Electroabsorption and electrooptic effect in SiGe-Si quantum wells: realization of low-voltage optical modulators

We have calculated the behavior of the band-to-band absorption coefficient in square, coupled, and graded bandgap Si/sub 0.6/Ge/sub 0.4/-Si quantum wells as a function of the transverse electric field. It is seen that due to the weak confinement of the electrons (/spl Delta/E/sub c//spl les/20 meV) the absorption of photons with energy equal to the interband transition energy can be reduced at very small values of the transverse electric field. This phenomenon lends itself to the design of efficient amplitude modulators. In addition, the resulting change in the refractive index is also large and the corresponding linear electrooptic coefficient is calculated to be as large as 1.9/spl times/10/sup -10/ m/V in square wells. This effect could prove to be the basis for the realization of efficient Si-based electrooptic modulators. Device designs are discussed.