Design And Fabrication Of Nanostructures For Light Manipulation In Solar Cells And Microelectromechanical Systems

This work is about nano-structuring of semiconductor devices for the improvement of their optical properties. The phenomenon of guided resonances in photonic crystal (PC) slabs will be introduced and ways of engineering these guided resonances for a variety of applications such as improved reflectivity in micro-electro-mechanical (MEMS) mirrors and increased absorption in thin-film solar cells will be discussed. The first part of the work focuses on the application of PCs in MEMS. A new process (GOPHER) that was developed to make low stress PCs out of monolithic silicon will be discussed. The advantage and ease of making multilayer PCs with Gopher will be illustrated and experimental results showing Gopher structures having spectra with broadband reflectivity (suitable for mirror applications) as well as sharp peaks (suitable for sensors) will be presented. Other applications of Gopher such as making waveguides etc., will be briefly discussed. The integration of a high quality PC mirror with a 1D resonant MEMS scanner will be demonstrated. Finally, the results of fabrication of a prototype wafer-scale encapsulated optical device will be shown. The Gopher process has a lot of potential for facilitating the integration of micro- and nano-scale photonics with CMOS circuitry. The second part of the work focuses on the application of PCs for light-trapping in solar cells. Thin-film photovoltaics has the potential to reduce cost by reducing the amount of photoactive material required and allowing for the use of material of poorer quality. Crystalline Silicon (c-Si) is an attractive material for photovoltaic cells due to its natural abundance, nearly ideal band gap, and leverage of existing process and materials knowledge. However, the poor optical absorption in the near-infrared spectral range requires the use of very efficient light trapping techniques. One such technique that is explored is to pattern the active layer into a 2D PC. Electromagnetic simulations are performed to show that an enhancement in integrated short-circuit current by a factor of 3 is possible when compared to a planar slab of equivalent volume. This is because the PC supports guided resonances into which incident radiation can couple which increases the absorption. Finally, the fabrication of an ultrathin c-Si solar cell where the active material is patterned into a square-lattice 2D PC is demonstrated. Both short-circuit current and external quantum efficiency measurements show an enhancement in optical absorption, especially at longer wavelengths. Scanning photocurrent maps confirm the improved optical absorption in the photonic structure over an unpatterned reference. Future applications of nanostructuring to thin-film cells that can be commercially realized is discussed.