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- Genre: Masters Thesis
Description
Carrier lifetime is one of the few parameters which can give information about the low defect densities in today's semiconductors. In principle there is no lower limit to the defect density determined by lifetime measurements. No other technique can easily detect defect densities as low as 10-9 - 10-10 cm-3 in a simple, contactless room temperature measurement. However in practice, recombination lifetime τr measurements such as photoconductance decay (PCD) and surface photovoltage (SPV) that are widely used for characterization of bulk wafers face serious limitations when applied to thin epitaxial layers, where the layer thickness is smaller than the minority carrier diffusion length Ln. Other methods such as microwave photoconductance decay (µ-PCD), photoluminescence (PL), and frequency-dependent SPV, where the generated excess carriers are confined to the epitaxial layer width by using short excitation wavelengths, require complicated configuration and extensive surface passivation processes that make them time-consuming and not suitable for process screening purposes. Generation lifetime τg, typically measured with pulsed MOS capacitors (MOS-C) as test structures, has been shown to be an eminently suitable technique for characterization of thin epitaxial layers. It is for these reasons that the IC community, largely concerned with unipolar MOS devices, uses lifetime measurements as a "process cleanliness monitor." However when dealing with ultraclean epitaxial wafers, the classic MOS-C technique measures an effective generation lifetime τg eff which is dominated by the surface generation and hence cannot be used for screening impurity densities. I have developed a modified pulsed MOS technique for measuring generation lifetime in ultraclean thin p/p+ epitaxial layers which can be used to detect metallic impurities with densities as low as 10-10 cm-3. The widely used classic version has been shown to be unable to effectively detect such low impurity densities due to the domination of surface generation; whereas, the modified version can be used suitably as a metallic impurity density monitoring tool for such cases.
ContributorsElhami Khorasani, Arash (Author) / Alford, Terry (Thesis advisor) / Goryll, Michael (Committee member) / Bertoni, Mariana (Committee member) / Arizona State University (Publisher)
Created2013
Description
The goal of this research work is to develop a particle-based device simulator for modeling strained silicon devices. Two separate modules had to be developed for that purpose: A generic bulk Monte Carlo simulation code which in the long-time limit solves the Boltzmann transport equation for electrons; and an extension to this code that solves for the bulk properties of strained silicon. One scattering table is needed for conventional silicon, whereas, because of the strain breaking the symmetry of the system, three scattering tables are needed for modeling strained silicon material. Simulation results for the average drift velocity and the average electron energy are in close agreement with published data. A Monte Carlo device simulation tool has also been employed to integrate the effects of self-heating into device simulation for Silicon on Insulator devices. The effects of different types of materials for buried oxide layers have been studied. Sapphire, Aluminum Nitride (AlN), Silicon dioxide (SiO2) and Diamond have been used as target materials of interest in the analysis and the effects of varying insulator layer thickness have also been investigated. It was observed that although AlN exhibits the best isothermal behavior, diamond is the best choice when thermal effects are accounted for.
ContributorsQazi, Suleman (Author) / Vasileska, Dragica (Thesis advisor) / Goodnick, Stephen (Committee member) / Tao, Meng (Committee member) / Arizona State University (Publisher)
Created2013
Description
In the nano-regime many materials exhibit properties that are quite different from their bulk counterparts. These nano-properties have been shown to be useful in a wide range of applications with nanomaterials being used for catalysts, in energy production, as protective coatings, and in medical treatment. While there is no shortage of exciting and novel applications, the world of nanomaterials suffers from a lack of large scale manufacturing techniques. The current methods and equipment used for manufacturing nanomaterials are generally slow, expensive, potentially dangerous, and material specific. The research and widespread use of nanomaterials has undoubtedly been hindered by this lack of appropriate tooling. This work details the effort to create a novel nanomaterial synthesis and deposition platform capable of operating at industrial level rates and reliability.
