Negative spin polarization observed by the Hanle effect in quantum wells grown on processed surfaces

A series of quantum well (QW) structures of Al sub(0.3)Ga sub(0.7)As/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 growthEEE: in situ etching the GaAs buffer with Cl sub(2) at 70 CDDD: 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 t and the spin lifetime t sub()sof the electrons were determined. A negative spin polarization was observed if the photon energy of the exciting light exceeds the difference between the spin-orbit band and the conduction band. This inverse circular polarization could be related to the quality of the sample.