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- All Subjects: Electrical Engineering
- Creators: Yu, Hongbin
Description
Fluxgate sensors are magnetic field sensors that can measure DC and low frequency AC magnetic fields. They can measure much lower magnetic fields than other magnetic sensors like Hall effect sensors, magnetoresistive sensors etc. They also have high linearity, high sensitivity and low noise. The major application of fluxgate sensors is in magnetometers for the measurement of earth's magnetic field. Magnetometers are used in navigation systems and electronic compasses. Fluxgate sensors can also be used to measure high DC currents. Integrated micro-fluxgate sensors have been developed in recent years. These sensors have much lower power consumption and area compared to their PCB counterparts. The output voltage of micro-fluxgate sensors is very low which makes the analog front end more complex and results in an increase in power consumption of the system. In this thesis a new analog front-end circuit for micro-fluxgate sensors is developed. This analog front-end circuit uses charge pump based excitation circuit and phase delay based read-out chain. With these two features the power consumption of analog front-end is reduced. The output is digital and it is immune to amplitude noise at the output of the sensor. Digital output is produced without using an ADC. A SPICE model of micro-fluxgate sensor is used to verify the operation of the analog front-end and the simulation results show very good linearity.
ContributorsPappu, Karthik (Author) / Bakkaloglu, Bertan (Thesis advisor) / Christen, Jennifer Blain (Committee member) / Yu, Hongbin (Committee member) / Arizona State University (Publisher)
Created2013
Description
As crystalline silicon solar cells continue to get thinner, the recombination of carriers at the surfaces of the cell plays an ever-important role in controlling the cell efficiency. One tool to minimize surface recombination is field effect passivation from the charges present in the thin films applied on the cell surfaces. The focus of this work is to understand the properties of charges present in the SiNx films and then to develop a mechanism to manipulate the polarity of charges to either negative or positive based on the end-application. Specific silicon-nitrogen dangling bonds (·Si-N), known as K center defects, are the primary charge trapping defects present in the SiNx films. A custom built corona charging tool was used to externally inject positive or negative charges in the SiNx film. Detailed Capacitance-Voltage (C-V) measurements taken on corona charged SiNx samples confirmed the presence of a net positive or negative charge density, as high as +/- 8 x 1012 cm-2, present in the SiNx film. High-energy (~ 4.9 eV) UV radiation was used to control and neutralize the charges in the SiNx films. Electron-Spin-Resonance (ESR) technique was used to detect and quantify the density of neutral K0 defects that are paramagnetically active. The density of the neutral K0 defects increased after UV treatment and decreased after high temperature annealing and charging treatments. Etch-back C-V measurements on SiNx films showed that the K centers are spread throughout the bulk of the SiNx film and not just near the SiNx-Si interface. It was also shown that the negative injected charges in the SiNx film were stable and present even after 1 year under indoor room-temperature conditions. Lastly, a stack of SiO2/SiNx dielectric layers applicable to standard commercial solar cells was developed using a low temperature (< 400 °C) PECVD process. Excellent surface passivation on FZ and CZ Si substrates for both n- and p-type samples was achieved by manipulating and controlling the charge in SiNx films.
ContributorsSharma, Vivek (Author) / Bowden, Stuart (Thesis advisor) / Schroder, Dieter (Committee member) / Honsberg, Christiana (Committee member) / Roedel, Ronald (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2013
Description
This thesis summarizes modeling and simulation of plasmonic waveguides and nanolasers. The research includes modeling of dielectric constants of doped semiconductor as a potential plasmonic material, simulation of plasmonic waveguides with different configurations and geometries, simulation and design of plasmonic nanolasers. In the doped semiconductor part, a more accurate model accounting for dielectric constant of doped InAs was proposed. In the model, Interband transitions accounted for by Adachi's model considering Burstein-Moss effect and free electron effect governed by Drude model dominate in different spectral regions. For plasmonic waveguide part, Insulator-Metal-Insulator (IMI) waveguide, silver nanowire waveguide with and without substrate, Metal-Semiconductor-Metal (MSM) waveguide and Metal-Insulator-Semiconductor-Insulator-Metal (MISIM) waveguide were investigated respectively. Modal analysis was given for each part. Lastly, a comparative study of plasmonic and optical modes in an MSM disk cavity was performed by FDTD simulation for room temperature at the telecommunication wavelength. The results show quantitatively that plasmonic modes have advantages over optical modes in the scalability down to small size and the cavity Quantum Electrodynamics(QED) effects due to the possibility of breaking the diffraction limit. Surprisingly for lasing characteristics, though plasmonic modes have large loss as expected, minimal achievable threshold can be attained for whispering gallery plasmonic modes with azimuthal number of 2 by optimizing cavity design at 1.55µm due to interplay of metal loss and radiation loss.
