottom-up techniques for synthesizing thin films are incapable of achieving the microstructural control required to explicitly tune their properties. This dissertation focuses on developing a novel technique to synthesize metallic alloy thin films with precisely controlled microstructures and subsequently characterizing their mechanical properties using in situ transmission electron microscopy (TEM). Control over the grain size and distribution was achieved by controlling the recrystallization process of amorphous films by the use of thin crystalline seed layers. The novel technique was used to manipulate the microstructure of structural (TiAl) and functional (NiTi) thin films thereby exhibiting its capability and versatility. Following the synthesis of thin films with tailored microstructures, in situ TEM techniques were employed to probe their mechanical properties. Firstly, a novel technique was developed to measure local atomic level elastic strains in metallic glass thin films during in situ TEM straining. This technique was used to detect structural changes and anelastic deformation in metallic glass thin films. Finally, as the electron beam (e-beam) in TEMs is known to cause radiation damage to specimen, systematic experiments were carried out to quantify the effect of the e-beam on the stress-strain response of nc metals. Experiments conducted on Al and Au films revealed that the e-beam enhances dislocation activity leading to stress relaxation.
ContributorsSarkar, Rohit (Author) / Rajagopalan, Jagannathan (Thesis advisor) / Peralta, Pedro (Committee member) / Sieradzki, Karl (Committee member) / Crozier, Peter (Committee member) / Arizona State University (Publisher)
Created2017
Description
The research of this dissertation has involved the nanoscale quantitative characterization of patterned magnetic nanostructures and devices using off-axis electron holography and Lorentz microscopy. The investigation focused on different materials of interest, including monolayer Co nanorings, multilayer Co/Cu/Py (Permalloy, Ni81Fe19) spin-valve nanorings, and notched Py nanowires, which were fabricated via a standard electron-beam lithography (EBL) and lift-off process. Magnetization configurations and reversal processes of Co nanorings, with and without slots, were observed. Vortex-controlled switching behavior with stepped hysteresis loops was identified, with clearly defined onion states, vortex states, flux-closure (FC) states, and Omega states. Two distinct switching mechanisms for the slotted nanorings, depending on applied field directions relative to the slot orientations, were attributed to the vortex chirality and shape anisotropy. Micromagnetic simulations were in good agreement with electron holography observations of the Co nanorings, also confirming the switching field of 700-800 Oe. Co/Cu/Py spin-valve slotted nanorings exhibited different remanent states and switching behavior as a function of the different directions of the applied field relative to the slots. At remanent state, the magnetizations of Co and Py layers were preferentially aligned in antiparallel coupled configuration, with predominant configurations in FC or onion states. Two-step and three-step hysteresis loops were quantitatively determined for nanorings with slots perpendicular, or parallel to the applied field direction, respectively, due to the intrinsic coercivity difference and interlayer magnetic coupling between Co and Py layers. The field to reverse both layers was on the order of ~800 Oe. Domain-wall (DW) motion within Py nanowires (NWs) driven by an in situ magnetic field was visualized and quantified. Different aspects of DW behavior, including nucleation, injection, pinning, depinning, relaxation, and annihilation, occurred depending on applied field strength. A unique asymmetrical DW pinning behavior was recognized, depending on DW chirality relative to the sense of rotation around the notch. The transverse DWs relaxed into vortex DWs, followed by annihilation in a reversed field, which was in agreement with micromagnetic simulations. Overall, the success of these studies demonstrated the capability of off-axis electron holography to provide valuable insights for understanding magnetic behavior on the nanoscale.
ContributorsHe, Kai (Author) / McCartney, Martha R. (Thesis advisor) / Smith, David J. (Thesis advisor) / Chamberlin, Ralph V. (Committee member) / Crozier, Peter A. (Committee member) / Drucker, Jeff (Committee member) / Arizona State University (Publisher)
Created2010
Description
Supported catalytic nanoparticles undergo rapid structural transformations faster than many transmission electron microscopes (TEMs) can track. This is the case with platinum nanoparticles supported on cerium oxide (Pt/CeO2) in a CO and O2 gaseous environment. By furthering our understanding of the structural dynamics of the Pt/CeO2 system, improved catalyst design principles may be derived to enhance the efficiency of this catalyst. Developing static models of a 2 nm Pt nanoparticle supported on CeO2 and simulating TEM images of the models was found to create similar images to those seen in experimental TEM time-resolved series of the system. Rotations of static models on a ceria support provides a way to understand the experimental samples in three dimensions, which is difficult in two dimensional TEM images. This project expands the possibilities of interpreting TEM images of catalytic systems.
