Browsing by Author "Veena Misra, Committee Chair"

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Applications of Redox Active Molecules in Solid State Electronics Devices and Organic Photovoltaic Cells
(2009-12-08) Chen, Zhong; Veena Misra, Committee Chair; Carl Osburn, Committee Member; Michael Escuti, Committee Member; Orlin Velev, Committee Member
Redox active molecules have been studied for several years due to their interesting charge storage properties for future application in molecular memory devices with multi-bit low-voltage operation and ultimate scalability. Hybrid Si-Molecular devices with liquid electrolyte as gate contact have been demonstrated for future DRAM and FLASH memories. The focus of this dissertation work has been on developing the completely solid state hybrid Si-Molecular devices embedded with redox active molecules to facilitate ease of integration. In addition, redox active molecules have also been explored in potential application in organic photovoltaic cells. This work exploits the charge storage properties of redox molecules in solid state devices and the absorption profile of the redox polymers. The charge transfer and charge screening process in conventional electrochemical cell with liquid electrolyte has been characterized using cyclic voltammetry measurements. The role of the liquid electrolyte and the electrical double layer in electrochemical cell for the redox process has been discussed. The solid state approach to hybrid Si-Molecular devices has been proposed. The solid state dielectric layer has been considered to replace electrical double layer. The requirements of dielectric layer and deposition methods in solid state molecular memory device are investigated for preservation and characterization of the redox molecules. AlN, Al2O3 and HfO2 are deposited and examined on top of redox active polymers or redox monolayer in metal-insulator-molecules-silicon (MIMS) or metal-insulator-molecules-metal (MIMM) capacitors. The CyV measurements indicate that the redox properties are preserved after dielectric layer deposition on molecules. The leakage of MIMM capacitors has been greatly improved after the optimization on HfO2 atomic layer deposition conditions and the W/WN gate stack. The low leakage current of MIMM structure provide a reliable test platform for redox molecules as well as the other molecule with more functionalities. The redox process in the ionic cell has been modeled with equivalent circuits. The capacitance at different frequencies for EMS capacitors is simulated to study the electrical properties of the solid state molecular device. The frequency dispersion of capacitance measurements for MIMM capacitance has been explored for understanding the redox charging or trapping in the molecular layer. Si nano-membranes are fabricated as an alternative contact to molecules. Solid state molecular transistors are demonstrated for the possible application in FLASH memory. Redox polymers are investigated for the application in renewable energy area. The organic nanoparticles-polymer solar cell has been compared with conventional bulk Si solar cell. The integration of redox polymers helps to improve the absorption profile of active layer in organic solar cells. Organic solar cells with P3HT and PCBM are fabricated and characterized for the future incorporation of redox molecules. In summary, this work provides the fundamental insights and realistic approaches for the applications of redox active molecules with unique charge storage properties into solid state electronic devices and organic photovoltaic cells, which may enable the solutions for the low cost multi-bit low-voltage scalable molecular memories as well as the high efficiency organic solar cell embedded with redox molecules.
Approach towards Hybrid Silicon/Molecular Electronics for Memory Applications
(2005-01-24) Li, Qiliang; Jonathan S. Lindsey, Committee Member; Douglas W. Barlage, Committee Member; Veena Misra, Committee Chair; Carlton M. Osburn, Committee Member
As CMOS technology extends to and beyond 65-nm technology node, many challenges to MOSFET were raised. The industrial and academic communities are aggressively searching for solutions to meet these challenges: (1) non-classical CMOS to extend the life of CMOS technology, and (2) fundamentally new technologies to replace CMOS technology including molecular electronics. The approach of hybrid silicon/molecular electronics is to provide a smooth transition technology by integrating molecular intrinsic scalability and diverse properties with the vast infrastructure of traditional MOS technology. The focus of this research is on integrating redox-active, organic molecules into Si-based structures to first, characterize and understand the properties of molecules; second, generate a new class of hybrid silicon/molecular devices for memory applications; and third, open a new way to develop purely molecular-scale devices. This dissertation has concentrated on the fabrication, characterization and simulation of hybrid CMOS/molecular devices for memory applications: (1) Specific procedures have been successfully developed for attaching redox-active, tightly-bonded, well-packed, molecular self-assembled monolayers on Si and SiO2 surfaces via solution-phase or vapor-phase deposition. (2) An electrolyte/molecule/Si structure has been implemented for electrical characterizations and theoretical simulations to understand the molecules and their application in memory devices, including DRAM and FLASH devices. (3) Two different strategies to achieve multibit memory have been developed and optimized using the methods of attaching mixed monolayers and stacked multilayer films. (4) Molecular multilayer films with very high surface coverage have been achieved for application in memory devices. Metal/molecule/Si sandwich structures using molecular multilayer films were fabricated and exhibited nonvolatile electrical switching properties. A set of control experiments indicate that these switching properties are due to the interaction of metal/molecule interface instead of the redox-related processes. In conclusion, this thesis has focused on hybrid silicon/molecular electronics and has investigated the intrinsic properties of molecules and proposed feasible ways to apply molecules in memory devices.
