Influence of buffer surface preparation on the quality of Al _x Ga _1-xAs/GaAs quantum wells studied by optical orientation experiments

Quantum well (QW) structures of Al _xGa _1-xAs/GaAs with x = 0.3 were characterized by photoluminescence spectroscopy (PL) with circularly polarized excitation at a temperature of 1.6 K. The samples contained three QWs with thickness of 7, 5, and 3 nm grown by molecular beam epitaxy (MBE) on a 500 nm thick buffer layer. Four samples with identical geometry but different surface treatments (in-situ etching the GaAs buffer with Cl ₂ at different temperatures, and air-exposed buffer, respectively) were compared. The degree of circular polarization of the PL and its decrease in a magnetic field applied perpendicularly to the direction of propagation of light (Hanle effect) allows the determination of the interband lifetime τ and the spin lifetime τ _s of the electrons. These lifetimes were different in the different QWs and strongly depend on the growth procedure.