Matching Items (14)
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- All Subjects: Semiconductors
- Creators: Vasileska, Dragica
- Creators: Smith, David
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
ABSTRACT Group III-nitride semiconductor materials have been commercially used in fabrication of light-emitting diodes (LEDs) and laser diodes (LDs) covering the spectral range from UV to visible and infrared, and exhibit unique properties suitable for modern optoelectronic applications. Great advances have recently happened in the research and development in high-power and high-efficiency blue-green-white LEDs, blue LDs and other optoelectronic applications. However, there are still many unsolved challenges with these materials. In this dissertation, several issues concerning structural, electronic and optical properties of III-nitrides have been investigated using a combination of transmission electron microscopy (TEM), electron holography (EH) and cathodoluminescence (CL) techniques. First, a trend of indium chemical inhomogeneity has been found as the indium composition increases for the InGaN epitaxial layers grown by hydride vapor phase epitaxy. Second, different mechanisms contributing to the strain relaxation have been studied for non-polar InGaN epitaxial layers grown on zinc oxide (ZnO) substrate. Third, various structural morphologies of non-polar InGaN epitaxial layers grown on free-standing GaN substrate have been investigated. Fourth, the effect of the growth temperature on the performance of GaN lattice-matched InAlN electron blocking layers has been studied. Finally, the electronic and optical properties of GaN nanowires containing a AlN/GaN superlattice structure have been investigated showing relatively small internal electric field and superlattice- and defect-related emissions along the nanowires.
ContributorsSun, Kewei (Author) / Ponce, Fernando (Thesis advisor) / Smith, David (Committee member) / Treacy, Michael (Committee member) / Drucker, Jeffery (Committee member) / Schmidt, Kevin (Committee member) / Arizona State University (Publisher)
Created2011
Description
Electromigration in metal interconnects is the most pernicious failure mechanism in semiconductor integrated circuits (ICs). Early electromigration investigations were primarily focused on aluminum interconnects for silicon-based ICs. An alternative metallization compatible with gallium arsenide (GaAs) was required in the development of high-powered radio frequency (RF) compound semiconductor devices operating at higher current densities and elevated temperatures. Gold-based metallization was implemented on GaAs devices because it uniquely forms a very low resistance ohmic contact and gold interconnects have superior electrical and thermal conductivity properties. Gold (Au) was also believed to have improved resistance to electromigration due to its higher melting temperature, yet electromigration reliability data on passivated Au interconnects is scarce and inadequate in the literature. Therefore, the objective of this research was to characterize the electromigration lifetimes of passivated Au interconnects under precisely controlled stress conditions with statistically relevant quantities to obtain accurate model parameters essential for extrapolation to normal operational conditions. This research objective was accomplished through measurement of electromigration lifetimes of large quantities of passivated electroplated Au interconnects utilizing high-resolution in-situ resistance monitoring equipment. Application of moderate accelerated stress conditions with a current density limited to 2 MA/cm2 and oven temperatures in the range of 300°C to 375°C avoided electrical overstress and severe Joule-heated temperature gradients. Temperature coefficients of resistance (TCRs) were measured to determine accurate Joule-heated Au interconnect film temperatures. A failure criterion of 50% resistance degradation was selected to prevent thermal runaway and catastrophic metal ruptures that are problematic of open circuit failure tests. Test structure design was optimized to reduce resistance variation and facilitate failure analysis. Characterization of the Au microstructure yielded a median grain size of 0.91 ìm. All Au lifetime distributions followed log-normal distributions and Black's model was found to be applicable. An activation energy of 0.80 ± 0.05 eV was measured from constant current electromigration tests at multiple temperatures. A current density exponent of 1.91 was extracted from multiple current densities at a constant temperature. Electromigration-induced void morphology along with these model parameters indicated grain boundary diffusion is dominant and the void nucleation mechanism controlled the failure time.
