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- All Subjects: Physics
- Creators: Drucker, Jeffery
- Creators: Bennett, Peter
Description
The theory of quantum electrodynamics predicts that beta decay of the neutron into a proton, electron, and anti-neutrino should be accompanied by a continuous spectrum of photons. A recent experiment, RDK I, reported the first detection of radiative decay photons from neutron beta decay with a branching ratio of (3.09 ± 0.32) × 10-3 in the energy range of 15 keV to 340 keV. This was achieved by prompt coincident detection of an electron and photon, in delayed coincidence with a proton. The photons were detected by using a single bar of bismuth germanate scintillating crystal coupled to an avalanche photodiode. This thesis deals with the follow-up experiment, RDK II, to measure the branching ratio at the level of approximately 1% and the energy spectrum at the level of a few percent. The most significant improvement of RDK II is the use of a photon detector with about an order of magnitude greater solid angle coverage than RDK I. In addition, the detectable energy range has been extended down to approximately 250 eV and up to the endpoint energy of 782 keV. This dissertation presents an overview of the apparatus, development of a new data analysis technique for radiative decay, and results for the ratio of electron-proton-photon coincident Repg to electron-proton coincident Rep events.
ContributorsO'Neill, Benjamin (Author) / Alarcon, Ricardo (Thesis advisor) / Drucker, Jeffery (Committee member) / Lebed, Richard (Committee member) / Comfort, Joseph (Committee member) / Chamberlin, Ralph (Committee member) / Arizona State University (Publisher)
Created2012
Description
Off-axis electron holography (EH) has been used to characterize electrostatic potential, active dopant concentrations and charge distribution in semiconductor nanostructures, including ZnO nanowires (NWs) and thin films, ZnTe thin films, Si NWs with axial p-n junctions, Si-Ge axial heterojunction NWs, and Ge/LixGe core/shell NW.
The mean inner potential (MIP) and inelastic mean free path (IMFP) of ZnO NWs have been measured to be 15.3V±0.2V and 55±3nm, respectively, for 200keV electrons. These values were then used to characterize the thickness of a ZnO nano-sheet and gave consistent values. The MIP and IMFP for ZnTe thin films were measured to be 13.7±0.6V and 46±2nm, respectively, for 200keV electrons. A thin film expected to have a p-n junction was studied, but no signal due to the junction was observed. The importance of dynamical effects was systematically studied using Bloch wave simulations.
The built-in potentials in Si NWs across the doped p-n junction and the Schottky junction due to Au catalyst were measured to be 1.0±0.3V and 0.5±0.3V, respectively. Simulations indicated that the dopant concentrations were ~1019cm-3 for donors and ~1017 cm-3 for acceptors. The effects of positively charged Au catalyst, a possible n+-n--p junction transition region and possible surface charge, were also systematically studied using simulations.
Si-Ge heterojunction NWs were studied. Dopant concentrations were extracted by atom probe tomography. The built-in potential offset was measured to be 0.4±0.2V, with the Ge side lower. Comparisons with simulations indicated that Ga present in the Si region was only partially activated. In situ EH biasing experiments combined with simulations indicated the B dopant in Ge was mostly activated but not the P dopant in Si. I-V characteristic curves were measured and explained using simulations.
The Ge/LixGe core/shell structure was studied during lithiation. The MIP for LixGe decreased with time due to increased Li content. A model was proposed to explain the lower measured Ge potential, and the trapped electron density in Ge core was calculated to be 3×1018 electrons/cm3. The Li amount during lithiation was also calculated using MIP and volume ratio, indicating that it was lower than the fully lithiated phase.
The mean inner potential (MIP) and inelastic mean free path (IMFP) of ZnO NWs have been measured to be 15.3V±0.2V and 55±3nm, respectively, for 200keV electrons. These values were then used to characterize the thickness of a ZnO nano-sheet and gave consistent values. The MIP and IMFP for ZnTe thin films were measured to be 13.7±0.6V and 46±2nm, respectively, for 200keV electrons. A thin film expected to have a p-n junction was studied, but no signal due to the junction was observed. The importance of dynamical effects was systematically studied using Bloch wave simulations.
The built-in potentials in Si NWs across the doped p-n junction and the Schottky junction due to Au catalyst were measured to be 1.0±0.3V and 0.5±0.3V, respectively. Simulations indicated that the dopant concentrations were ~1019cm-3 for donors and ~1017 cm-3 for acceptors. The effects of positively charged Au catalyst, a possible n+-n--p junction transition region and possible surface charge, were also systematically studied using simulations.
