Exploring the limits of materials for energy and charge storage applications [electronic resource]

This thesis seeks to contribute to the solution of two critical challenges in our society - energy storage and charge storage - by creating a material or a combination of materials engineered at the nanoscale level. Research on alternative energy sources to replace fossil fuels has received considerable attention recently due to concerns about the environmental and geopolitical stability. As alternative energy sources become more popular, finding an efficient electrical or electrochemical energy storage system also become necessary as a replacement for chemical energy storage based on fossil fuels. Capacitive energy storage is attractive because it is a pure electronic storage mechanism, which in turn offers safety, longevity and compact size. However, although its power density is high, the energy density of such devices needs considerable improvement. Capacitive storage is also critical for another prominent challenge in our society, charge storage in integrated circuits. The downscaling of integrated circuits poses many challenges. In such devices, capacitance is directly proportional to the active area and inversely proportional to the thickness. The thickness, however, cannot be decreased indefinitely due to leakage by Fowler-Nordheim tunneling and ultimately due to the size of an atomic monolayer. This poses a significant restriction on, for example, the design of memory capacitors in dynamic random access memories (DRAMs). To overcome these challenges, this work focuses on optimizing the properties of a dielectric material to minimize charge loss by leakage, sustain a high electric field and maximize the dielectric constant by taking advantage of atomic layer deposition (ALD), in order to create capacitive energy and charge storage devices. ALD is introduced in the first part of this work as an enabling technology to deposit ultrathin, high-quality thin films and nanostructures. Its self-limiting chemistry is explained by contrasting thermal and plasma-enhanced ALD. Two techniques to pattern ALD films in-situ are designed and experimentally demonstrated. The breakdown behavior of silicon dioxide (SiO2) is studied in detail as a high-bandgap, high-breakdown strength material that may be used as a barrier layer for bandgap engineering. An extremely high hard breakdown strength reaching above 2.0 V/nm was observed along with a window where Fowler-Nordheim tunneling becomes the dominant leakage mode. The laminated capacitors of hafnium dioxide (HfO2) and aluminum oxide (Al2O3) are then presented as an example of a bandgap-engineered device to improve the breakdown strength. In the seven-layer grated structure, a breakdown strength of 0.58 V/nm was achieved due to suppression of crystallization and reduction of the electric field in HfO2. The area-to-thickness ratio is presently the highest reported to the author's knowledge, proving the quality of ultrathin dielectric films deposited by ALD. Device-level studies of energy and charge storage are then presented. A new type of capacitive energy storage mechanism utilizing platinum quantum dots embedded in a dielectric matrix was designed, fabricated and tested. Although no conclusive evidence of energy storage was witnessed, indications of charge transfer between the embedded platinum structures were observed. Finally, a titanium dioxide (TiO2) based charge storage device that has a low leakage current, high dielectric constant and uniform step coverage is presented. Doping and bandgap engineering were proved to be beneficial in reducing the leakage current. Plasma exposure during plasma-enhanced ALD was shown to influence the microstructure of the deposited thin film, which may allow fine-tuning of the dielectric constant. Crystalline TiO2 was deposited on a trench structure of 1:50 aspect ratio using plasma-enhanced ALD with 88 % step coverage. Such functional materials and devices engineered at the nanoscale by ALD may contribute to improving present energy and charge storage technologies.

Keywords: Mechanical Engineering.
Advisor: Prinz F. B.
Kenny Thomas William advisor. Thesis advisor
Sinclair Robert (Materials scientist) advisor. Thesis advisor