Matching Items (7)
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- All Subjects: Computational Physics
- Creators: Vasileska, Dragica
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
The goal of this research work is to develop an understanding as well as modelling thermal effects in Si based nano-scale devices using a multiscale simulator tool. This tool has been developed within the research group at Arizona State University led by Professor Dr. Dragica Vasileska. Another research group, headed by Professor Dr. Thornton, also at Arizona State University, provided support with software tools, by not only laying out the physical experimental device, but also provided experimental data to verify the correctness and accuracy of the developed simulation tool. The tool consists of three separate but conjoined modules at different scales of representation. 1) A particle based, ensemble Monte Carlo (MC) simulation tool, which, in the long-time (electronic motion) limit, solves the Boltzmann transport equation (BTE) for electrons, coupled with an iterative solution to a two-dimensional (2D) Poisson’s equation, at the base device level. 2) Another device level thermal modeling tool which solves the energy balance equations accounting for carrier-phonon and phonon-phonon interactions and is integrated with the MC tool. 3) Lastly, a commercial technology computer aided design (TCAD) software, Silvaco is employed to incorporate the results from the above two tools to a circuit level, common-source dual-transistor circuit, where one of the devices acts a heater and the other as a sensor, to study the impacts of thermal heating. The results from this tool are fed back to the previous device level tools to iterate on, until a stable, unified electro-thermal equilibrium/result is obtained. This coupled electro-thermal approach was originally developed for an individual n-channel MOSFET (NMOS) device by Prof. Katerina Raleva and was extended to allow for multiple devices in tandem, thereby providing a platform for better and more accurate modeling of device behavior, analyzing circuit performance, and understanding thermal effects. Simulating this dual device circuit and analyzing the extracted voltage transfer and output characteristics verifies the efficacy of this methodology as the results obtained from this multi-scale, electro-thermal simulator tool, are found to be in good general agreement with the experimental data.
ContributorsQazi, Suleman Sami (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen M (Committee member) / Thornton, Trevor J (Committee member) / Ferry, David K (Committee member) / Arizona State University (Publisher)
Created2021
Description
Gallium Nitride (GaN) is uniquely suited for Radio Frequency (RF) and power electronic applications due to its intrinsically high saturation velocity and high mobility compared to Silicon and Silicon Carbide (SiC). High Electron Mobility Transistors (HEMTs) have remained the primary topology for GaN transistors in RF applications. However, GaN HEMTs suffer from a variety of issues such as current crowding, lack of enhancement mode (E-Mode) operation and non-linearity. These drawbacks slow the widespread adoption of GaN devices for ultra-low voltage (ULV) applications such as voltage regulators, automotive and computing applications. E-mode operation is especially desired in low-voltage high frequency switching applications. In this context, Fin Field Effect Transistors (FinFETs) offer an alternative topology for ULV applications as opposed to conventional HEMTs. Recent advances in material processing, high aspect ratio epitaxial growth and etching methods has led to an increased interest in 3D nanostructures such as Nano-FinFETs and Nanowire FETs. A typical 3D nano-FinFET is the AlGaN/GaN Metal Insulator Semiconductor (MIS) FET wherein a layer of Al2O3 surrounds the AlGaN/GaN fin. The presence of the side gates leads to additional lateral confinement of the 2D Electron Gas (2DEG). Theoretical calculations of transport properties in confined systems such as AlGaN/GaN Finfets are scarce compared to those of their planar HEMT counterparts.
A novel simulator is presented in this dissertation, which employs self-consistent solution of the coupled 1D Boltzmann – 2D Schrödinger – 3D Poisson problem, to yield the channel electrostatics and the low electric field transport characteristics of AlGaN/GaN MIS FinFETs. The low field electron mobility is determined by solving the Boltzmann transport equation in the Quasi-1D region using 1D Ensemble Monte Carlo method. Three electron-phonon scattering mechanisms (acoustic, piezoelectric and polar optical phonon scattering) and interface roughness scattering at the AlGaN/GaN interface are considered in this theoretical model. Simulated low-field electron mobility and its temperature dependence are in agreement with experimental data reported in the literature.
A quasi-1D version of alloy clustering model is derived and implemented and the limiting effect of alloy clustering on the low-field electron mobility is investigated for the first time for MIS FinFET device structures.
