Matching Items (11)
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- All Subjects: Monte Carlo
- Creators: Vasileska, Dragica
Description
This work is focused on modeling the reliability concerns in GaN HEMT technology. The two main reliability concerns in GaN HEMTs are electromechanical coupling and current collapse. A theoretical model was developed to model the piezoelectric polarization charge dependence on the applied gate voltage. As the sheet electron density in the channel increases, the influence of electromechanical coupling reduces as the electric field in the comprising layers reduces. A Monte Carlo device simulator that implements the theoretical model was developed to model the transport in GaN HEMTs. It is observed that with the coupled formulation, the drain current degradation in the device varies from 2%-18% depending on the gate voltage. Degradation reduces with the increase in the gate voltage due to the increase in the electron gas density in the channel. The output and transfer characteristics match very well with the experimental data. An electro-thermal device simulator was developed coupling the Monte Caro-Poisson solver with the energy balance solver for acoustic and optical phonons. An output current degradation of around 2-3 % at a drain voltage of 5V due to self-heating was observed. It was also observed that the electrostatics near the gate to drain region of the device changes due to the hot spot created in the device from self heating. This produces an electric field in the direction of accelerating the electrons from the channel to surface states. This will aid to the current collapse phenomenon in the device. Thus, the electric field in the gate to drain region is very critical for reliable performance of the device. Simulations emulating the charging of the surface states were also performed and matched well with experimental data. Methods to improve the reliability performance of the device were also investigated in this work. A shield electrode biased at source potential was used to reduce the electric field in the gate to drain extension region. The hot spot position was moved away from the critical gate to drain region towards the drain as the shield electrode length and dielectric thickness were being altered.
ContributorsPadmanabhan, Balaji (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen M (Committee member) / Alford, Terry L. (Committee member) / Venkatraman, Prasad (Committee member) / Arizona State University (Publisher)
Created2013
Description
The research described in this dissertation has involved the use of transmission electron microcopy (TEM) to characterize the structural properties of II-VI and III-V compound semiconductor heterostructures and superlattices. The microstructure of thick ZnTe epilayers (~2.4 µm) grown by molecular beam epitaxy (MBE) under virtually identical conditions on GaSb, InAs, InP and GaAs (100) substrates were compared using TEM. High-resolution electron micrographs revealed a highly coherent interface for the ZnTe/GaSb sample, and showed extensive areas with well-separated interfacial misfit dislocations for the ZnTe/InAs sample. Lomer edge dislocations and 60o dislocations were commonly observed at the interfaces of the ZnTe/InP and ZnTe/GaAs samples. The amount of residual strain at the interfaces was estimated to be 0.01% for the ZnTe/InP sample and -0.09% for the ZnTe/GaAs sample. Strong PL spectra for all ZnTe samples were observed from 80 to 300 K. High quality GaSb grown by MBE on ZnTe/GaSb (001) virtual substrates with a temperature ramp at the beginning of the GaSb growth has been demonstrated. High-resolution X-ray diffraction (XRD) showed clear Pendellösung thickness fringes from both GaSb and ZnTe epilayers. Cross-section TEM images showed excellent crystallinity and smooth morphology for both ZnTe/GaSb and GaSb/ZnTe interfaces. Plan-view TEM image revealed the presence of Lomer dislocations at the interfaces and threading dislocations in the top GaSb layer. The defect density was estimated to be ~1 x107/cm2. The PL spectra showed improved optical properties when using the GaSb transition layer grown on ZnTe with a temperature ramp. The structural properties of strain-balanced InAs/InAs1-xSbx SLs grown on GaSb (001) substrates by metalorganic chemical vapor deposition (MOCVD) and MBE, have been studied using XRD and TEM. Excellent structural quality of the InAs/InAs1-xSbx SLs grown by MOCVD has been demonstrated. Well-defined ordered-alloy structures within individual InAs1-xSbx layers were observed for samples grown by modulated MBE. However, the ordering disappeared when defects propagating through the SL layers appeared during growth. For samples grown by conventional MBE, high-resolution images revealed that interfaces for InAs1-xSbx grown on InAs layers were sharper than for InAs grown on InAs1-xSbx layers, most likely due to a Sb surfactant segregation effect.
