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Description
The work contained in this dissertation is focused on the optical properties of direct band gap semiconductors which crystallize in a wurtzite structure: more specifically, the III-nitrides and ZnO. By using cathodoluminescence spectroscopy, many of their properties have been investigated, including band gaps, defect energy levels, carrier lifetimes, strain states, exciton binding energies, and effects of electron irradiation on luminescence. Part of this work is focused on p-type Mg-doped GaN and InGaN. These materials are extremely important for the fabrication of visible light emitting diodes and diode lasers and their complex nature is currently not entirely understood. The luminescence of Mg-doped GaN films has been correlated with electrical and structural measurements in order to understand the behavior of hydrogen in the material. Deeply-bound excitons emitting near 3.37 and 3.42 eV are observed in films with a significant hydrogen concentration during cathodoluminescence at liquid helium temperatures. These radiative transitions are unstable during electron irradiation. Our observations suggest a hydrogen-related nature, as opposed to a previous assignment of stacking fault luminescence. The intensity of the 3.37 eV transition can be correlated with the electrical activation of the Mg acceptors. Next, the acceptor energy level of Mg in InGaN is shown to decrease significantly with an increase in the indium composition. This also corresponds to a decrease in the resistivity of these films. In addition, the hole concentration in multiple quantum well light emitting diode structures is much more uniform in the active region when Mg-doped InGaN (instead of Mg-doped GaN) is used. These results will help improve the efficiency of light emitting diodes, especially in the green/yellow color range. Also, the improved hole transport may prove to be important for the development of photovoltaic devices. Cathodoluminescence studies have also been performed on nanoindented ZnO crystals. Bulk, single crystal ZnO was indented using a sub-micron spherical diamond tip on various surface orientations. The resistance to deformation (the "hardness") of each surface orientation was measured, with the c-plane being the most resistive. This is due to the orientation of the easy glide planes, the c-planes, being positioned perpendicularly to the applied load. The a-plane oriented crystal is the least resistive to deformation. Cathodoluminescence imaging allows for the correlation of the luminescence with the regions located near the indentation. Sub-nanometer shifts in the band edge emission have been assigned to residual strain the crystals. The a- and m-plane oriented crystals show two-fold symmetry with regions of compressive and tensile strain located parallel and perpendicular to the ±c-directions, respectively. The c-plane oriented crystal shows six-fold symmetry with regions of tensile strain extending along the six equivalent a-directions.
ContributorsJuday, Reid (Author) / Ponce, Fernando A. (Thesis advisor) / Drucker, Jeff (Committee member) / Mccartney, Martha R (Committee member) / Menéndez, Jose (Committee member) / Shumway, John (Committee member) / Arizona State University (Publisher)
Created2013
Description
he accurate simulation of many-body quantum systems is a challenge for computational physics. Quantum Monte Carlo methods are a class of algorithms that can be used to solve the many-body problem. I study many-body quantum systems with Path Integral Monte Carlo techniques in three related areas of semiconductor physics: (1) the role of correlation in exchange coupling of spins in double quantum dots, (2) the degree of correlation and hyperpolarizability in Stark shifts in InGaAs/GaAs dots, and (3) van der Waals interactions between 1-D metallic quantum wires at finite temperature. The two-site model is one of the simplest quantum problems, yet the quantitative mapping from a three-dimensional model of a quantum double dot to an effective two-site model has many subtleties requiring careful treatment of exchange and correlation. I calculate exchange coupling of a pair of spins in a double dot from the permutations in a bosonic path integral, using Monte Carlo method. I also map this problem to a Hubbard model and find that exchange and correlation renormalizes the model parameters, dramatically decreasing the effective on-site repulsion at larger separations. Next, I investigated the energy, dipole moment, polarizability and hyperpolarizability of excitonic system in InGaAs/GaAs quantum dots of different shapes and successfully give the photoluminescence spectra for different dots with electric fields in both the growth and transverse direction. I also showed that my method can deal with the higher-order hyperpolarizability, which is most relevant for fields directed in the lateral direction of large dots. Finally, I show how van der Waals interactions between two metallic quantum wires change with respect to the distance between them. Comparing the results from quantum Monte Carlo and the random phase approximation, I find similar power law dependance. My results for the calculation in quasi-1D and exact 1D wires include the effect of temperature, which has not previously been studied.
