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Atomic force microscope-based lithography for site-selective surface grafting of semiconductor nanocrystals on patterned silicon Wang, Tian Si

Abstract

A three dimensional optical microcavity was designed in a textured silicon planar waveguide to have a resonant wavelength near 1.5 μm, and a quality factor of ~4000. The design is based on planar photonic bandgap concepts, and was achieved using a commercial finite-difference-time-domain electromagnetic simulation tool. This microcavity is intended to be integrated with PbSe semiconductor nanocrystals synthesized by collaborators to exhibit a strong excitonic resonance also near 1.5 μm. To study the coupling of this excitonic resonance, with the optical microcavity mode, a process for locally attaching the PbSe nanocrystals on the silicon microcavity surface in a region of high electric field intensity had to be developed. Accordingly, a lithographic process was developed based on the use a conducting atomic force microscope (AFM) tip to oxidize the silicon surface at a precise location with respect to the photonic crystal microcavity. The AFM lithography has to be done after the silicon microcavity is first uniformly coated with an organic monolayer designed to repel the PbSe nanocrystals. Two different organic monolayers, OTS (Octadecyltrichlorosilane), and octadecene, were used in this study. Two different AFM microscopes were adapted for this process development work. By varying the applied bias voltage and pulse duration on bare silicon, and silicon-on-insulator substrates, and on monolayer-coated versions of the same substrates, the optimum conditions for oxide dot formation were determined. While it is relatively easy to form oxide dots with diameters down to ~50 nm on bare silicon and silicon-on-insulator substrates using ~ -5 V, 100 ms pulses, higher voltages and longer pulse durations were required to form oxide dots on the monolayer coated samples, and the uniformity of the oxide dots formed on the monolayer samples was found to be poorer than on the bare samples. An AFM-oxidized dot with ~190 nm diameter was successfully formed within 50 nm of the centre of an octadecene-coated microcavity formed in a silicon-on-insulator substrate. This proves that this process is capable of site-selectively defining a site for nanocrystal attachment on the microcavities, but further work is needed to make the process "routine".

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