Caltech Nanofabrication Group

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Sameer Walavalkar

Personal Information:

Position: Alumni (Visiting Associates) Walavalkar, Sameer
Research Area: Nanophotonics
Phone: 626.395.4578
Location: Research Scientist, USC Translational Imaging


Silicon Nanostructures

I work on making sub-10nm sized silicon nanostructures using CMOS compatible, top-down, 3D etching techniques. Constricting silicon to this size fundamentally changes its properties. At this scale silicon bends like a rubber band, emits light, and can be used to make room-temperature quantum devices. The image above is a rendering of a Coulomb Blockade, double tunnel junction device that we have fabricated.

Resonant Tunneling Devices

The images below show the fabrication of a double-tunnel junction silicon nanopillar device. Panel (a) shows the nanopillar just after etching. We have worked out a novel etching method that lets us sculpt the silicon as we etch it. (Scale bar is 100nm) Panel (b) shows a TEM of a similar pillar after oxidation. (Scale bar is 50nm) We utilize a unique self-terminating oxidation to pick the final diameter of the silicon core inside the oxide sheath. Panel (c) shows a schematic of what the bandgap of the resulting silicon nano-whisker should look like inside the oxidized nanopillar. Spatially restricting the size and putting the silicon understrain should significantly widen the bandgap. By modulating this effect in the vertical direction we can make custom energy landscapes -- a method we have called 'geometric bandgap engineering.' Panel (d) is a schematic of how we make contact to the device and panel (e) shows a completed device. (Scale bar is 500nm)

Fab flow of DTJ device

This device and its gated counterpart displays clear Coulomb Blockade behavior at liquid nitrogen temperature. It is our goal to push the device size to a smaller scale in order to retain this novel behavior at room temperature.

Three Dimensional Etching

Recently we published an article in Nanoscale explaining a new etching method that lets us shape silicon with single nanometer precision. With this method we can make structures that were previously impossible to create, using simple silicon wafers and single masking step. Here are a few examples:

For more information on this technique please take a look at our article: Walavalkar et al. Nanoscale (2013)

Silicon Luminescence

Although silicon has been the electronic material of choice of the semiconductor industry it has been dismissed as an optical gain material due to its lack of luminescent behavior. This poor optical property stems from a band misalignment in momentum between the electrons in the conduction band and the holes in the valence band. We have found, however, by squeezing silicon structures down to between 2 and 10 nm we can re-orient the bandstructure and 're-align' the conductance and valence band. We used this property to create bright silicon nanopillars that could emit light anywhere from the near-IR through the visible spectrum. The plot below shows the measured photoluminescence from arrays of pillars with different diameters:

PL plot

For more information on this novel behavior please take a look at our article: Walavalkar et al. Nanoletters (2010)

Nanostructures in Biology

More info coming soon!

Fiat Lux!









Administrative and Financial Contact

Kate Finigan
MC 200-36, Caltech
1200 E California Blvd
Pasadena, CA 91125

Office:  212 Sloan Annex
Phone:  626.395.4585
Fax: 626.577.8442