ASU Electronic Theses and Dissertations
This collection includes most of the ASU Theses and Dissertations from 2011 to present. ASU Theses and Dissertations are available in downloadable PDF format; however, a small percentage of items are under embargo. Information about the dissertations/theses includes degree information, committee members, an abstract, supporting data or media.
In addition to the electronic theses found in the ASU Digital Repository, ASU Theses and Dissertations can be found in the ASU Library Catalog.
Dissertations and Theses granted by Arizona State University are archived and made available through a joint effort of the ASU Graduate College and the ASU Libraries. For more information or questions about this collection contact or visit the Digital Repository ETD Library Guide or contact the ASU Graduate College at gradformat@asu.edu.
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- Creators: Smith, David J.
Description
The research described in this dissertation has involved the use of transmission electron microcopy (TEM) to characterize the structural properties of II-VI and III-V compound semiconductor heterostructures and superlattices. The microstructure of thick ZnTe epilayers (~2.4 µm) grown by molecular beam epitaxy (MBE) under virtually identical conditions on GaSb, InAs, InP and GaAs (100) substrates were compared using TEM. High-resolution electron micrographs revealed a highly coherent interface for the ZnTe/GaSb sample, and showed extensive areas with well-separated interfacial misfit dislocations for the ZnTe/InAs sample. Lomer edge dislocations and 60o dislocations were commonly observed at the interfaces of the ZnTe/InP and ZnTe/GaAs samples. The amount of residual strain at the interfaces was estimated to be 0.01% for the ZnTe/InP sample and -0.09% for the ZnTe/GaAs sample. Strong PL spectra for all ZnTe samples were observed from 80 to 300 K. High quality GaSb grown by MBE on ZnTe/GaSb (001) virtual substrates with a temperature ramp at the beginning of the GaSb growth has been demonstrated. High-resolution X-ray diffraction (XRD) showed clear Pendellösung thickness fringes from both GaSb and ZnTe epilayers. Cross-section TEM images showed excellent crystallinity and smooth morphology for both ZnTe/GaSb and GaSb/ZnTe interfaces. Plan-view TEM image revealed the presence of Lomer dislocations at the interfaces and threading dislocations in the top GaSb layer. The defect density was estimated to be ~1 x107/cm2. The PL spectra showed improved optical properties when using the GaSb transition layer grown on ZnTe with a temperature ramp. The structural properties of strain-balanced InAs/InAs1-xSbx SLs grown on GaSb (001) substrates by metalorganic chemical vapor deposition (MOCVD) and MBE, have been studied using XRD and TEM. Excellent structural quality of the InAs/InAs1-xSbx SLs grown by MOCVD has been demonstrated. Well-defined ordered-alloy structures within individual InAs1-xSbx layers were observed for samples grown by modulated MBE. However, the ordering disappeared when defects propagating through the SL layers appeared during growth. For samples grown by conventional MBE, high-resolution images revealed that interfaces for InAs1-xSbx grown on InAs layers were sharper than for InAs grown on InAs1-xSbx layers, most likely due to a Sb surfactant segregation effect.
ContributorsOuyang, Lu (Author) / Smith, David J. (Thesis advisor) / McCartney, Martha (Committee member) / Ponce, Fernando (Committee member) / Chamberlin, Ralph (Committee member) / Menéndez, Jose (Committee member) / Arizona State University (Publisher)
Created2012
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
Multifunctional oxide thin-films grown on silicon and several oxide substrates have been characterized using High Resolution (Scanning) Transmission Electron Microscopy (HRTEM), Energy-Dispersive X-ray Spectroscopy (EDX), and Electron Energy-Loss Spectroscopy (EELS). Oxide thin films grown on SrTiO3/Si pseudo-substrate showed the presence of amorphised SrTiO3 (STO) at the STO/Si interface. Oxide/oxide interfaces were observed to be atomically clean with very few defects.
Al-doped SrTiO3 thin films grown on Si were of high crystalline quality. The Ti/O ratio estimated from EELS line scans revealed that substitution of Ti by Al created associated O vacancies. The strength of the crystal field in STO was measured using EELS, and decreased by ~1.0 eV as Ti4+ was substituted by Al3+. The damping of O-K EELS peaks confirmed the rise in oxygen vacancies. For Co-substituted STO films grown on Si, the EDS and EELS spectra across samples showed Co doping was quite random. The substitution of Ti4+ with Co3+ or Co2+ created associated oxygen vacancies for charge balance. Presence of oxygen vacancies was also confirmed by shift of Ti-L EELS peaks towards lower energy by ~0.4 eV. The crystal-field strength decreased by ~0.6 eV as Ti4+ was partially substituted by Co3+ or Co2+.
Spinel Co3O4 thin films grown on MgAl2O4 (110) were observed to have excellent crystalline quality. The structure of the Co3O4/MgAl2O4 interface was determined using HRTEM and image simulations. It was found that MgAl2O4 substrate is terminated with Al and oxygen. Stacking faults and associated strain fields in spinel Co3O4 were found along [111], [001], and [113] using Geometrical Phase Analysis.
NbO2 films on STO (111) were observed to be tetragonal with lattice parameter of 13.8 Å and NbO films on LSAT (111) were observed to be cubic with lattice parameter of 4.26 Å. HRTEM showed formation of high quality NbOx films and excellent coherent interface. HRTEM of SrAl4 on LAO (001) confirmed an island growth mode. The SrAl4 islands were highly crystalline with excellent epitaxial registry with LAO. By comparing HRTEM images with image simulations, the interface structure was determined to consist of Sr-terminated SrAl4 (001) on AlO2-terminated LAO (001).
Al-doped SrTiO3 thin films grown on Si were of high crystalline quality. The Ti/O ratio estimated from EELS line scans revealed that substitution of Ti by Al created associated O vacancies. The strength of the crystal field in STO was measured using EELS, and decreased by ~1.0 eV as Ti4+ was substituted by Al3+. The damping of O-K EELS peaks confirmed the rise in oxygen vacancies. For Co-substituted STO films grown on Si, the EDS and EELS spectra across samples showed Co doping was quite random. The substitution of Ti4+ with Co3+ or Co2+ created associated oxygen vacancies for charge balance. Presence of oxygen vacancies was also confirmed by shift of Ti-L EELS peaks towards lower energy by ~0.4 eV. The crystal-field strength decreased by ~0.6 eV as Ti4+ was partially substituted by Co3+ or Co2+.
Spinel Co3O4 thin films grown on MgAl2O4 (110) were observed to have excellent crystalline quality. The structure of the Co3O4/MgAl2O4 interface was determined using HRTEM and image simulations. It was found that MgAl2O4 substrate is terminated with Al and oxygen. Stacking faults and associated strain fields in spinel Co3O4 were found along [111], [001], and [113] using Geometrical Phase Analysis.
NbO2 films on STO (111) were observed to be tetragonal with lattice parameter of 13.8 Å and NbO films on LSAT (111) were observed to be cubic with lattice parameter of 4.26 Å. HRTEM showed formation of high quality NbOx films and excellent coherent interface. HRTEM of SrAl4 on LAO (001) confirmed an island growth mode. The SrAl4 islands were highly crystalline with excellent epitaxial registry with LAO. By comparing HRTEM images with image simulations, the interface structure was determined to consist of Sr-terminated SrAl4 (001) on AlO2-terminated LAO (001).
ContributorsDhamdhere, Ajit (Author) / Smith, David J. (Thesis advisor) / McCartney, Martha R. (Committee member) / Chamberlin, Ralph (Committee member) / Ponce, Fernando (Committee member) / Arizona State University (Publisher)
Created2015