Over the past 60 years silicon devices have evolved to form the backbone of the micro-electronics industry. Unfortunately, the indirect nature of the silicon band-gap has hampered the material with highly inefficient light emission properties, thus impeding its spread into the optoelectronic field. Recent work, however, has focused on utilizing the physical changes seen in low dimensional nanostructures to breathe life into the use of silicon as an active optical material. The most popular nanostructuring technique, precipitating nanocrystals, comes with some key limitations such as the fact that the spatial and size distribution of these dots cannot be controlled accurately.
Our recent work has been to fabricate and characterize singular etched quantum dots. These top-down devices have demonstrated narrow linewidths and ‘blinking’ behavior associated with emission from single nanocrystals. We have demonstrated the ability to fabricate uniform arrays of stacked pillars of silicon quantum dots whose size (and thus peak emission wavelength) can be precisely tuned via etching parameters and oxidation conditions. These vertical quantum dots exhibit bright photoluminescence (PL) from the visible to the near infrared and can be predictably and repeatably placed through lithographic techniques.
The samples were fabricated using a “pseudo-Bosch” etch recipe by oscillating the etching conditions to controllably undercut and overpassivate the silicon. The oscillations were designed to produce three uniform corrugations per pillar. The following step in the fabrication is a self-terminating furnace oxidation. Then the presence of a silicon quantum dot was confirmed via TEM as well as PL testing.
The ability to fabricate silicon quantum dots with predictable sizes could prove to be useful when attempting to incorporate them with existing silicon photonic structures such as waveguides or photonic crystals. Instead of using a stochastic distribution of nanocrystals, these can be placed lithographically to coincide with the peak of the optical mode. The ability to tailor the peak emission of the quantum dots allows one to select the emission wavelength to suit a task or create a vertical stack of different dot sizes to allow for broad spectral emission. We have presented a CMOS compatible method to fabricate narrow band, luminescent silicon quantum dots. These devices and their behavior may have important applications in both future nanophotonic and nanoelectronics devices.
- Walavalkar, S., Homyk, A., Hoffmann, C. E., Henry, M. D., Atwater, H. A. & Scherer, A. (2011). Size Tunable Visible and Near-Infrared Photoluminescence from Vertically Etched Silicon Quantum Dots. Applied Physics Letters, 98(15), 153114.