Si-Ge heterojunction NWs were studied. Dopant concentrations were extracted by atom probe tomography. The built-in potential offset was measured to be 0.4±0.2V, with the Ge side lower. Comparisons with simulations indicated that Ga present in the Si region was only partially activated. In situ EH biasing experiments combined with simulations indicated the B dopant in Ge was mostly activated but not the P dopant in Si. I-V characteristic curves were measured and explained using simulations.
The Ge/LixGe core/shell structure was studied during lithiation. The MIP for LixGe decreased with time due to increased Li content. A model was proposed to explain the lower measured Ge potential, and the trapped electron density in Ge core was calculated to be 3×1018 electrons/cm3. The Li amount during lithiation was also calculated using MIP and volume ratio, indicating that it was lower than the fully lithiated phase.
ContributorsGan, Zhaofeng (Author) / Mccartney, Martha R (Thesis advisor) / Smith, David J. (Thesis advisor) / Drucker, Jeffery (Committee member) / Bennett, Peter A (Committee member) / Arizona State University (Publisher)
Created2015
Description
Group III-nitride semiconductors have attracted much attention for applications on high brightness light-emitting diodes (LEDs) and laser diodes (LDs) operating in the visible and ultra-violet spectral range using indium gallium nitride in the active layer. However, the device efficiency in the green to red range is limited by quantum-confined Stark effects resulting from the lattice mismatch between GaN and InGaN. In this dissertation, the optical and micro-structural properties of GaN-based light emitting structures have been analyzed and correlated by utilizing cathodoluminescence and transmission electron microscopy techniques. In the first section, optimization of the design of GaN-based lasers diode structures is presented. The thermal strain present in the GaN underlayer grown on sapphire substrates causes a strain-induced wavelength shift. The insertion of an InGaN waveguide mitigates the mismatch strain at the interface between the InGaN quantum well and the GaN quantum barrier. The second section of the thesis presents a study of the characteristics of thick non-polar m-plane InGaN films and of LED structures containing InGaN quantum wells, which minimize polarization-related electric fields. It is found that in some cases the in-plane piezoelectric fields can still occur due to the existence of misfit dislocations which break the continuity of the film. In the final section, the optical and structural properties of InGaAlN quaternary alloys are analyzed and correlated. The composition of the components of the film is accurately determined by Rutherford backscattering spectroscopy.
ContributorsHuang, Yu (Author) / Ponce, Fernando A. (Thesis advisor) / Tsen, Kong-Thon (Committee member) / Treacy, Michael (Committee member) / Drucker, Jeffery (Committee member) / Culbertson, Robert (Committee member) / Arizona State University (Publisher)
Created2011
Description
With the advent of the X-ray free-electron laser (XFEL), an opportunity has arisen to break the nexus between radiation dose and spatial resolution in diffractive imaging, by outrunning radiation damage altogether when using single X-ray pulses so brief that they terminate before atomic motion commences. This dissertation concerns the application of XFELs to biomolecular imaging in an effort to overcome the severe challenges associated with radiation damage and macroscopic protein crystal growth. The method of femtosecond protein nanocrystallography (fsPNX) is investigated, and a new method for extracting crystallographic structure factors is demonstrated on simulated data and on the first experimental fsPNX data obtained at an XFEL. Errors are assessed based on standard metrics familiar to the crystallography community. It is shown that resulting structure factors match the quality of those measured conventionally, at least to 9 angstrom resolution. A new method for ab-initio phasing of coherently-illuminated nanocrystals is then demonstrated on simulated data. The method of correlated fluctuation small-angle X-ray scattering (CFSAXS) is also investigated as an alternative route to biomolecular structure determination, without the use of crystals. It is demonstrated that, for a constrained two-dimensional geometry, a projection image of a single particle can be formed, ab-initio and without modeling parameters, from measured diffracted intensity correlations arising from disordered ensembles of identical particles illuminated simultaneously. The method is demonstrated experimentally, based on soft X-ray diffraction from disordered but identical nanoparticles, providing the first experimental proof-of-principle result. Finally, the fundamental limitations of CFSAXS is investigated through both theory and simulations. It is found that the signal-to-noise ratio (SNR) for CFSAXS data is essentially independent of the number of particles exposed in each diffraction pattern. The dependence of SNR on particle size and resolution is considered, and realistic estimates are made (with the inclusion of solvent scatter) of the SNR for protein solution scattering experiments utilizing an XFEL source.
