| AVS 58th Annual International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Thursday Sessions |
| Session EN+NS-ThA |
| Session: | Nanostructures for Energy Storage and Fuel Cells II |
| Presenter: | Lauren Haspert, University of Maryland, College Park |
| Authors: | L.C. Haspert, University of Maryland, College Park G.W. Rubloff, University of Maryland, College Park S.B. Lee, University of Maryland, College Park |
| Correspondent: | Click to Email |
Increasing energy demands require innovative nanofabrication techniques for efficiently storing and supplying available energy. This talk discusses how anodic aluminum oxide (AAO) and atomic layer deposition (ALD) technologies are implemented and designed for creating high performance nanoelectrostatic metal-insulator-metal (MIM) capacitors. The densely porous (~1010pores/cm2) self-aligned, self-assembled AAO nanostructure serves as a complex nanostructured template in which the self-limiting and conformal ALD process can uniformly coat this complex 3D structure. Thus, combining these two technologies results is a nano-capacitor with high power density and increased energy density, comparable to electrochemical batteries.
AAO template fabrication is a two-step anodization process, in which pores self-order in the first anodization. Then, the oxide is removed, leaving in a pre-patterned scalloped Al surface. Peak asperities are rounded with a barrier anodic alumina (BAA) and the rounded structures are retained during the subsequent anodization. Mild anodization (MA) chemistries provide interpore spacings, Dint (in nm), equal to 2.5x the anodization voltage, Vanod (in V), whereas hard anodization chemistries provide Dint~2xVanod. In this work, oxalic acid MA results in pores spaced 100nm apart and 40nm in diameter. A final step etches pore sidewalls, increasing pore diameters up to 85nm. MIM layers are deposited by sequentially depositing 10nm of Al-doped ZnO (AZO), 8nm of Al2O3 and ~100nm AZO.
The porous structure increases the available surface area on which charge is stored, thus increasing the energy density since E=½CV2. The capacitance increases with increasing depth, where planar, 1µm, 1.5µm and 2µm pore depth have capacitance of ~1, 11, 19 and 26µF/cm2, respectively. Introducing the BAA reduces leakage currents to ~10-10A/cm2 and breakdown fields are increased to 9.3MV/cm. A model simulates performance of the 3D nanogeometry, distributed resistances and dielectric capacitances, and internal non-linear resistance of the capacitor as a function of voltage. Additionally, trade-offs between pore size vs. layer thickness, AAO template interpore spacings vs. capacitance, pore depth vs. electrode series resistance are considered.
The ability to create scalable nano-structured devices is highly desirable for integrating with energy harvesting technologies. The fully self-aligned, self-assembled and self-limiting MIM nanocapacitors fabricated with ALD deposition in AAO templates demonstrate excellent electrical performance. Simulating device performance will aid in further increasing device performance and energy densities.