Optical transitions between valley split subbands in biased Si quantum wells

By a tight-binding sp(3)d(5)s(*) model, we study numerically the optical transitions involving the lowest conduction states confined in strained [001] Si quantum wells. These states belong to the fundamental and to the first excited quantum well (QW) subbands, each one split into a doublet by intervalley interaction. Both hard wall and finite SiGe barriers boundary conditions for the QWs are considered. Amplitudes of the doublet splittings as a function of the well width and of a uniform electric field superimposed along the growth direction are first investigated. Then, we study atomic contributions and parity character of the doublet wave functions to derive selection rules for interdoublet optical transitions. Finally, we demonstrate the role of intervalley coupling and the effectiveness of the selection rules here presented, for the interpretation of the absorption spectrum of a n-type Si QW between SiGe barriers, evaluated at different temperatures.