Electron Transport in Bulk-Si NMOSFETs in Presence of High-k Insulator-charge Trapping and Mobility
(2006-11-29) Maitra, Kingsuk; Mark Johnson, Committee Member; Douglas Barlage, Committee Member; Carl Osburn, Committee Member; Veena Misra, Committee Chair
Recent advancements in gate stack engineering has led to the development of aggressively scaled, high mobility, high-k dielectric based NMOSFETs with metal gates. Most of the current literature on the subject also stressed on the need for a high temperature process step to attain the high mobility under minimal change of effective oxide thickness. However, the physical origin of high mobility is not well understood. In this work, fundamental insight into the necessity of the high temperature process step is provided. Novel experimental strategies are developed to understand the impact of interface states and bulk traps separately and exclusively on channel mobility. It is conjectured that the interface states at the SiO2⁄(100) bulk-Si interface are identical in nature (as far as coupling with the channel electrons is concerned) to those at the high-k⁄SiO2⁄(100) bulk-Si interface. Thus, the response of interface states on channel electrons in high-k insulator based NMOSFETs is properly calibrated by a novel thermal desorption of hydrogen experiment on SiO2⁄(100) bulk-Si NMOSFETs to yield a highly accurate parameterized equation. The value of interface state response parameter determined by the aforementioned experiment is compared with theoretical predictions, and independently determined projections from electrical stress measurements. The impact of transient charging on transport in the channel is investigated. It is conclusively shown that remote charge has minimal impact on mobility in the channel. The role of nitrogen induced fixed oxide charge is studied on a set of Hf-silicate samples. Role of soft optical phonon scattering and the beneficial impact of metal gates on soft optical phonon limited mobility are thoroughly investigated both theoretically and experimentally. Conclusions are drawn on the fundamental limit of mobility attainable in high-k dielectric based NMOSFETs.
Investigation of MOS Interfaces with Atomic-Layer-Deposited High-k Gate Dielectrics on III-V Semiconductors.
(2010-08-06) Suri, Rahul; Veena Misra, Committee Chair; Mark Johnson, Committee Member; Salah M. Bedair, Committee Member; Mehmet Ozturk, Committee Member; Dan Lichtenwalner, Committee Member
Investigation of multi-state charge-storage properties of redox-active organic molecules in silicon-molecular hybrid devices for DRAM and Flash applications
(2008-01-08) Gowda, Srivardhan Shivappa; Leda Lunardi, Committee Member; Eric Rotenberg, Committee Member; Veena Misra, Committee Chair; Jonathan S. Lindsey, Committee Member
Metal alloys and Gate Stack Engineering for CMOS Gate Electrode Application
(2006-10-24) Chen, Bei; D. W. Barlage, Committee Member; Mark Johnson, Committee Member; Veena Misra, Committee Chair; Carl Osburn, Committee Member
The purpose of this research has been to search for proper metallic gate electrodes for CMOS devices. This dissertation covers several binary alloy metal gate research topics. First, intermetallic binary alloy RuY was investigated. From C-V analysis we obtained the effective work function of Ru-Y thin films to range from 5.0eV to 3.9eV which is suitable for dual metal gate CMOS. The rich Y film was found to be not stable on SiO2 dielectrics because of the high oxygen affinity of Y. RuxYy thin film may still be a candidate for low temperature process, especially due to its large range of work function. More over, RuY has smaller grain size than Ru which demonstrates one of the advantages of alloy by reducing grain size to achieve more uniform gate film and more uniform effective work function for the nano-size device applications. Chapter 3 presents MoxTay as a potential candidate for dual metal CMOS applications. The electrical characterization results of MoTa alloy indicates that the effective work function can be controlled to around 4.3 eV on SiO2 and is suitable for NMOS gate electrode application. The MoTa alloy forms a solid solution instead of an intermetallic compound. We report that the MoTa solid solution can achieve low work function values and is stable up to 900°C. X-ray diffraction results indicated only a single MoTa alloy phase. Moreover, from Auger electron spectroscopy and Rutherford backscattering spectroscopy analysis, MoTa was found to be stable on SiO2 under high temperature anneals and no metal diffusion into substrate Si channel was detected. This indicates that MoxTay is a good candidate for CMOS metal gate applications. Chapter 4 evaluates Ru and W capping layer for MoTa metal gate electrodes in Metal Oxide Semiconductor capacitor applications. We report that the oxygen diffusion from the capping layer plays an important role in determining the MoTa alloy effective work function value on SiO2. MoTa alloy metal gate with Ru capping exhibit stable effective work function up to 900°C anneal but is not stable with W capping. Auger electron spectroscopy and Rutherford backscattering spectroscopy analysis shows minimal oxygen diffusion into the MoTa gate stacks with Ru capping while severe oxygen diffusion into the gate is observed with W capping metal after 900°C anneal. In chapter 5, We have studied the φm behavior of AlTa alloys with varying compositions ranging from pure Al to pure Ta. The effective work function of AlTa alloy increased up to 4.45 eV as compared to pure Al work function (~4.1eV) or pure Ta work function (~4.2eV) on SiO2 at 400°C FGA. We ascribe the φm increase due to an interface dipole originating from a thin negative charged reaction layer formed between the AlTa alloy and dielectric layer. In order to further increase the stability of the AlTa alloy while still obtaining φm tuning, N was added to make AlTaN. These alloy electrodes resulted in effective work function values of ~5.13 eV after a 1000°C anneal making them suitable candidates for PMOS electrodes. Chapter 6 shows a new method that can tune the effective work function utilizing dipole layers has been demonstrated. Continued work function values can be expected by modifying the dipole strengths. This routine can potentially provide a new method for the metal gate work function research for the future wide gape semiconductor device.