The tool, referred to as Deppy, deposits material via hypersonic impaction, a two chamber process that takes advantage of compressible fluids operating in the choked flow regime to accelerate particles to up several thousand meters per second before they impact and stick to the substrate. This allows for the energetic separation of the synthesis and deposition processes while still behaving as a continuous flow reactor giving Deppy the unique ability to independently control the particle properties and the deposited film properties. While the ultimate goal is to design a tool capable of producing a broad range of nanomaterial films, this work will showcase Deppy's ability to produce silicon nano-particle films as a proof of concept.
By adjusting parameters in the upstream chamber the particle composition was varied from completely amorphous to highly crystalline as confirmed by Raman spectroscopy. By adjusting parameters in the downstream chamber significant variation of the film's density was achieved. Further it was shown that the system is capable of making these adjustments in each chamber without affecting the operation of the other.
The tool, referred to as Deppy, deposits material via hypersonic impaction, a two chamber process that takes advantage of compressible fluids operating in the choked flow regime to accelerate particles to up several thousand meters per second before they impact and stick to the substrate. This allows for the energetic separation of the synthesis and deposition processes while still behaving as a continuous flow reactor giving Deppy the unique ability to independently control the particle properties and the deposited film properties. While the ultimate goal is to design a tool capable of producing a broad range of nanomaterial films, this work will showcase Deppy's ability to produce silicon nano-particle films as a proof of concept.
By adjusting parameters in the upstream chamber the particle composition was varied from completely amorphous to highly crystalline as confirmed by Raman spectroscopy. By adjusting parameters in the downstream chamber significant variation of the film's density was achieved. Further it was shown that the system is capable of making these adjustments in each chamber without affecting the operation of the other.
ContributorsFirth, Peter (Author) / Holman, Zachary C (Thesis advisor) / Kozicki, Michael (Committee member) / Goryll, Michael (Committee member) / Arizona State University (Publisher)
Created2015
Description
High photoluminescence (PL) quantum yields reported from amorphous (a-Si) and crystalline (c-Si) nanoparticles have opened up lots of possibilities for use of silicon in optical applications such as light emitting diodes (LEDs), photonics and solar cells with added processing and cost benefits. However, the PL response and the mechanisms behind it are highly dependent on the matrix in which the nanoparticles are grown and on the growth method. While, the bottom-up approach for deposition of free standing nanoparticles seem to be perfectly suited for large area deposition for LED and solar cell applications, the dominant growth techniques (laser ablation and pyrolysis) have been shown to suffer from limitations in control over size distribution of nanoparticles and the requirement of equipment capable of withstanding high temperature. This led to the exploration of plasma based synthesis methods in this work.
Towards this effort, the development and automation of a novel tool “Anny” for synthesis of silicon nanoparticles using non-thermal plasma chamber is reported. These nanoparticles are then accelerated due to choked flow through a nozzle leading to substrate independent deposition. The nanoparticle properties are characterized against precursor gas flow rates and RF power to identify the optimum growth conditions for a stable, continuous deposition. It is found that amorphous nanoparticles offer a wide variety of chamber conditions for growth with a high throughput, stable plasma for continuous, long term operations.
The quantum confinement model for crystalline and spatial confinement models for amorphous nanoparticles in our size regime (6-8nm) are suggested for free standing nanoparticles and we report a high PL output from well passivated amorphous nanoparticles.
The PL output and its dependence on stability of surface hydrogen passivation is explored using Fourier Transform Infrared spectroscopy (FTIR). It is shown that the amorphous nanoparticles have a better and more stable passivation compared to crystalline nanoparticles grown under similar conditions. Hence, we show a-Si nanoparticles as exciting alternatives for optical applications to c-Si nanoparticles.
Towards this effort, the development and automation of a novel tool “Anny” for synthesis of silicon nanoparticles using non-thermal plasma chamber is reported. These nanoparticles are then accelerated due to choked flow through a nozzle leading to substrate independent deposition. The nanoparticle properties are characterized against precursor gas flow rates and RF power to identify the optimum growth conditions for a stable, continuous deposition. It is found that amorphous nanoparticles offer a wide variety of chamber conditions for growth with a high throughput, stable plasma for continuous, long term operations.