ContributorsWang, Haotong (Author) / Ning, Cunzheng (Thesis advisor) / Palais, Joseph (Committee member) / Yu, Hongbin (Committee member) / Arizona State University (Publisher)
Created2014
Description
Semiconductor nanowires are important candidates for highly scaled three dimensional electronic devices. It is very advantageous to combine their scaling capability with the high yield of planar CMOS technology by integrating nanowire devices into planar circuits. The purpose of this research is to identify the challenges associated with the fabrication of vertically oriented Si and Ge nanowire diodes and modeling their electrical behavior so that they can be utilized to create unique three dimensional architectures that can boost the scaling of electronic devices into the next generation. In this study, vertical Ge and Si nanowire Schottky diodes have been fabricated using bottom-up vapor-liquid-solid (VLS) and top-down reactive ion etching (RIE) approaches respectively. VLS growth yields nanowires with atomically smooth sidewalls at sub-50 nm diameters but suffers from the problem that the doping increases radially outwards from the core of the devices. RIE is much faster than VLS and does not suffer from the problem of non-uniform doping. However, it yields nanowires with rougher sidewalls and gets exceedingly inefficient in yielding vertical nanowires for diameters below 50 nm. The I-V characteristics of both Ge and Si nanowire diodes cannot be adequately fit by the thermionic emission model. Annealing in forming gas which passivates dangling bonds on the nanowire surface is shown to have a considerable impact on the current through the Si nanowire diodes indicating that fixed charges and traps on the surface of the devices play a major role in determining their electrical behavior. Also, due to the vertical geometry of the nanowire diodes, electric field lines originating from the metal and terminating on their sidewalls can directly modulate their conductivity. Both these effects have to be included in the model aimed at predicting the current through vertical nanowire diodes. This study shows that the current through vertical nanowire diodes cannot be predicted accurately using the thermionic emission model which is suitable for planar devices and identifies the factors needed to build a comprehensive analytical model for predicting the current through vertically oriented nanowire diodes.
ContributorsChandra, Nishant (Author) / Goodnick, Stephen M (Thesis advisor) / Tracy, Clarence J. (Committee member) / Yu, Hongbin (Committee member) / Ferry, David K. (Committee member) / Arizona State University (Publisher)
Created2014
Description
Increasing the conversion efficiencies of photovoltaic (PV) cells beyond the single junction theoretical limit is the driving force behind much of third generation solar cell research. Over the last half century, the experimental conversion efficiency of both single junction and tandem solar cells has plateaued as manufacturers and researchers have optimized various materials and structures. While existing materials and technologies have remarkably good conversion efficiencies, they are approaching their own limits. For example, tandem solar cells are currently well developed commercially but further improvements through increasing the number of junctions struggle with various issues related to material interfacial defects. Thus, there is a need for novel theoretical and experimental approaches leading to new third generation cell structures. Multiple exciton generation (MEG) and intermediate band (IB) solar cells have been proposed as third generation alternatives and theoretical modeling suggests they can surpass the detailed balance efficiency limits of single junction and tandem solar cells. MEG or IB solar cell has a variety of advantages enabling the use of low bandgap materials. Integrating MEG and IB with other cell types to make novel solar cells (such as MEG with tandem, IB with tandem or MEG with IB) potentially offers improvements by employing multi-physics effects in one device. This hybrid solar cell should improve the properties of conventional solar cells with a reduced number of junction, increased light-generated current and extended material selections. These multi-physics effects in hybrid solar cells can be achieved through the use of nanostructures taking advantage of the carrier confinement while using existing solar cell materials with excellent characteristics. This reduces the additional cost to develop novel materials and structures. In this dissertation, the author develops thermodynamic models for several novel types of solar cells and uses these models to optimize and compare their properties to those of existing PV cells. The results demonstrate multiple advantages from combining MEG and IB technology with existing solar cell structures.
