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- All Subjects: Condensed Matter Physics
- All Subjects: Nanowires
- Creators: Vasileska, Dragica
- Creators: Zhang, Yong-Hang
Description
This dissertation aims to demonstrate a new approach to fabricating solar cells for spectrum-splitting photovoltaic systems with the potential to reduce their cost and complexity of manufacturing, called Monolithically Integrated Laterally Arrayed Multiple Band gap (MILAMB) solar cells. Single crystal semiconductor alloy nanowire (NW) ensembles are grown with the alloy composition and band gap changing continuously across a broad range over the surface of a single substrate in a single, inexpensive growth step by the Dual-Gradient Method. The nanowire ensembles then serve as the absorbing materials in a set of solar cells for spectrum-splitting photovoltaic systems.
Preliminary design and simulation studies based on Anderson's model band line-ups were undertaken for CdPbS and InGaN alloys. Systems of six subcells obtained efficiencies in the 32-38% range for CdPbS and 34-40% for InGaN at 1-240 suns, though both materials systems require significant development before these results could be achieved experimentally. For an experimental demonstration, CdSSe was selected due to its availability. Proof-of-concept CdSSe nanowire ensemble solar cells with two subcells were fabricated simultaneously on one substrate. I-V characterization under 1 sun AM1.5G conditions yielded open-circuit voltages (Voc) up to 307 and 173 mV and short-circuit current densities (Jsc) up to 0.091 and 0.974 mA/cm2 for the CdS- and CdSe-rich cells, respectively. Similar thin film cells were also fabricated for comparison. The nanowire cells showed substantially higher Voc than the film cells, which was attributed to higher material quality in the CdSSe absorber. I-V measurements were also conducted with optical filters to simulate a simple form of spectrum-splitting. The CdS-rich cells showed uniformly higher Voc and fill factor (FF) than the CdSe-rich cells, as expected due to their larger band gaps. This suggested higher power density was produced by the CdS-rich cells on the single-nanowire level, which is the principal benefit of spectrum-splitting. These results constitute a proof-of-concept experimental demonstration of the MILAMB approach to fabricating multiple cells for spectrum-splitting photovoltaics. Future systems based on this approach could help to reduce the cost and complexity of manufacturing spectrum-splitting photovoltaic systems and offer a low cost alternative to multi-junction tandems for achieving high efficiencies.
Preliminary design and simulation studies based on Anderson's model band line-ups were undertaken for CdPbS and InGaN alloys. Systems of six subcells obtained efficiencies in the 32-38% range for CdPbS and 34-40% for InGaN at 1-240 suns, though both materials systems require significant development before these results could be achieved experimentally. For an experimental demonstration, CdSSe was selected due to its availability. Proof-of-concept CdSSe nanowire ensemble solar cells with two subcells were fabricated simultaneously on one substrate. I-V characterization under 1 sun AM1.5G conditions yielded open-circuit voltages (Voc) up to 307 and 173 mV and short-circuit current densities (Jsc) up to 0.091 and 0.974 mA/cm2 for the CdS- and CdSe-rich cells, respectively. Similar thin film cells were also fabricated for comparison. The nanowire cells showed substantially higher Voc than the film cells, which was attributed to higher material quality in the CdSSe absorber. I-V measurements were also conducted with optical filters to simulate a simple form of spectrum-splitting. The CdS-rich cells showed uniformly higher Voc and fill factor (FF) than the CdSe-rich cells, as expected due to their larger band gaps. This suggested higher power density was produced by the CdS-rich cells on the single-nanowire level, which is the principal benefit of spectrum-splitting. These results constitute a proof-of-concept experimental demonstration of the MILAMB approach to fabricating multiple cells for spectrum-splitting photovoltaics. Future systems based on this approach could help to reduce the cost and complexity of manufacturing spectrum-splitting photovoltaic systems and offer a low cost alternative to multi-junction tandems for achieving high efficiencies.