ContributorsBlock, Claire (Author) / Crozier, Peter (Thesis director) / Muhich, Christopher (Committee member) / Materials Science and Engineering Program (Contributor, Contributor) / Watts College of Public Service & Community Solut (Contributor) / Barrett, The Honors College (Contributor)
Created2021-05
Description
Titanium dioxide is an essential material under research for energy and environmental applications, chiefly through its photocatalytic properties. These properties allow it to be used for water-splitting, detoxification, and photovoltaics, in addition to its conventional uses in pigmentation and sunscreen. Titanium dioxide exists in several polymorphic structures, of which the most common are rutile and anatase. We focused on anatase for the purposes of this research, due to its promising results for hydrolysis.
Anatase exists often in its reduced form (TiO2-x), enabling it to perform redox reactions through the absorption and release of oxygen into/from the crystal lattice. These processes result in structural changes, induced by defects in the material, which can theoretically be observed using advanced characterization methods. In situ electron microscopy is one of such methods, and can provide a window into these structural changes. However, in order to interpret the structural evolution caused by defects in materials, it is often necessary and pertinent to use atomistic simulations to compare the experimental images with models.
In this thesis project, we modeled the defect structures in anatase, around oxygen vacancies and at surfaces, using molecular dynamics, benchmarked with density functional theory. Using a “reactive” forcefield designed for the simulation of interactions between anatase and water that can model and treat bonding through the use of bond orders, different vacancy structures were analyzed and simulated. To compare these theoretical, generated models with experimental data, the “multislice approach” to TEM image simulation was used. We investigated a series of different vacancy configurations and surfaces and generated fingerprints for comparison with TEM experiments. This comparison demonstrated a proof of concept for a technique suggesting the possibility for the identification of oxygen vacancy structures directly from TEM images. This research aims to improve our atomic-level understanding of oxide materials, by providing a methodology for the analysis of vacancy formation from very subtle phenomena in TEM images.
Anatase exists often in its reduced form (TiO2-x), enabling it to perform redox reactions through the absorption and release of oxygen into/from the crystal lattice. These processes result in structural changes, induced by defects in the material, which can theoretically be observed using advanced characterization methods. In situ electron microscopy is one of such methods, and can provide a window into these structural changes. However, in order to interpret the structural evolution caused by defects in materials, it is often necessary and pertinent to use atomistic simulations to compare the experimental images with models.
In this thesis project, we modeled the defect structures in anatase, around oxygen vacancies and at surfaces, using molecular dynamics, benchmarked with density functional theory. Using a “reactive” forcefield designed for the simulation of interactions between anatase and water that can model and treat bonding through the use of bond orders, different vacancy structures were analyzed and simulated. To compare these theoretical, generated models with experimental data, the “multislice approach” to TEM image simulation was used. We investigated a series of different vacancy configurations and surfaces and generated fingerprints for comparison with TEM experiments. This comparison demonstrated a proof of concept for a technique suggesting the possibility for the identification of oxygen vacancy structures directly from TEM images. This research aims to improve our atomic-level understanding of oxide materials, by providing a methodology for the analysis of vacancy formation from very subtle phenomena in TEM images.
ContributorsShindel, Benjamin Noam (Author) / Crozier, Peter (Thesis director) / Anwar, Shahriar (Committee member) / Singh, Arunima (Committee member) / Materials Science and Engineering Program (Contributor) / Barrett, The Honors College (Contributor)
Created2019-05
Description
Non-stoichiometric oxides play a critical role in many catalytic, energy, and sensing technologies, providing the ability to reversibly exchange oxygen with the ambient environment through the creation and annihilation of surface oxygen vacancies. Oxygen exchange at the surfaces of these materials is strongly influenced by atomic structure, which varies significantly across nanoparticle surfaces. The studies presented herein elucidate the relationship between surface structure behaviors and oxygen exchange reactions on ceria (CeO2) catalyst materials. In situ aberration-corrected transmission electron microscopy (AC-TEM) techniques were developed and employed to correlate dynamic atomic-level structural heterogeneities to local oxygen vacancy activity.
