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ContributorsChang, Ruihong (Performer) / ASU Library. Music Library (Publisher)
Created2018-03-29
Description
There has been recent interest in demonstrating solar cells which approach the detailed-balance or thermodynamic efficiency limit in order to establish a model system for which mass-produced solar cells can be designed. Polycrystalline CdS/CdTe heterostructures are currently one of many competing solar cell material systems. Despite being polycrystalline, efficiencies up to 21 % have been demonstrated by the company First Solar. However, this efficiency is still far from the detailed-balance limit of 32.1 % for CdTe. This work explores the use of monocrystalline CdTe/MgCdTe and ZnTe/CdTe/MgCdTe double heterostructures (DHs) grown on (001) InSb substrates by molecular beam epitaxy (MBE) for photovoltaic applications.
Undoped CdTe/MgCdTe DHs are first grown in order to determine the material quality of the CdTe epilayer and to optimize the growth conditions. DH samples show strong photoluminescence with over double the intensity as that of a GaAs/AlGaAs DH with an identical layer structure. Time-resolved photoluminescence of the CdTe/MgCdTe DH gives a carrier lifetime of up to 179 ns for a 2 µm thick CdTe layer, which is more than one order of magnitude longer than that of polycrystalline CdTe films. MgCdTe barrier layers are found to be effective at confining photogenerated carriers and have a relatively low interface recombination velocity of 461 cm/s. The optimal growth temperature and Cd/Te flux ratio is determined to be 265 °C and 1.5, respectively.
Monocrystalline ZnTe/CdTe/MgCdTe P-n-N DH solar cells are designed, grown, processed into solar cell devices, and characterized. A maximum efficiency of 6.11 % is demonstrated for samples without an anti-reflection coating. The low efficiency is mainly due to the low open-circuit voltage (Voc), which is attributed to high dark current caused by interface recombination at the ZnTe/CdTe interface. Low-temperature measurements show a linear increase in Voc with decreasing temperature down to 77 K, which suggests that the room-temperature operation is limited by non-radiative recombination. An open-circuit voltage of 1.22 V and an efficiency of 8.46 % is demonstrated at 77 K. It is expected that a coherently strained MgCdTe/CdTe/MgCdTe DH solar cell design will produce higher efficiency and Voc compared to the ZnTe/CdTe/MgCdTe design with relaxed ZnTe layer.
Undoped CdTe/MgCdTe DHs are first grown in order to determine the material quality of the CdTe epilayer and to optimize the growth conditions. DH samples show strong photoluminescence with over double the intensity as that of a GaAs/AlGaAs DH with an identical layer structure. Time-resolved photoluminescence of the CdTe/MgCdTe DH gives a carrier lifetime of up to 179 ns for a 2 µm thick CdTe layer, which is more than one order of magnitude longer than that of polycrystalline CdTe films. MgCdTe barrier layers are found to be effective at confining photogenerated carriers and have a relatively low interface recombination velocity of 461 cm/s. The optimal growth temperature and Cd/Te flux ratio is determined to be 265 °C and 1.5, respectively.
Monocrystalline ZnTe/CdTe/MgCdTe P-n-N DH solar cells are designed, grown, processed into solar cell devices, and characterized. A maximum efficiency of 6.11 % is demonstrated for samples without an anti-reflection coating. The low efficiency is mainly due to the low open-circuit voltage (Voc), which is attributed to high dark current caused by interface recombination at the ZnTe/CdTe interface. Low-temperature measurements show a linear increase in Voc with decreasing temperature down to 77 K, which suggests that the room-temperature operation is limited by non-radiative recombination. An open-circuit voltage of 1.22 V and an efficiency of 8.46 % is demonstrated at 77 K. It is expected that a coherently strained MgCdTe/CdTe/MgCdTe DH solar cell design will produce higher efficiency and Voc compared to the ZnTe/CdTe/MgCdTe design with relaxed ZnTe layer.
ContributorsDiNezza, Michael John (Author) / Zhang, Yong-Hang (Thesis advisor) / Johnson, Shane (Committee member) / Tao, Meng (Committee member) / Holman, Zachary (Committee member) / Arizona State University (Publisher)
Created2014
Description
This thesis mainly focuses on the study of quantum efficiency (QE) and its measurement, especially for nanowires (NWs). First, a brief introduction of nano-technology and nanowire is given to describe my initial research interest. Next various fundamental kinds of recombination mechanisms are described; both for radiative and non-radiative processes. This is an introduction for defining the internal quantum efficiency (IQE). A relative IQE measurement method is shown following that. Then it comes to the major part of the thesis discussing a procedure of quantum efficiency measurement using photoluminescence (PL) method and an integrating sphere, which has not been much applied to nanowires (NWs). In fact this is a convenient and useful approach for evaluating the quality of NWs since it considers not only the PL emission but also the absorption of NWs. The process is well illustrated and performed with both wavelength-dependent and power-dependent measurements. The measured PLQE is in the range of 0.3% ~ 5.4%. During the measurement, a phenomenon called photodegradation is observed and examined by a set of power-dependence measurements. This effect can be a factor for underestimating the PLQE and a procedure is introduced during the sample preparation process which managed to reduce this effect for some degree.
