Theoretical and experimental studies on optical spin orientation of electrons in direct and indirect bandgap AlGaAs epitaxial layers

We report the optical spin orientation of electrons in AlGaAs epitaxial layers with bandgap varying across direct-indirect cross over. Optical spin orientation spectra of Al_xGa_1-xAs layers with ‘x’ varying from 0.26 (direct) to 0.63 (indirect) are recorded by measuring the degree of circular polarization (DCP) of photoluminescence signal emanating from an adjacent GaAs quantum well (QW) layer. Energy of incident photons is tuned over a wide range (1.8 - 2.9 eV), to enable excitation from various valence bands in the barrier layer. A reversal of sign in DCP at a particular excitation energy is observed when the nature of band-gap is changed from direct to indirect. It is explained by proposing a theoretical model based on the capture of electrons in GaAs QW layer via different valleys in AlGaAs barrier layer. The corresponding rate equations are solved to estimate the steady state spin polarized carrier density in barrier and QW layers. Timescales of different relaxation phenomena (spin, energy, momentum) are included in the rate equations in order to explain the experimental observations. It is seen that the linear k spin splitting of X-valley governs the shape of the optical orientation spectra in indirect bandgap Al_xGa_1-xAs epitaxial layers.