ContributorsKumar, Viswanathan Naveen (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen (Committee member) / Nemanich, Robert (Committee member) / Povolotskyi, Michael (Committee member) / Esqueda, Ivan Sanchez (Committee member) / Arizona State University (Publisher)
Created2022
Description
Amorphous materials can be uniformly deposited over a large area at lower cost compared to crystalline semiconductors (Silicon or Germanium). This property along with its high resistivity and wide band-gap found many applications in devices like rectifiers, xerography, xero-radiography, ultrahigh sensitivity optical cameras, digital radiography, and mammography (2D and 3D tomosynthesis). Amorphous selenium is the only amorphous material that undergoes impact ionization where only holes avalanche at high electric fields. This leads to a small excess noise factor which is a very important performance comparison matrix for avalanche photodetectors. Thus, there is a need to model high field avalanche process in amorphous selenium. At high fields, the transport in amorphous selenium changes from low values of activated trap-limited drift mobility to higher values of band transport mobility, via extended states. When the transport shifts from activated mobility with a high degree of localization to extended state band transport, the wavefunction of the amorphous material resembles that of its crystalline counterpart. To that effect, crystalline monoclinic selenium which has the closest resemblance to vapor deposited amorphous selenium has been studied. Modelling a crystalline semiconductor makes calculations simpler. The transport phenomena in crystalline monoclinic selenium is studied by using a bulk Monte Carlo technique to solve the semi-classical Boltzman Transport equation and thus calculate vital electrical parameters like mobility, critical field and mobility variations against temperatures. The band structure and the density of states function for monoclinic selenium was obtained by using an atomistic simulation tool, the Atomistic Toolkit in the Virtual Nano Lab, Quantum Wise, Copenhagen, Denmark. Moreover, the velocity and energy against time characteristics have been simulated for a wide range of electric fields (1-1000 $\frac{kV}{cm}$), which is further used to find the hole drift mobility. The low field mobility is obtained from the slope of the velocity vs. electric field plot. The low field hole mobility was calculated to be 5.51 $\frac{cm^{2}}{Vs}$ at room temperature. The experimental value for low field hole mobility is 7.29 $\frac{cm^{2}}{Vs}$. The energy versus electric field simulation at high fields is used to match the experimental onset of avalanche (754 $\frac{kV}{cm}$) for an ionization threshold energy of 2.1 eV. The Arrhenius plot for mobility against temperature is simulated and compared with published experimental data. The experimental and simulation results show a close match, thus validating the study.
ContributorsMukherjee, Atreyo (Author) / Vasileska, Dragica (Thesis advisor) / Goldan, Amirhossein (Thesis advisor) / Goodnick, Stephen (Committee member) / Arizona State University (Publisher)
Created2017
Description
The quest to find efficient algorithms to numerically solve differential equations isubiquitous in all branches of computational science. A natural approach to address
this problem is to try all possible algorithms to solve the differential equation and
choose the one that is satisfactory to one's needs. However, the vast variety of algorithms
in place makes this an extremely time consuming task. Additionally, even
after choosing the algorithm to be used, the style of programming is not guaranteed
to result in the most efficient algorithm. This thesis attempts to address the same
problem but pertinent to the field of computational nanoelectronics, by using PETSc
linear solver and SLEPc eigenvalue solver packages to efficiently solve Schrödinger
and Poisson equations self-consistently.
In this work, quasi 1D nanowire fabricated in the GaN material system is considered
as a prototypical example. Special attention is placed on the proper description
of the heterostructure device, the polarization charges and accurate treatment of the
free surfaces. Simulation results are presented for the conduction band profiles, the
electron density and the energy eigenvalues/eigenvectors of the occupied sub-bands
for this quasi 1D nanowire. The simulation results suggest that the solver is very
efficient and can be successfully used for the analysis of any device with two dimensional
confinement. The tool is ported on www.nanoHUB.org and as such is freely
available.
this problem is to try all possible algorithms to solve the differential equation and
choose the one that is satisfactory to one's needs. However, the vast variety of algorithms
in place makes this an extremely time consuming task. Additionally, even
after choosing the algorithm to be used, the style of programming is not guaranteed
to result in the most efficient algorithm. This thesis attempts to address the same
problem but pertinent to the field of computational nanoelectronics, by using PETSc
linear solver and SLEPc eigenvalue solver packages to efficiently solve Schrödinger
and Poisson equations self-consistently.
In this work, quasi 1D nanowire fabricated in the GaN material system is considered
as a prototypical example. Special attention is placed on the proper description
of the heterostructure device, the polarization charges and accurate treatment of the
free surfaces. Simulation results are presented for the conduction band profiles, the
electron density and the energy eigenvalues/eigenvectors of the occupied sub-bands
for this quasi 1D nanowire. The simulation results suggest that the solver is very
efficient and can be successfully used for the analysis of any device with two dimensional
confinement. The tool is ported on www.nanoHUB.org and as such is freely
available.