ContributorsKilgore, Stephen (Author) / Adams, James (Thesis advisor) / Schroder, Dieter (Thesis advisor) / Krause, Stephen (Committee member) / Gaw, Craig (Committee member) / Arizona State University (Publisher)
Created2013
Description
The energy band gap of a semiconductor material critically influences the operating wavelength of an optoelectronic device. Realization of any desired band gap, or even spatially graded band gaps, is important for applications such as lasers, light-emitting diodes (LEDs), solar cells, and detectors. Compared to thin films, nanowires offer greater flexibility for achieving a variety of alloy compositions. Furthermore, the nanowire geometry permits simultaneous incorporation of a wide range of compositions on a single substrate. Such controllable alloy composition variation can be realized either within an individual nanowire or between distinct nanowires across a substrate. This dissertation explores the control of spatial composition variation in ternary alloy nanowires. Nanowires were grown by the vapor-liquid-solid (VLS) mechanism using chemical vapor deposition (CVD). The gas-phase supersaturation was considered in order to optimize the deposition morphology. Composition and structure were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD). Optical properties were investigated through photoluminescence (PL) measurements. The chalcogenides selected as alloy endpoints were lead sulfide (PbS), cadmium sulfide (CdS), and cadmium selenide (CdSe). Three growth modes of PbS were identified, which included contributions from spontaneously generated catalyst. The resulting wires were found capable of lasing with wavelengths over 4000 nm, representing the longest known wavelength from a sub-wavelength wire. For CdxPb1-xS nanowires, it was established that the cooling process significantly affects the alloy composition and structure. Quenching was critical to retain metastable alloys with x up to 0.14, representing a new composition in nanowire form. Alternatively, gradual cooling caused phase segregation, which created heterostructures with light emission in both the visible and mid-infrared regimes. The CdSSe alloy system was fully explored for spatial composition variation. CdSxSe1-x nanowires were grown with composition variation across the substrate. Subsequent contact printing preserved the designed composition gradient and led to the demonstration of a variable wavelength photodetector device. CdSSe axial heterostructure nanowires were also achieved. The growth process involved many variables, including a deliberate and controllable change in substrate temperature. As a result, both red and green light emission was detected from single nanowires.
ContributorsNichols, Patricia (Author) / Ning, Cun-Zheng (Thesis advisor) / Carpenter, Ray (Committee member) / Bennett, Peter (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2012
Description
A dual chamber molecular beam epitaxy (MBE) system was rebuilt for the growth of 6.1 Angstrom II-VI and III-V compound semiconductor materials that are to be used in novel optoelectronic devices that take advantage of the nearly continuous bandgap availability between 0 eV and 3.4 eV. These devices include multijunction solar cells and multicolor detectors. The MBE system upgrade involved the conversion of a former III-V chamber for II-VI growth. This required intensive cleaning of the chamber and components to prevent contamination. Special features including valved II-VI sources and the addition of a cold trap allowed for the full system to be baked to 200 degrees Celsius to improve vacuum conditions and reduce background impurity concentrations in epilayers. After the conversion, the system was carefully calibrated and optimized for the growth of ZnSe and ZnTe on GaAs (001) substrates. Material quality was assessed using X-ray diffraction rocking curves. ZnSe layers displayed a trend of improving quality with decreasing growth temperature reaching a minimum full-width half-maximum (FWHM) of 113 arcsec at 278 degrees Celsius. ZnTe epilayer quality increased with growth temperature under Zn rich conditions attaining a FWHM of 84 arcsec at 440 degrees Celsius. RHEED oscillations were successfully observed and used to obtain growth rate in situ for varying flux and temperature levels. For a fixed flux ratio, growth rate decreased with growth temperature as the desorption rate increased. A directly proportional dependence of growth rate on Te flux was observed for Zn rich growth. Furthermore, a method for determining the flux ratio necessary for attaining the stoichiometric condition was demonstrated.
