Optical, Mechanical, and Electronic Properties of Etched Silicon Nanopillars

Citation

Walavalkar, Sameer Sudhir (2011) Optical, Mechanical, and Electronic Properties of Etched Silicon Nanopillars. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/QCW6-0C39. https://resolver.caltech.edu/CaltechTHESIS:05162011-142708481

Abstract

This work focuses on the fabrication, characterization and applications of silicon nanopillars. We explain the techniques involved in creating sub 50 nm diameter pillars with aspect ratios of 60:1. Original work encompassed the use of a novel etch mask made of reactive ion sputtered aluminum oxide, 'pseudo-Bosch' inductively coupled reactive ion etching (ICP-RIE) to etch structures on the nanoscale. These methods demonstrate a unique approach to the largely 'bottom-up' technology used in nanowire fabrication.

We also explored the self-terminating oxidation behavior of convex, two-dimension silicon structures. It was found that during the oxidation process, strain built up at the moving Si-SiO₂ interface eventually led to a cessation of oxidation. This was used to predictably reduce the diameter of these pillars to 2 nm, making 'nanowhiskers.' We were able to characterize the results of this oxidation non-destructively by utilizing reflection mode transmission electron microscopy (R-TEM).

Using spun-on PMMA and an electron beam to constrict it and bend the pillars, we were able to incorporate as much as 25% strain. More interestingly this deformation appeared to be elastic, as the pillars, once freed from the polymer, would snap back to their upright position.

A consequence of the creation of silicon nanowhiskers was that silicon, a normally poor light emitter due to its indirect bandgap, became photoluminescent. As we reduced the diameter we noticed that the bandgap became direct and the emission peak was blue-shifted. We were able to utilize a tight-binding model (TBM) that was modified by the oxidation induced strain. This modified model predicted the blue-shift in peak emission wavelength with decreasing pillar diameter. The strain induced in the pillar during the oxidation played a significant role in the peak emission wavelength and shape of the bandstructure. By corrugating the pillars with an oscillating etch technique we were able to turn our nanopillars into quantum dots which also proved to photoluminesce.

Finally we look at the possibilities of creating a silicon light emitting diode. By creating a double-gated structure it is possible to overcome the difficulties encountered with sub 5 nm diameter pillars. A possible fabrication process, and the current work done to implement it, is presented as well as a simulation explaining the behavior of this device in the future.

Thesis Files