ContributorsBaikadi, Pranay Kumar Reddy (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen (Committee member) / Povolotskyi, Mykhailo (Committee member) / Arizona State University (Publisher)
Created2020
Description
Silicon photonic technology continues to dominate the solar industry driven by steady improvement in device and module efficiencies. Currently, the world record conversion efficiency (~26.6%) for single junction silicon solar cell technologies is held by silicon heterojunction (SHJ) solar cells based on hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si). These solar cells utilize the concept of carrier selective contacts to improve device efficiencies. A carrier selective contact is designed to optimize the collection of majority carriers while blocking the collection of minority carriers. In the case of SHJ cells, a thin intrinsic a-Si:H layer provides crucial passivation between doped a-Si:H and the c-Si absorber that is required to create a high efficiency cell. There has been much debate regarding the role of the intrinsic a-Si:H passivation layer on the transport of photogenerated carriers, and its role in optimizing device performance. In this work, a multiscale model is presented which utilizes different simulation methodologies to study interfacial transport across the intrinsic a-Si:H/c-Si heterointerface and through the a-Si:H passivation layer. In particular, an ensemble Monte Carlo simulator was developed to study high field behavior of photogenerated carriers at the intrinsic a-Si:H/c-Si heterointerface, a kinetic Monte Carlo program was used to study transport of photogenerated carriers across the intrinsic a-Si:H passivation layer, and a drift-diffusion model was developed to model the behavior in the quasi-neutral regions of the solar cell. This work reports de-coupled and self-consistent simulations to fully understand the role and effect of transport across the a-Si:H passivation layer in silicon heterojunction solar cells, and relates this to overall solar cell device performance.
ContributorsMuralidharan, Pradyumna (Author) / Goodnick, Stephen M (Thesis advisor) / Vasileska, Dragica (Thesis advisor) / Honsberg, Christiana (Committee member) / Ringhofer, Christian (Committee member) / Arizona State University (Publisher)
Created2019
Description
In this dissertation, I investigate the electronic properties of two important silicon(Si)-based heterojunctions 1) hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) which has already been commercialized in Heterojunction with Intrinsic Thin-layer (HIT) cells and 2) gallium phosphide/silicon (GaP/Si) which has been suggested to be a good candidate for replacing a-Si:H/c-Si in HIT cells in order to boost the HIT cell’s efficiency.
In the first part, the defect states of amorphous silicon (a-Si) and a-Si:H material are studied using density functional theory (DFT). I first employ simulated annealing using molecular dynamics (MD) to create stable configurations of a-Si:H, and then analyze the atomic and electronic structure to investigate which structural defects interact with H, and how the electronic structure changes with H addition. I find that H atoms decrease the density of mid-gap states and increase the band gap of a-Si by binding to Si atoms with strained bonds. My results also indicate that Si atoms with strained bonds creates high-localized orbitals in the mobility gap of a-Si, and the binding of H atoms to them can dramatically decrease their degree of localization.
In the second part, I explore the effect of the H binding configuration on the electronic properties of a-Si:H/c-Si heterostructure using density functional theory studies of models of the interface between a-Si:H and c-Si. The electronic properties from DFT show that depending on the energy difference between configurations, the electronic properties are sensitive to the H binding configurations.
In the last part, I examine the electronic structure of GaP/Si(001) heterojunctions and the effect of hydrogen H passivation at the interface in comparison to interface mixing, through DFT calculations. My calculations show that due to the heterovalent mismatch nature of the GaP/Si interface, there is a high density of localized states at the abrupt GaP/Si interface due to the excess charge associated with heterovalent bonding, as reported elsewhere. I find that the addition of H leads to additional bonding at the interface which mitigates the charge imbalance, and greatly reduces the density of localized states, leading to a nearly ideal heterojunction.
In the first part, the defect states of amorphous silicon (a-Si) and a-Si:H material are studied using density functional theory (DFT). I first employ simulated annealing using molecular dynamics (MD) to create stable configurations of a-Si:H, and then analyze the atomic and electronic structure to investigate which structural defects interact with H, and how the electronic structure changes with H addition. I find that H atoms decrease the density of mid-gap states and increase the band gap of a-Si by binding to Si atoms with strained bonds. My results also indicate that Si atoms with strained bonds creates high-localized orbitals in the mobility gap of a-Si, and the binding of H atoms to them can dramatically decrease their degree of localization.
In the second part, I explore the effect of the H binding configuration on the electronic properties of a-Si:H/c-Si heterostructure using density functional theory studies of models of the interface between a-Si:H and c-Si. The electronic properties from DFT show that depending on the energy difference between configurations, the electronic properties are sensitive to the H binding configurations.
In the last part, I examine the electronic structure of GaP/Si(001) heterojunctions and the effect of hydrogen H passivation at the interface in comparison to interface mixing, through DFT calculations. My calculations show that due to the heterovalent mismatch nature of the GaP/Si interface, there is a high density of localized states at the abrupt GaP/Si interface due to the excess charge associated with heterovalent bonding, as reported elsewhere. I find that the addition of H leads to additional bonding at the interface which mitigates the charge imbalance, and greatly reduces the density of localized states, leading to a nearly ideal heterojunction.
ContributorsVatan Meidanshahi, Reza (Author) / Goodnick, Stephen Marshall (Thesis advisor) / Vasileska, Dragica (Committee member) / Bowden, Stuart (Committee member) / Honsberg, Christiana (Committee member) / Arizona State University (Publisher)
Created2019