ContributorsDettlaff, W. Hank G (Author) / Zhang, Yong-Hang (Thesis advisor) / Vasileska, Dragica (Committee member) / Johnson, Shane (Committee member) / Arizona State University (Publisher)
Created2012
Description
Group III-nitride semiconductors have wide application in optoelectronic devices. Spontaneous and piezoelectric polarization effects have been found to be critical for electric and optical properties of group III-nitrides. In this dissertation, firstly, the crystal orientation dependence of the polarization is calculated and in-plane polarization is revealed. The in-plane polarization is sensitive to the lateral characteristic dimension determined by the microstructure. Specific semi-polar plane growth is suggested for reducing quantum-confined Stark effect. The macroscopic electrostatic field from the polarization discontinuity in the heterostructures is discussed, b ased on that, the band diagram of InGaN/GaN quantum well/barrier and AlGaN/GaN heterojunction is obtained from the self-consistent solution of Schrodinger and Poisson equations. New device design such as triangular quantum well with the quenched polarization field is proposed. Electron holography in the transmission electron microscopy is used to examine the electrostatic potential under polarization effects. The measured potential energy profiles of heterostructure are compared with the band simulation, and evidences of two-dimensional hole gas (2DHG) in a wurtzite AlGaN/ AlN/ GaN superlattice, as well as quasi two-dimensional electron gas (2DEG) in a zinc-blende AlGaN/GaN are found. The large polarization discontinuity of AlN/GaN is the main source of the 2DHG of wurtzite nitrides, while the impurity introduced during the growth of AlGaN layer provides the donor states that to a great extent balance the free electrons in zinc-blende nitrides. It is also found that the quasi-2DEG concentration in zinc-blende AlGaN/GaN is about one order of magnitude lower than the wurtzite AlGaN/GaN, due to the absence of polarization. Finally, the InAlN/GaN lattice-matched epitaxy, which ideally has a zero piezoelectric polarization and strong spontaneous polarization, is experimentally studied. The breakdown in compositional homogeneity is triggered by threading dislocations with a screw component propagating from the GaN underlayer, which tend to open up into V-grooves at a certain thickness of the InxAl1-xN layer. The V-grooves coalesce at 200 nm and are filled with material that exhibits a significant drop in indium content and a broad luminescence peak. The structural breakdown is due to heterogeneous nucleation and growth at the facets of the V-grooves.
ContributorsWei, Qiyuan (Author) / Ponce, Fernando A. (Thesis advisor) / Tsen, Kong-Thon (Committee member) / Shumway, John (Committee member) / Menéndez, Jose (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2012
Description
The semiconductor industry looks to constantly improve the efficiency of research and development in order to reduce costs and time to market. One such method was designed in order to decrease time spent inducing warpage in integrated circuits in an Intel research process. Intel's Atom product line seeks to compete with ARM architecture by entering the mobile devices CPU market. Due to the fundamental differences between the Atom's Bonnell architecture and the ARM architecture, the Intel Atom product line must utilize such improved research and development methods. Until power consumption is drastically lowered while maintaining processing speed, the Atom product line will not be able to effectively break into the mobile devices CPU market.
ContributorsLandseidel, Jack Adam (Author) / Adams, James (Thesis director) / Krause, Stephen (Committee member) / Anwar, Shahriar (Committee member) / Barrett, The Honors College (Contributor) / School of Mathematical and Statistical Sciences (Contributor) / Materials Science and Engineering Program (Contributor)
Created2013-05
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
From 2D planar MOSFET to 3D FinFET, the geometry of semiconductor devices is getting more and more complex. Correspondingly, the number of mesh grid points increases largely to maintain the accuracy of carrier transport and heat transfer simulations. By substituting the conventional uniform mesh with non-uniform mesh, one can reduce the number of grid points. However, the problem of how to solve governing equations on non-uniform mesh is then imposed to the numerical solver. Moreover, if a device simulator is integrated into a multi-scale simulator, the problem size will be further increased. Consequently, there exist two challenges for the current numerical solver. One is to increase the functionality to accommodate non-uniform mesh. The other is to solve governing physical equations fast and accurately on a large number of mesh grid points.
This research rst discusses a 2D planar MOSFET simulator and its numerical solver, pointing out its performance limit. By analyzing the algorithm complexity, Multigrid method is proposed to replace conventional Successive-Over-Relaxation method in a numerical solver. A variety of Multigrid methods (standard Multigrid, Algebraic Multigrid, Full Approximation Scheme, and Full Multigrid) are discussed and implemented. Their properties are examined through a set of numerical experiments. Finally, Algebraic Multigrid, Full Approximation Scheme and Full Multigrid are integrated into one advanced numerical solver based on the exact requirements of a semiconductor device simulator. A 2D MOSFET device is used to benchmark the performance, showing that the advanced Multigrid method has higher speed, accuracy and robustness.
