Complementary Symmetry Nanowire Logic Circuits: Experimental Demonstrations and in Silico Optimizations

Complementary symmetry (CS) Boolean logic utilizes both p- and n-type field-effect transistors (FETs) so that an input logic voltage signal will turn one or more p- or n-type FETs on, while turning an equal number of n- or p-type FETs off. The voltage powering the circuit is prevented from having a direct pathway to ground, making the circuit energy efficient. CS circuits are thus attractive for nanowire logic, although they are challenging to implement. CS logic requires a relatively large number of FETs per logic gate, the output logic levels must be fully restored to the input logic voltage level, and the logic gates must exhibit high gain and robust noise margins. We report on CS logic circuits constructed from arrays of 16 nm wide silicon nanowires. Gates up to a complexity of an XOR gate (6 p-FETs and 6 n-FETs) containing multiple nanowires per transistor exhibit signal restoration and can drive other logic gates, implying that large scale logic can be implemented using nanowires. In silico modeling of CS inverters, using experimentally derived look-up tables of individual FET properties, is utilized to provide feedback for optimizing the device fabrication process. Based upon this feedback, CS inverters with a gain approaching 50 and robust noise margins are demonstrated. Single nanowire-based logic gates are also demonstrated, but are found to exhibit significant device-to-device fluctuations

DEVELOPMENT OF A SIMPLIFIED, MASS PRODUCIBLE HYBRIDIZED AMBIENT, LOW FREQUENCY, LOW INTENSITY VIBRATION ENERGY SCAVENGER (HALF-LIVES)

Scavenging energy from environmental sources is an active area of research to enable remote sensing and microsystems applications. Furthermore, as energy demands soar, there is a significant need to explore new sources and curb waste. Vibration energy scavenging is one environmental source for remote applications and a candidate for recouping energy wasted by mechanical sources that can be harnessed to monitor and optimize operation of critical infrastructure (e.g. Smart Grid). Current vibration scavengers are limited by volume and ancillary requirements for operation such as control circuitry overhead and battery sources. This dissertation, for the first time, reports a mass producible hybrid energy scavenger system that employs both piezoelectric and electrostatic transduction on a common MEMS device. The piezoelectric component provides an inherent feedback signal and pre-charge source that enables electrostatic scavenging operation while the electrostatic device provides the proof mass that enables low frequency operation. The piezoelectric beam forms the spring of the resonant mass-spring transducer for converting vibration excitation into an AC electrical output. A serially poled, composite shim, piezoelectric bimorph produces the highest output rectified voltage of over 3.3V and power output of 145uW using ¼ g vibration acceleration at 120Hz. Considering solely the volume of the piezoelectric beam and tungsten proof mass, the volume is 0.054cm3, resulting in a power density of 2.68mW/cm3. Incorporation of a simple parallel plate structure that provides the proof mass for low frequency resonant operation in addition to cogeneration via electrostatic energy scavenging provides a 19.82 to 35.29 percent increase in voltage beyond the piezoelectric generated DC rails. This corresponds to approximately 2.1nW additional power from the electrostatic scavenger component and demonstrates the first instance of hybrid energy scavenging using both piezoelectric and synchronous electrostatic transduction. Furthermore, it provides a complete system architecture and development platform for additional enhancements that will enable in excess of 100uW additional power from the electrostatic scavenger

Performance Comparison of Phase Change Materials and Metal-Insulator Transition Materials for Direct Current and Radio Frequency Switching Applications

Advanced understanding of the physics makes phase change materials (PCM) and metal-insulator transition (MIT) materials great candidates for direct current (DC) and radio frequency (RF) switching applications. In the literature, germanium telluride (GeTe), a PCM, and vanadium dioxide (VO2), an MIT material have been widely investigated for DC and RF switching applications due to their remarkable contrast in their OFF/ON state resistivity values. In this review, innovations in design, fabrication, and characterization associated with these PCM and MIT material-based RF switches, have been highlighted and critically reviewed from the early stage to the most recent works. We initially report on the growth of PCM and MIT materials and then discuss their DC characteristics. Afterwards, novel design approaches and notable fabrication processes; utilized to improve switching performance; are discussed and reviewed. Finally, a brief vis-á-vis comparison of resistivity, insertion loss, isolation loss, power consumption, RF power handling capability, switching speed, and reliability is provided to compare their performance to radio frequency microelectromechanical systems (RF MEMS) switches; which helps to demonstrate the current state-of-the-art, as well as insight into their potential in future applications