ContributorsOuyang, Lu (Author) / Smith, David J. (Thesis advisor) / McCartney, Martha (Committee member) / Ponce, Fernando (Committee member) / Chamberlin, Ralph (Committee member) / Menéndez, Jose (Committee member) / Arizona State University (Publisher)
Created2012
Description
In semiconductor physics, many properties or phenomena of materials can be brought to light through certain changes in the materials. Having a tool to define new material properties so as to highlight certain phenomena greatly increases the ability to understand that phenomena. The generalized Monte Carlo tool allows the user to do that by keeping every parameter used to define a material, within the non-parabolic band approximation, a variable in the control of the user. A material is defined by defining its valleys, energies, valley effective masses and their directions. The types of scattering to be included can also be chosen. The non-parabolic band structure model is used. With the deployment of the generalized Monte Carlo tool onto www.nanoHUB.org the tool will be available to users around the world. This makes it a very useful educational tool that can be incorporated into curriculums. The tool is integrated with Rappture, to allow user-friendly access of the tool. The user can freely define a material in an easy systematic way without having to worry about the coding involved. The output results are automatically graphed and since the code incorporates an analytic band structure model, it is relatively fast. The versatility of the tool has been investigated and has produced results closely matching the experimental values for some common materials. The tool has been uploaded onto www.nanoHUB.org by integrating it with the Rappture interface. By using Rappture as the user interface, one can easily make changes to the current parameter sets to obtain even more accurate results.
ContributorsHathwar, Raghuraj (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen M (Committee member) / Saraniti, Marco (Committee member) / Arizona State University (Publisher)
Created2011
Description
Infrared photodetectors, used in applications for sensing and imaging, such as military target recognition, chemical/gas detection, and night vision enhancement, are predominantly comprised of an expensive II-VI material, HgCdTe. III-V type-II superlattices (SLs) have been studied as viable alternatives for HgCdTe due to the SL advantages over HgCdTe: greater control of the alloy composition, resulting in more uniform materials and cutoff wavelengths across the wafer; stronger bonds and structural stability; less expensive substrates, i.e., GaSb; mature III-V growth and processing technologies; lower band-to-band tunneling due to larger electron effective masses; and reduced Auger recombination enabling operation at higher temperatures and longer wavelengths. However, the dark current of InAs/Ga1-xInxSb SL detectors is higher than that of HgCdTe detectors and limited by Shockley-Read-Hall (SRH) recombination rather than Auger recombination. This dissertation work focuses on InAs/InAs1-xSbx SLs, another promising alternative for infrared laser and detector applications due to possible lower SRH recombination and the absence of gallium, which simplifies the SL interfaces and growth processes. InAs/InAs1-xSbx SLs strain-balanced to GaSb substrates were designed for the mid- and long-wavelength infrared (MWIR and LWIR) spectral ranges and were grown using MOCVD and MBE by various groups. Detailed characterization using high-resolution x-ray diffraction, atomic force microscopy, photoluminescence (PL), and photoconductance revealed the excellent structural and optical properties of the MBE materials. Two key material parameters were studied in detail: the valence band offset (VBO) and minority carrier lifetime. The VBO between InAs and InAs1-xSbx strained on GaSb with x = 0.28 - 0.41 was best described by Qv = ÄEv/ÄEg = 1.75 ± 0.03. Time-resolved PL experiments on a LWIR SL revealed a lifetime of 412 ns at 77 K, one order of magnitude greater than that of InAs/Ga1-xInxSb LWIR SLs due to less SRH recombination. MWIR SLs also had 100's of ns lifetimes that were dominated by radiative recombination due to shorter periods and larger wave function overlaps. These results allow InAs/InAs1-xSbx SLs to be designed for LWIR photodetectors with minority carrier lifetimes approaching those of HgCdTe, lower dark currents, and higher operating temperatures.
ContributorsSteenbergen, Elizabeth H (Author) / Zhang, Yong-Hang (Thesis advisor) / Brown, Gail J. (Committee member) / Vasileska, Dragica (Committee member) / Johnson, Shane R. (Committee member) / Arizona State University (Publisher)
Created2012
Description
Thermal effects in nano-scaled devices were reviewed and modeling methodologies to deal with this issue were discussed. The phonon energy balance equations model, being one of the important previous works regarding the modeling of heating effects in nano-scale devices, was derived. Then, detailed description was given on the Monte Carlo (MC) solution of the phonon Boltzmann Transport Equation. The phonon MC solver was developed next as part of this thesis. Simulation results of the thermal conductivity in bulk Si show good agreement with theoretical/experimental values from literature.
