Interação plasmon-fônon LO em superredes semicondutoras

Abstract

This work presents a Raman investigation of the optical vibrations in highly doped In-GaAs/InP and GaAs/AlGaAs superlattices (SL s). The InGaAs/InP SL s grown with different periods were analyzed using polarized Raman techniques. No Raman selection rules were found in the long period InGaAs/InP SL s due to the structural defects in the bulk materials constituting the layers. With the decrease of the SL period the selection rules emerges and considerable blue shift of the longitudinal optical mode originated in the barriers were observed, as a manifestation of the coupled plasmon-LO phonon vibrations propagating along the growth axis. The observed effect can be attributed to the formation of the miniband electron energy structure. A quantitative analysis showed that the selection rules noticed in the longitudinal optical vibrations of the short period SL s occur due to the increasing of the coherence length of the coupled modes with respect to the coherence length of the optical phonon. The GaAs/AlGaAs SL s studied were growth with different disorder strengths. In weakly disordered SL s we also observed the coupling between the vertical plasmon and the longitudinal optical phonons of the barriers. The lineshape analysis of the sprectra measured in the strongly disordered SL s allowed us to obtain the plasmon damping constant, proving that in these samples the plasmon presents an overdamping behavior, that provokes the decoupling between the plasmon and the LO-phonon. For the first time we have shown aphysical system in which we can control the plasmon-LO phonon interaction, changing from a coupled regime to a decoupled one, by varying the disorder strength. Plotting both the coherence lengths of the AlAs-like coupled modes and the plasmon damping constants versus the disorder strengths we could draw some conclusion about the process of the transition from the coupled to uncoupled phase and to obtain the behavior of the plasmon relaxation as a function of the disorder.