Piezoelectric microelectromechanical systems (PiezoMEMS) are attractive for developing next generation self-powered microsystems. PiezoMEMS promises to eliminate the costly assembly for microsensors/microsystems and provide various mechanisms for recharging the batteries, thereby, moving us closer towards batteryless wireless sensors systems and networks. In order to achieve practical implementation of this technology, a fully assembled energy harvester on the order of a quarter size dollar coin (diameter=24.26 mm, thickness=1.75 mm) should be able to generate about 100 μW continuous power from low frequency ambient vibrations (below 100 Hz). This paper reviews the state-of-the-art in microscale piezoelectric energy harvesting, summarizing key metrics such as power density and bandwidth of reported structures at low frequency input. This paper also describes the recent advancements in piezoelectric materials and resonator structures. Epitaxial growth and grain texturing of piezoelectric materials is being developed to achieve much higher energy conversion efficiency. For embedded medical systems, lead-free piezoelectric thin films are being developed and MEMS processes for these new classes of materials are being investigated. Non-linear resonating beams for wide bandwidth resonance are also reviewed as they would enable wide bandwidth and low frequency operation of energy harvesters. Particle/granule spray deposition techniques such as aerosol-deposition (AD) and granule spray in vacuum (GSV) are being matured to realize the meso-scale structures in a rapid manner. Another important element of an energy harvester is a power management circuit, which should maximize the net energy harvested. Towards this objective, it is essential for the power management circuit of a small-scale energy harvester to dissipate minimal power, and thus it requires special circuit design techniques and a simple maximum power point tracking scheme. Overall, the progress made by the research and industrial community has brought the energy harvesting technology closer to the practical applications in near future.

The oxidation states, interface, and band alignment properties of physical vapor deposited CeO 2 films on epitaxial (100), (110), and (111)Ge were investigated by x-ray photoelectron spectroscopy (XPS). The cross-sectional transmission electron microscopy demonstrated the polycrystalline nature of the CeO 2 film. XPS analysis showed multiple Ce3d and Ce4d oxidation states with a mixture of Ce 3þ and Ce 4þ components existing in CeO 2 . Angular resolved XPS investigations indicate that the CeO 2 films mostly consist of Ce 4þ oxidation states while the Ce 3þ oxidation states are preferentially present near the surface. The CeO 2 /(100)Ge, CeO 2 /(110)Ge, and CeO 2 /(111)Ge structures showed almost identical valence band offset (VBO) values of 1.6, 1.5, and 1.6 eV, respectively, using XPS measurements from Ce3d core level (CL) peaks. These (VBO) values were also supported by XPS measurements from shallow Ce4d CL binding energy peaks. The conduction band offset values between CeO 2 /Ge were $1.3 eV using the measured optical bandgap of CeO 2 . The XPS spectral analysis of cerium oxidation states and the measured band offset parameters for carrier confinement would offer an important path for the future design of Ge-based metal-oxide semiconductor devices.

Piezoelectric microelectromechanical systems (PiezoMEMS) are attractive for developing next generation self-powered microsystems. PiezoMEMS promises to eliminate the costly assembly for microsensors/microsystems and provide various mechanisms for recharging the batteries, thereby, moving us closer towards batteryless wireless sensors systems and networks. In order to achieve practical implementation of this technology, a fully assembled energy harvester on the order of a quarter size dollar coin (diameter = 24.26 mm, thickness = 1.75 mm) should be able to generate about 100 μW continuous power from low frequency ambient vibrations (below 100 Hz). This paper reviews the state-of-the-art in microscale piezoelectric energy harvesting, summarizing key metrics such as power density and bandwidth of reported structures at low frequency input. This paper also describes the recent advancements in piezoelectric materials and resonator structures. Epitaxial growth and grain texturing of piezoelectric materials is being developed to achieve much higher energy conversion efficiency. For embedded medical systems, lead-free piezoelectric thin films are being developed and MEMS processes for these new classes of materials are being investigated. Nonlinear resonating beams for wide bandwidth resonance are also reviewed as they would enable wide bandwidth and low frequency operation of energy harvesters. Particle/granule spray deposition techniques such as aerosol-deposition (AD) and granule spray in vacuum (GSV) are being matured to realize the meso-scale structures in a rapid manner. Another important element of an energy harvester is a power management circuit, which should maximize the net energy harvested. Towards this objective, it is essential for the power management circuit of a small-scale energy harvester to dissipate minimal power, and thus it requires special circuit design techniques and a simple maximum power point tracking scheme. Overall, the progress made by the research and industrial community has brought the energy harvesting technology closer to the practical applications in near future.

We wrapped 150 nm period aluminum wire grid polarizer (WGP) with AlSiOx by using atomic layer deposition at 250 degrees C. The nanometer precision coating defined the spacer to double the spatial frequency of the 100 mm diameter grating fabricated by using a legacy immersion holography setup at 351 nm wavelength. Half-pitch grating of approximately 38 nm was demonstrated with good pattern uniformity, excellent repeatability, and a wide processing window. We believe 10 nm half-pitch grating over even larger areas are viable, overcoming one major hurdle to commercialize nanoimprint.

We developed an integrated circular polarizer based on stacking an aluminum nano-wire grid polarizer with a dielectric nano-grating-based quarter waveplate. The polarizer consists of 65 nm wide and 130 nm tall aluminum wires with a period of 148 nm. For integration, the aluminum nanowires were buried into a silicon dioxide matrix by a trench filling and planarization technology. The buried nanowire polarizer achieved excellent optical performance in a broad wavelength range from 400 nm to >900nm. On top of the buried and planarized nanowire polarizer, a visible quarter waveplate based on a 200 nm period silicon nitride nano-grating was fabricated. Both the 148 nm period aluminum grating and the 200 nm period silicon nitride grating were fabricated by an ultraviolet (UV)-nanoimprint lithography. The ability to integrate multiple nanostructure-based optical layers opens a path for novel integrated optical devices, as well as a new strategy for driving both miniaturization and cost.