Conduction intersubband transitions at normal incidence inSi1−xGex quantum well devices

Abstract

We show theoretically that it is possible to design SiGe-based quantum well structures in whichconduction intersubband transitions are induced by normal incidence infrared radiation. Asp3d5s* tight binding model has been adopted to evaluate the electronic states and opticaltransitions between lowest conduction confined states of a superlattice composed of onepure Ge quantum well separated by SiGe alloys, grown along the [001] direction. We findthat significant optical coupling between confined states in the Ge wells is achieved atnormal incidence radiation by the off-diagonal elements of the mass tensor. The minimumenergy Ge conduction valleys are, in fact, tilted with respect to the [001] growth axis. Forcomparison we show that no such coupling can be realized for the conductionstates confined in a similar structure composed by Si quantum wells because theellipsoids of the lowest conduction valleys are oriented along the growth direction.