A model Ni/CeO2 catalyst was used to probe the role of a ceria support during hydrocarbon reforming reactions, and it was revealed that carbon formation was inhibited on Ni metal nanoparticles due to the removal of lattice oxygen from the ceria support and subsequent oxidation of adsorbed decomposed hydrocarbon products. Atomic resolution observations of surface oxygen vacancy creation and annihilation were performed on CeO2 nanoparticle surfaces using a novel time-resolved in situ AC-TEM approach. Cation displacements were found to be related to oxygen vacancy creation and annihilation, and the most reactive surface oxygen sites were identified by monitoring the frequency of cation displacements. In addition, the dynamic evolution of CeO2 surface structures was characterized with high temporal resolution AC-TEM imaging, which resulted in atomic column positions and occupancies to be determined with a combination of spatial precision and temporal resolution that had not previously been achieved. As a result, local lattice expansions and contractions were observed on ceria surfaces, which were likely related to cyclic oxygen vacancy activity. Finally, local strain fields on CeO2 surfaces were quantified, and it was determined that local strain enhanced the ability of a surface site to create oxygen vacancies. Through the characterization of dynamic surface structures with advanced AC-TEM techniques, an improvement in the fundamental understanding of how ceria surfaces influence and control oxygen exchange reactions was obtained.
A model Ni/CeO2 catalyst was used to probe the role of a ceria support during hydrocarbon reforming reactions, and it was revealed that carbon formation was inhibited on Ni metal nanoparticles due to the removal of lattice oxygen from the ceria support and subsequent oxidation of adsorbed decomposed hydrocarbon products. Atomic resolution observations of surface oxygen vacancy creation and annihilation were performed on CeO2 nanoparticle surfaces using a novel time-resolved in situ AC-TEM approach. Cation displacements were found to be related to oxygen vacancy creation and annihilation, and the most reactive surface oxygen sites were identified by monitoring the frequency of cation displacements. In addition, the dynamic evolution of CeO2 surface structures was characterized with high temporal resolution AC-TEM imaging, which resulted in atomic column positions and occupancies to be determined with a combination of spatial precision and temporal resolution that had not previously been achieved. As a result, local lattice expansions and contractions were observed on ceria surfaces, which were likely related to cyclic oxygen vacancy activity. Finally, local strain fields on CeO2 surfaces were quantified, and it was determined that local strain enhanced the ability of a surface site to create oxygen vacancies. Through the characterization of dynamic surface structures with advanced AC-TEM techniques, an improvement in the fundamental understanding of how ceria surfaces influence and control oxygen exchange reactions was obtained.
ContributorsLawrence, Ethan Lee (Author) / Crozier, Peter A. (Thesis advisor) / Lin, Jerry (Committee member) / Liu, Jingyue (Committee member) / Petuskey, William (Committee member) / Arizona State University (Publisher)
Created2019
Description
III-V-bismide semiconductor alloys are a class of materials with applications in the mid and long wave infrared spectrum. The quaternary alloy InAsSbBi is attractive because it can be grown lattice-matched to commercially available GaSb substrates, and the adjustment of the Bi and Sb mole fractions enables both lattice constant and bandgap to be tuned independently. This dissertation provides a comprehensive study of the surface morphology and the structural and chemical properties of InAsSbBi alloys grown by molecular beam epitaxy.
210 nm thick InAsSbBi layers grown at temperatures from 280 °C to 430 °C on (100) on-axis, (100) offcut 1° to (011), and (100) offcut 4° to (111)A GaSb substrates are investigated using Rutherford back scattering, X-ray diffraction, transmission electron microscopy, Nomarski optical microscopy, atomic force microscopy, and photoluminescence spectroscopy. The results indicate that the layers are coherently strained and contain dilute Bi mole fractions.
Large surface droplets with diameters and densities on the order of 3 µm and 106 cm-2 are observed when the growth is performed with As overpressures around 1%. Preferential orientation of the droplets occurs along the [011 ̅] step edges offcut (100) 1° to (011) substrate. The surface droplets are not observed when the As overpressure is increased to 4%. Small crystalline droplets with diameters and densities on the order of 70 nm and 1010 cm-2 are observed between the large droplets for the growth at 430°C. Analysis of one of the small droplets indicates a misoriented zinc blende structure composed of In, Sb, and Bi, with a 6.543 ± 0.038 Å lattice constant.
Lateral variation in the Bi mole fraction is observed in InAsSbBi grown at high temperature (400 °C, 420 °C) on (100) on-axis and (100) offcut 4° to (111)A substrates, but is not observed for growth at 280 °C or on (100) substrates that are offcut 1° to (011). Improved crystal and optical quality is observed in the high temperature grown InAsSbBi and CuPtB type atomic ordering on the {111}B planes is observed in the low temperature grown InAsSbBi. Strain induced tilt is observed in coherently strained InAsSbBi grown on offcut substrates.