This research rst discusses a 2D planar MOSFET simulator and its numerical solver, pointing out its performance limit. By analyzing the algorithm complexity, Multigrid method is proposed to replace conventional Successive-Over-Relaxation method in a numerical solver. A variety of Multigrid methods (standard Multigrid, Algebraic Multigrid, Full Approximation Scheme, and Full Multigrid) are discussed and implemented. Their properties are examined through a set of numerical experiments. Finally, Algebraic Multigrid, Full Approximation Scheme and Full Multigrid are integrated into one advanced numerical solver based on the exact requirements of a semiconductor device simulator. A 2D MOSFET device is used to benchmark the performance, showing that the advanced Multigrid method has higher speed, accuracy and robustness.
ContributorsGuo, Xinchen (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen (Committee member) / Ferry, David (Committee member) / Arizona State University (Publisher)
Created2015
Description
This dissertation presents research findings on the three materials systems: lateral Si nanowires (SiNW), In2Se3/Bi2Se3 heterostructures and graphene. The first part of the thesis was focused on the growth and characterization of lateral SiNW. Lateral here refers to wires growing along the plane of substrate; vertical NW on the other hand grow out of the plane of substrate. It was found, using the Au-seeded vapor – liquid – solid technique, that epitaxial single-crystal SiNW can be grown laterally along Si(111) substrates that have been miscut toward [11− 2]. The ratio of lateral-to-vertical NW was found to increase as the miscut angle increased and as disilane pressure and substrate temperature decreased. Based on this observation, growth parameters were identified whereby all of the deposited Au seeds formed lateral NW. Furthermore, the nanofaceted substrate guided the growth via a mechanism that involved pinning of the trijunction at the liquid/solid interface of the growing nanowire.
Next, the growth of selenide heterostructures was explored. Specifically, molecular beam epitaxy was utilized to grow In2Se3 and Bi2Se3 films on h-BN, highly oriented pyrolytic graphite and Si(111) substrates. Growth optimizations of In2Se3 and Bi2Se3 films were carried out by systematically varying the growth parameters. While the growth of these films was demonstrated on h-BN and HOPG surface, the majority of the effort was focused on growth on Si(111). Atomically flat terraces that extended laterally for several hundred nm, which were separated by single quintuple layer high steps characterized surface of the best In2Se3 films grown on Si(111). These In2Se3 films were suitable for subsequent high quality epitaxy of Bi2Se3 .
The last part of this dissertation was focused on a recently initiated and ongoing study of graphene growth on liquid metal surfaces. The initial part of the study comprised a successful modification of an existing growth system to accommodate graphene synthesis and process development for reproducible graphene growth. Graphene was grown on Cu, Au and AuCu alloys at varioua conditions. Preliminary results showed triangular features on the liquid part of the Cu metal surface. For Au, and AuCu alloys, hexagonal features were noticed both on the solid and liquid parts.
Next, the growth of selenide heterostructures was explored. Specifically, molecular beam epitaxy was utilized to grow In2Se3 and Bi2Se3 films on h-BN, highly oriented pyrolytic graphite and Si(111) substrates. Growth optimizations of In2Se3 and Bi2Se3 films were carried out by systematically varying the growth parameters. While the growth of these films was demonstrated on h-BN and HOPG surface, the majority of the effort was focused on growth on Si(111). Atomically flat terraces that extended laterally for several hundred nm, which were separated by single quintuple layer high steps characterized surface of the best In2Se3 films grown on Si(111). These In2Se3 films were suitable for subsequent high quality epitaxy of Bi2Se3 .
The last part of this dissertation was focused on a recently initiated and ongoing study of graphene growth on liquid metal surfaces. The initial part of the study comprised a successful modification of an existing growth system to accommodate graphene synthesis and process development for reproducible graphene growth. Graphene was grown on Cu, Au and AuCu alloys at varioua conditions. Preliminary results showed triangular features on the liquid part of the Cu metal surface. For Au, and AuCu alloys, hexagonal features were noticed both on the solid and liquid parts.
ContributorsRathi, Somilkumar J (Author) / Drucker, Jeffery (Thesis advisor) / Smith, David (Committee member) / Chen, Tingyong (Committee member) / Arizona State University (Publisher)
Created2014