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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
This dissertation describes the characterization of optoelectronic and electronic materials being considered for next generation semiconductor devices, primarily using electron microscopy techniques. The research included refinement of growth parameters for optimizing material quality, and investigation of heterostructured interfaces. The results provide better understanding of the fundamental materials science and should lead to future improvements in device applications.
A microstructural study of tin selenide and tin manganese selenide thin films grown by molecular beam epitaxy (MBE) on GaAs (111)B substrates with different Se:Sn flux ratios and Mn concentrations was carried out. Low flux ratios lead to highly defective films, mostly consisting of SnSe, whereas higher flux ratios gave higher quality, single-phase SnSe2. The ternary (Sn,Mn)Se films evolved quasi-coherently, as the Mn concentration increased, from SnSe2 into a complex lattice, and then into MnSe with 3D rock-salt structure. These structural transformations should underlie the evolution of magnetic properties of this ternary system reported earlier in the literature.
II-VI/III-V compound semiconductor heterostructures have been characterized for growth in both single- and dual-chamber MBE systems. Three groups of lattice-matched materials have been investigated: i) 5.65Å materials based on GaAs, ii) 6.1Å materials based on InAs or GaSb, and iii) 6.5Å materials based on InSb. High quality II-VI materials grown on III-V substrates were demonstrated for ZnTe/GaSb and CdTe/InSb. III-V materials grown on II-VI buffer layers present additional challenges and were grown with varying degrees of success. InAsSb quantum wells in between ZnTe barriers were nearly defect-free, but showed 3D island growth. All other materials demonstrated flat interfaces, despite low growth temperature, but with stacking faults in the II-VI materials.
Femtosecond laser-induced defects (LIDs) in silicon solar cells were characterized using a variety of electron microscopy techniques. Scanning electron microscope (SEM) images showed that the intersections of laser lines, finger and busbar intersections, exhibited LIDs with the potential to shunt the contacts. SEM and transmission electron microscope (TEM) images correlated these LIDs with ablated c-Si and showed these defects to come in two sizes ~40nm and ~.5µm. The elemental profiles across defective and non-defective regions were found using energy dispersive x-ray spectroscopy.
A microstructural study of tin selenide and tin manganese selenide thin films grown by molecular beam epitaxy (MBE) on GaAs (111)B substrates with different Se:Sn flux ratios and Mn concentrations was carried out. Low flux ratios lead to highly defective films, mostly consisting of SnSe, whereas higher flux ratios gave higher quality, single-phase SnSe2. The ternary (Sn,Mn)Se films evolved quasi-coherently, as the Mn concentration increased, from SnSe2 into a complex lattice, and then into MnSe with 3D rock-salt structure. These structural transformations should underlie the evolution of magnetic properties of this ternary system reported earlier in the literature.
II-VI/III-V compound semiconductor heterostructures have been characterized for growth in both single- and dual-chamber MBE systems. Three groups of lattice-matched materials have been investigated: i) 5.65Å materials based on GaAs, ii) 6.1Å materials based on InAs or GaSb, and iii) 6.5Å materials based on InSb. High quality II-VI materials grown on III-V substrates were demonstrated for ZnTe/GaSb and CdTe/InSb. III-V materials grown on II-VI buffer layers present additional challenges and were grown with varying degrees of success. InAsSb quantum wells in between ZnTe barriers were nearly defect-free, but showed 3D island growth. All other materials demonstrated flat interfaces, despite low growth temperature, but with stacking faults in the II-VI materials.
Femtosecond laser-induced defects (LIDs) in silicon solar cells were characterized using a variety of electron microscopy techniques. Scanning electron microscope (SEM) images showed that the intersections of laser lines, finger and busbar intersections, exhibited LIDs with the potential to shunt the contacts. SEM and transmission electron microscope (TEM) images correlated these LIDs with ablated c-Si and showed these defects to come in two sizes ~40nm and ~.5µm. The elemental profiles across defective and non-defective regions were found using energy dispersive x-ray spectroscopy.
ContributorsTracy, Brian David (Author) / Smith, David J. (Thesis advisor) / Bennett, Peter A (Committee member) / Drucker, Jeffery (Committee member) / Mccartney, Martha R (Committee member) / Zhang, Yong-Hang (Committee member) / Arizona State University (Publisher)
Created2018
Description
This dissertation covers my doctoral research on the cathodoluminescence (CL) study of the optical properties of III-niride semiconductors.