ContributorsKirian, Richard A (Author) / Spence, John C. H. (Committee member) / Doak, R. Bruce (Committee member) / Weierstall, Uwe (Committee member) / Bennett, Peter (Committee member) / Treacy, Michael M. J. (Committee member) / Arizona State University (Publisher)
Created2011
Description
In this project, a novel method is presented for measuring the resistivity of nanoscale metallic conductors (nanowires) using a variable-spacing 2-point method with a modified ultrahigh vacuum scanning tunneling microscope. An auxiliary field emission imaging method that allows for scanning insulating surfaces using a large gap distance (20nm) is also presented. Using these methods, the resistivity of self-assembled endotaxial FeSi2 nanowires (NWs) on Si(110) was measured. The resistivity was found to vary inversely with NW width, being rhoNW = 200 uOhm cm at 12 nm and 300 uOhm cm at 2 nm. The increase at small w is attributed to boundary scattering, and is fit to the Fuchs-Sondheimer model, yielding values of rho0 = 150 uOhm cm and lambda = 2.4 nm, for specularity parameter p = 0.5. These results are attributed to a high concentration of point defects in the FeSi2 structure, with a correspondingly short inelastic electron scattering length. It is remarkable that the defect concentration persists in very small structures, and is not changed by surface oxidation.
ContributorsTobler, Samuel (Author) / Bennett, Peter (Thesis advisor) / McCartney, Martha (Committee member) / Tao, Nongjian (Committee member) / Doak, Bruce (Committee member) / Chen, Tingyong (Committee member) / Arizona State University (Publisher)
Created2011
Description
Ge1-ySny alloys represent a new class of photonic materials for integrated optoelectronics on Si. In this work, the electrical and optical properties of Ge1-ySny alloy films grown on Si, with concentrations in the range 0 ≤ y ≤ 0.04, are studied via a variety of methods. The first microelectronic devices from GeSn films were fabricated using newly developed CMOS-compatible protocols, and the devices were characterized with respect to their electrical properties and optical response. The detectors were found to have a detection range that extends into the near-IR, and the detection edge is found to shift to longer wavelengths with increasing Sn content, mainly due to the compositional dependence of the direct band gap E0. With only 2 % Sn, all of the telecommunication bands are covered by a single detector. Room temperature photoluminescence was observed from GeSn films with Sn content up to 4 %. The peak wavelength of the emission was found to shift to lower energies with increasing Sn content, corresponding to the decrease in the direct band gap E0 of the material. An additional peak in the spectrum was assigned to the indirect band gap. The separation between the direct and indirect peaks was found to decrease with increasing Sn concentration, as expected. Electroluminescence was also observed from Ge/Si and Ge0.98Sn0.02 photodiodes under forward bias, and the luminescence spectra were found to match well with the observed photoluminescence spectra. A theoretical expression was developed for the luminescence due to the direct band gap and fit to the data.
ContributorsMathews, Jay (Author) / Menéndez, Jose (Thesis advisor) / Kouvetakis, John (Thesis advisor) / Drucker, Jeffery (Committee member) / Chizmeshya, Andrew (Committee member) / Ponce, Fernando (Committee member) / Arizona State University (Publisher)
Created2011
Description
Studying charge transport through single molecules tethered between two metal electrodes is of fundamental importance in molecular electronics. Over the years, a variety of methods have been developed in attempts of performing such measurements. However, the limitation of these techniques is still one of the factors that prohibit one from gaining a thorough understanding of single molecule junctions. Firstly, the time resolution of experiments is typically limited to milli to microseconds, while molecular dynamics simulations are carried out on the time scale of pico to nanoseconds. A huge gap therefore persists between the theory and the experiments. This thesis demonstrates a nanosecond scale measurement of the gold atomic contact breakdown process. A combined setup of DC and AC circuits is employed, where the AC circuit reveals interesting observations in nanosecond scale not previously seen using conventional DC circuits. The breakdown time of gold atomic contacts is determined to be faster than 0.1 ns and subtle atomic events are observed within nanoseconds. Furthermore, a new method based on the scanning tunneling microscope break junction (STM-BJ) technique is developed to rapidly record thousands of I-V curves from repeatedly formed single molecule junctions. 2-dimensional I-V and conductance-voltage (G-V) histograms constructed using the acquired data allow for more meaningful statistical analysis to single molecule I-V characteristics. The bias voltage adds an additional dimension to the conventional single molecule conductance measurement. This method also allows one to perform transition voltage spectra (TVS) for individual junctions and to study the correlation between the conductance and the tunneling barrier height. The variation of measured conductance values is found to be primarily determined by the poorly defined contact geometry between the molecule and metal electrodes, rather than the tunnel barrier height. In addition, the rapid I-V technique is also found useful in studying thermoelectric effect in single molecule junctions. When applying a temperature gradient between the STM tip and substrate in air, the offset current at zero bias in the I-V characteristics is a measure of thermoelectric current. The rapid I-V technique allows for statistical analysis of such offset current at different temperature gradients and thus the Seebeck coefficient of single molecule junctions is measured. Combining with single molecule TVS, the Seebeck coefficient is also found to be a measure of tunnel barrier height.