210 nm thick InAsSbBi layers grown at temperatures from 280 °C to 430 °C on (100) on-axis, (100) offcut 1° to (011), and (100) offcut 4° to (111)A GaSb substrates are investigated using Rutherford back scattering, X-ray diffraction, transmission electron microscopy, Nomarski optical microscopy, atomic force microscopy, and photoluminescence spectroscopy. The results indicate that the layers are coherently strained and contain dilute Bi mole fractions.
Large surface droplets with diameters and densities on the order of 3 µm and 106 cm-2 are observed when the growth is performed with As overpressures around 1%. Preferential orientation of the droplets occurs along the [011 ̅] step edges offcut (100) 1° to (011) substrate. The surface droplets are not observed when the As overpressure is increased to 4%. Small crystalline droplets with diameters and densities on the order of 70 nm and 1010 cm-2 are observed between the large droplets for the growth at 430°C. Analysis of one of the small droplets indicates a misoriented zinc blende structure composed of In, Sb, and Bi, with a 6.543 ± 0.038 Å lattice constant.
Lateral variation in the Bi mole fraction is observed in InAsSbBi grown at high temperature (400 °C, 420 °C) on (100) on-axis and (100) offcut 4° to (111)A substrates, but is not observed for growth at 280 °C or on (100) substrates that are offcut 1° to (011). Improved crystal and optical quality is observed in the high temperature grown InAsSbBi and CuPtB type atomic ordering on the {111}B planes is observed in the low temperature grown InAsSbBi. Strain induced tilt is observed in coherently strained InAsSbBi grown on offcut substrates.
ContributorsKosireddy, Rajeev Reddy (Author) / Johnson, Shane R (Thesis advisor) / Smith, David J. (Committee member) / Alford, Terry L. (Committee member) / Soignard, Emmanuel (Committee member) / Arizona State University (Publisher)
Created2020
Description
Extended crystal defects often play a critical role in determining semiconductor device performance. This dissertation describes the application of transmission electron microscopy (TEM) and aberration-corrected scanning TEM (AC-STEM) to study defect clusters and the atomic-scale structure of defects in compound semiconductors.
An extensive effort was made to identify specific locations of crystal defects in epitaxial CdTe that might contribute to degraded light-conversion efficiency. Electroluminescence (EL) mapping and the creation of surface etch pits through chemical treatment were combined in attempts to identify specific structural defects for subsequent TEM examination. Observations of these specimens revealed only surface etch pits, without any visible indication of extended defects near their base. While chemical etch pits could be helpful for precisely locating extended defects that intersect with the treated surface, this study concluded that surface roughness surrounding etch pits would likely mitigate against their usefulness.
Defect locations in GaAs solar-cell devices were identified using combinations of EL, photoluminescence, and Raman scattering, and then studied more closely using TEM. Observations showed that device degradation was invariably associated with a cluster of extended defects, rather than a single defect, as previously assumed. AC-STEM observations revealed that individual defects within each cluster consisted primarily of intrinsic stacking faults terminated by 30° and 90° partial dislocations, although other defect structures were also identified. Lomer dislocations were identified near locations where two lines of strain contrast intersected in a large cluster, and a comparatively shallow cluster, largely constrained to the GaAs emitter layer, contained 60° perfect dislocations associated with localized strain contrast.
In another study, misfit dislocations at II-VI/III-V heterovalent interfaces were investigated and characterized using AC-STEM. Misfit strain at ZnTe/GaAs interfaces, which have relatively high lattice mismatch (7.38%), was relieved primarily through Lomer dislocations, while ZnTe/InP interfaces, with only 3.85% lattice mismatch, were relaxed by a mixture of 60° perfect dislocations, 30° partial dislocations, and Lomer dislocations. These results were consistent with the previous findings that misfit strain was relaxed primarily through 60° perfect dislocations that had either dissociated into partial dislocations or interacted to form Lomer dislocations as the amount of misfit strain increased.
