We directly measure the three-dimensional movement of intrinsic point defects driven by applied electric fields inside ZnO nano- and micro-wire metal–semiconductor–metal device structures. Using ...depth- and spatially resolved cathodoluminescence spectroscopy (CLS) in situ to map the spatial distributions of local defect densities with increasing applied bias, we drive the reversible conversion of metal–ZnO contacts from rectifying to Ohmic and back. These results demonstrate how defect movements systematically determine Ohmic and Schottky barriers to ZnO nano- and microwires and how they can account for the widely reported instability in nanowire transport. Exceeding a characteristic threshold voltage, in situ CLS reveals a current-induced thermal runaway that drives the radial diffusion of defects toward the nanowire free surface, causing VO defects to accumulate at the metal–semiconductor interfaces. In situ post- vs pre-breakdown CLS reveal micrometer-scale wire asperities, which X-ray photoelectron spectroscopy (XPS) finds to have highly oxygen-deficient surface layers that can be attributed to the migration of preexisting VO species. These findings show the importance of in-operando intrinsic point-defect migration during nanoscale electric field measurements in general. This work also demonstrates a novel method for ZnO nanowire refinement and processing.
We have studied the optical band and defect transitions of ScN, a group IIIB transition metal nitride semiconductor with electronic and optoelectronic applications. Recent works have focused on the ...degenerate nature of ScN by substitutional impurities ON and FN, which shift the direct (X–X) gap transition to higher energies via the Burstein–Moss effect. We used cathodoluminescence spectroscopy (CLS) to observe optical signatures of both the midgap VN precursor to ON doping as well as above the direct (X–X) bandgap corresponding to band-to-band transitions from four separate conduction bands near the Γ point with the valence band minimum, in agreement with the calculated band structure diagrams. Thin film ScN grown by reactive magnetron sputtering displays mild degenerate doping by substitutional oxygen as indicated by elevated (X–X) transition energies and the presence of Sc–O bonding determined via x-ray photoelectron spectroscopy (XPS), while ScN grown by physical vapor transport exhibited the intrinsic, non-degenerate (X–X) bandgap predicted by theory. CLS reveals a sharp, sub-bandgap emission at 1.26 eV for sputter grown ScN on GaN, which we attribute to nitrogen vacancies (VN) based on surface sensitive CLS and XPS chemical trends. This finding is in strong agreement with theoretical calculations for VN predicting the formation of a defect energy level within the gap.
We report on the observation of an unusual negative magnetic hysteresis loop in ZnO thin film codoped with cobalt and aluminum (Co-Al:ZnO), while other transition-metal-doped ZnO films, such as ...Cu-doped ZnO and Mn-doped ZnO, exhibit normal hysteresis loops. The unusual magnetic behavior is ascribed to the presence of double magnetic layers with different magnetic moments due to the change of structural defects across the film layers. Positron annihilation measurements confirmed the presence of unique microstructural changes in the Co-Al:ZnO film. This study shows that defects in diluted magnetic semiconductors may induce not only ferromagnetism but also novel magnetic behaviors.
Wide bandgap semiconductors (WBGS) are an exceptionally important class of materials for next-generation microelectronics that exhibit a diverse set of physical phenomena spanning piezo- and ...ferroelectricity, ferromagnetism, superconductivity, and 2-dimensional electron and hole gases. Oxide and nitride WBGS are particularly well-positioned to accelerate the advancement of renewable energy technologies and improve power electronic efficiencies and life-cycle costs. The identification and capacity for control of optically and electrically active point defects are central in transitioning these materials from research laboratory to practical applications.This dissertation investigates the optoelectronic property-structure relationship of native point defects in wide bandgap semiconductors. By combining spatially resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy (SPS) with surface sensitive x-ray photoelectron spectroscopy (XPS), native anionic point defects and cation defect complexes are identified in ternary ZnGeN2 and binary ScN. The control and refinement of these defects with material growth is essential for coherent integration with complementary GaN architectures. Direct control of point defects on the nanoscale is demonstrated in oxide semiconducting Ga2O3 vertical devices and ZnO nanowire structures by applying electric fields to stimulate the migration of intrinsic defect species. Redistribution of oxygen vacancies (VO) in high power Ga2O3 is demonstrated for the first time by nanoscale hyperspectral imaging after strong reverse biasing while manipulation of oxygen vacancies in suspended ZnO nanowires is selectively driven by applied bias through nanoscale contacts. The density of positively charged interfacial VO is tuned via electric fields to control Schottky barrier heights and reversibly convert metal-ZnO interfaces between Ohmic and Schottky behavior, highlighting the need to consider intrinsic point defect migration in-operando while demonstrating a promising new avenue for nanoscale oxide refinement and processing.
A combination of depth-resolved cathodoluminescence spectroscopy (DRCLS) and X-ray photoemission depth profiling (XPS) measured the pronounced changes in both the electronic density of states and ...lithium composition near the nanoscale Li x V2O5/LiPON interface. DRCLS studies of electrochemically lithiated bare V2O5 and the sputter-deposited V2O5 plus LiPON overlayer electrochemically lithiated in stages both showed that in the bulk the luminescence intensity of the “split-off” hybridized bonding density of states was anticorrelated with XPS-measured Li content, decreasing as the Li content increased. However, the LiPON overlayer was found to modify the band structure of the underlying Li x V2O5 (LVO) to a depth of at least 30 nm beneath the V2O5 interface. DRCLS spectra near the electrochemically lithiated LiPON/LVO interface showed a significant intensity of the split-off band, implying a low Li content. However, XPS depth profiling revealed a pronounced negative gradient of Li extending from a maximum Li content at the intimate LiPON boundary to its lowest content of ∼30 nm into the V2O5 in the same region, indicating a strong interaction between band structure and Li electrochemical potential near this heterojunction. These results provide evidence for substantial effects on the local band structure near an electrolyte/cathode interface and insights into the electrochemical interface behavior of solid-state batteries in general.
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