ContributorsLee, Jongwon (Author) / Honsberg, C. (Christiana B.) (Thesis advisor) / Bowden, Stuart (Committee member) / Roedel, Ronald (Committee member) / Goodnick, Stephen (Committee member) / Schroder, Dieter (Committee member) / Arizona State University (Publisher)
Created2014
Description
With increasing demand for System on Chip (SoC) and System in Package (SiP) design in computer and communication technologies, integrated inductor which is an essential passive component has been widely used in numerous integrated circuits (ICs) such as in voltage regulators and RF circuits. In this work, soft ferromagnetic core material, amorphous Co-Zr-Ta-B, was incorporated into on-chip and in-package inductors in order to scale down inductors and improve inductors performance in both inductance density and quality factor. With two layers of 500 nm Co-Zr-Ta-B films a 3.5X increase in inductance and a 3.9X increase in quality factor over inductors without magnetic films were measured at frequencies as high as 1 GHz. By laminating technology, up to 9.1X increase in inductance and more than 5X increase in quality factor (Q) were obtained from stripline inductors incorporated with 50 nm by 10 laminated films with a peak Q at 300 MHz. It was also demonstrated that this peak Q can be pushed towards high frequency as far as 1GHz by a combination of patterning magnetic films into fine bars and laminations. The role of magnetic vias in magnetic flux and eddy current control was investigated by both simulation and experiment using different patterning techniques and by altering the magnetic via width. Finger-shaped magnetic vias were designed and integrated into on-chip RF inductors improving the frequency of peak quality factor from 400 MHz to 800 MHz without sacrificing inductance enhancement. Eddy current and magnetic flux density in different areas of magnetic vias were analyzed by HFSS 3D EM simulation. With optimized magnetic vias, high frequency response of up to 2 GHz was achieved. Furthermore, the effect of applied magnetic field on on-chip inductors was investigated for high power applications. It was observed that as applied magnetic field along the hard axis (HA) increases, inductance maintains similar value initially at low fields, but decreases at larger fields until the magnetic films become saturated. The high frequency quality factor showed an opposite trend which is correlated to the reduction of ferromagnetic resonant absorption in the magnetic film. In addition, experiments showed that this field-dependent inductance change varied with different patterned magnetic film structures, including bars/slots and fingers structures. Magnetic properties of Co-Zr-Ta-B films on standard organic package substrates including ABF and polyimide were also characterized. Effects of substrate roughness and stress were analyzed and simulated which provide strategies for integrating Co-Zr-Ta-B into package inductors and improving inductors performance. Stripline and spiral inductors with Co-Zr-Ta-B films were fabricated on both ABF and polyimide substrates. Maximum 90% inductance increase in hundreds MHz frequency range were achieved in stripline inductors which are suitable for power delivery applications. Spiral inductors with Co-Zr-Ta-B films showed 18% inductance increase with quality factor of 4 at frequency up to 3 GHz.