ContributorsYoo, Seung Kyung (Author) / Vasileska, Dragica (Thesis advisor) / Ferry, David (Committee member) / Goodnick, Stephen (Committee member) / Arizona State University (Publisher)
Created2015
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
High-performance III-V semiconductors based on ternary alloys and superlattice systems are fabricated, studied, and compared for infrared optoelectronic applications. InAsBi is a ternary alloy near the GaSb lattice constant that is not as thoroughly investigated as other III-V alloys and that is challenging to produce as Bi has a tendency to surface segregate and form droplets during growth rather than incorporate. A growth window is identified within which high-quality droplet-free bulk InAsBi is produced and Bi mole fractions up to 6.4% are obtained. Photoluminescence with high internal quantum efficiency is observed from InAs/InAsBi quantum wells. The high structural and optical quality of the InAsBi materials examined demonstrates that bulk, quantum well, and superlattice structures utilizing InAsBi are an important design option for efficient infrared coverage.
Another important infrared material system is InAsSb and the strain-balanced InAs/InAsSb superlattice on GaSb. Detailed examination of X-ray diffraction, photoluminescence, and spectroscopic ellipsometry data provides the temperature and composition dependent bandgap of bulk InAsSb. The unintentional incorporation of approximately 1% Sb into the InAs layers of the superlattice is measured and found to significantly impact the analysis of the InAs/InAsSb band alignment. In the analysis of the absorption spectra, the ground state absorption coefficient and transition strength of the superlattice are proportional to the square of the electron-hole wavefunction overlap; wavefunction overlap is therefore a major design parameter in terms of optimizing absorption in these materials. Furthermore in addition to improvements through design optimization, the optical quality of the materials studied is found to be positively enhanced with the use of Bi as a surfactant during molecular beam epitaxy growth.
A software tool is developed that calculates and optimizes the miniband structure of semiconductor superlattices, including bismide-based designs. The software has the capability to limit results to designs that can be produced with high structural and optical quality, and optimized designs in terms of maximizing absorption are identified for several infrared superlattice systems at the GaSb lattice constant. The accuracy of the software predictions are tested with the design and growth of an optimized mid-wave infrared InAs/InAsSb superlattice which exhibits superior optical and absorption properties.
Another important infrared material system is InAsSb and the strain-balanced InAs/InAsSb superlattice on GaSb. Detailed examination of X-ray diffraction, photoluminescence, and spectroscopic ellipsometry data provides the temperature and composition dependent bandgap of bulk InAsSb. The unintentional incorporation of approximately 1% Sb into the InAs layers of the superlattice is measured and found to significantly impact the analysis of the InAs/InAsSb band alignment. In the analysis of the absorption spectra, the ground state absorption coefficient and transition strength of the superlattice are proportional to the square of the electron-hole wavefunction overlap; wavefunction overlap is therefore a major design parameter in terms of optimizing absorption in these materials. Furthermore in addition to improvements through design optimization, the optical quality of the materials studied is found to be positively enhanced with the use of Bi as a surfactant during molecular beam epitaxy growth.
A software tool is developed that calculates and optimizes the miniband structure of semiconductor superlattices, including bismide-based designs. The software has the capability to limit results to designs that can be produced with high structural and optical quality, and optimized designs in terms of maximizing absorption are identified for several infrared superlattice systems at the GaSb lattice constant. The accuracy of the software predictions are tested with the design and growth of an optimized mid-wave infrared InAs/InAsSb superlattice which exhibits superior optical and absorption properties.
ContributorsWebster, Preston Thomas (Author) / Johnson, Shane R (Thesis advisor) / Zhang, Yong-Hang (Committee member) / Menéndez, Jose (Committee member) / Vasileska, Dragica (Committee member) / Arizona State University (Publisher)
Created2016
Description
Self-heating degrades the performance of devices in advanced technology nodes. Understanding of self-heating effects is necessary to improve device performance. Heat generation in these devices occurs at nanometer scales but heat transfer is a microscopic phenomena. Hence a multi-scale modeling approach is required to study the self-heating effects. A state of the art Monte Carlo device simulator and the commercially available Giga 3D tool from Silvaco are used in our study to understand the self heating effects. The Monte Carlo device simulator solves the electrical transport and heat generation for nanometer length scales accurately while the Giga 3D tool solves for thermal transport over micrometer length scales. The approach used is to understand the self-heating effects in a test device structure, composed of a heater and a sensor, fabricated and characterized by IMEC. The heater is the Device Under Test(DUT) and the sensor is used as a probe. Therefore, the heater is biased in the saturation region and the sensor is biased in the sub-threshold regime. Both are planar MOSFETs of gate length equal to 22 nm. The simulated I-V characteristics of the sensor match with the experimental behavior at lower applied drain voltages but differ at higher applied biases.