ContributorsZhang, Lei (Author) / Shumway, John (Thesis advisor) / Schmidt, Kevin (Committee member) / Bennet, Peter (Committee member) / Menéndez, Jose (Committee member) / Drucker, Jeff (Committee member) / Arizona State University (Publisher)
Created2011
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
Silicon nanowires were grown epitaxially on Si (100) and (111) surfaces using the Vapor-Liquid-Solid (VLS) mechanism under both thermal and plasma enhanced growth conditions. Nanowire morphology was investigated as a function of temperature, time, disilane partial pressure and substrate preparation. Silicon nanowires synthesized in low temperature plasma typically curved compared to the linear nanowires grown under simple thermal conditions. The nanowires tended bend more with increasing disilane partial gas pressure up to 25 x10-3 mTorr. The nanowire curvature measured geometrically is correlated with the shift of the main silicon peak obtained in Raman spectroscopy. A mechanistic hypothesis was proposed to explain the bending during plasma activated growth. Additional driving forces related to electrostatic and Van der Waals forces were also discussed. Deduced from a systematic variation of a three-step experimental protocol, the mechanism for bending was associated with asymmetric deposition rate along the outer and inner wall of nanowire. The conditions leading to nanowire branching were also examined using a two-step growth process. Branching morphologies were examined as a function of plasma powers between 1.5 W and 3.5 W. Post-annealing thermal and plasma-assisted treatments in hydrogen were compared to understand the influences in the absence of an external silicon source (otherwise supplied by disilane). Longer and thicker nanowires were associated with longer annealing times due to an Ostwald-like ripening effect. The roles of surface diffusion, gas diffusion, etching and deposition rates were examined.
ContributorsJoun, Hee-Joung (Author) / Petuskey, William T. (Thesis advisor) / Drucker, Jeff (Committee member) / Chizmeshya, Andrew (Committee member) / Arizona State University (Publisher)
Created2011
Description
Chemical Vapor Deposition (CVD) is the most widely used method to grow large-scale single layer graphene. However, a systematic experimental study of the relationship between growth parameters and graphene film morphology, especially in the industrially preferred cold wall CVD, has not been undertaken previously. This research endeavored to address this and provide comprehensive insight into the growth physics of graphene on supported solid and liquid Cu films using cold wall CVD.
A multi-chamber UHV system was customized and transformed into a cold wall CVD system to perform experiments. The versatile growth process was completely custom-automated by controlling the process parameters with LabVIEW. Graphene growth was explored on solid electrodeposited, recrystallized and thin sputter deposited Cu films as well as on liquid Cu supported on W/Mo refractory substrates under ambient pressure using Ar, H₂ and CH₄ mixtures.
The results indicate that graphene grown on Cu films using cold wall CVD follows a classical two-dimensional nucleation and growth mechanism. The nucleation density decreases and average size of graphene crystallites increases with increasing dilution of the CH₄/H₂ mixture by Ar, decrease in total flow rate and decrease in CH₄:H₂ ratio at a fixed substrate temperature and chamber pressure. Thus, the resulting morphological changes correspond with those that would be expected if the precursor deposition rate was varied at a fixed substrate temperature for physical deposition using thermal evaporation. The evolution of graphene crystallite boundary morphology with decreasing effective C deposition rate indicates the effect of edge diffusion of C atoms along the crystallite boundaries, in addition to H₂ etching, on graphene crystallite shape.
The roles of temperature gradient, chamber pressure and rapid thermal heating in C precursor-rich environment on graphene growth morphology on thin sputtered Cu films were explained. The growth mechanisms of graphene on substrates annealed under reducing and non-reducing environment were explained from the scaling functions of graphene island size distribution in the pre-coalescence regime. It is anticipated that applying the pre-coalescence size distribution method presented in this work to other 2D material systems may be useful for elucidating atomistic mechanisms of film growth that are otherwise difficult to obtain.
A multi-chamber UHV system was customized and transformed into a cold wall CVD system to perform experiments. The versatile growth process was completely custom-automated by controlling the process parameters with LabVIEW. Graphene growth was explored on solid electrodeposited, recrystallized and thin sputter deposited Cu films as well as on liquid Cu supported on W/Mo refractory substrates under ambient pressure using Ar, H₂ and CH₄ mixtures.
The results indicate that graphene grown on Cu films using cold wall CVD follows a classical two-dimensional nucleation and growth mechanism. The nucleation density decreases and average size of graphene crystallites increases with increasing dilution of the CH₄/H₂ mixture by Ar, decrease in total flow rate and decrease in CH₄:H₂ ratio at a fixed substrate temperature and chamber pressure. Thus, the resulting morphological changes correspond with those that would be expected if the precursor deposition rate was varied at a fixed substrate temperature for physical deposition using thermal evaporation. The evolution of graphene crystallite boundary morphology with decreasing effective C deposition rate indicates the effect of edge diffusion of C atoms along the crystallite boundaries, in addition to H₂ etching, on graphene crystallite shape.
