OVERDAMPED PLASMON-LO PHONON MODES IN INTENTIONALLY DISORDERED GaAs/AlGaAs SUPERLATTICES

In polar highly doped semiconductors, the collective oscillations of the free carriers (plasmons) can interact with the longitudinal-optical (LO) phonons, screening their macroscopic electric fields. The so called coupled LO-plasmon mode presents low-energy (omega(-)) and high-energy (omega(+)) branches, with frequencies that depend on the free-carrier concentrations. For a given free-carrier concentration, as the wave vector uncertainty increases due to the increasing disorder, omega(+) vanishes (it becomes overdamped) and the omega(-) mode frequency approaches the transverse optical phonon frequency. In the strongly disordered case, when the plasmon damping is larger than the plasmon frequency, the free-carriers are no longer able to screen the electric field associated with the LO phonon. As a consequence the plasmon and the LO phonon are uncoupled. In this work we present a Raman spectroscopy study of a set of samples of disordered (GaAs)(m)(Al0.3Ga0.7As)(6):Si superlattices (SL) with N-Si = 1.2x 10(18)cm(-3); the samples were grown with different disorder levels such that both carrier-phonon interaction regimes (coupled and uncoupled plasmons) described above can be present, depending on the disorder strength. By means of the fittings of the Raman spectra in the region of the AlAs-like modes (omega(+)) we could probe the coupled character of theses modes in the weakly disordered SL's. The analysis of the spectra lineshapes in the case of the strongly disordered SL's allows us to obtain the plasmon damping constant, proving that in these samples the overdamping behavior is present.