ContributorsGuo, Shaoyin (Author) / Tao, Nongjian (Thesis advisor) / Bennett, Peter (Committee member) / Ning, Cun-Zheng (Committee member) / Yu, Hongbin (Committee member) / Arizona State University (Publisher)
Created2012
Description
A distinct characteristic of ferroelectric materials is the existence of a reversible spontaneous polarization with the application of an electric field. The relevant properties ferroelectric lithium niobate surfaces include a low density of defects and external screening of the bound polarization charge. These properties result in unique surface electric field distribution with a strong electric field in the vicinity of domain boundaries, while away from the boundaries, the field decreases rapidly. In this work, ferroelectric lithium niobate (LN) is used as a template to direct the assembly of metallic nanostructures via photo-induced reduction and a substrate for deposition of ZnO semiconducting thin films via plasma enhanced atomic layer deposition (PE-ALD). To understand the mechanism the photo-induced deposition process the following effects were considered: the illumination photon energy and intensity, the polarization screening mechanism of the lithium niobate template and the chemical concentration. Depending on the UV wavelength, variation of Ag deposition rate and boundary nanowire formation are observed and attributed to the unique surface electric field distribution of the polarity patterned template and the penetration depth of UV light. Oxygen implantation is employed to transition the surface from external screening to internal screening, which results in depressed boundary nanowire formation. The ratio of the photon flux and Ag ion flux to the surface determine the deposition pattern. Domain boundary deposition is enhanced with a high photon/Ag ion flux ratio while domain boundary deposition is depressed with a low photon/Ag ion flux ratio. These results also support the photo-induced deposition model where the process is limited by carrier generation, and the cation reduction occurs at the surface. These findings will provide a foundational understanding to employ ferroelectric templates for assembly and patterning of inorganic, organic, biological, and integrated structures. ZnO films deposited on positive and negative domain surfaces of LN demonstrate different I-V curve behavior at different temperatures. At room temperature, ZnO deposited on positive domains exhibits almost two orders of magnitude greater conductance than on negative domains. The conductance of ZnO on positive domains decreases with increasing temperature while the conductance of ZnO on negative domains increases with increasing temperature. The observations are interpreted in terms of the downward or upward band bending at the ZnO/LN interface which is induced by the ferroelectric polarization charge. Possible application of this effect in non-volatile memory devices is proposed for future work.
ContributorsSun, Yang (Author) / Nemanich, Robert (Thesis advisor) / Bennett, Peter (Committee member) / Sukharev, Maxim (Committee member) / Ros, Robert (Committee member) / McCartney, Martha (Committee member) / Arizona State University (Publisher)
Created2011
Description
ABSTRACT Group III-nitride semiconductor materials have been commercially used in fabrication of light-emitting diodes (LEDs) and laser diodes (LDs) covering the spectral range from UV to visible and infrared, and exhibit unique properties suitable for modern optoelectronic applications. Great advances have recently happened in the research and development in high-power and high-efficiency blue-green-white LEDs, blue LDs and other optoelectronic applications. However, there are still many unsolved challenges with these materials. In this dissertation, several issues concerning structural, electronic and optical properties of III-nitrides have been investigated using a combination of transmission electron microscopy (TEM), electron holography (EH) and cathodoluminescence (CL) techniques. First, a trend of indium chemical inhomogeneity has been found as the indium composition increases for the InGaN epitaxial layers grown by hydride vapor phase epitaxy. Second, different mechanisms contributing to the strain relaxation have been studied for non-polar InGaN epitaxial layers grown on zinc oxide (ZnO) substrate. Third, various structural morphologies of non-polar InGaN epitaxial layers grown on free-standing GaN substrate have been investigated. Fourth, the effect of the growth temperature on the performance of GaN lattice-matched InAlN electron blocking layers has been studied. Finally, the electronic and optical properties of GaN nanowires containing a AlN/GaN superlattice structure have been investigated showing relatively small internal electric field and superlattice- and defect-related emissions along the nanowires.