The roles of temperature gradient, chamber pressure and rapid thermal heating in C precursor-rich environment on graphene growth morphology on thin sputtered Cu films were explained. The growth mechanisms of graphene on substrates annealed under reducing and non-reducing environment were explained from the scaling functions of graphene island size distribution in the pre-coalescence regime. It is anticipated that applying the pre-coalescence size distribution method presented in this work to other 2D material systems may be useful for elucidating atomistic mechanisms of film growth that are otherwise difficult to obtain.
ContributorsDas, Shantanu, Ph.D (Author) / Drucker, Jeff (Thesis advisor) / Alford, Terry (Committee member) / Chen, Tingyong (Committee member) / Arizona State University (Publisher)
Created2018
Description
In this experiment, an attempt was made to measure the index of refraction of a thin glass microscope slide, with a known thickness of 1.01 mm. A monochromatic laser with wavelength of 532nm was employed to generate the interference pattern through the use of a Michelson interferometer. The slide was placed in the path of one of the beams. The slide could then be rotated through a series of angles, and, from the resulting changes in the interference pattern, the index of refraction of the slide could be extracted. The index of refraction was found to be 1.5±0.02.
ContributorsSwenson, Jordan (Author) / Sukharev, Maxim (Thesis director) / Bennett, Peter (Committee member) / Barrett, The Honors College (Contributor) / Department of Physics (Contributor)
Created2014-05
Description
Within the context of the Finite-Difference Time-Domain (FDTD) method of simulating interactions between electromagnetic waves and matter, we adapt a known absorbing boundary condition, the Convolutional Perfectly-Matched Layer (CPML) to a background of Drude-dispersive medium. The purpose of this CPML is to terminate the virtual grid of scattering simulations by absorbing all outgoing radiation. In this thesis, we exposit the method of simulation, establish the Perfectly-Matched Layer as a domain which houses a spatial-coordinate transform to the complex plane, construct the CPML in vacuum, adapt the CPML to the Drude medium, and conclude with tests of the adapted CPML for two different scattering geometries.
ContributorsThornton, Brandon Maverick (Author) / Sukharev, Maxim (Thesis director) / Goodnick, Stephen (Committee member) / School of Mathematical and Statistical Sciences (Contributor) / Department of Physics (Contributor) / Barrett, The Honors College (Contributor)
Created2018-05
Description
The importance of lasing cannot be overstated – from improving medicine through surgery uses and industry through laser cutting and micro-wielding (just to name a few), to the development of laser cooling to isolate the first Bose-Einstein condensate in 1995. Not only do the technological benefits encourage research but, as could probably be deduced, lasers are expensive devices. From Ruby crystals to Rubidium gasses, the materials required to construct lasers can be rare and highly specialized. Since the advancement of computer technology, computational physics has proved exceedingly useful. As a combination of both theoretical and experimental physcis, computational physics proves itself invaluable for allowing the testing of various theories and running of experiments in a time efficient and far less expensive way. For the purpose of this paper, having a clear understanding of the computational lasing system allows for simulations that are incredibly expensive or might not even be possible yet, to be conducted and the groundwork to be laid for future theory, experiment, or product.
The response of a molecular sheet with varying densities of simple, two-level system without lasing due to an ultra-short, wideband pulse centered at 2 eV is first investigated. The Fabry-Pérot modes rising from interference are observed, as well as the expected redshift in the transmission and reflection frequencies in the thin molecular sheet regime. Cautions regarding numerical instability and implementation of the Fast Fourier Transform are discussed. Upon activating the lasing levels of the molecules (creating a four-level system), the transmission and refection responses are measured for four combinations of molecular density and molecular sheet thickness. Lasing threshold and saturation phenomenon are observed and a clear lasing region is seen in the Power input/output analysis.
Population inversion is driving force that triggers lasing through stimulated emission. To investigate this, the populations of each of the four molecular energies levels are tracked for the same combinations of parameters in the previous tests. The population inversions and the threshold/saturation phenomena do not correspond to within reasonable limits. Inspection of the population data reveals a highly varied distribution within the molecular, suggesting that the system does not reach steady-state, and therefore and alternate method of analysis will need to be developed.
Having experimented with the simulations above, both the development of appropriate population analysis framework and the investigation of higher order dimensions (2-D and 3-D) will be pursed.
