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Description
This dissertation is on the study of structural and optical properties of some III-V and II-VI compound semiconductors. The first part of this dissertation is a study of the deformation mechanisms associated with nanoindentation and nanoscratching of InP, GaN, and ZnO crystals. The second part is an investigation of some fundamental issues regarding compositional fluctuations and microstructure in GaInNAs and InAlN alloys. In the first part, the microstructure of (001) InP scratched in an atomic force microscope with a small diamond tip has been studied as a function of applied normal force and crystalline direction in order to understand at the nanometer scale the deformation mechanisms in the zinc-blende structure. TEM images show deeper dislocation propagation for scratches along <110> compared to <100>. High strain fields were observed in <100> scratches, indicating hardening due to locking of dislocations gliding on different slip planes. Reverse plastic flow have been observed in <110> scratches in the form of pop-up events that result from recovery of stored elastic strain. In a separate study, nanoindentation-induced plastic deformation has been studied in c-, a-, and m-plane ZnO single crystals and c-plane GaN respectively, to study the deformation mechanism in wurtzite hexagonal structures. TEM results reveal that the prime deformation mechanism is slip on basal planes and in some cases, on pyramidal planes, and strain built up along particular directions. No evidence of phase transformation or cracking was observed in both materials. CL imaging reveals quenching of near band-edge emission by dislocations. In the second part, compositional inhomogeneity in quaternary GaInNAs and ternary InAlN alloys has been studied using TEM. It is shown that exposure to antimony during growth of GaInNAs results in uniform chemical composition in the epilayer, as antimony suppresses the surface mobility of adatoms that otherwise leads to two-dimensional growth and elemental segregation. In a separate study, compositional instability is observed in lattice-matched InAlN films grown on GaN, for growth beyond a certain thickness. Beyond 200 nm of thickness, two sub-layers with different indium content are observed, the top one with lower indium content.
ContributorsHuang, Jingyi (Author) / Ponce, Fernando A. (Thesis advisor) / Carpenter, Ray W (Committee member) / Smith, David J. (Committee member) / Yu, Hongbin (Committee member) / Treacy, Michael Mj (Committee member) / Arizona State University (Publisher)
Created2013
Description
Semiconductor nanostructures are promising building blocks for light management in thin silicon solar cells and silicon-based tandems due their tunable optical properties. The present dissertation is organized along three main research areas: (1) characterization and modeling of III-V nanowires as active elements of solar cell tandems, (2) modeling of silicon nanopillars for reduced optical losses in ultra-thin silicon solar cells, and (3) characterization and modeling of nanoparticle-based optical coatings for light management.
First, the recombination mechanisms in polytype GaAs nanowires are studied through photoluminescence measurements coupled with rate equation analysis. When photons are absorbed in polytype nanowires, electrons and holes quickly thermalize to the band-edges of the zinc-blende and wurtzite phases, recombining indirectly in space across the type-II offset. Using a rate equation model, different configurations of polytype defects along the nanowire are investigated, which compare well with experiment considering spatially indirect recombination between different polytypes, and defect-related recombination due to twin planes and other defects. The presented analysis is a path towards predicting the performance of nanowire-based solar cells.
Following this topic, the optical mechanisms in silicon nanopillar arrays are investigated using full-wave optical simulations in comparison to measured reflectance data. The simulated electric field energy density profiles are used to elucidate the mechanisms contributing to the reduced front surface reflectance. Strong forward scattering and resonant absorption are observed for shorter- and longer- aspect ratio nanopillars, respectively, with the sub-wavelength periodicity causing additional diffraction. Their potential for light-trapping is investigated using full-wave optical simulation of an ultra-thin nanostructured substrate, where the conventional light-trapping limit is exceeded for near-bandgap wavelengths.
Finally, the correlation between the optical properties of silicon nanoparticle layers to their respective pore size distributions is investigated using optical and structural characterization coupled with full-wave optical simulation. The presence of
scattering is experimentally correlated to wider pore size distributions obtained from nitrogen adsorption measurements. The correlation is validated with optical simulation of random and clustered structures, with the latter approximating experimental. Reduced structural inhomogeneity in low-refractive-index nanoparticle inter-layers at the metal/semiconductor interface improves their performance as back reflectors, while reducing parasitic absorption in the metal.
First, the recombination mechanisms in polytype GaAs nanowires are studied through photoluminescence measurements coupled with rate equation analysis. When photons are absorbed in polytype nanowires, electrons and holes quickly thermalize to the band-edges of the zinc-blende and wurtzite phases, recombining indirectly in space across the type-II offset. Using a rate equation model, different configurations of polytype defects along the nanowire are investigated, which compare well with experiment considering spatially indirect recombination between different polytypes, and defect-related recombination due to twin planes and other defects. The presented analysis is a path towards predicting the performance of nanowire-based solar cells.