The first part of this thesis focuses on the optical properties of Mg-doped gallium nitride (GaN:Mg) epitaxial films. GaN is an emerging material for power electronics, especially for high power and high frequency applications. Compared to traditional Si-based devices, GaN-based devices offer superior breakdown properties, faster switching speed, and reduced system size. Some of the current device designs involve lateral p-n junctions which require selective-area doping. Dopant distribution in the selectively-doped regions is a critical issue that can impact the device performance. While most studies on Mg doping in GaN have been reported for epitaxial grown on flat c-plane substrates, questions arise regarding the Mg doping efficiency and uniformity in selectively-doped regions, where growth on surfaces etched away from the exact c-plane orientation is involved. Characterization of doping concentration distribution in lateral structures using secondary ion mass spectroscopy lacks the required spatial resolution. In this work, visualization of acceptor distribution in GaN:Mg epilayers grown by metalorganic chemical vapor deposition (MOCVD) was achieved at sub-micron scale using CL imaging. This was enabled by establishing a correlation among the luminescence characteristics, acceptor concentration, and electrical conductivity of GaN:Mg epilayers. Non-uniformity in acceptor distribution has been observed in epilayers grown on mesa structures and on miscut substrates. It is shown that non-basal-plane surfaces, such as mesa sidewalls and surface step clusters, promotes lateral growth along the GaN basal planes with a reduced Mg doping efficiency. The influence of surface morphology on the Mg doping efficiency in GaN has been studied.
The second part of this thesis focuses on the optical properties of InGaN for photovoltaic applications. The effects of thermal annealing and low energy electron beam irradiation (LEEBI) on the optical properties of MOCVD-grown In0.14Ga0.86N films were studied. A multi-fold increase in luminescence intensity was observed after 800 °C thermal annealing or LEEBI treatment. The mechanism leading to the luminescence intensity increase has been discussed. This study shows procedures that significantly improve the luminescence efficiency of InGaN, which is important for InGaN-based optoelectronic devices.
The first part of this thesis focuses on the optical properties of Mg-doped gallium nitride (GaN:Mg) epitaxial films. GaN is an emerging material for power electronics, especially for high power and high frequency applications. Compared to traditional Si-based devices, GaN-based devices offer superior breakdown properties, faster switching speed, and reduced system size. Some of the current device designs involve lateral p-n junctions which require selective-area doping. Dopant distribution in the selectively-doped regions is a critical issue that can impact the device performance. While most studies on Mg doping in GaN have been reported for epitaxial grown on flat c-plane substrates, questions arise regarding the Mg doping efficiency and uniformity in selectively-doped regions, where growth on surfaces etched away from the exact c-plane orientation is involved. Characterization of doping concentration distribution in lateral structures using secondary ion mass spectroscopy lacks the required spatial resolution. In this work, visualization of acceptor distribution in GaN:Mg epilayers grown by metalorganic chemical vapor deposition (MOCVD) was achieved at sub-micron scale using CL imaging. This was enabled by establishing a correlation among the luminescence characteristics, acceptor concentration, and electrical conductivity of GaN:Mg epilayers. Non-uniformity in acceptor distribution has been observed in epilayers grown on mesa structures and on miscut substrates. It is shown that non-basal-plane surfaces, such as mesa sidewalls and surface step clusters, promotes lateral growth along the GaN basal planes with a reduced Mg doping efficiency. The influence of surface morphology on the Mg doping efficiency in GaN has been studied.
The second part of this thesis focuses on the optical properties of InGaN for photovoltaic applications. The effects of thermal annealing and low energy electron beam irradiation (LEEBI) on the optical properties of MOCVD-grown In0.14Ga0.86N films were studied. A multi-fold increase in luminescence intensity was observed after 800 °C thermal annealing or LEEBI treatment. The mechanism leading to the luminescence intensity increase has been discussed. This study shows procedures that significantly improve the luminescence efficiency of InGaN, which is important for InGaN-based optoelectronic devices.
ContributorsLiu, Hanxiao (Author) / Ponce, Fernando A. (Thesis advisor) / Zhao, Yuji (Committee member) / Newman, Nathan (Committee member) / Fischer, Alec M (Committee member) / Arizona State University (Publisher)
Created2020