An extensive effort was made to identify specific locations of crystal defects in epitaxial CdTe that might contribute to degraded light-conversion efficiency. Electroluminescence (EL) mapping and the creation of surface etch pits through chemical treatment were combined in attempts to identify specific structural defects for subsequent TEM examination. Observations of these specimens revealed only surface etch pits, without any visible indication of extended defects near their base. While chemical etch pits could be helpful for precisely locating extended defects that intersect with the treated surface, this study concluded that surface roughness surrounding etch pits would likely mitigate against their usefulness.
Defect locations in GaAs solar-cell devices were identified using combinations of EL, photoluminescence, and Raman scattering, and then studied more closely using TEM. Observations showed that device degradation was invariably associated with a cluster of extended defects, rather than a single defect, as previously assumed. AC-STEM observations revealed that individual defects within each cluster consisted primarily of intrinsic stacking faults terminated by 30° and 90° partial dislocations, although other defect structures were also identified. Lomer dislocations were identified near locations where two lines of strain contrast intersected in a large cluster, and a comparatively shallow cluster, largely constrained to the GaAs emitter layer, contained 60° perfect dislocations associated with localized strain contrast.
In another study, misfit dislocations at II-VI/III-V heterovalent interfaces were investigated and characterized using AC-STEM. Misfit strain at ZnTe/GaAs interfaces, which have relatively high lattice mismatch (7.38%), was relieved primarily through Lomer dislocations, while ZnTe/InP interfaces, with only 3.85% lattice mismatch, were relaxed by a mixture of 60° perfect dislocations, 30° partial dislocations, and Lomer dislocations. These results were consistent with the previous findings that misfit strain was relaxed primarily through 60° perfect dislocations that had either dissociated into partial dislocations or interacted to form Lomer dislocations as the amount of misfit strain increased.
ContributorsMcKeon, Brandon (Author) / Smith, David J. (Thesis advisor) / McCartney, Martha R. (Thesis advisor) / Liu, Jingyue (Committee member) / Zhang, Yong-Hang (Committee member) / Arizona State University (Publisher)
Created2020
Description
Photocatalytic water splitting over suspended nanoparticles represents a potential solution for achieving CO2-neutral energy generation and storage. To design efficient photocatalysts, a fundamental understanding of the material’s structure, electronic properties, defects, and how these are controlled via synthesis is essential. Both bulk and nanoscale materials characterization, in addition to various performance metrics, can be combined to elucidate functionality at multiple length scales. In this work, two promising visible light harvesting systems are studied in detail: Pt-functionalized graphitic carbon nitrides (g-CNxHys) and TiO2-supported CeO2-x composites.
Electron energy-loss spectroscopy (EELS) is used to sense variations in the local concentration of amine moieties (defects believed to facilitate interfacial charge transfer) at the surface of a g-CNxHy flake. Using an aloof-beam configuration, spatial resolution is maximized while minimizing damage thus providing nanoscale vibrational fingerprints similar to infrared absorption spectra. Structural disorder in g-CNxHys is further studied using transmission electron microscopy at low electron fluence rates. In-plane structural fluctuations revealed variations in the local azimuthal orientation of the heptazine building blocks, allowing planar domain sizes to be related to the average polymer chain length. Furthermore, competing factors regulating photocatalytic performance in a series of Pt/g-CNxHys is elucidated. Increased polymer condensation in the g-CNxHy support enhances the rate of charge transfer to reactants owing to higher electronic mobility. However, active site densities are over 3x lower on the most condensed g-CNxHy which ultimately limits its H2 evolution rate (HER). Based on these findings, strategies to improve the cocatalyst configuration on intrinsically active supports are given.
In TiO2/CeO2-x photocatalysts, the effect of the support particle size on the bulk
anoscale properties and photocatalytic performance is investigated. Small anatase supports facilitate highly dispersed CeO2-x species, leading to increased visible light absorption and HERs resulting from a higher density of mixed metal oxide (MMO) interfaces with Ce3+ species. Using monochromated EELS, bandgap states associated with MMO interfaces are detected, revealing electronic transitions from 0.5 eV up to the bulk bandgap onset of anatase. Overall, the electron microscopy/spectroscopy techniques developed and applied herein sheds light onto the relevant defects and limiting processes operating within these photocatalyst systems thus suggesting rational design strategies.
