Raman scattering from superlattices of diluted magnetic semiconductors

Abstract

Collective and localized excitations in superlattices of the Mn-based II-VI diluted magnetic semiconductors ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te/${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{y}}$${\mathrm{Mn}}_{\mathrm{y}}$Te and ZnSe/${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se have been investigated with use of Raman scattering. The new periodicity introduced by the superlattice causes zone folding of the acoustic phonons allowing modes with large q vectors, normally inactive in light scattering, to become Raman active; their frequencies can be understood in terms of the elastic constants of the two constituents. In the optical-phonon region, the Raman spectra of the superlattices with superlattice axis along [001] show optical phonons ``confined'' to the well or the barrier layer when the corresponding dispersion curves do not overlap, whereas ``propagating'' optical phonons are observed when they do. In [111] ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te/${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{y}}$${\mathrm{Mn}}_{\mathrm{y}}$Te superlattices, interface optical phonons are selectively favored in Raman scattering by the intermediate states associated with excitons localized at the heterointerfaces.In ZnSe/${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se superlattices, the strain arising from lattice mismatch gives rise to shifts in the optical-phonon frequencies. The large magnetic field shifts in the photoluminescence associated with the electronic transitions in the well demonstrate the existence of large exchange interaction between the band electrons and ${\mathrm{Mn}}^{2+}$, as in bulk crystals. This effect can be exploited in the magnetically tuned resonance enhancement of the Raman spectrum of optical phonons. In the presence of an external magnetic field, as in the bulk, Raman electron paramagnetic resonance of ${\mathrm{Mn}}^{2+}$ is observed. The absence of a magnon feature is indicative of the absence of magnetic ordering in superlattices. In the [111] ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te/${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{y}}$${\mathrm{Mn}}_{\mathrm{y}}$Te superlattice, a new Raman line is observed which can be interpreted as a transition in the isolated ${\mathrm{Mn}}^{2+}$ pairs; its position yields a value of the nearest-neighbor exchange interaction \ensuremath{\Vert}${J}_{\mathrm{NN}\mathrm{\ensuremath{\Vert}}=(6.8\ifmmode\pm\else\textpm\fi{}0.2}$ K)${k}_{B}$.