The quantum confinement model for crystalline and spatial confinement models for amorphous nanoparticles in our size regime (6-8nm) are suggested for free standing nanoparticles and we report a high PL output from well passivated amorphous nanoparticles.
The PL output and its dependence on stability of surface hydrogen passivation is explored using Fourier Transform Infrared spectroscopy (FTIR). It is shown that the amorphous nanoparticles have a better and more stable passivation compared to crystalline nanoparticles grown under similar conditions. Hence, we show a-Si nanoparticles as exciting alternatives for optical applications to c-Si nanoparticles.
ContributorsGarg, Prateek (Author) / Holman, Zachary C (Thesis advisor) / Zhang, Yong H (Committee member) / Bertoni, Mariana (Committee member) / Arizona State University (Publisher)
Created2015
Description
Layers of intrinsic hydrogenated amorphous silicon and amorphous silicon carbide
were prepared on a polished, intrinsic crystalline silicon substrate via plasma-enhanced chemical vapor deposition to simulate heterojunction device relevant stacks of various materials. The minority carrier lifetime, optical band gap and FTIR spectra were observed at incremental stages of thermal annealing. By observing the changes in the lifetimes the sample structure responsible for the most thermally robust surface passivation could be determined. These results were correlated to the optical band gap and the position and relative area of peaks in the FTIR spectra related to to silicon-hydrogen bonds in the layers. It was found that due to an increased presence of hydrogen bonded to silicon at voids within the passivating layer, hydrogenated amorphous silicon carbide at the interface of the substrate coupled with a hydrogenated amorphous silicon top layer provides better passivation after high temperature annealing than other device structures.
were prepared on a polished, intrinsic crystalline silicon substrate via plasma-enhanced chemical vapor deposition to simulate heterojunction device relevant stacks of various materials. The minority carrier lifetime, optical band gap and FTIR spectra were observed at incremental stages of thermal annealing. By observing the changes in the lifetimes the sample structure responsible for the most thermally robust surface passivation could be determined. These results were correlated to the optical band gap and the position and relative area of peaks in the FTIR spectra related to to silicon-hydrogen bonds in the layers. It was found that due to an increased presence of hydrogen bonded to silicon at voids within the passivating layer, hydrogenated amorphous silicon carbide at the interface of the substrate coupled with a hydrogenated amorphous silicon top layer provides better passivation after high temperature annealing than other device structures.
ContributorsJackson, Alec James (Author) / Holman, Zachary (Thesis advisor) / Bertoni, Mariana (Committee member) / Kozicki, Michael (Committee member) / Arizona State University (Publisher)
Created2016
Description
Static random-access memories (SRAM) are integral part of design systems as caches and data memories that and occupy one-third of design space. The work presents an embedded low power SRAM on a triple well process that allows body-biasing control. In addition to the normal mode operation, the design is embedded with Physical Unclonable Function (PUF) [Suh07] and Sense Amplifier Test (SA Test) mode. With PUF mode structures, the fabrication and environmental mismatches in bit cells are used to generate unique identification bits. These bits are fixed and known as preferred state of an SRAM bit cell. The direct access test structure is a measurement unit for offset voltage analysis of sense amplifiers. These designs are manufactured using a foundry bulk CMOS 55 nm low-power (LP) process. The details about SRAM bit-cell and peripheral circuit design is discussed in detail, for certain cases the circuit simulation analysis is performed with random variations embedded in SPICE models. Further, post-silicon testing results are discussed for normal operation of SRAMs and the special test modes. The silicon and circuit simulation results for various tests are presented.