Electron energy-loss spectroscopy (EELS) is used to sense variations in the local concentration of amine moieties (defects believed to facilitate interfacial charge transfer) at the surface of a g-CNxHy flake. Using an aloof-beam configuration, spatial resolution is maximized while minimizing damage thus providing nanoscale vibrational fingerprints similar to infrared absorption spectra. Structural disorder in g-CNxHys is further studied using transmission electron microscopy at low electron fluence rates. In-plane structural fluctuations revealed variations in the local azimuthal orientation of the heptazine building blocks, allowing planar domain sizes to be related to the average polymer chain length. Furthermore, competing factors regulating photocatalytic performance in a series of Pt/g-CNxHys is elucidated. Increased polymer condensation in the g-CNxHy support enhances the rate of charge transfer to reactants owing to higher electronic mobility. However, active site densities are over 3x lower on the most condensed g-CNxHy which ultimately limits its H2 evolution rate (HER). Based on these findings, strategies to improve the cocatalyst configuration on intrinsically active supports are given.
In TiO2/CeO2-x photocatalysts, the effect of the support particle size on the bulk
anoscale properties and photocatalytic performance is investigated. Small anatase supports facilitate highly dispersed CeO2-x species, leading to increased visible light absorption and HERs resulting from a higher density of mixed metal oxide (MMO) interfaces with Ce3+ species. Using monochromated EELS, bandgap states associated with MMO interfaces are detected, revealing electronic transitions from 0.5 eV up to the bulk bandgap onset of anatase. Overall, the electron microscopy/spectroscopy techniques developed and applied herein sheds light onto the relevant defects and limiting processes operating within these photocatalyst systems thus suggesting rational design strategies.
ContributorsHaiber, Diane Michelle (Author) / Crozier, Peter (Thesis advisor) / Chan, Candace (Committee member) / Liu, Jingyue (Committee member) / Treacy, Michael (Committee member) / Arizona State University (Publisher)
Created2019
Description
The evolution of defects at different stages of strain relaxation in low-mismatched GaAs/GaAs1-xSbx/GaAs(001) (x ~ 0.08) heterostructures, and the underlying relaxation mechanisms, have been comprehensively studied primarily using transmission electron microscopy (TEM). Aberration-corrected scanning transmission electron microscopy (STEM) has been used for atomic-scale study of interfacial defects in low-mismatched GaAs(001)-based and high-mismatched GaSb/GaAs(001) heterostructures.Three distinct stages of strain relaxation were identified in GaAs/GaAs1-xSbx/GaAs(001) (x ~ 0.08) heterostructures with GaAsSb film thicknesses in the range of 50 to 4000 nm capped with 50-nm-thick GaAs layers. Diffraction contrast analysis with conventional TEM revealed that although 60° dislocations were primarily formed during the initial sluggish Stage-I relaxation, 90° dislocations were also created. Many curved dislocations, the majority of which extended into the substrate, were formed during Stage-II and Stage-III relaxation. The capping layers of heterostructures with larger film thickness (500 nm onwards) exhibited only Stage-I relaxation. A decrease in dislocation density was observed at the cap/film interface of the heterostructure with 4000-nm-thick film compared to that with 2000-nm-thick film, which correlated with smoothening of surface cross-hatch morphology. Detailed consideration of plausible dislocation sources for the capping layer led to the conclusion that dislocation half-loops nucleated at surface troughs were the main source of threading dislocations in these heterostructures.
Aberration-corrected STEM imaging revealed that interfacial 60° dislocations in GaAs/GaAsSb/GaAs(001) and GaAs/GaAsP/GaAs(001) heterostructures were dissociated to form intrinsic stacking faults bounded by 90° and 30° Shockley partial dislocations. The cores of the 30° partials contained single atomic columns indicating that these dislocations primarily belonged to glide set. Apart from isolated dissociated 60° dislocations, Lomer-Cottrell locks, Lomer dislocations and a novel type of dissociated 90° dislocation were observed in GaAs/GaAsSb/GaAs heterostructures.
The core structure of interfacial defects in GaSb/GaAs(001) heterostructure was also investigated using aberration-corrected STEM. 90° Lomer dislocations were primarily formed; however, glide-set perfect 60° and dissociated 60° dislocations were also observed. The 5-7 atomic-ring shuffle-set dislocation, the left-displaced 6-8 atomic-ring glide-set and the right-displaced 6-8 atomic-ring glide-set dislocations were three types of Lomer dislocations that were identified, among which the shuffle-set type was most common.
ContributorsGangopadhyay, Abhinandan (Author) / Smith, David J. (Thesis advisor) / Bertoni, Mariana (Committee member) / Crozier, Peter A. (Committee member) / King, Richard R. (Committee member) / McCartney, Martha R. (Committee member) / Arizona State University (Publisher)
Created2021