Spire recently fabricated the first yellow and orange- emitting heterojunction light emitting diode (LED) based on silicon. These devices were fabricated by depositing a transparent n-type semiconductor (indium- tinoxide) on the porous surface of electrochemically etched p-type Si wafers. The key to the visible emission is believed to be the quantum confinement phenomenon in the nanometer-sized Si crystallites in the porous Si layers. As crystallites get smaller, the emission spectrum shifts to shorter wavelengths (blue shift). However, we have noted that Si crystallites with light emission below yellow wavelengths are physically too fragile and susceptible to environmental effects and thus are not suitable for device fabrication. We propose a unique solution to the above problem; since crystalline carbon (diamond) has a large band gap of 5.47 eV, alloys of only a few percent C in Si will readily increase the base band gap of the new material into the range where quantum structures of stable dimensionality can be prepared. We shall use non- equilibrium deposition conditions to assure epitaxial growth of Si 1-x Cx alloys without the formation of SiC phase. In Phase I, we will use the chemical vapor deposition (CVD) process to grow epitaxial Si 1-x Cx layers on Si substrates at low temperatures. These layers will then be electrochemically etched to produce porous layers capable of light emission in the blue wavelength range. The material and structural quality of the epitaxial and porous layers will be evaluated using various characterization techniques. The blue photoluminescence in porous Si 1-x Cx layers will be demonstrated. We will fabricate high efficiency Si- based blue LEDs in Phase II.
