Negative spin polarization observed by the Hanle effect in Al _xGa_1-xAs/GaAs quantum wells grown on processed surfaces

A series of quantum well (QW) structures of Al_0.3Ga _0.7As/GaAs were characterized by photoluminescence spectroscopy with circularly polarized excitation at 1.6 K. The samples were grown by molecular beam epitaxy on a 500 nm thick GaAs buffer layer and contained three QW structures with thicknesses of 7, 5, and 3 nm, respectively. Four samples were prepared with different processing of the GaAs buffer layer surface (i: continuous growth, ii: in situ etching the GaAs buffer with Cl₂ at 70 °C, iii: at 200 °C, respectively and iv: air-exposed buffer). The influence of buffer surface preparation on sample characteristics was analyzed. Using the Hanle effect the interband lifetime τ and the spin lifetime τs of the electrons were determined. A negative spin polarization was observed if the photon energy of the exciting light exceeds the difference between the spinorbit band and the conduction band. This inverse circular polarization could be related to the quality of the sample.