Resonant Raman scattering by acceptors in GaAs/AlxGa1-xAs multiple quantum wells: A probe of exciton localization

We report a study of the electronic Raman scattering (ERS) by residual acceptors and of LO-phonon resonant Raman scattering in undoped multiquantum wells (MQW's). The samples studied are GaAs/AlxGa1-xAs MQW's with x=0.3 and 1 and well widths L=40 and 70. The spectral range under study is the vicinity of the (e1:hh1)1S exciton band (where hh denotes heavy hole) and the experiments were carried out at low temperatures (T=2 K). The scattering transition involved in the ERS is the hole transition 1S3/2(67). The ERS shows a superlinear dependence on the exciting laser power. This is due to the creation of neutral acceptors by the exciting laser beam, which increases the concentration of scattering centers. For a laser intensity of 300 W/cm2, we find that the ERS cross section is at least eight orders of magnitude larger than the LO-phonon Raman-scattering cross section. The ERS resonates at energies in the low-energy tail of the intrinsic (e1:hh1)1S exciton band. We identify this spectral range as the acceptor-bound-exciton band (A0,X). On the other hand, the LO-phonon Raman scattering resonates at an energy corresponding to the delocalized intrinsic (e1:hh1)1S exciton band as measured in photoluminescence excitation experiments. The ERS resonance profile has an asymmetric shape with a cutoff on its high-energy side. We show that this sharp decrease in the scattering cross section is due to an increase of the (A0,X) damping factor, which reflects the tunneling between (A0,X) and intrinsic (e1:hh1)1S exciton states. We present a model that accounts for the measured ERS resonance profile in which the (A0,X) damping factor is assumed to be proportional to the (e1:hh1)1S exciton density of states.