ContributorsCaselli, Derek (Author) / Ning, Cun-Zheng (Thesis advisor) / Tao, Meng (Committee member) / Yu, Hongbin (Committee member) / Vasileska, Dragica (Committee member) / Arizona State University (Publisher)
Created2014
Description
One of the challenges in future semiconductor device design is excessive rise of power dissipation and device temperatures. With the introduction of new geometrically confined device structures like SOI, FinFET, nanowires and continuous incorporation of new materials with poor thermal conductivities in the device active region, the device thermal problem is expected to become more challenging in coming years. This work examines the degradation in the ON-current due to self-heating effects in 10 nm channel length silicon nanowire transistors. As part of this dissertation, a 3D electrothermal device simulator is developed that self-consistently solves electron Boltzmann transport equation with 3D energy balance equations for both the acoustic and the optical phonons. This device simulator predicts temperature variations and other physical and electrical parameters across the device for different bias and boundary conditions. The simulation results show insignificant current degradation for nanowire self-heating because of pronounced velocity overshoot effect. In addition, this work explores the role of various placement of the source and drain contacts on the magnitude of self-heating effect in nanowire transistors. This work also investigates the simultaneous influence of self-heating and random charge effects on the magnitude of the ON current for both positively and negatively charged single charges. This research suggests that the self-heating effects affect the ON-current in two ways: (1) by lowering the barrier at the source end of the channel, thus allowing more carriers to go through, and (2) via the screening effect of the Coulomb potential. To examine the effect of temperature dependent thermal conductivity of thin silicon films in nanowire transistors, Selberherr's thermal conductivity model is used in the device simulator. The simulations results show larger current degradation because of self-heating due to decreased thermal conductivity . Crystallographic direction dependent thermal conductivity is also included in the device simulations. Larger degradation is observed in the current along the [100] direction when compared to the [110] direction which is in agreement with the values for the thermal conductivity tensor provided by Zlatan Aksamija.
ContributorsHossain, Arif (Author) / Vasileska, Dragica (Thesis advisor) / Ahmed, Shaikh (Committee member) / Bakkaloglu, Bertan (Committee member) / Goodnick, Stephen (Committee member) / Arizona State University (Publisher)
Created2011
Description
Nanowires (NWs) have attracted many interests due to their advance in synthesis and their unique structural, electrical and optical properties. NWs have been realized as promising candidates for future photonic platforms. In this work, erbium chloride silicate (ECS), CdS and CdSSe NWs growth by vapor-liquid-solid mechanism and their characterization were demonstrated. In the ECS NWs part, systematic experiments were performed to investigate the relation between growth temperature and NWs structure. Scanning electron microscopy, Raman spectroscopy, X-ray diffraction and photoluminescence characterization were used to study the NWs morphology, crystal quality and optical properties. At low growth temperature, there was strong Si Raman signal observed indicating ECS NWs have Si core. At high growth temperature, the excess Si signal was disappeared and the NWs showed better crystal quality and optical properties. The growth temperature is the key parameter that will induce the transition from Si/ECS core-shell NWs structure to solid ECS NWs. With the merits of high Er concentration and long PL lifetime, ECS NWs can serve as optical gain material with emission at 1.5 μm for communications and amplifiers. In the CdS, CdSSe NWs part, the band gap engineering of CdSSe NWs with spatial composition tuning along single NWs were demonstrated. The first step of realizing CdSSe NWs was the controlled growth of CdS NWs. It showed that overall pressure would largely affect the lengths of the CdS NWs. NWs with longer length can be obtained at higher pressure. Then, based on CdS NWs growth and by adding CdSe step by step, composition graded CdSSe alloy NWs were successfully synthesized. The temperature control over the source vapor concentration plays the key role for the growth.
