Matching Items (73)
Description
The research of alternative materials and new device architectures to exceed the limits of conventional silicon-based devices has been sparked by the persistent pursuit of semiconductor technology scaling. The development of tungsten diselenide (WSe2) and molybdenum disulfide (MoS2), well-known member of the transition metal dichalcogenide (TMD) family, has made great strides towards ultrascaled two-dimensional (2D) field-effect-transistors (FETs). The scaling issues facing silicon-based complementary metal-oxide-semiconductor (CMOS) technologies can be solved by 2D FETs, which show extraordinary potential.This dissertation provides a comprehensive experimental analysis relating to improvements in p-type metal-oxide-semiconductor (PMOS) FETs with few-layer WSe2 and high-κ metal gate (HKMG) stacks. Compared to this works improved methods, standard metallization (more damaging to underlying channel) results in significant Fermi-level pinning, although Schottky barrier heights remain small (< 100 meV) when using high work function metals. Temperature-dependent analysis reveals a dominant contribution to contact resistance from the damaged channel access region. Thus, through less damaging metallization methods combined with strongly scaled HKMG stacks significant improvements were achieved in contact resistance and PMOS FET overall performance. A clean contact/channel interface was achieved through high-vacuum evaporation and temperature-controlled stepped deposition. Theoretical analysis using a Landauer transport adapted to WSe2 Schottky barrier FETs (SB-FETs) elucidates the prospects of nanoscale 2D PMOS FETs indicating high-performance towards the ultimate CMOS scaling limit.
Next, this dissertation discusses how device electrical characteristics are affected by scaling of equivalent oxide thickness (EOT) and by adopting double-gate FET architectures, as well as how this might support CMOS scaling. An improved gate control over the channel is made possible by scaling EOT, improving on-off current ratios, carrier mobility, and subthreshold swing. This study also elucidates the impact of EOT scaling on FET gate hysteresis attributed to charge-trapping effects in high-κ-dielectrics prepared by atomic layer deposition (ALD). These developments in 2D FETs offer a compelling alternative to conventional silicon-based devices and a path for continued transistor scaling. This research contributes to ongoing efforts in 2D materials for future semiconductor technologies. Finally, this work introduces devices based on emerging Janus TMDs and bismuth oxyselenide (Bi2O2Se) layered semiconductors.
ContributorsPatoary, Md Naim Hossain (Author) / Sanchez Esqueda, Ivan (Thesis advisor) / Tongay, Sefaattin (Committee member) / Vasileska, Dragica (Committee member) / Goodnick, Stephen (Committee member) / Arizona State University (Publisher)
Created2023
Description
This work presents a thorough analysis of reconstruction of global wave fields (governed by the inhomogeneous wave equation and the Maxwell vector wave equation) from sensor time series data of the wave field. Three major problems are considered. First, an analysis of circumstances under which wave fields can be fully reconstructed from a network of fixed-location sensors is presented. It is proven that, in many cases, wave fields can be fully reconstructed from a single sensor, but that such reconstructions can be sensitive to small perturbations in sensor placement. Generally, multiple sensors are necessary. The next problem considered is how to obtain a global approximation of an electromagnetic wave field in the presence of an amplifying noisy current density from sensor time series data. This type of noise, described in terms of a cylindrical Wiener process, creates a nonequilibrium system, derived from Maxwell’s equations, where variance increases with time. In this noisy system, longer observation times do not generally provide more accurate estimates of the field coefficients. The mean squared error of the estimates can be decomposed into a sum of the squared bias and the variance. As the observation time $\tau$ increases, the bias decreases as $\mathcal{O}(1/\tau)$ but the variance increases as $\mathcal{O}(\tau)$. The contrasting time scales imply the existence of an ``optimal'' observing time (the bias-variance tradeoff). An iterative algorithm is developed to construct global approximations of the electric field using the optimal observing times. Lastly, the effect of sensor acceleration is considered. When the sensor location is fixed, measurements of wave fields composed of plane waves are almost periodic and so can be written in terms of a standard Fourier basis. When the sensor is accelerating, the resulting time series is no longer almost periodic. This phenomenon is related to the Doppler effect, where a time transformation must be performed to obtain the frequency and amplitude information from the time series data. To obtain frequency and amplitude information from accelerating sensor time series data in a general inhomogeneous medium, a randomized algorithm is presented. The algorithm is analyzed and example wave fields are reconstructed.
