RAMAN-SCATTERING BY GAINAS-INP QUANTUM-WELLS - EFFECTS OF FREE-CARRIERS AND IMPURITIES

We 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 Ga//0//. //4//7In//0//. //5//3As and Ga//0//. //4//7In//0//. //5//3As-InP single quantum wells. In the bulk alloy high carrier densities caus 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 behavior is in contrast to that of the InP capping layer where we 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 us to study effects of this layer on the Ga//0//. //4//7In//0//. //5//3As phonons. In addition, the quantum-well Raman spectra indicate the presence of impurities due to cross contamination during growth.