Carrier effects on Raman spectra from ZnSe/GaAs heterostructures

Non-resonant micro-Raman spectra from heterostructures of ZnSe on semi-insulating (SI-), n+-, and p+-GaAs using various excitation densities are reported. The effects of the carriers generated by both doping and photo-excitation have been investigated and the results are compared with those from bare GaAs substrates. The scattering from unscreened longitudinal optical phonons and coupled phonon-plasma modes in GaAs are observed in all three types of sample, but with different characteristics. The plasma was found to be electron gas in n+-GaAs but hole gas in SI- and p+-GaAs. Depending on the type of substrate and the density of excitation, the plasma effects induced by doping and photo-excitation in the surface and in the deeper regions are demonstrated. As compared to the case for bare GaAs substrates, the density of photo-carriers generated by a given excitation was significantly enhanced in ZnSe/GaAs heterostructures, demonstrating a longer lifetime and a lower recombination rate of photo-carriers in the latter type of structure. The band diagrams of the ZnSe/GaAs heterojunctions in all three cases were obtained from the Raman spectra. In contrast to the carrier effects, no electric field-induced scattering was observed in forbidden configurations.