The self-heating model assumes that the heat transport within the device follows Energy Balance model which may not be accurate. To properly study heat transport within the device, a state of the art Monte Carlo device simulator is necessary. In this regard, the Phonon Monte Carlo(PMC) simulator is developed. Phonons are treated as quasi particles that carry heat energy. Like electrons, phonons obey a corresponding Boltzmann Transport Equation(BTE) which can be used to study their transport. The direct solution of the BTE for phonons is possible, but it is difficult to incorporate all scattering mechanisms. In the Monte Carlo based solution method, it is easier to incorporate different relevant scattering mechanisms. Although the Monte Carlo method is computationally intensive, it provides good insight into the physical nature of the transport problem. Hence Monte Carlo based techniques are used in the present work for studying phonon transport. Monte Carlo simulations require calculating the scattering rates for different scattering processes. In the present work, scattering rates for three phonon interactions are calculated from different approaches presented in the literature. Optical phonons are also included in the transport problem. Finally, the temperature dependence of thermal conductivity for silicon is calculated in the range from 100K to 900K and is compared to available experimental data.
The self-heating model assumes that the heat transport within the device follows Energy Balance model which may not be accurate. To properly study heat transport within the device, a state of the art Monte Carlo device simulator is necessary. In this regard, the Phonon Monte Carlo(PMC) simulator is developed. Phonons are treated as quasi particles that carry heat energy. Like electrons, phonons obey a corresponding Boltzmann Transport Equation(BTE) which can be used to study their transport. The direct solution of the BTE for phonons is possible, but it is difficult to incorporate all scattering mechanisms. In the Monte Carlo based solution method, it is easier to incorporate different relevant scattering mechanisms. Although the Monte Carlo method is computationally intensive, it provides good insight into the physical nature of the transport problem. Hence Monte Carlo based techniques are used in the present work for studying phonon transport. Monte Carlo simulations require calculating the scattering rates for different scattering processes. In the present work, scattering rates for three phonon interactions are calculated from different approaches presented in the literature. Optical phonons are also included in the transport problem. Finally, the temperature dependence of thermal conductivity for silicon is calculated in the range from 100K to 900K and is compared to available experimental data.
ContributorsShaik, Abdul Rawoof (Author) / Vasileska, Dragica (Thesis advisor) / Ferry, David (Committee member) / Goodnick, Stephan (Committee member) / Arizona State University (Publisher)
Created2016
Description
In this work, transport in nanowire materials and nanowire field effect transistors is studied using a full band Monte Carlo simulator within the tight binding basis. Chapter 1 is dedicated to the importance of nanowires and nanoscale devices in present day electronics and the necessity to use a computationally efficient tool to simulate transport in these devices. Chapter 2 discusses the calculation of the full band structure of nanowires based on an atomistic tight binding approach, particularly noting the use of the exact same tight binding parameters for bulk band structures as well as the nanowire band structures. Chapter 3 contains the scattering rate formula for deformation potential, polar optical phonon, ionized impurity and impact ionization scattering in nanowires using Fermi’s golden rule and the tight binding basis to describe the wave functions. A method to calculate the dielectric screening in 1D systems within the tight binding basis is also described. Importantly, the scattering rates of nanowires tends to the bulk scattering rates at high energies, enabling the use of the same parameter set that were fitted to bulk experimental data to be used in the simulation of nanowire transport. A robust and efficient method to model interband tunneling is discussed in chapter 4 and its importance in nanowire transport is highlighted. In chapter 5, energy relaxation of excited electrons is studied for free standing nanowires and cladded nanowires. Finally, in chapter 6, a full band Monte Carlo particle based solver is created which treats confinement in a full quantum way and the current voltage characteristics as well as the subthreshold swing and percentage of ballistic transport is analyzed for an In0.7Ga0.3As junctionless nanowire field effect transistor.
ContributorsHathwar, Raghuraj (Author) / Goodnick, Stephen M (Committee member) / Saraniti, Marco (Committee member) / Vasileska, Dragica (Committee member) / Ferry, David K. (Committee member) / Arizona State University (Publisher)
Created2016
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