ContributorsNing, Hao (Author) / Ning, Cunzheng (Thesis advisor) / Yu, Hongbin (Committee member) / Zhang, Yong-Hang (Committee member) / Arizona State University (Publisher)
Created2012
Description
This dissertation addresses challenges pertaining to multi-junction (MJ) solar cells from material development to device design and characterization. Firstly, among the various methods to improve the energy conversion efficiency of MJ solar cells using, a novel approach proposed recently is to use II-VI (MgZnCd)(SeTe) and III-V (AlGaIn)(AsSb) semiconductors lattice-matched on GaSb or InAs substrates for current-matched subcells with minimal defect densities. CdSe/CdTe superlattices are proposed as a potential candidate for a subcell in the MJ solar cell designs using this material system, and therefore the material properties of the superlattices are studied. The high structural qualities of the superlattices are obtained from high resolution X-ray diffraction measurements and cross-sectional transmission electron microscopy images. The effective bandgap energies of the superlattices obtained from the photoluminescence (PL) measurements vary with the layer thicknesses, and are smaller than the bandgap energies of either the constituent material. Furthermore, The PL peak position measured at the steady state exhibits a blue shift that increases with the excess carrier concentration. These results confirm a strong type-II band edge alignment between CdSe and CdTe. The valence band offset between unstrained CdSe and CdTe is determined as 0.63 eV±0.06 eV by fitting the measured PL peak positions using the Kronig-Penney model. The blue shift in PL peak position is found to be primarily caused by the band bending effect based on self-consistent solutions of the Schrödinger and Poisson equations. Secondly, the design of the contact grid layout is studied to maximize the power output and energy conversion efficiency for concentrator solar cells. Because the conventional minimum power loss method used for the contact design is not accurate in determining the series resistance loss, a method of using a distributed series resistance model to maximize the power output is proposed for the contact design. It is found that the junction recombination loss in addition to the series resistance loss and shadowing loss can significantly affect the contact layout. The optimal finger spacing and maximum efficiency calculated by the two methods are close, and the differences are dependent on the series resistance and saturation currents of solar cells. Lastly, the accurate measurements of external quantum efficiency (EQE) are important for the design and development of MJ solar cells. However, the electrical and optical couplings between the subcells have caused EQE measurement artifacts. In order to interpret the measurement artifacts, DC and small signal models are built for the bias condition and the scan of chopped monochromatic light in the EQE measurements. Characterization methods are developed for the device parameters used in the models. The EQE measurement artifacts are found to be caused by the shunt and luminescence coupling effects, and can be minimized using proper voltage and light biases. Novel measurement methods using a pulse voltage bias or a pulse light bias are invented to eliminate the EQE measurement artifacts. These measurement methods are nondestructive and easy to implement. The pulse voltage bias or pulse light bias is superimposed on the conventional DC voltage and light biases, in order to control the operating points of the subcells and counterbalance the effects of shunt and luminescence coupling. The methods are demonstrated for the first time to effectively eliminate the measurement artifacts.
ContributorsLi, Jingjing (Author) / Zhang, Yong-Hang (Thesis advisor) / Tao, Meng (Committee member) / Schroder, Dieter (Committee member) / Vasileska, Dragica (Committee member) / Arizona State University (Publisher)
Created2012
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
Semiconductor nanowires have the potential to emerge as the building blocks of next generation field-effect transistors, logic gates, solar cells and light emitting diodes. Use of Gallium Nitride (GaN) and other wide bandgap materials combines the advantages of III-nitrides along with the enhanced mobility offered by 2-dimensional confinement present in nanowires. The focus of this thesis is on developing a low field mobility model for a GaN nanowire using Ensemble Monte Carlo (EMC) techniques. A 2D Schrödinger-Poisson solver and a one-dimensional Monte Carlo solver is developed for an Aluminum Gallium Nitride/Gallium Nitride Heterostructure nanowire. A GaN/AlN/AlGaN heterostructure device is designed which creates 2-dimensional potential well for electrons. The nanowire is treated as a quasi-1D system in this work. A self-consistent 2D Schrödinger-Poisson solver is designed which determines the subband energies and the corresponding wavefunctions of the confined system. Three scattering mechanisms: acoustic phonon scattering, polar optical phonon scattering and piezoelectric scattering are considered to account for the electron phonon interactions in the system. Overlap integrals and 1D scattering rate expressions are derived for all the mechanisms listed. A generic one-dimensional Monte Carlo solver is also developed. Steady state results from the 1D Monte Carlo solver are extracted to determine the low field mobility of the GaN nanowires.
