PhD Oral Examination - Design and Fabrication of Nanostructures for Light Manipulation in Solar Cells and MicroElectroMechanical Systems (Mon. Oct10, 3:00pm, CIS-X 101)

Shrestha Basu Mallick sbasumal at
Mon Oct 3 11:09:27 PDT 2011

Department of Applied Physics
University PhD Dissertation Defense

Design and Fabrication of Nanostructures for Light Manipulation in Solar
Cells and MicroElectroMechanical Systems

Shrestha Basu Mallick
Research Advisor: Professor Mark Brongersma

Monday October 10, 2011 @3:00 pm
(Refreshments served @2:45 pm)

Location: Allen Building, (Formerly CIS-X), Room 101

This talk 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 discussed 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 talk will focus 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 nanoscale photonics with CMOS
**        ****        ****        **The second part of the talk will focus
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 crystal regions. Future applications of nanostructuring to
thin-film cells that can be commercially realized will be discussed.
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