PHOTOEXCITED PLASMON-LO-PHONON MODES AT THE ZNSE/GAAS INTERFACE

ZnSe epitaxial layers grown on GaAs(100) substrates were studied by Raman spectroscopy. The intensity of the incident light was varied over three orders of magnitude in the range from about 1 kW/cm2 to 0.5 MW/cm2. The spectra recorded at low laser power exhibit the pure phonon features of GaAs and ZnSe, i.e. only scattering by the longitudinal optical (LO) phonons is observed. While the lineshape of the ZnSe LO phonon peak remains constant upon increasing laser power, drastic changes can be seen in the GaAs Raman features. These changes depend only on the laser power and are found to be completely reversible. The relative GaAs LO intensity decreases and simultaneously a new peak appears close to the GaAs TO phonon position. This behaviour is explained in terms of scattering by photogenerated coupled plasmon-LO-phonon (PLP) modes. Free carrier concentrations above 10(18) cm-3 are deduced from the PLP mode frequencies. At the highest laser powers used the spectra are completely dominated by the PLP modes. The long carrier lifetime necessary for the generation of such high concentrations suggests that the ZnSe/GaAs interface has to be of high structural quality leading to low recombination velocities. Indeed ZnSe/GaAs heterostructures of lower quality showed the effect of photogenerated carriers only to a lesser extent. Thus Raman scattering by the photoexcited free carriers is likely to serve as an excellent new tool for interface quality assessment.