ContributorsKumar, Viswanathan Naveen (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen (Committee member) / Zhao, Yuji (Committee member) / Arizona State University (Publisher)
Created2017
Description
In recent years, there has been increased interest in the Indium Gallium Nitride (InGaN) material system for photovoltaic (PV) applications. The InGaN alloy system has demonstrated high performance for high frequency power devices, as well as for optical light emitters. This material system is also promising for photovoltaic applications due to broad range of bandgaps of InxGa1-xN alloys from 0.65 eV (InN) to 3.42 eV (GaN), which covers most of the electromagnetic spectrum from ultraviolet to infrared wavelengths. InGaN’s high absorption coefficient, radiation resistance and thermal stability (operating with temperature > 450 ℃) makes it a suitable PV candidate for hybrid concentrating solar thermal systems as well as other high temperature applications. This work proposed a high efficiency InGaN-based 2J tandem cell for high temperature (450 ℃) and concentration (200 X) hybrid concentrated solar thermal (CSP) application via numerical simulation. In order to address the polarization and band-offset issues for GaN/InGaN hetero-solar cells, band-engineering techniques are adopted and a simple interlayer is proposed at the hetero-interface rather than an Indium composition grading layer which is not practical in fabrication. The base absorber thickness and doping has been optimized for 1J cell performance and current matching has been achieved for 2J tandem cell design. The simulations also suggest that the issue of crystalline quality (i.e. short SRH lifetime) of the nitride material system to date is a crucial factor limiting the performance of the designed 2J cell at high temperature. Three pathways to achieve ~25% efficiency have been proposed under 450 ℃ and 200 X. An anti-reflection coating (ARC) for the InGaN solar cell optical management has been designed. Finally, effective mobility model for quantum well solar cells has been developed for efficient quasi-bulk simulation.
ContributorsFang, Yi, Ph.D (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen (Thesis advisor) / Ponce, Fernando (Committee member) / Nemanich, Robert (Committee member) / Arizona State University (Publisher)
Created2017
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
Wide bandgap (WBG) semiconductors GaN (3.4 eV), Ga2O3 (4.8 eV) and AlN (6.2 eV), have gained considerable interests for energy-efficient optoelectronic and electronic applications in solid-state lighting, photovoltaics, power conversion, and so on. They can offer unique device performance compared with traditional semiconductors such as Si. Efficient GaN based light-emitting diodes (LEDs) have increasingly displaced incandescent and fluorescent bulbs as the new major light sources for lighting and display. In addition, due to their large bandgap and high critical electrical field, WBG semiconductors are also ideal candidates for efficient power conversion.
In this dissertation, two types of devices are demonstrated: optoelectronic and electronic devices. Commercial polar c-plane LEDs suffer from reduced efficiency with increasing current densities, knowns as “efficiency droop”, while nonpolar/semipolar LEDs exhibit a very low efficiency droop. A modified ABC model with weak phase space filling effects is proposed to explain the low droop performance, providing insights for designing droop-free LEDs. The other emerging optoelectronics is nonpolar/semipolar III-nitride intersubband transition (ISBT) based photodetectors in terahertz and far infrared regime due to the large optical phonon energy and band offset, and the potential of room-temperature operation. ISBT properties are systematically studied for devices with different structures parameters.
In terms of electronic devices, vertical GaN p-n diodes and Schottky barrier diodes (SBDs) with high breakdown voltages are homoepitaxially grown on GaN bulk substrates with much reduced defect densities and improved device performance. The advantages of the vertical structure over the lateral structure are multifold: smaller chip area, larger current, less sensitivity to surface states, better scalability, and smaller current dispersion. Three methods are proposed to boost the device performances: thick buffer layer design, hydrogen-plasma based edge termination technique, and multiple drift layer design. In addition, newly emerged Ga2O3 and AlN power electronics may outperform GaN devices. Because of the highly anisotropic crystal structure of Ga2O3, anisotropic electrical properties have been observed in Ga2O3 electronics. The first 1-kV-class AlN SBDs are demonstrated on cost-effective sapphire substrates. Several future topics are also proposed including selective-area doping in GaN power devices, vertical AlN power devices, and (Al,Ga,In)2O3 materials and devices.
In this dissertation, two types of devices are demonstrated: optoelectronic and electronic devices. Commercial polar c-plane LEDs suffer from reduced efficiency with increasing current densities, knowns as “efficiency droop”, while nonpolar/semipolar LEDs exhibit a very low efficiency droop. A modified ABC model with weak phase space filling effects is proposed to explain the low droop performance, providing insights for designing droop-free LEDs. The other emerging optoelectronics is nonpolar/semipolar III-nitride intersubband transition (ISBT) based photodetectors in terahertz and far infrared regime due to the large optical phonon energy and band offset, and the potential of room-temperature operation. ISBT properties are systematically studied for devices with different structures parameters.