ContributorsDosi, Ankita (Author) / Clark, Lawrence (Thesis advisor) / Seo, Jae-Sun (Committee member) / Brunhaver, John (Committee member) / Arizona State University (Publisher)
Created2017
Finite element analysis of silicon thin films on soft substrates as anodes for lithium ion batteries
Description
The wide-scale use of green technologies such as electric vehicles has been slowed due to insufficient means of storing enough portable energy. Therefore it is critical that efficient storage mediums be developed in order to transform abundant renewable energy into an on-demand source of power. Lithium (Li) ion batteries are seeing a stream of improvements as they are introduced into many consumer electronics, electric vehicles and aircraft, and medical devices. Li-ion batteries are well suited for portable applications because of their high energy-to-weight ratios, high energy densities, and reasonable life cycles. Current research into Li-ion batteries is focused on enhancing its energy density, and by changing the electrode materials, greater energy capacities can be realized. Silicon (Si) is a very attractive option because it has the highest known theoretical charge capacity. Current Si anodes, however, suffer from early capacity fading caused by pulverization from the stresses induced by large volumetric changes that occur during charging and discharging. An innovative system aimed at resolving this issue is being developed. This system incorporates a thin Si film bonded to an elastomeric substrate which is intended to provide the desired stress relief. Non-linear finite element simulations have shown that a significant amount of deformation can be accommodated until a critical threshold of Li concentration is reached; beyond which buckling is induced and a wavy structure appears. When compared to a similar system using rigid substrates where no buckling occurs, the stress is reduced by an order of magnitude, significantly prolonging the life of the Si anode. Thus the stress can be released at high Li-ion diffusion induced strains by buckling the Si thin film. Several aspects of this anode system have been analyzed including studying the effects of charge rate and thin film plasticity, and the results are compared with preliminary empirical measurements to show great promise. This study serves as the basis for a radical resolution to one of the few remaining barriers left in the development of high performing Si based electrodes for Li-ion batteries.
ContributorsShaffer, Joseph (Author) / Jiang, Hanqing (Thesis advisor) / Rajan, Subramaniam D. (Committee member) / Peralta, Pedro (Committee member) / Arizona State University (Publisher)
Created2011
Description
The maximum theoretical efficiency of a terrestrial non-concentrated silicon solar cell is 29.4%, as obtained from detailed balance analysis. Over 90% of the current silicon photovoltaics market is based on solar cells with diffused junctions (Al-BSF, PERC, PERL, etc.), which are limited in performance by increased non-radiative recombination in the doped regions. This limitation can be overcome through the use of passivating contacts, which prevent recombination at the absorber interfaces while providing the selectivity to efficiently separate the charge carriers generated in the absorber. This thesis aims at developing an understanding of how the material properties of the contact affect device performance through simulations.The partial specific contact resistance framework developed by Onno et al. aims to link material behavior to device performance specifically at open circuit. In this thesis, the framework is expanded to other operating points of a device, leading to a model for calculating the partial contact resistances at any current flow. The error in calculating these resistances is irrelevant to device performance resulting in an error in calculating fill factor from resistances below 0.1% when the fill factors of the cell are above 70%, i.e., for cells with good passivation and selectivity.
Further, silicon heterojunction (SHJ) and tunnel-oxide based solar cells are simulated in 1D finite-difference modeling package AFORS-HET. The effects of material property changes on device performance are investigated using novel contact materials like Al0.8Ga0.2As (hole contact for SHJ) and ITO (electron contact for tunnel-oxide cells). While changing the bandgap and electron affinity of the contact affect the height of the Schottky barrier and hence contact resistivity, increasing the doping of the contact will increase its selectivity. In the case of ITO, the contact needs to have a work function below 4.2 eV to be electron selective, which suggests that other low work function TCOs (like AZO) will be more applicable as alternative dopant-free electron contacts. The AFORS-HET model also shows that buried doped regions arising from boron diffusion in the absorber can damage passivation and decrease the open circuit voltage of the device.
Further, silicon heterojunction (SHJ) and tunnel-oxide based solar cells are simulated in 1D finite-difference modeling package AFORS-HET. The effects of material property changes on device performance are investigated using novel contact materials like Al0.8Ga0.2As (hole contact for SHJ) and ITO (electron contact for tunnel-oxide cells). While changing the bandgap and electron affinity of the contact affect the height of the Schottky barrier and hence contact resistivity, increasing the doping of the contact will increase its selectivity. In the case of ITO, the contact needs to have a work function below 4.2 eV to be electron selective, which suggests that other low work function TCOs (like AZO) will be more applicable as alternative dopant-free electron contacts. The AFORS-HET model also shows that buried doped regions arising from boron diffusion in the absorber can damage passivation and decrease the open circuit voltage of the device.