ContributorsBarclay, Bryce Matthew (Author) / Mahalov, Alex (Thesis advisor) / Kostelich, Eric J (Thesis advisor) / Moustaoui, Mohamed (Committee member) / Motsch, Sebastien (Committee member) / Platte, Rodrigo (Committee member) / Arizona State University (Publisher)
Created2023
Description
Doping is the cornerstone of Semiconductor technology, enabling the functionalities of modern digital electronics. Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have tunable direct bandgaps, strong many-body interactions, and promising applications in future quantum information sciences, optoelectronic, spintronic, and valleytronic devices. However, their wafer-scale synthesis and precisely controllable doping are challenging. Moreover, there is no fixed framework to identify the doping concentration, which impedes their process integration for future commercialization. This work utilizes the Neutron Transmutation Doping technique to control the doping uniformly and precisely in TMDCs. Rhenium and Tin dopants are introduced in Tungsten- and Indium-based Chalcogenides, respectively. Fine-tuning over 0.001% doping level is achieved. Precise analytical techniques such as Gamma spectroscopy and Secondary Ion Mass Spectrometry are used to quantify ultra-low doping levels ranging from 0.005-0.01% with minimal error. Dopants in 2D TMDCs often exhibit a broad stokes-shifted emission, with high linewidths, due to extrinsic effects such as substrate disorder and surface adsorbates. A well-defined bound exciton emission induced by Rhenium dopants in monolayer WSe2 and WS2 at liquid nitrogen temperatures is reported along with specific annealing regimes to minimize the defects induced in the Neutron Transmutation process. This work demonstrates a framework for Neutron Doping in 2D materials, which can be a scalable process for controlling doping and doping-induced effects in 2D materials.
ContributorsLakhavade, Sushant Sambhaji (Author) / Tongay, Sefaattin (Thesis advisor) / Alford, Terry (Committee member) / Yang, Sui (Committee member) / Arizona State University (Publisher)
Created2023
Description
In the past decade, 2D materials especially transition metal dichalcogenides (TMDc), have been studied extensively for their remarkable optical and electrical properties arising from their reduced dimensionality. A new class of materials developed based on 2D TMDc that has gained great interest in recent years is Janus crystals. In contrast to TMDc, Janus monolayer consists of two different chalcogen atomic layers between which the transition metal layer is sandwiched. This structural asymmetry causes strain buildup or a vertically oriented electric field to form within the monolayer. The presence of strain brings questions about the materials' synthesis approach, particularly when strain begins to accumulate and whether it causes defects within monolayers.The initial research demonstrated that Janus materials could be synthesized at high temperatures inside a chemical vapor deposition (CVD) furnace. Recently, a new method (selective epitaxy atomic replacement - SEAR) for plasma-based room temperature Janus crystal synthesis was proposed. In this method etching and replacing top layer chalcogen atoms of the TMDc monolayer happens with reactive hydrogen and sulfur radicals. Based on Raman and photoluminescence studies, the SEAR method produces high-quality Janus materials. Another method used to create Janus materials was the pulsed laser deposition (PLD) technique, which utilizes the interaction of sulfur/selenium plume with monolayer to replace the top chalcogen atomic layer in a single step.
The goal of this analysis is to characterize microscale defects that appear in 2D Janus materials after they are synthesized using SEAR and PLD techniques. Various microscopic techniques were used for this purpose, as well as to understand the
mechanism of defect formation. The main mechanism of defect formation was proposed to be strain release phenomena. Furthermore, different chalcogen atom positions within the monolayer result in different types of defects, such as the appearance of cracks or wrinkles across monolayers. In addition to investigating sample topography, Kelvin probe force microscopy (KPFM) was used to examine its electrical properties to see if the formation of defects impacts work function. Further study directions have been suggested for identifying and characterizing defects and their formation mechanism in the Janus crystals to understand their fundamental properties.