ContributorsWu, Hao (Author) / Yu, Hongbin (Thesis advisor) / Bakkaloglu, Bertan (Committee member) / Cao, Yu (Committee member) / Chickamenahalli, Shamala (Committee member) / Arizona State University (Publisher)
Created2013
Description
Zinc oxide (ZnO), a naturally n-type semiconductor has been identified as a promising candidate to replace indium tin oxide (ITO) as the transparent electrode in solar cells, because of its wide bandgap (3.37 eV), abundant source materials and suitable refractive index (2.0 at 600 nm). Spray deposition is a convenient and low cost technique for large area and uniform deposition of semiconductor thin films. In particular, it provides an easier way to dope the film by simply adding the dopant precursor into the starting solution. In order to reduce the resistivity of undoped ZnO, many works have been done by doping in the ZnO with either group IIIA elements or VIIA elements using spray pyrolysis. However, the resistivity is still too high to meet TCO's resistivity requirement. In the present work, a novel co-spray deposition technique is developed to bypass a fundamental limitation in the conventional spray deposition technique, i.e. the deposition of metal oxides from incompatible precursors in the starting solution. With this technique, ZnO films codoped with one cationic dopant, Al, Cr, or Fe, and an anionic dopant, F, have been successfully synthesized, in which F is incompatible with all these three cationic dopants. Two starting solutions were prepared and co-sprayed through two separate spray heads. One solution contained only the F precursor, NH 4F. The second solution contained the Zn and one cationic dopant precursors, Zn(O 2CCH 3) 2 and AlCl 3, CrCl 3, or FeCl 3. The deposition was carried out at 500 °C; on soda-lime glass in air. Compared to singly-doped ZnO thin films, codoped ZnO samples showed better electrical properties. Besides, a minimum sheet resistance, 55.4 Ω/sq, was obtained for Al and F codoped ZnO films after vacuum annealing at 400 °C;, which was lower than singly-doped ZnO with either Al or F. The transmittance for the Al and F codoped ZnO samples was above 90% in the visible range. This co-spray deposition technique provides a simple and cost-effective way to synthesize metal oxides from incompatible precursors with improved properties.
ContributorsZhou, Bin (Author) / Tao, Meng (Thesis advisor) / Goryll, Michael (Committee member) / Vasileska, Dragica (Committee member) / Yu, Hongbin (Committee member) / Arizona State University (Publisher)
Created2013
Description
In the interest of expediting future pilot line start-ups for solar cell research, the development of Arizona State University's student-led pilot line at the Solar Power Laboratory is discussed extensively within this work. Several experiments and characterization techniques used to formulate and optimize a series of processes for fabricating diffused-junction, screen-printed silicon solar cells are expounded upon. An experiment is conducted in which the thickness of a PECVD deposited anti-reflection coating (ARC) is varied across several samples and modeled as a function of deposition time. Using this statistical model in tandem with reflectance measurements for each sample, the ARC thickness is optimized to increase light trapping in the solar cells. A response surface model (RSM) experiment is conducted in which 3 process parameters are varied on the PECVD tool for the deposition of the ARCs on several samples. A contactless photoconductance decay (PCD) tool is used to measure the dark saturation currents of these samples. A statistical analysis is performed using JMP in which optimum deposition parameters are found. A separate experiment shows an increase in the passivation quality of the a-SiNx:H ARCs deposited on the solar cells made on the line using these optimum parameters. A RSM experiment is used to optimize the printing process for a particular silver paste in a similar fashion, the results of which are confirmed by analyzing the series resistance of subsequent cells fabricated on the line. An in-depth explanation of a more advanced analysis using JMP and PCD measurements on the passivation quality of 3 aluminum back-surface fields (BSF) is given. From this experiment, a comparison of the means is conducted in order to choose the most effective BSF paste for cells fabricated on the line. An experiment is conducted in parallel which confirms the results via Voc measurements. It is shown that in a period of 11 months, the pilot line went from producing a top cell efficiency of 11.5% to 17.6%. Many of these methods used for the development of this pilot line are equally applicable to other cell structures, and can easily be applied to other solar cell pilot lines.
