While silicon electronics has been a success in modem technologies, silicon photonics is still in development and in need of a laser source. Many approaches have been explored, from anodized silicon luminescence, to generating direct emissions by quantum-confinement, and to indirect down-conversion of a shorter wavelength laser light via silicon's nonlinear dielectric responses. One approach that was developed has led to the demonstration of laser emission in silicon-on-insulator at cryogenic temperatures (<85K).

Silicon's inability to emit light and to 'lase' is rooted in the particular atomic arrangement (lattice) of silicon atoms in their crystalline form. As such, the creation of an all-silicon laser or merely an efficient all-silicon light emitter would necessarily begin at the atomic level. Emissive deformation centers (or 'designer defects') were created in the silicon lattice. These emissive centers exist naturally in silicon. In electronics, they are either detrimental to device performance or are a source of unwanted variation.

One example of such centers is called the G-center, which is formed by moving a silicon atom from its normal lattice site and substituting a carbon atom in its place. When the substituted carbon pairs up with a second carbon atom nearby, a local lattice deformation or emissive center is created and an electron captured at the site can then emit light directly. One way to make silicon more optically active is to increase the density of these G-centers, without adversely increasing electrical and optical losses, so as to allow laser action.

A nano-patterning technique was developed that uses an etch mask made of a regular array of oxide (AAO). In the etching process, this AAO mask is placed directly on a slice of silicon. The etching through the mask results in a pattern of silicon structures. The extreme uniformity of the approach helped to keep optical losses low, the large field size provided sufficient total optical gain, the small feature sizes minimized scattering loss, and the hardness of the AAO stood up well against the deep etching process and protected the underlying silicon everywhere except where the nano-pores were to be etched.

The same nano-patterned etching pro - cess also created local lattice deformation and strain field in the side-wall region (-4 nm thick) and a band-gap narrowing. Both benefited optical emission by facilitating the gathering of electron-hole pairs from the surrounding silicon to the emission centers in the side-wall layer of the etched pore. This itself also benefited from the fact that the nano-pores are much closer together than the electron diffusion length within crystalline silicon.

A future option is to combine patterned implantation with the strain effects that can aid in optical activity, both through direct lattice deformation and indirect selection and stabilization of the desired emissive deformation centers.

This work was done by Jimmy Xu of Brown University for the Office of Naval Research.


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Lasing Consequences of Silicon Nanostructures

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This article first appeared in the February, 2009 issue of Defense Tech Briefs Magazine.

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