ContributorsSinha, Shantanu (Author) / Tongay, Sefaattin (Thesis advisor) / Alford, Terry (Committee member) / Yang, Sui (Committee member) / Arizona State University (Publisher)
Created2022
Description
Complex perovskite materials, including Ba(Zn1/3Ta2/3)O3 (BZT), are commonly used to make resonators and filters in communication systems because of their low dielectric loss and high-quality factors (Q). Transition metal additives are introduced (i.e., Ni2+, Co2+, Mn2+) to act as sintering agents and tune their temperature coefficient to zero or near-zero. However, losses in these commercial dielectric materials at cryogenic temperatures increase markedly due to spin-excitation resulting from the presence of paramagnetic defects. Applying a large magnetic field (e.g., 5 Tesla) quenches these losses and has allowed the study of other loss mechanisms present at low temperatures. Work was performed on Fe3+ doped LaAlO3. At high magnetic fields, the residual losses versus temperature plots exhibit Debye peaks at ~40 K, ~75 K, and ~215 K temperature and can be tentatively associated with defect reactions O_i^x+V_O^x→O_i^'+V_O^•, Fe_Al^x+V_Al^"→Fe_Al^'+V_Al^' and Al_i^x+Al_i^(••)→〖2Al〗_i^•, respectively. Peaks in the loss tangent versus temperature graph of Zn-deficient BZT indicate a higher concentration of defects and appear to result from conduction losses.Guided by the knowledge gained from this study, a systematic study to develop high-performance microwave materials for ultra-high performance at cryogenic temperatures was performed. To this end, the production and characterization of perovskite materials that were either undoped or contained non-paramagnetic additives were carried out. Synthesis of BZT ceramic with over 98% theoretical density was obtained using B2O3 or BaZrO3 additives. At 4 K, the highest Q x f product of 283,000 GHz was recorded for 5% BaZrO3 doped BZT.
A portable, inexpensive open-air spectrometer was designed, built, and tested to make the electron paramagnetic resonance (EPR) technique more accessible for high-school and university lab instruction. In this design, the sample is placed near a dielectric resonator and does not need to be enclosed in a cavity, as is used in commercial EPR spectrometers. Permanent magnets used produce fields up to 1500 G, enabling EPR measurements up to 3 GHz.
ContributorsGajare, Siddhesh Girish (Author) / Newman, Nathan (Thesis advisor) / Alford, Terry (Committee member) / Tongay, Sefaattin (Committee member) / Chamberlin, Ralph (Committee member) / Arizona State University (Publisher)
Created2022
Description
Many important technologies, including electronics, computing, communications, optoelectronics, and sensing, are built on semiconductors. The band gap is a crucial factor in determining the electrical and optical properties of semiconductors. Beyond graphene, newly found two-dimensional (2D) materials have semiconducting bandgaps that range from the ultraviolet in hexagonal boron nitride to the terahertz and mid-infrared in bilayer graphene and black phosphorus, visible in transition metal dichalcogenides (TMDs). These 2D materials were shown to have highly controllable bandgaps which can be controlled by alloying. Only a small number of TMDs and monochalcogenides have been alloyed, though, because alloying compromised the material's Van der Waals (Vdw) property and the stability of the host crystal lattice phase. Phase transition in 2D materials is an interesting phenomenon where work has been done only on few TMDs namely MoTe2, MoS2, TaS2 etc.In order to change the band gaps and move them towards the UV (ultraviolet) and IR (infrared) regions, this work has developed new 2D alloys in InSe by alloying them with S and Te at 10% increasing concentrations. As the concentration of the chalcogens (S and Te) increased past a certain point, a structural phase transition in the alloys was observed. However, pinpointing the exact concentration for phase change and inducing phase change using external stimuli will be a thing of the future. The resulting changes in the crystal structure and band gap were characterized using some basic characterization techniques like scanning electron microscopy (SEM), X-ray Diffraction (XRD), Raman and photoluminescence spectroscopy.