In terms of electronic devices, vertical GaN p-n diodes and Schottky barrier diodes (SBDs) with high breakdown voltages are homoepitaxially grown on GaN bulk substrates with much reduced defect densities and improved device performance. The advantages of the vertical structure over the lateral structure are multifold: smaller chip area, larger current, less sensitivity to surface states, better scalability, and smaller current dispersion. Three methods are proposed to boost the device performances: thick buffer layer design, hydrogen-plasma based edge termination technique, and multiple drift layer design. In addition, newly emerged Ga2O3 and AlN power electronics may outperform GaN devices. Because of the highly anisotropic crystal structure of Ga2O3, anisotropic electrical properties have been observed in Ga2O3 electronics. The first 1-kV-class AlN SBDs are demonstrated on cost-effective sapphire substrates. Several future topics are also proposed including selective-area doping in GaN power devices, vertical AlN power devices, and (Al,Ga,In)2O3 materials and devices.
ContributorsFu, Houqiang (Author) / Zhao, Yuji (Thesis advisor) / Vasileska, Dragica (Committee member) / Goodnick, Stephen (Committee member) / Yu, Hongbin (Committee member) / Wang, Liping (Committee member) / Arizona State University (Publisher)
Created2019
Description
Conductance fluctuations associated with quantum transport through quantumdot systems are currently understood to depend on the nature of the corresponding classical dynamics, i.e., integrable or chaotic. There are a couple of interesting phenomena about conductance fluctuation and quantum tunneling related to geometrical shapes of graphene systems. Firstly, in graphene quantum-dot systems, when a magnetic field is present, as the Fermi energy or the magnetic flux is varied, both regular oscillations and random fluctuations in the conductance can occur, with alternating transitions between the two. Secondly, a scheme based on geometrical rotation of rectangular devices to effectively modulate the conductance fluctuations is presented. Thirdly, when graphene is placed on a substrate of heavy metal, Rashba spin-orbit interaction of substantial strength can occur. In an open system such as a quantum dot, the interaction can induce spin polarization. Finally, a problem using graphene systems with electron-electron interactions described by the Hubbard Hamiltonian in the setting of resonant tunneling is investigated.
Another interesting problem in quantum transport is the effect of disorder or random impurities since it is inevitable in real experiments. At first, for a twodimensional Dirac ring, as the disorder density is systematically increased, the persistent current decreases slowly initially and then plateaus at a finite nonzero value, indicating remarkable robustness of the persistent currents, which cannot be discovered in normal metal and semiconductor rings. In addition, in a Floquet system with a ribbon structure, the conductance can be remarkably enhanced by onsite disorder.
Recent years have witnessed significant interest in nanoscale physical systems, such as semiconductor supperlattices and optomechanical systems, which can exhibit distinct collective dynamical behaviors. Firstly, a system of two optically coupled optomechanical cavities is considered and the phenomenon of synchronization transition associated with quantum entanglement transition is discovered. Another useful issue is nonlinear dynamics in semiconductor superlattices caused by its key potential application lies in generating radiation sources, amplifiers and detectors in the spectral range of terahertz. In such a system, transition to multistability, i.e., the emergence of multistability with chaos as a system parameter passes through a critical point, is found and argued to be abrupt.
Another interesting problem in quantum transport is the effect of disorder or random impurities since it is inevitable in real experiments. At first, for a twodimensional Dirac ring, as the disorder density is systematically increased, the persistent current decreases slowly initially and then plateaus at a finite nonzero value, indicating remarkable robustness of the persistent currents, which cannot be discovered in normal metal and semiconductor rings. In addition, in a Floquet system with a ribbon structure, the conductance can be remarkably enhanced by onsite disorder.
Recent years have witnessed significant interest in nanoscale physical systems, such as semiconductor supperlattices and optomechanical systems, which can exhibit distinct collective dynamical behaviors. Firstly, a system of two optically coupled optomechanical cavities is considered and the phenomenon of synchronization transition associated with quantum entanglement transition is discovered. Another useful issue is nonlinear dynamics in semiconductor superlattices caused by its key potential application lies in generating radiation sources, amplifiers and detectors in the spectral range of terahertz. In such a system, transition to multistability, i.e., the emergence of multistability with chaos as a system parameter passes through a critical point, is found and argued to be abrupt.
ContributorsYing, Lei (Author) / Lai, Ying-Cheng (Thesis advisor) / Vasileska, Dragica (Committee member) / Chen, Tingyong (Committee member) / Yao, Yu (Committee member) / Arizona State University (Publisher)
Created2016