Valley splitting and optical intersubband transitions at parallel and normal incidence in [001]-Ge/SiGe quantum wells

We investigate intervalley splitting in the conduction band of strained [001]-Ge quantum well (QW) systems with finite SiGe alloy barriers by means of a sp(3)d(5)s(*) tight-binding Hamiltonian. We find that interaction between germanium bulk L minima splits each confined subband into a doublet. We first characterize this splitting as a function of the well width and of the strength of a uniform electric field superimposed along the growth direction. Varying the well width, an oscillating behavior of the splitting magnitude similar to that predicted for Si QW systems is observed and explained. Then we focus on the optical intersubband transitions occurring between states belonging to the fundamental and the first-excited doublets. Selection rules for intersubband transitions at normal and parallel incidence are discussed exploiting the parity character of the involved doublet states. Numerical results for infrared-absorption spectra, evaluated for both symmetric and biased Ge QWs, support our findings.