The response of a molecular sheet with varying densities of simple, two-level system without lasing due to an ultra-short, wideband pulse centered at 2 eV is first investigated. The Fabry-Pérot modes rising from interference are observed, as well as the expected redshift in the transmission and reflection frequencies in the thin molecular sheet regime. Cautions regarding numerical instability and implementation of the Fast Fourier Transform are discussed. Upon activating the lasing levels of the molecules (creating a four-level system), the transmission and refection responses are measured for four combinations of molecular density and molecular sheet thickness. Lasing threshold and saturation phenomenon are observed and a clear lasing region is seen in the Power input/output analysis.
Population inversion is driving force that triggers lasing through stimulated emission. To investigate this, the populations of each of the four molecular energies levels are tracked for the same combinations of parameters in the previous tests. The population inversions and the threshold/saturation phenomena do not correspond to within reasonable limits. Inspection of the population data reveals a highly varied distribution within the molecular, suggesting that the system does not reach steady-state, and therefore and alternate method of analysis will need to be developed.
Having experimented with the simulations above, both the development of appropriate population analysis framework and the investigation of higher order dimensions (2-D and 3-D) will be pursed.
ContributorsBrewer, Andre (Author) / Sukharev, Maxim (Thesis director) / Treacy, Michael (Committee member) / Barrett, The Honors College (Contributor)
Created2016-05
Description
The interaction of light with nanoscale structures consisting of metal and two-level quantum emitters is investigated computationally. A method of tilting the incoming electromagnetic wave is used to demonstrate coupling between a sinusoidal grating and two-level quantum emitters. A system consisting of metallic v-grooves and two-level emitters is thoroughly explored in the linear regime, where the spatially uniform fields provide a unique means of characterizing the coupling between the v-grooves and emitters. Furthermore, subwavelength spatial effects in the ground state population of emitters in the v-grooves are observed and analyzed in the non-linear regime. Finally, photon echoes are explored in the case of a one-dimensional ensemble of interacting two-level emitters as well as two-level emitters coupled to metallic slits, demonstrating the influence of collective effects on the echo amplitude in the former and the modifcation of the photon echo due to interaction with surface plasmons on the slits in the latter.
ContributorsBlake, Adam H (Author) / Sukharev, Maxim (Thesis advisor) / Treacy, Mike (Committee member) / Shovkovy, Igor (Committee member) / Drucker, Jeff (Committee member) / Arizona State University (Publisher)
Created2016
Description
The study of subwavelength behavior of light and nanoscale lasing has broad
potential applications in various forms of computation i.e. optical and quantum, as well as
in energy engineering. Although this field has been under active research, there has been
little work done on describing the behaviors of threshold and saturation. Particularly, how
the gain-molecule behavior affects the lasing behavior has yet to be investigated.
In this work, the interaction of surface-plasmon-polaritons (SPPs) and molecules is
observed in lasing. Various phenomenologies are observed related to the appearance of the
threshold and saturation regions. The lasing profile, as a visual delimiter of lasing threshold
and saturation, is introduced and used to study various parametrical dependencies of lasing,
including the number-density of molecules, the molecular thickness and the frequency
detuning between the molecular transition frequency and the SPP resonant frequency. The
molecular population distributions are studied in terminal and dynamical methods and are
found to contain unexpected and theoretically challenging properties. Using an average
dynamical analysis, the simulated spontaneous emission cascade can be clearly seen.
Finally, theoretical derivations of simple 1D strands of dipoles are presented in both
the exact and mean-field approximation, within the density matrix formalism. Some
preliminary findings are presented, detailing the observed behaviors of some simple
systems.
potential applications in various forms of computation i.e. optical and quantum, as well as
in energy engineering. Although this field has been under active research, there has been
little work done on describing the behaviors of threshold and saturation. Particularly, how
the gain-molecule behavior affects the lasing behavior has yet to be investigated.
In this work, the interaction of surface-plasmon-polaritons (SPPs) and molecules is
observed in lasing. Various phenomenologies are observed related to the appearance of the
threshold and saturation regions. The lasing profile, as a visual delimiter of lasing threshold
and saturation, is introduced and used to study various parametrical dependencies of lasing,
including the number-density of molecules, the molecular thickness and the frequency
detuning between the molecular transition frequency and the SPP resonant frequency. The
molecular population distributions are studied in terminal and dynamical methods and are
found to contain unexpected and theoretically challenging properties. Using an average
dynamical analysis, the simulated spontaneous emission cascade can be clearly seen.
Finally, theoretical derivations of simple 1D strands of dipoles are presented in both
the exact and mean-field approximation, within the density matrix formalism. Some
preliminary findings are presented, detailing the observed behaviors of some simple
systems.
ContributorsBrewer, Andre J (Author) / Sukharev, Maxim (Thesis advisor) / Rivera, Daniel E (Thesis advisor) / Menéndez, Jose (Committee member) / Arizona State University (Publisher)
Created2017