| Monolithic integration of electronic and optical devices has long been the ultimate goal of many researchers. Potential advantages include reducing packaging cost and improving performance. However, an ideal system has yet to be found. The key problem in such an optoelectronic integration is that different epitaxial layers are required for the optimum performance of each device type. Silicon is still the dominant material system for electronic devices. However, because of the indirect nature of its energy band structure, bulk silicon has a very low luminescent efficiency and is unsuitable as a light emitting device. The discovery of intense room-temperature visible photoluminescence from porous silicon produced by electrochemical etching of bulk silicon has reinvigorated the attempt to make silicon optically active. Since Canham's discovery of this phenomenon in 1990 and his proposed quantum confinement model for explaining the luminescence mechanism, over 1000 scientific papers have been published on this subject. Despite all these, the exact nature of the luminescence from porous silicon is still a hotly debated issue.; The aim of our research was to develop an alternative approach for studying the properties of silicon nano-structures. Using a combination of high resolution electron beam lithography, anisotropic reactive ion etching, and thermal oxidation, we have fabricated sub-5 nm silicon nano-wires in a controllable fashion. A high resolution transmission electron microscopy technique was also developed to characterize the oxidation properties of these nano-wires. Important oxidation properties discovered with this technique include the self-limiting and the faceting effects. A viscous flow oxidation model was shown to yield adequate description of the oxidation rate prior to the onset of the self-limiting effect. This represents the first systematic oxidation study of lithographically defined silicon columnar structures in the nanometer regime. Optical characterizations including photoluminescence and Raman spectroscopy were also performed on these oxidized nano-wire structures. Interesting Raman signals due to confinement and stress effects were obtained. However, the internal quantum efficiency of the photoluminescence was estimated to be below 1% for these nano-wires. These results along with other potential applications such as field emission tip array and infrared sensor are discussed. |
