Ferroelectricity in hafnium zirconium oxide (Hf1−xZrxO2) and the factors that impact it have been a popular research topic since its discovery in 2011. Although the general trends are known, the interactions between fabrication parameters and their effect on the ferroelectricity of Hf1−xZrxO2 require further investigation. In this paper, we present a statistical study and a model that relates Zr concentration (x), film thickness (tf), and annealing temperature (Ta) with the remanent polarization (Pr) in tungsten (W)-capped Hf1−xZrxO2. This work involved the fabrication and characterization of 36 samples containing multiple sets of metal-ferroelectric-metal capacitors while varying x (0.26, 0.48, and 0.57), tf (10 and 19 nm), and Ta (300, 400, 500, and 600 °C). In addition to the well-understood effects of x and Ta on the ferroelectricity of Hf1−xZrxO2, the statistical analysis showed that thicker Hf1−xZrxO2 films or films with higher x require lower Ta to crystallize and demonstrated that there is no statistical difference between samples annealed to 500 and 600 °C, thus suggesting that most films fully crystallize with Ta ∼ 500 °C for 60 s. Our model explains 95% of the variability in the Pr data for the films fabricated, presents the estimates of the phase composition of the film, and provides a starting point for selecting fabrication parameters when a specific Pr is desired.

Concentrated photovoltaic (PV) technology represents a growing market in the field of terrestrial solar energy production. As the demand for renewable energy technologies increases, further importance is placed on the modeling, design, and simulation of these systems. Given the cultural shift toward energy awareness and conservation, several concentrated PV systems have been installed across the world. This research presents a new model for carrier concentration within a solar cell. The goal of this innovation is to facilitate the determination of the steady-state operating temperature as a function of the concentration factor for the optical part of the concentrated PV system, to calculate the optimum concentration that maximizes power output and efficiency. This model will be shown to produce a more realistic estimate of the current through a solar cell, which will enable further research into dynamic thermal modeling.

As technology continues to move forward, the demand for smaller, more efficient radio frequency (RF) devices capable of operating at higher voltages, temperatures, and frequencies also increases. Devices made using the beta phase of gallium oxide ($\beta$-Ga2O3) have the potential to operate at a high voltage and temperature. Combining the capacitance enhancement available in negative capacitance (NC) ferroelectric dielectrics with the benefits of $\beta$-Ga2O3 shows promise for the development of the next-generation RF devices. In this paper, we work towards the integration of ferroelectric Hf0.8Zr0.2O2 films on $\beta$-Ga2O3 and fabricate different capacitive structures. We characterize these capacitors using capacitance-voltage and polarization-voltage measurements and study the impact of annealing temperature on the polarization of the Hf0.8Zr0.2O2 films. Results show that saturation polarization, remnant polarization, and capacitance of the ferroelectric film increase with higher annealing temperatures. Considering that temperature could be a key factor in the fabrication and operation of RF devices made with $\beta$-Ga2O3, our study paves the way for the integration of these NC-ferroelectric materials on $\beta$-Ga2O3.

Despite the discovery of secondary electron emission over a century ago, repeatability in secondary electron emission measurements remains challenging. This work discusses the transient effects associated with sensing low-level currents during SEY measurements. Operations in the low-level range are shown to be prone to long settling times, transmission line effects, and capacitive coupling between isolated circuits. By programming our measurement system to avoid transients, our system was able to perform SEY measurements with currents as low as 140 fA.

Multipactor and vacuum breakdown are serious problems for RF systems that utilize vacuum electronic devices. These devices contain metallic surfaces that serve as the source of unwanted particles released into vacuum. The primary culprits are desorbed gas molecules and secondary electrons. Outgassing results from stimulated or thermal desorption of molecular species in metals and can lead to undesired plasma formation in the vacuum gap. Secondary electrons (SEs) can lead to the multipactor effect when an avalanche of SEs obtain resonance with the internal RF field. Here we discuss surface treatments for mitigating these problems. Laser surface melting (LSM) entails irradiating a metallic sample with the output of a high energy, continuous laser beam, thereby causing melting, flow and re-solidification of the material. LSM processing reduces H outgassing by decreasing the number of grain boundaries through which H can diffuse. We have demonstrated a 50x reduction over an untreated stainless steel surface1. For SE reduction, both surface roughening and patterning have been investigated. Surface roughening was accomplished by using specific grades of metallographic polishing pads imparting controlled levels of roughness and surface features. Electron beam bombardment experiments showed that the secondary electron yield (SEY) increased with surface roughening2. Surface patterning with micro-pores has also been shown to reduce SEY. Modeling has shown that the amount of reduction depends on the aspect ratio of the pore (the ratio of the pore height to the pore diameter), with the greatest reductions occurring for aspect ratios less than two. In order to validate these models, a variety of micro-porous gold surfaces were designed and fabricated using photolithography and electroplating processes, and the models were validated with experimental results3, These results will be discussed.

Multipactor is a critical problem in satellites and vacuum electron devices (VEDs). Described as an "avalanche" of electrons in radio frequency (RF) and microwave devices under vacuum, multipactor is caused by repeated secondary electron emission (SEE) stimulated by a time- varying electric field. Its effects range in severity from a temporary disruption in device operation to arcing, melting, cracking, or destruction of the device. A new and promising field of multipactor suppression research is engineering the internal surface topography of a VED to limit the secondary electron yield (SEY) to unity or less. Such low values of SEY render impossible the growth in electron population that is necessary to initiate the electron avalanche in a multipactor. We have developed a new model to predict the SEY of a porous surface which is useful to determine optimal topographies to control SEY. In order to assess how porous surfaces will be effected by extreme temperatures encountered in space we performed thermomechanical simulations of single gold pores. Finally, we discuss the design and fabrication of various microporous and nanoporous surfaces that will be used to validate the model in a future experimental SEY study.

