Photovoltaic effect in unipolar multivalley semiconductors

Abstract

The theory of the nonequilibrium charge carrier transport in unipolar multivalley semiconductors is developed. It is shown that the diffusion of photoexcited nonequilibrium heavy and light electrons in multivalley semiconductors is a correlated process, like the ambipolar diffusion in the case of the electron-hole plasma. The light-induced intervalley transitions, resulting in the imbalance between the subsystems of the light and heavy electrons, give rise to the electromotive force (emf) through the mechanism of the Dember photovoltaic effect. The value of the emf occurring in the ‘metal-semiconductor-metal’ structure is calculated in the linear approximation in terms of the light intensity as a small parameter. It is shown that the emf is determined by the conductivity of heavy and light electron subsystems, as well as by the surface conductivity of the metal-semiconductor interface.     

Electron–hole symmetry and spin–orbit splitting in IV–VI asymmetric quantum wells

It is shown that, due to the electron–hole symmetry of the fundamental gap of the lead–salts (PbTe, PbSe and PbS), the Rashba spin splitting in their flat band asymmetric quantum wells is much reduced with the usual equal conduction and valence band-offsets. Different from the III–V case, we find that the important structure inversion asymmetry for the Rashba splitting in IV–VI quantum wells with different left and right barriers is not a material property (i.e., barrier height, effective mass or band gap) but results from the band alignment. This is shown by specific envelope function calculations of the spin-dependent subband structure of Pb_1−xEu_xTe/PbTe/Pb_1−yEu_yTe asymmetric quantum wells (x≠y), based on a simple but accurate four-band kp model for the bulk band structure near the gap, which takes into account band anisotropy, nonparabolicity and multi-valley effects.