ContributorsYarra, Anvesh Sai (Author) / Tongay, Sefaattin (Thesis advisor) / Yang, Sui (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2022
Description
Climate is a critical determinant of agricultural productivity, and the ability to accurately predict this productivity is necessary to provide guidance regarding food security and agricultural management. Previous predictions vary in approach due to the myriad of factors influencing agricultural productivity but generally suggest long-term declines in productivity and agricultural land suitability under climate change. In this paper, I relate predicted climate changes to yield for three major United States crops, namely corn, soybeans, and wheat, using a moderate emissions scenario. By adopting data-driven machine learning approaches, I used the following machine learning methods: random forest (RF), extreme gradient boosting (XGB), and artificial neural networks (ANN) to perform comparative analysis and ensemble methodology. I omitted the western US due to the region's susceptibility to water stress and the prevalence of artificial irrigation as a means to compensate for dry conditions. By considering only climate, the model's results suggest an ensemble mean decline in crop yield of 23.4\% for corn, 19.1\% for soybeans, and 7.8\% for wheat between the years of 2017 and 2100. These results emphasize potential negative impacts of climate change on the current agricultural industry as a result of shifting bio-climactic conditions.
ContributorsSwarup, Shray (Author) / Eikenberry, Steffen (Thesis director) / Mahalov, Alex (Committee member) / Barrett, The Honors College (Contributor) / Computer Science and Engineering Program (Contributor)
Created2023-05
Description
Thin film solar cells are based on polycrystalline materials that contain a high concentration of intrinsic and extrinsic defects. Improving the device efficiency in such systems relies on understanding the nature of defects - whether they are positive, negative, or neutral in their influence - and their sources in order to engineer optimized absorbers. Oftentimes, these are studied individually, as characterization techniques are limited in their ability to directly relate material properties in individual layers to their impact on the actual device performance. Expanding the tools available for increased understanding of materials and devices has been critical for reducing the translation time of laboratory-scale research to changes in commercial module manufacturing lines. The use of synchrotron X-ray fluorescence (XRF) paired with X-ray beam induced current and voltage (XBIC, XBIV respectively) has proven to be an effective technique for understanding the impact of material composition and inhomogeneity on solar cell device functioning. The combination of large penetration depth, small spot size, and high flux allows for the measurement of entire solar cell stacks with high spatial resolution and chemical sensitivity. In this work, I combine correlative XRF/XBIC/XBIV with other characterization approaches across varying length scales, such as micro-Raman spectroscopy and photoluminescence, to understand how composition influences device performance in thin films. The work described here is broken into three sections. Firstly, understanding the influence of KF post-deposition treatment (PDT) and the use of Ag-alloying to reduce defect density in the Ga-free material system, CuInSe2 (CIS). Next, applying a similar characterization workflow to industrially relevant Ga-containing Cu(In1-xGax)Se2 (CIGS) modules with Ag and KF-PDT. The influence of light soaking and dark heat exposure on the modules are also studied in detail. Results show that Ag used with KF-PDT in CIS causes undesirable cation ordering at the CdS interface and affects the device through increased potential fluctuations. The results also demonstrate the importance of tuning the concentration of KF-PDT used when intended to be used in Ag-alloyed devices. Commercially-processed modules with optimized Ag and KF concentrations are shown to have the device performance instead be dominated by variations in the CIGS composition itself. In particular, changes in Cu and Se concentrations are found to be most influential on the device response to accelerated stressors such as dark heat exposure and light soaking. In the final chapter, simulations of nano-scale XBIC and XBIV are done to contribute to the understanding of these measurements.
