Enhanced quantum-confined pockels effect in SiGe superlattices

An improved virtual crystal approximation in the empirical pseudopotential formalism is proposed with the accumulated strain effect taken into proper account. Then as applied to the long-period trilayer superlattices, Si/Si0.75Ge0.25/Si0.5Ge0.5, the enhanced optical anisotropy in this strained structure in the presence of an external electric field, namely, the giant quantum confined Pockels effect, is confirmed. This enhanced Pockels effect is attributed to the type-II indirect optical transitions associated with two different chemical bonds with different orientations at the Si0.5Ge0.5-Si interface. Varying the structure parameters, the dependence of the Pockels coefficient on the thickness of each constituent layer is explained, and the optimized structure is obtained. Moreover, a new structure, the graded-SixGe1-x/Si superlattice with varying profiles of x in the graded layers, is proposed, in which the whole graded region can contribute to the Pockels effect; thus a Pockels coefficient as large as 10(-9) cm/V is predicted. Three types of graded-SixGe1-x/Si superlattices, i.e., the x profile as functions of sawtooth, parabola, and antiparabola, are investigated, and the most promising structure is obtained, and explained by the competition between the quantum confinement of carriers and the spatial variation rate of the composition x in graded layers.