ContributorsDasgupta, Sagnik (Author) / Holman, Zachary (Thesis advisor) / Onno, Arthur (Committee member) / Wang, Qing Hua (Committee member) / Arizona State University (Publisher)
Created2020
Description
With the InSight mission deploying a seismometer , Martian bulk chemical compositional models are more important than ever. Three largely consistent models for the Martian mantle have been suggested over the past two decades. Of these three, two are fairly similar and one is dramatically different. Of these three, the EH70 (Sanloup et al., 1999) models have the systematically lower divalent cation to silicon ratios as compared to the other model, the DW85 (Dreibus and Wanke, 1985) model. However, impact of such a low (Mg+Fe+Ca)/Si ratio on mineralogy has not been experimentally investigated. Measurements have been made of the mineralogy of the EH70 bulk mantle composition (Sanloup et al., 1999)) through in-situ laser-heated diamond anvil cell (LHDAC) and large volume press (LVP). Majorite-garnet (Mj) dominated mineralogy has been observed up to 25 GPa. Bridgmanite (Bm) begins to appear from 25.2 GPa and continues in a mixed phase with Mj up to 27 GPa at which point only Bm and calcium perovskite (CaPv) remain. Akimotoite (Ak) is stable up to 1873 K, higher by ≈300 K compared to numerical calculations (Connolly, 2009). This may result in an Ak layer in the Martian mantle, something missing in Earth’s mantle. The overall ratio of pyroxene to olivine polymorphs by volume is high, approaching pure pyroxene. This agrees with numerical calculations. Additionally, ferropericlase (Fp) is stable at lower temperatures, suggesting a higher dependence on temperature for its stability, something that is different from Perple_X calculations which show a strong dependence on pressure. Furthermore, Mj, which make up a majority of the volume of EH70 mantles, was measured to increase in Fe content as pressure increases. The more oxidizing conditions coupled with the silicon-rich composition resulted in three times higher Fe3+ content in Mj as opposed to a pyrolite model. This increased Fe3+ meant our Mj composition approached that of skiagite (Ski,Fe2+ 3 Fe3+ 2 Si3O12) and this caused Mj to have a very low compressibility of only 152.8 GPa, lower than any other Mj compositions in literature. This result suggests that a mantle with EH70 bulk composition would have lower than predicted seismic wave velocities , lower than Perple_X predicts. The Al content of Mj was also found to suppress the first derivative of compressibility to 4.45, lower than that of Ski100 at 6.7. Such differences compared with pyrolitic composition are important to estimate the velocity profiles and to model the dynamics of the Martian mantle. This dataset of mineralogy and composition can also model terrestrial exoplanetary mantles. Current measurements of stellar abundances show a wide range of compositions, and especially compositions with (Mg+Fe+Ca)/Si ratios approaching 1 (Brewer and Fischer, 2016). This experimental study of EH70 composition can fill-in this gap.
ContributorsDolinschi, Jonathan David (Author) / Shim, Sang-Heon D. (Thesis advisor) / Desch, Steven (Committee member) / Lee, Mingming (Committee member) / Arizona State University (Publisher)
Created2019
Description
Accurate knowledge and understanding of thermal conductivity is very important in awide variety of applications both at microscopic and macroscopic scales. Estimation,however varies widely with respect to scale and application. At a lattice level, calcu-lation of thermal conductivity of any particular alloy require very heavy computationeven for a relatively small number of atoms. This thesis aims to run conventionalmolecular dynamic simulations for a particular supercell and then employ a machinelearning based approach and compare the two in hopes of developing a method togreatly reduce computational costs as well as increase the scale and time frame ofthese systems. Conventional simulations were run using interatomic potentials basedon density function theory-basedab initiocalculations. Then deep learning neuralnetwork based interatomic potentials were used run similar simulations to comparethe two approaches.
ContributorsDabir, Anirudh (Author) / Zhuang, Houlong (Thesis advisor) / Nian, Qiong (Committee member) / Jiao, Yang (Committee member) / Arizona State University (Publisher)
Created2021