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Raman scattering by GaInAs-InP quantum wells: effects of free carriers and impurities

The authors report a Raman scattering study of the effects of free carriers, produced by both photo-excitation and remote doping, on the LO phonons in both bulk Ga0.47In0.53As and Ga0.47In0.53As-InP single quantum wells. In the bulk alloy high carrier densities cause both a weakening and a shift to lower energy of the dominant GaAs-like LO phonon. In the quantum-well structures this effect is greatly enhanced due to the efficient trapping of photo-excited carriers in the well. This behaviour is in contrast to that of the InP capping layer where they observe a coupled LO-plasmon mode on the high-energy side of the LO phonon. Removal of the InP capping layer by selective etching allows them to study effects of this layer on the Ga0.47In0.53As phonons. In addition, the quantum-well Raman spectra indicate the presence of impurities due to cross contamination during growth.