ContributorsPickett, Guy (Author) / Bowden, Stuart (Thesis advisor) / Honsberg, Christiana (Committee member) / Bertoni, Mariana (Committee member) / Arizona State University (Publisher)
Created2014
Description
Silicon (Si) solar cells are the dominant technology used in the Photovoltaics industry. Field-effect passivation by means of electrostatic charges stored in an overlying insulator on a silicon solar cell has been proven to be a significantly efficient way to reduce effective surface recombination velocity and increase minority carrier lifetime. Silicon nitride (SiNx) films have been extensively used as passivation layers. The capability to store charges makes SiNx a promising material for excellent feild effect passivation. In this work, symmetrical Si/SiO2/SiNx stacks are developed to study the effect of charges in SiNx films. SiO2 films work as barrier layers. Corona charging technique showed the ability to inject charges into the SiNx films in a short time. Minority carrier lifetimes of the Czochralski (CZ) Si wafers increased significantly after either positive or negative charging. A fast and contactless method to characterize the charged overlying insulators on Si wafer through lifetime measurements is proposed and studied in this work, to overcome the drawbacks of capacitance-voltage (CV) measurements such as time consuming, induction of contanmination and hysteresis effect, etc. Analytical simulations showed behaviors of inverse lifetime (Auger corrected) vs. minority carrier density curves depend on insulator charge densities (Nf). From the curve behavior, the Si surface condition and region of Nf can be estimated. When the silicon surface is at high strong inversion or high accumulation, insulator charge density (Nf) or surface recombination velocity parameters (Sn0 and Sp0) can be determined from the slope of inverse lifetime curves, if the other variable is known. If Sn0 and Sp0 are unknown, Nf values of different samples can be compared as long as all have similar Sn0 and Sp0 values. Using the saturation current density (J0) and intercept fit extracted from the lifetime measurement, the bulk lifetime can be calculated. Therefore, this method is feasible and promising for charged insulator characterization.
ContributorsYang, Qun (Author) / Bowden, Stuart (Thesis advisor) / Honsberg, Christiana (Committee member) / Tracy, Clarence (Committee member) / Arizona State University (Publisher)
Created2014
Description
Nanolasers represents the research frontier in both the areas of photonics and nanotechnology for its interesting properties in low dimension physics, its appealing prospects in integrated photonics, and other on-chip applications. In this thesis, I present my research work on fabrication and characterization of a new type of nanolasers: metallic cavity nanolasers. The last ten years witnessed a dramatic paradigm shift from pure dielectric cavity to metallic cavity in the research of nanolasers. By using low loss metals such as silver, which is highly reflective at near infrared, light can be confined in an ultra small cavity or waveguide with sub-wavelength dimensions, thus enabling sub-wavelength cavity lasers. Based on this idea, I fabricated two different kinds of metallic cavity nanolasers with rectangular and circular geometries with InGaAs as the gain material and silver as the metallic shell. The lasing wavelength is around 1.55 μm, intended for optical communication applications. Continuous wave (CW) lasing at cryogenic temperature under current injection was achieved on devices with a deep sub-wavelength physical cavity volume smaller than 0.2 λ3. Improving device fabrication process is one of the main challenges in the development of metallic cavity nanolasers due to its ultra-small size. With improved fabrication process and device design, CW lasing at room temperature was demonstrated as well on a sub-wavelength rectangular device with a physical cavity volume of 0.67 λ3. Experiments verified that a small circular nanolasers supporting TE¬01 mode can generate an azimuthal polarized laser beam, providing a compact such source under electrical injection. Sources with such polarizations could have many special applications. Study of digital modulation of circular nanolasers showed that laser noise is an important factor that will affect the data rate of the nanolaser when used as the light source in optical interconnects. For future development, improving device fabrication processes is required to improve device performance. In addition, techniques need to be developed to realize nanolaser/Si waveguide integration. In essence, resolving these two critical issues will finally pave the way for these nanolasers to be used in various practical applications.
ContributorsDing, Kang (Author) / Ning, Cun-Zheng (Thesis advisor) / Yu, Hongbin (Committee member) / Palais, Joseph (Committee member) / Zhang, Yong-Hang (Committee member) / Arizona State University (Publisher)
Created2014