Photomodulated reflectivity of Zn_1-xMn_xTe/ZnTe multiple-quantum wells with below-bandgap excitation

A series of Zn_0.93Mn_0.07Te/ZnTe multiple-quantum well (MQW) structures of different quantum well widths has been studied at 77 K using the conventional modulation technique of photoreflectivity (PR) with above-bandgap light providing the modulation as well as a novel version of the technique which employs below-bandgap excitation. Photoluminescence (PL) spectra of the structures are also presented. The appearance of the PR spectra obtained with below-bandgap excitation differs from that of the PR spectra with above-bandgap excitation, but fitting the spectra with appropriate lineshapes gives the same spectroscopic information about the energy positions of the excitonic transitions in the well and the bandgaps of the ZnTe buffer layer and the Zn_0.93Mn_0.07Te barriers. The fits to the PR spectra yield energy positions of the lowest excitonic transitions which are consistent with the PL data. From the transition energies calculated with a one-dimensional transfer matrix model in which strain effects are taken into account, we conclude that the structures are to a good approximation strained to the ZnTe buffer layers.