ContributorsNietzold, Tara (Author) / Bertoni, Mariana I. (Thesis advisor) / Holt, Martin (Committee member) / Shafarman, William N. (Committee member) / Tongay, Sefaattin (Committee member) / Arizona State University (Publisher)
Created2021
Description
The application of silicon thin films in solar cells has evolved from their use in amorphous silicon solar cells to their use as passivating and carrier-selective contacts in crystalline silicon solar cells. Their use as carrier-selective contacts has enabled crystalline silicon solar cell efficiencies above 26%, just 3% shy of the theoretical efficiency limit. The two cell architectures that have exceeded 26% are the silicon heterojunction and tunnel oxide passivating contact cell. These two cell architectures use two different forms of silicon thin films. In the case of the silicon heterojunction, the crystalline wafer is sandwiched between layers of intrinsic amorphous silicon, which acts as the passivation layer, and doped amorphous silicon, which acts as the carrier-selective layer. On the other hand, the tunnel oxide passivating contact cell uses a thin silicon oxide passivation layer and a doped polycrystalline silicon layer as the carrier-selective layer. Both cell structures have their distinct advantages and disadvantages when it comes to production. The processing of the silicon heterojunction relies on a low thermal budget and leads to high open-circuit voltages, but the cost of high-vacuum processing equipment presents a major hurdle for industrial scale production while the tunnel oxide passivating contact can be easily integrated into current industrial lines, yet it requires a higher thermal budgets and does not produce as high of an open-circuit voltage as the silicon heterojunction. This work focuses on using both forms of silicon thin films applied as passivating and carrier-selective contacts to crystalline silicon thin films.First, a thorough analysis of the series resistivity in silicon heterojunction solar cells is conducted. In particular, variations in the thickness and doping of the individual
ii
contact layers are performed to reveal their effect on the contact resistivity and in turn the total series resistivity of the cell. Second, a tunnel oxide passivated contact is created using a novel deposition method for the silicon oxide layer. A 21% efficient proof-of-concept device is presented demonstrating the potential of this deposition method. Finally, recommendations to further improve the efficiency of these cells is presented.
ContributorsWeigand, William (Author) / Holman, Zachary (Thesis advisor) / Yu, Zhengshan (Committee member) / Bertoni, Mariana (Committee member) / Tongay, Sefaattin (Committee member) / Arizona State University (Publisher)
Created2023
Description
Metal-Oxide-Semiconductor (MOS) is essential to modern VLSI devices. In the past decades, a wealth of literature has been created to understand the impact of the radiation-induced charges on the devices, i.e., the creation of electron-hole pairs in the oxide layer which is the most sensitive part of MOS structure to the radiation effect. In this work, both MOS and MNOS devices were fabricated at ASU NanoFab to study the total ionizing dose effect using capacitance-voltage (C-V) electrical characterization by observing the direction and amounts of the shift in C-V curves and electron holography observation to directly image the charge buildup at the irradiated oxide film of the oxide-only MOS device.C-V measurements revealed the C-V curves shifted to the left after irradiation (with a positive bias applied) because of the net positive charges trapped at the oxide layer for the oxide-only sample. On the other hand, for nitride/oxide samples with positive biased during irradiation, the C-V curve shifted to the right due to the net negative charges trapped at the oxide layer. It was also observed that the C-V curve has less shift in voltage for MNOS than MOS devices after irradiation due to the less charge buildup after irradiation.
Off-axis electron holography was performed to map the charge distribution across the MOSCAP sample. Compared with both pre-and post-irradiated samples, a larger potential drop at the Si/SiO2 was noticed in post-irradiation samples, which indicates the presence of greater amounts of positive charges that buildup the Si/SiO2 interface after the TID exposure.
TCAD modeling was used to extract the density of charges accumulated near the SiO2/Si and SiO2/ Metal interface by matching the simulation results to the potential data from holography. The increase of near-interface positive charges in post-irradiated samples is consistent with the C-V results.
ContributorsChang, Ching Tao (Author) / Barnaby, Hugh (Thesis advisor) / Holbert, Keith (Committee member) / Tongay, Sefaattin (Committee member) / Arizona State University (Publisher)
Created2023