Quantum Dot Enhanced Epitaxial Lift-Off Solar Cells

Embedded nanostructures such as quantum dots (QDs) have been studied for many applications in solar cells including enhanced mini-band absorption in intermediate-band solar cells and current matching in multi junction cells. The major drawbacks of using such techniques to decrease intrinsic solar cell loss mechanisms are twofold: first, it is difficult to maintain partially populated states using QDs due to a quick thermal extraction of carriers; second, QDs have a weak absorption which necessitates a near-perfect control of QD growth mechanisms to carefully ensure a balance between dot size and density. One avenue for improving absorption into QDs is to utilize a thin cell with a back surface reflector in order to increase the effective optical path length (OPL) of light through the QD region, which has the potential to increase absorption into QD states. One method for the processing of thin solar cells that has been experimentally demonstrated on large 4-6” wafers is epitaxial lift-off, which takes advantage of an inverted growth and a wet chemical etch of a sacrificial release layer to remove the substrate. In this thesis, 0.25 cm2 InAs/GaAs QD cells were grown on 4” wafers, fabricated, and processed by epitaxial lift off, creating thin and flexible devices. Materials and optical characterization techniques such as atomic force microscopy and photoluminescence were used on test structures prior to and following ELO, and analysis indicated that QD optical coherence and material quality after ELO processing were preserved, although non-uniform. This was concluded to be caused by the radial thermal profile of the growth reactor, through which spatial dependence led to local variations in QD quality and size across the 4” wafer, indicative of the high temperature sensitivity of QDs. Transmission electron microscopy measurements were used to investigate defects and dislocations throughout the QD device structure that would impact performance, and showed a higher concentration of defects in regions of the wafer subject to a higher temperature during growth. A similar pattern of radial dependence was observed in solar cell devices by electrical characterization. Current-voltage measurements under one-sun AM0 illumination were taken on several cells around the wafer, showing a statistical variation in solar cell device metrics dependent on wafer position. Spectral responsivity measurements show an established cavity mode pattern in sub-host bandgap wavelengths, which is discussed as an enhancement due to the thinning of the device. Integrated external quantum efficiency shows a QD contribution to the short circuit current density of 0.23 mA/cm2. In addition to optical, materials, and electrical characterization, QD and baseline ELO devices were exposed to alpha radiation to gauge the effects of a harmful environment on cell performance. The QD device exhibited a remaining factor increase of 2 % (absolute) in conversion efficiency over the baseline device at an end of life alpha particle fluence of 5x109α/cm2/s. In addition, linear temperature coefficients for solar cell figures of merit were extracted as a function of increasing α fluence. At a fluence of 5x108α/cm2/s, the QD device showed an efficiency temperature coefficient 0.2 %/°C higher (absolute) than the baseline, indicating that the inclusion of QDs could improve the radiation and temperature tolerance of solar cell devices used for space applications

Optimization of electron beam lithography and lift-off process for nanofabrication of sub-50 NM gold nanostructures

Since the demonstration of the first integrated circuit in the late 1950s, the microelectronics industry has witnessed a vast transformation with transistor densities doubling roughly every two years as a result of continuous scaling down of device dimensions, referred to as miniaturization. The fundamental concept of miniaturization has not only been employed for the realization of ultra large scale integrated (ULSI) circuits with reduced manufacturing costs, lower power consumption, higher speed and computational power; but also, for developing novel transducer elements and energy storage devices by harnessing the unique physical effects that arise at micro/nanoscales such as higher surface-to-volume ratios. One of the most important technologies in micro/nano device fabrication, if not the single most important, is lithography. The broad range of lithographic techniques ranging from conventional optical lithography methods (e.g. ultraviolet-UV, deep ultraviolet-DUV, extreme ultraviolet-EUV) to unconventional ones (e.g. electron beam lithography, x-ray lithography, ion-beam lithography, stereolithography, scanning probe lithography, nanoimprint lithography, directed self-assembly) can be used to create features with microns to tens of nanometer resolution and below. Among these, electron beam lithography (EBL) stands out as a powerful direct-write tool offering nanometer scale patterning capability and is especially useful in low volume R&D prototyping. However, patterning with EBL requires careful balance of process parameters which need to be considered in conjunction with the pattern transfer technology that can be either etching or lift-off specifically for the case metallic layers. Accordingly, this thesis provides a systematic study to address the gap in process optimization of lift-off process based on EBL patterning of sub-50 nm metallic nanostructures using a lower cost PMMA/PMMA positive tone bilayer resist spin approach. The governing parameters in EBL including exposure dose, bake temperature, develop time, developer solution, substrate effect, proximity effect (PE) are experimentally studied and their effects on nanopatterning are characterized by field emission scanning electron microscopy (FE-SEM) of fabricated nanostructure

Exploration of Miniature Flexible Devices Empowered by Van Der Waals Material

This research mainly focuses on the fabrication of miniature flexible devices empowered by van der Waals materials. Through the extensive experiments contained in this thesis, by exploring the characteristics of van der Waals materials, optimizing the manufacturing process of lithography technology, and characterizing the photoelectric performance of micro devices, this thesis has promoted the development of micro flexible device manufacturing and expanded its applications in the fields of biological detection, medical treatment, and environmental monitoring. We introduced a miniature van der Waals semiconductor empowered vertical color sensor, which saves three times the volume space compared to the traditional planer color sensor and includes multiple optical aberration correction functions as well. Such a small red, green, and blue (RGB) color sensor can be applied in bionic eyes, breaking through the limitations of existing black and white recognition. On this basis, we further explored the stretchability of two-dimensional materials represented by MoS2. We proposed a chemical treatment method combined with gold nanoparticles and (3-mercaptopropyl)trimethoxysilane (MPTMS) to realize the relocation of flexible micro devices. This method improves the adhesion between the material layer and the flexible substrate (PDMS), which significantly increases the flexible device stretchability, and prolongs its service life. Through the above work, this thesis explores the van der Waals materials’ properties, and optimizes the manufacturing process of micro devices, further exerts the advantages of material flexibility, therefore provides more possibilities for the development of smart wearable devices, biomedical detection, and other fields