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.
Displaying 1 - 3 of 3
Filtering by
- Creators: Smith, David
Description
In this dissertation, the surface interactions of fluorine were studied during atomic layer deposition (ALD) and atomic layer etching (ALE) of wide band gap materials. To enable this research two high vacuum reactors were designed and constructed for thermal and plasma enhanced ALD and ALE, and they were equipped for in-situ process monitoring. Fluorine surface interactions were first studied in a comparison of thermal and plasma enhanced ALD (TALD and PEALD) of AlF3 thin films prepared using hydrogen fluoride (HF), trimethylaluminum (TMA), and H2-plasma. The ALD AlF3 films were compared ¬in-situ using ellipsometry and X-ray photoelectron spectroscopy (XPS). Ellipsometry showed a growth rate of 1.1 Å/ cycle and 0.7 Å/ cycle, at 100°C, for the TALD and PEALD AlF3 processes, respectively. XPS indicated the presence of Al-rich clusters within the PEALD film. The formation of the Al-rich clusters is thought to originate during the H2-plasma step of the PEALD process. The Al-rich clusters were not detected in the TALD AlF3 films. This study provided valuable insight on the role of fluorine in an ALD process.
Reactive ion etching is a common dry chemical etch process for fabricating GaN devices. However, the use of ions can induce various defects, which can degrade device performance. The development of low-damage post etch processes are essential for mitigating plasma induced damage. As such, two multistep ALE methods were implemented for GaN based on oxidation, fluorination, and ligand exchange. First, GaN surfaces were oxidized using either water vapor or O2-plasma exposures to produce a thin oxide layer. The oxide layer was addressed using alternating exposures of HF and TMG, which etch Ga2O3 films. Each ALE process was characterized using in-situ using ellipsometry and XPS and ex-situ transmission electron microscopy (TEM). XPS indicated F and O impurities remained on the etched surfaces. Ellipsometry and TEM showed a slight reduction in thickness. The very low ALE rate was interpreted as the inability of the Ga2O3 ALE process to fluorinate the ordered surface oxide on GaN (0001).
Overall, these results indicate HF is effective for the ALD of metal fluorides and the ALE of metal oxides.
ContributorsMessina, Daniel C (Author) / Nemanich, Robert J (Thesis advisor) / Goodnick, Stephen (Committee member) / Ponce, Fernando A (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2021
Description
The development of high efficiency III-V solar cells is needed to meet the demands of a promising renewable energy source. Intermediate band solar cells (IBSCs) using semiconductor quantum dots (QDs) have been proposed to exceed the Shockley-Queisser efficiency limit [1]. The introduction of an IB in the forbidden gap of host material generates two additional carrier transitions for sub-bandgap photon absorption, leading to increased photocurrent of IBSCs while simultaneously allowing an open-circuit voltage of the highest band gap. To realize a high efficiency IBSC, QD structures should have high crystal quality and optimized electronic properties. This dissertation focuses on the investigation and optimization of the structural and optical properties of InAs/GaAsSb QDs and the development of InAs/GaAsSb QD-based IBSCs.
In the present dissertation, the interband optical transition and carrier lifetime of InAs/GaAsSb QDs with different silicon delta-doping densities have been first studied by time-integrated and time-resolved photoluminescence (PL). It is found that an optimized silicon delta-doping density in the QDs enables to fill the QD electronic states with electrons for sub-bandgap photon absorption and to improve carrier lifetime of the QDs.
After that, the crystal quality and QD morphology of single- and multi-stack InAs/GaAsSb QDs with different Sb compositions have been investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The TEM studies reveal that QD morphology of single-stack QDs is affected by Sb composition due to strain reducing effect of Sb incorporation. The XRD studies confirm that the increase of Sb composition increases the lattice mismatch between GaAs matrix and GaAsSb spacers, resulting in increase of the strain relaxation in GaAsSb of the multi-stack QDs. Furthermore, the increase of Sb composition causes a PL redshift and increases carrier lifetime of QDs.
