| Porous silicon quantum dots have been extensively studied due to the rapid progress in silicon nanotechnology. Their simple fabrication process can be economically translated into mass-production, and their development is relevant for the progress in microelectronic devices for VLSI circuits and in photonic applications for data networks. Understanding their remarkable physical, electrical and optical properties is therefore required. To manipulate these properties, one must control both the silicon crystallite size and its surface passivation. As predicted by quantum confinement theory, stable red, orange, yellow, green and blue photoluminescence from porous silicon quantum dots with decreasing sizes has been obtained. However, this tunability can only be achieved if the surface is passivated with silicon-hydrogen bonds. After exposure to oxygen, silicon-oxide is formed on the surface, and the photoluminescence is red shifted by as much as 1 eV. A theoretical model, in which new electronic states appear in the bandgap due to oxidation explains the experimental results, and provides a global explanation to the recombination process in silicon quantum dots. Photoluminescence dynamics and polarization memory spectroscopy further support the new recombination model. To modulate light with silicon, a novel optoelectronic device is constructed by integrating porous silicon mirrors with liquid crystal molecules, which are commonly used in various switching devices. The mirrors are made of a multilayer stack of porous silicon films, which reflect an incident light by nearly 100%. When a voltage is applied to this composite device, the liquid crystal molecules are reoriented, the refractive index of each layer is changed, and the mirror's reflection is electrically modulated. |