This work seeks to understand how the topography of a surface can be engineered to control secondary electron emission (SEE) for multipactor suppression. Two unique, semi-empirical models for the secondary electron yield (SEY) of a micro-porous surface are derived and compared. The first model is based on a two-dimensional (2D) pore geometry. The second model is based on a three-dimensional (3D) pore geometry. The SEY of both models is shown to depend on two categories of surface parameters: chemistry and topography. An important parameter in these models is the probability of electron emissions to escape the surface pores. This probability is shown by both models to depend exclusively on the aspect ratio of the pore (the ratio of the pore height to the pore diameter). The increased accuracy of the 3D model (compared to the 2D model) results in lower electron escape probabilities with the greatest reductions occurring for aspect ratios less than two. In order to validate these models, a variety of micro-porous gold surfaces were designed and fabricated using photolithography and electroplating processes. The use of an additive metal-deposition process (instead of the more commonly used subtractive metal-etch process) provided geometrically ideal pores which were necessary to accurately assess the 2D and 3D models. Comparison of the experimentally measured SEY data with model predictions from both the 2D and 3D models illustrates the improved accuracy of the 3D model. For a micro-porous gold surface consisting of pores with aspect ratios of two and a 50% pore density, the 3D model predicts that the maximum total SEY will be one. This provides optimal engineered surface design objectives to pursue for multipactor suppression using gold surfaces.

A significant problem for space-based systems is multipactor - an avalanche of electrons caused by repeated secondary electron emission (SEE). The consequences of multipactor range from altering the operation of radio frequency (RF) devices to permanent device damage. Existing efforts to suppress multipactor rely heavily on limiting power levels below a multipactor threshold [1]. This research applies surface micromachining techniques to create porous surfaces to control the secondary electron yield (SEY) of a material for multipactor suppression. Surface characteristics of interest include pore aspect ratio and density. A discussion is provided on the advantage of using electroplating (vice etching) to create porous surfaces for studying the relationships between SEY and pore aspect ratio & density (i.e. porosity). Preventing multipactor through SEY reduction will allow power level restrictions to be eased, leading to more powerful and capable space-based systems.

How surface geometries can be selectively manipulated through nanosphere lithography (NSL) is discussed. Self-assembled monolayers and multilayers of nanospheres have been studied for decades and have been applied to lithography for almost as long. When compared to the most modern, state-of-the-art techniques, NSL offers comparable feature resolution with many advantages over competing technologies. Several high-resolution alternatives require scan-based implementation (i.e., focused ion beams and e-beam lithography) while NSL is much more of a batch operation, allowing for full wafer or possibly even multiple wafer processing, potentially saving time and increasing throughput in a manufacturing environment. Additionally, NSL has continued to be of interest because it does not require expensive, complex equipment to be researched and realized, which continues to fuel interest in this approach. In spite of these advantages, applying NSL to specific, realizable devices is limited in the literature. The reason for this lack of application is not only unreliability in the self-assembly process, but also control of these patterned nanospheres within larger, multistep processes often required to fabricate most devices. Both of these items are addressed in this paper. The first issue was addressed through the development of a series of custom-designed nanosphere application vessels. These were designed based on the best published results from the literature, utilizing an alternate method of dip-coating but performed through draining the carrier fluid over the substrate rather than moving the substrate across the liquid–air boundary layer. This method is in the easier to perform, but arguably less-reliable spin-coating method also commonly employed. The key enabler in this effort lies in commercially available three-dimensional (3-D) printing technology, and how it was applied to rapidly prototype-improved deposition vessels. This was accomplished primarily with a single day turn-around between each 3-D printed design iteration. Each vessel design was incrementally improved, built, and tested to optimize the best performance in achieving the most reliable, repeatable self-aligned nanosphere layer formation. With an optimized design of this vessel in hand, the second challenge was addressed by using this vessel and the patterned nanosphere layers it produced with a patterned photoresist design to capture single layers of nanospheres in specifically designed locations and orientations. The hybrid mask produced from this approach can be integrated within virtually any multistep fabrication process. Additionally, other processing steps will be discussed, such as reactive ion etching (RIE), plasma ashing, and photoresist reflowing, and how they might be combined with these hybrid masks. Various results from combinations of these steps are presented. Finally, two potential applications which could benefit greatly from the resulting, engineered surface structures are discussed. These include a small-scale device application (engineering the contacting surfaces in a microswitch), as well as a much larger scale surface study application (surface engineering for controlling secondary electron emissions). The final results from this method allow for patterning groups of 500-nm polystyrene nanospheres formed in four to eight distinct rows each. These are positioned within patterned wells created in a 650-nm thick photoresist. The size and location of these wells are as precise as the photolithography process used to form them, in this case, ∼40-nm position error in the location of the edge of the laser using a Heidelberg laser lithography system. By combining multiple wells in close proximity, virtually any combination of nanosphere clusters become possible. Once patterned, postprocessing though RIE and deposition method selection together determine the final shape of the nanoscale features which result.

In this paper, we report on the electrical properties of the phase change (PC) material, Germanium Telluride (GeTe), and discuss the fabrication and testing of GeTe resistors. Vertical and horizontal GeTe resistors are fabricated and tested through resistance measurements before and after substrate heating and joule heating performed by voltage pulsing. Resistance changes of six orders of magnitude and four orders of magnitude are demonstrated by substrate heating and voltage pulsing, respectively. The horizontal resistor designs are shown to be impractical for low voltage applications due to the their large inter-electrode distances. Various sizes of GeTe resistors ranging from 15×15×0.4 μm3 to 3×3×0.09 μm3 are tested and a relationship is demonstrated between GeTe resistor length and the threshold voltage pulse required for phase change.