Finally, the spacer layer thickness of multi-stack InAs/GaAsSb QDs is optimized for the growth of InAs/GaAsSb QD solar cells (QDSCs). The InAs/GaAsSb QDSCs with GaP strain compensating layer are grown and their device performances are characterized. The increase of GaP coverage is beneficial to improve the conversion efficiency of the QDSCs. However, the conversion efficiency is reduced when using a relatively large GaP coverage.
In the present dissertation, the interband optical transition and carrier lifetime of InAs/GaAsSb QDs with different silicon delta-doping densities have been first studied by time-integrated and time-resolved photoluminescence (PL). It is found that an optimized silicon delta-doping density in the QDs enables to fill the QD electronic states with electrons for sub-bandgap photon absorption and to improve carrier lifetime of the QDs.
After that, the crystal quality and QD morphology of single- and multi-stack InAs/GaAsSb QDs with different Sb compositions have been investigated by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The TEM studies reveal that QD morphology of single-stack QDs is affected by Sb composition due to strain reducing effect of Sb incorporation. The XRD studies confirm that the increase of Sb composition increases the lattice mismatch between GaAs matrix and GaAsSb spacers, resulting in increase of the strain relaxation in GaAsSb of the multi-stack QDs. Furthermore, the increase of Sb composition causes a PL redshift and increases carrier lifetime of QDs.
Finally, the spacer layer thickness of multi-stack InAs/GaAsSb QDs is optimized for the growth of InAs/GaAsSb QD solar cells (QDSCs). The InAs/GaAsSb QDSCs with GaP strain compensating layer are grown and their device performances are characterized. The increase of GaP coverage is beneficial to improve the conversion efficiency of the QDSCs. However, the conversion efficiency is reduced when using a relatively large GaP coverage.
ContributorsKim, Yeongho (Author) / Honsberg, Christiana (Thesis advisor) / Goodnick, Stephen (Committee member) / Faleev, Nikolai (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2015
Description
A model of self-heating is incorporated into a Cellular Monte Carlo (CMC) particle-based device simulator through the solution of an energy balance equation (EBE) for phonons. The EBE self-consistently couples charge and heat transport in the simulation through a novel approach to computing the heat generation rate in the device under study. First, the moments of the Boltzmann Transport equation (BTE) are discussed, and subsequently the EBE of for phonons is derived. Subsequently, several tests are performed to verify the applicability and accuracy of a nonlinear iterative method for the solution of the EBE in the presence of convective boundary conditions, as compared to a finite element analysis solver as well as using the Kirchhoff transformation. The coupled electrothermal characterization of a GaN/AlGaN high electron mobility transistor (HEMT) is then performed, and the effects of non-ideal interfaces and boundary conditions are studied.
The proposed thermal model is then applied to a novel $\Pi$-gate architecture which has been suggested to reduce hot electron generation in the device, compared to the conventional T-gate. Additionally, small signal ac simulations are performed for the determination of cutoff frequencies using the thermal model as well.
Finally, further extensions of the CMC algorithm used in this work are discussed, including 1) higher-order moments of the phonon BTE, 2) coupling to phonon Monte Carlo simulations, and 3) application to other large-bandgap, and therefore high-power, materials such as diamond.
The proposed thermal model is then applied to a novel $\Pi$-gate architecture which has been suggested to reduce hot electron generation in the device, compared to the conventional T-gate. Additionally, small signal ac simulations are performed for the determination of cutoff frequencies using the thermal model as well.
Finally, further extensions of the CMC algorithm used in this work are discussed, including 1) higher-order moments of the phonon BTE, 2) coupling to phonon Monte Carlo simulations, and 3) application to other large-bandgap, and therefore high-power, materials such as diamond.
ContributorsMerrill, Ky (Author) / Saraniti, Marco (Thesis advisor) / Goodnick, Stephen (Committee member) / Smith, David (Committee member) / Wang, Robert (Committee member) / Arizona State University (Publisher)
Created2020