Silicon Colloids: From Microcavities to Photonic Sponges

Abstract

Silicon is a material of paramount importance in both microlectronics and photonics. Most silicon devices are based on planar technology. Because of the lack of fabrication methods for spheres at the mesoscale dimension, spherical geometries of silicon have been little studied. Here, we report on a method for obtaining silicon colloids with diameters from 0.5 lm to 5 lm. Because of their spherical shape and smooth surface, they work as optical microcavities with well-defined resonating modes in the near-infrared range. Silicon colloids may facilitate development of high-quality-factor optical microcavities with strong light confinement effects, allowing integration of fundamental electronic devices such as a p–n junction into a single system. Silicon colloids have also been used as the building blocks of a new microstructured material formed by treelike arrangements of polydisperse microspheres. We call this material “photonic sponge” because it can scatter light strongly over a wide range of wavelengths. Silicon microspheres have been obtained by chemical vapor deposition techniques. Under controlled chemical reaction conditions, silicon colloids nucleate and grow in disilane gas similar to the growing process of silica colloids in a liquid solution containing appropriate precursors. In the gas, the colloidal particles become highly spherical thanks to surface tension forces. The particles diffuse into the gas and produce a “rain” of micrometer-sized spheres that deposit on the walls of the gas reactor or onto any substrate previously introduced into the reactor. Microspheres were found isolated, as clusters of a few units, and also as agglomerates of many spheres that we term “photonic sponges”. Isolated microspheres can also be obtained from photonic sponges through a very mild grinding process that preserves the spherical quality of particles; however, some of the particles will sustain nicks at the contact points with other spheres. The obtained microspheres are polydisperse with diameters from 0.5 to 5 lm, as shown in the scanning electron microscopy (SEM) image in Figure 1. The inset of the figure shows a high-magnification image of a sphere 2 lm in diameter, illustrating the spherical perfection and smoothness of its surface.