ContributorsChen, Dongzi (Author) / Ning, Cun-Zheng (Thesis advisor) / Zhang, Yong-Hang (Committee member) / Yu, Hongbin (Committee member) / Arizona State University (Publisher)
Created2012
Description
Four Souvenirs for Violin and Piano was composed by Paul Schoenfeld (b.1947) in 1990 as a showpiece, spotlighting the virtuosity of both the violin and piano in equal measure. Each movement is a modern interpretation of a folk or popular genre, re- envisioned over intricate jazz harmonies and rhythms. The work was commissioned by violinist Lev Polyakin, who specifically requested some short pieces that could be performed in a local jazz establishment named Night Town in Cleveland, Ohio. The result is a work that is approximately fifteen minutes in length. Schoenfeld is a respected composer in the contemporary classical music community, whose Café Music (1986) for piano trio has recently become a staple of the standard chamber music repertoire. Many of his other works, however, remain in relative obscurity. It is the focus of this document to shed light on at least one other notable composition; Four Souvenirs for Violin and Piano. Among the topics to be discussed regarding this piece are a brief history behind the genesis of this composition, a structural summary of the entire work and each of its movements, and an appended practice guide based on interview and coaching sessions with the composer himself. With this project, I hope to provide a better understanding and appreciation of this work.
ContributorsJanczyk, Kristie Annette (Author) / Ryan, Russell (Thesis advisor) / Campbell, Andrew (Committee member) / Norton, Kay (Committee member) / Arizona State University (Publisher)
Created2015
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
Description
Group-IV semiconductor alloys are of interest for Si-integrated optoelectronic applications due to the band gap tunability and enhanced optical capabilities that can be achieved through compositional tuning. This work advances the field by presenting a systematic study of the optical and electronic properties of Ge1-ySny and analogous Ge1-x-ySixSny alloys.
The fundamental direct and indirect band gaps of Ge1-ySny materials are measured by room temperature photoluminescence in samples containing 0 ≤ y ≤ 0.11 and a transition to direct gap materials is found to occur at yc = 0.087. This result is enabled by the development of sample growth and processing protocols that produce high-quality materials epitaxially on Ge-buffered Si(100) substrates. Strategies to optimize the optical performance are explored by varying the film thickness, thermal and surface treatments, and n-type doping. The electrical and optical properties of diodes based on these materials are characterized by current-voltage, optical responsivity, and electroluminescence measurements. These show improved optical performance near yc with tunable emission out to 2500 nm. Measuring the carrier lifetimes in devices with strain relaxed and fully strained interfaces show significantly longer lifetimes in the fully strained case.
The direct and indirect band gaps of Sn-rich (y > x) Ge1-x-ySixSny materials are measured by room temperature photoluminescence on optimized samples. These data confirm a transition to direct gap materials occurs for the ternary alloy as well. Devices based on compositions 0.02 ≤ x ≤ 0.10 and 0.03 ≤ y ≤ 0.11 are characterized by current-voltage, optical responsivity, and electroluminescence measurements and show competitive performance with analogous devices based on Ge1-ySny materials. A detailed study of the direct gap in Ge1-xSix alloys gives parameters crucial en route to a global description of the Ge1-x-ySixSny fundamental band gaps.
Archetypal laser device designs on Si are explored by fabricating degenerate pn junction diodes and highly doped waveguide devices based on high-quality Ge1-ySny materials. The diodes showed tunnel-like current-voltage characteristics and tailored electroluminescence based on the doping profile. The waveguides demonstrate emission under optical stimulation.
The fundamental direct and indirect band gaps of Ge1-ySny materials are measured by room temperature photoluminescence in samples containing 0 ≤ y ≤ 0.11 and a transition to direct gap materials is found to occur at yc = 0.087. This result is enabled by the development of sample growth and processing protocols that produce high-quality materials epitaxially on Ge-buffered Si(100) substrates. Strategies to optimize the optical performance are explored by varying the film thickness, thermal and surface treatments, and n-type doping. The electrical and optical properties of diodes based on these materials are characterized by current-voltage, optical responsivity, and electroluminescence measurements. These show improved optical performance near yc with tunable emission out to 2500 nm. Measuring the carrier lifetimes in devices with strain relaxed and fully strained interfaces show significantly longer lifetimes in the fully strained case.
The direct and indirect band gaps of Sn-rich (y > x) Ge1-x-ySixSny materials are measured by room temperature photoluminescence on optimized samples. These data confirm a transition to direct gap materials occurs for the ternary alloy as well. Devices based on compositions 0.02 ≤ x ≤ 0.10 and 0.03 ≤ y ≤ 0.11 are characterized by current-voltage, optical responsivity, and electroluminescence measurements and show competitive performance with analogous devices based on Ge1-ySny materials. A detailed study of the direct gap in Ge1-xSix alloys gives parameters crucial en route to a global description of the Ge1-x-ySixSny fundamental band gaps.
Archetypal laser device designs on Si are explored by fabricating degenerate pn junction diodes and highly doped waveguide devices based on high-quality Ge1-ySny materials. The diodes showed tunnel-like current-voltage characteristics and tailored electroluminescence based on the doping profile. The waveguides demonstrate emission under optical stimulation.
ContributorsGallagher, James Dennis (Author) / Menéndez, Jose (Thesis advisor) / Kouvetakis, John (Thesis advisor) / Chizmeshya, Andrew (Committee member) / Ponce, Fernando (Committee member) / Arizona State University (Publisher)
Created2015
ContributorsWhite, Aaron (Performer) / Kim, Olga (Performer) / Hammond, Marinne (Performer) / Shaner, Hayden (Performer) / Yoo, Katie (Performer) / Shoemake, Crista (Performer) / Gebe, Vladimir, 1987- (Performer) / Wills, Grace (Performer) / McKinch, Riley (Performer) / Freshmen Four (Performer) / ASU Library. Music Library (Publisher)
Created2018-04-27
ContributorsRosenfeld, Albor (Performer) / Pagano, Caio, 1940- (Performer) / ASU Library. Music Library (Publisher)
Created2018-10-03