Following this topic, the optical mechanisms in silicon nanopillar arrays are investigated using full-wave optical simulations in comparison to measured reflectance data. The simulated electric field energy density profiles are used to elucidate the mechanisms contributing to the reduced front surface reflectance. Strong forward scattering and resonant absorption are observed for shorter- and longer- aspect ratio nanopillars, respectively, with the sub-wavelength periodicity causing additional diffraction. Their potential for light-trapping is investigated using full-wave optical simulation of an ultra-thin nanostructured substrate, where the conventional light-trapping limit is exceeded for near-bandgap wavelengths.
Finally, the correlation between the optical properties of silicon nanoparticle layers to their respective pore size distributions is investigated using optical and structural characterization coupled with full-wave optical simulation. The presence of
scattering is experimentally correlated to wider pore size distributions obtained from nitrogen adsorption measurements. The correlation is validated with optical simulation of random and clustered structures, with the latter approximating experimental. Reduced structural inhomogeneity in low-refractive-index nanoparticle inter-layers at the metal/semiconductor interface improves their performance as back reflectors, while reducing parasitic absorption in the metal.
ContributorsVulic, Natasa (Author) / Goodnick, Stephen M (Thesis advisor) / Honsberg, C. (Christiana B.) (Committee member) / Holman, Zachary C (Committee member) / Smith, David J. (Committee member) / Arizona State University (Publisher)
Created2019
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
A new nanoparticle deposition technique, Aerosol Impaction-Driven Assembly (AIDA), was extensively characterized for material structures and properties. Aerogel films can be deposited directly onto a substrate with AIDA without the long aging and drying steps in the sol-gel method. Electron microscopy, pore size analysis, thermal conductivity, and optical measurements show the nanoparticle (NP) films to be similar to typical silica aerogel. Haze of nanoparticle films modeled as scattering sites correlates strongly with pore size distribution. Supporting evidence was obtained from particle sizes and aggregates using electron microscopy and small-angle X-ray scattering. NP films showed interlayers of higher porosity and large aggregates formed by tensile film stress.
To better understand film stress and NP adhesion, chemical bonding analyses were performed for samples annealed up to 900 °C. Analysis revealed that about 50% of the NP surfaces are functionalized by hydroxyl (-OH) groups, providing for hydrogen bonding. Ellipsometric porosimetry was used to further understand the mechanical properties by providing a measure of strain upon capillary pressure from filling pores. Upon annealing to 200 °C, the films lost water resulting in closer bonding of NPs and higher Young’s modulus. Upon further annealing up to 900 °C, the films lost hydroxyl bonds while gaining siloxane bonds, reducing Young’s modulus. The application of ellipsometric porosimetry to hydrophilic coatings brings into question the validity of pore size distribution calculations for materials that hold onto water molecules and result in generally smaller calculated pore sizes.
Doped hydrogenated microcrystalline silicon was grown on crystalline silicon NPs, as a test case of an application for NP films to reduce parasitic absorption in silicon heterojunction solar cells. Parasitic absorption of blue light could be reduced because microcrystalline silicon has a mix of direct and indirect bandgap, giving lower blue absorption than amorphous silicon. Using Ultraviolet Raman spectroscopy, the crystallinity of films as thin as 13 nm was determined rapidly (in 1 minute) and non-destructively. A mono-layer of nanocrystals was applied as seeds for p-doped microcrystalline silicon growth and resulted in higher crystallinity films. Applications of the method could be explored for other nanocrystalline materials.
To better understand film stress and NP adhesion, chemical bonding analyses were performed for samples annealed up to 900 °C. Analysis revealed that about 50% of the NP surfaces are functionalized by hydroxyl (-OH) groups, providing for hydrogen bonding. Ellipsometric porosimetry was used to further understand the mechanical properties by providing a measure of strain upon capillary pressure from filling pores. Upon annealing to 200 °C, the films lost water resulting in closer bonding of NPs and higher Young’s modulus. Upon further annealing up to 900 °C, the films lost hydroxyl bonds while gaining siloxane bonds, reducing Young’s modulus. The application of ellipsometric porosimetry to hydrophilic coatings brings into question the validity of pore size distribution calculations for materials that hold onto water molecules and result in generally smaller calculated pore sizes.
Doped hydrogenated microcrystalline silicon was grown on crystalline silicon NPs, as a test case of an application for NP films to reduce parasitic absorption in silicon heterojunction solar cells. Parasitic absorption of blue light could be reduced because microcrystalline silicon has a mix of direct and indirect bandgap, giving lower blue absorption than amorphous silicon. Using Ultraviolet Raman spectroscopy, the crystallinity of films as thin as 13 nm was determined rapidly (in 1 minute) and non-destructively. A mono-layer of nanocrystals was applied as seeds for p-doped microcrystalline silicon growth and resulted in higher crystallinity films. Applications of the method could be explored for other nanocrystalline materials.
ContributorsCarpenter, Joe Victor (Author) / Holman, Zachary C (Thesis advisor) / Bertoni, Mariana I (Committee member) / Chan, Candace K. (Committee member) / Smith, David J. (Committee member) / Arizona State University (Publisher)
Created2020