Optimization of Metal Gate Electrode Stacks for Work Function Tuning
(2007-08-22) Lee, JaeHoon; Doug Barlage, Committee Member; Veena Misra, Committee Chair; Carlton Osburn, Committee Member; Gregory Parsons, Committee Member
Redox-active Organic Molecules on Silicon and Silicon Dioxide Surfaces for Hybrid Silicon-molecular Memory Devices.
(2006-11-17) Mathur, Guruvayurappan; Jonathan S. Lindsey, Committee Member; Eric Rotenberg, Committee Member; John R. Hauser, Committee Member; Veena Misra, Committee Chair
The focus of this dissertation is on creating electronic devices that utilize unique charge storage properties of redox-active organic molecules for memory applications. A hybrid silicon-molecular approach has been adopted to make use of the advantages of the existing silicon technology, as well as to study and exploit the interaction between the organic molecules and the bulk semiconductor. As technology heads into the nano regime, this hybrid approach may prove to be the bridge between the existing Si-only technology and a future molecule-only technology. Functionalized monolayers of redox-active molecules were formed on silicon surfaces of different doping types and densities. Electrolyte-molecule-silicon test structures were electrically characterized and studied using cyclic voltammetry and impedance spectroscopy techniques. The dependence of the oxidation and reduction processes on the silicon doping type and density were analyzed and explained using voltage balance equations and surface potentials of silicon. The role played by the silicon substrate on the operation of these memory devices was identified. Multiple bits in a single cell were achieved using either molecules exhibiting multiple stable redox states or mixed monolayer of different molecules. Self-assembled monolayers of redox-active molecules were also incorporated on varying thickness of silicon dioxide on n- and p- silicon substrates in an attempt to create non-volatile memory. The dependences of read/write/erase voltages and retention times of these devices were correlated to the SiO2 thickness by using a combination of Butler-Volmer and semiconductor theories. The region of operation of the silicon surface (accumulation, depletion or inversion) and the extent of tunneling current through the silicon dioxide were found to influence the charging and discharging of the molecules in the monolayer. Increased retention times due to the presence of SiO2 can be useful in realizing non-volatile memories. Polymeric films of molecules were formed on Si and SiO2 substrates and exhibited very high surface densities. Metal films were deposited directly on these films and the resultant devices were found to exhibit redox-independent behavior. A combination of metal gate and dielectric was deposited on molecules in an attempt to create solid-state hybrid silicon-molecular devices. The metal gate and dielectric can replace the electrolyte and electrolytic double-layer to create an electronic cell instead of an ionic cell. The redox properties of the molecules were retained after the deposition of dielectric and metal, which augurs well for a solid-state device. FET type structures were fabricated and molecules incorporated on them in order to modulate the characteristics of the FETs by charging and discharging the molecules. Drain current and transfer characteristics of electrolyte-gated "moleFETs" were modulated by oxidizing and reducing molecules on the channel region. Hybrid moleFET devices may be ideal tools for creating non-volatile FLASH type memory devices. This work has recognized the interaction of organic molecules and bulk silicon and utilized the advantages of current CMOS technology along with the unique properties of molecules, such as discrete quantum states, low voltage operation etc., to create a class of hybrid memory devices. A way to create solid-state molecular devices retaining the inherent properties of molecules has been proposed and demonstrated. This work might be useful in providing a smooth transition from silicon electronics to molecular electronics.