ContributorsSun, Kewei (Author) / Ponce, Fernando (Thesis advisor) / Smith, David (Committee member) / Treacy, Michael (Committee member) / Drucker, Jeffery (Committee member) / Schmidt, Kevin (Committee member) / Arizona State University (Publisher)
Created2011
Description
This dissertation presents research findings regarding the exploitation of localized surface plasmon (LSP) of epitaxial Ag islands as a means to enhance the photoluminescence (PL) of Germanium (Ge) quantum dots (QDs). The first step of this project was to investigate the growth of Ag islands on Si(100). Two distinct families of Ag islands have been observed. “Big islands” are clearly faceted and have basal dimensions in the few hundred nm to μm range with a variety of basal shapes. “Small islands” are not clearly faceted and have basal diameters in the 10s of nm range. Big islands form via a nucleation and growth mechanism, and small islands form via precipitation of Ag contained in a planar layer between the big islands that is thicker than the Stranski-Krastanov layer existing at room-temperature.
The pseudodielectric functions of epitaxial Ag islands on Si(100) substrates were investigated with spectroscopic ellipsometry. Comparing the experimental pseudodielectric functions obtained for Si with and without Ag islands clearly identifies a plasmon mode with its dipole moment perpendicular to the surface. This observation is confirmed using a simulation based on the thin island film (TIF) theory. Another mode parallel to the surface may be identified by comparing the experimental pseudodielectric functions with the simulated ones from TIF theory. Additional results suggest that the LSP energy of Ag islands can be tuned from the ultra-violet to the infrared range by an amorphous Si (α-Si) cap layer.
Heterostructures were grown that incorporated Ge QDs, an epitaxial Si cap layer and Ag islands grown atop the Si cap layer. Optimum growth conditions for distinct Ge dot ensembles and Si cap layers were obtained. The density of Ag islands grown on the Si cap layer depends on its thickness. Factors contributing to this effect may include the average strain and Ge concentration on the surface of the Si cap layer.
The effects of the Ag LSP on the PL of Ge coherent domes were investigated for both α-Si capped and bare Ag islands. For samples with low-doped substrates, the LSPs reduce the Ge dot-related PL when the Si cap layer is below some critical thickness and have no effect on the PL when the Si cap layer is above the critical thickness. For samples grown on highly-doped wafers, the LSP of bare Ag islands enhanced the PL of Ge QDs by ~ 40%.
The pseudodielectric functions of epitaxial Ag islands on Si(100) substrates were investigated with spectroscopic ellipsometry. Comparing the experimental pseudodielectric functions obtained for Si with and without Ag islands clearly identifies a plasmon mode with its dipole moment perpendicular to the surface. This observation is confirmed using a simulation based on the thin island film (TIF) theory. Another mode parallel to the surface may be identified by comparing the experimental pseudodielectric functions with the simulated ones from TIF theory. Additional results suggest that the LSP energy of Ag islands can be tuned from the ultra-violet to the infrared range by an amorphous Si (α-Si) cap layer.
Heterostructures were grown that incorporated Ge QDs, an epitaxial Si cap layer and Ag islands grown atop the Si cap layer. Optimum growth conditions for distinct Ge dot ensembles and Si cap layers were obtained. The density of Ag islands grown on the Si cap layer depends on its thickness. Factors contributing to this effect may include the average strain and Ge concentration on the surface of the Si cap layer.
The effects of the Ag LSP on the PL of Ge coherent domes were investigated for both α-Si capped and bare Ag islands. For samples with low-doped substrates, the LSPs reduce the Ge dot-related PL when the Si cap layer is below some critical thickness and have no effect on the PL when the Si cap layer is above the critical thickness. For samples grown on highly-doped wafers, the LSP of bare Ag islands enhanced the PL of Ge QDs by ~ 40%.
ContributorsKong, Dexin (Author) / Drucker, Jeffery (Thesis advisor) / Chen, Tingyong (Committee member) / Ros, Robert (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2015