Far Infrared Spectroscopy of Confined LO Phonons in [113] GaAs/AlAs Superlattices

Infrared 45° reflectometry of very thin films

“…1 of the experimental work. 16 A wide dip near the bulk value TO ϭ363.5 cm Ϫ1 7,16,28 of transverse optical phonons in AlAs is observed in both p-and s-polarization reflectivities. Because of the Berreman effect a resonance at the bulk value LO ϭ407 cm Ϫ1 16,7,28 of longitudinal optical phonons in AlAs is clearly discernible in the p-polarization reflectivity.…”

“…First, let us consider a system studied in Ref. 16, i.e., a GaAs/AlAs superlattice grown by MBE on ͓113͔-oriented substrate of n ϩ -doped GaAs. The thicknesses of the GaAs and AlAs layers are 18 and 11 Å, respectively.…”

“…16 There are additional features in the spectrum R p , which are associated with confined LO phonons in the ultrathin AlAs layers of the superlattice. The confinement of phonons in such AlAs layers quantizes their wave vector, 16,29 whose magnitude is given by q ϭm/d AlAs , where d AlAs is the thickness of the AlAs layers in the superlattice ͑here d AlAs Ϸ11 Å), m is the number of confined mode. As a rule only m-odd confined optical phonons are infrared active.…”

“…We should emphasize that the ⌬ 45 spectra for the three specimens above studied were obtained by digitizing the experimental curves for R p and R s shown in the articles. 13,14,16 In spite of the error introduced by copying experimental curves, the ⌬ 45 spectra allowed to determine directly the frequencies of TO and LO phonons in thin layers of those specimens. Obviously, the use of the original experimental data will give more accurate spectra ⌬ 45 .…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] The infrared spectroscopy ͑reflectivity and transmissivity͒ turns out to be very useful, particularly in the case of zinc-blende crystals with ͓100͔ growth axis and alkali halides, where Raman scattering encounters important limitations. 7 The frequencies of the TO and LO vibrational modes can be deduced by comparing experimental reflectivity spectra with theoretical curves [1][2][3][4]7,9,[11][12][13]18 which are calculated with a dielectric function having adjustable parameters: TO , the high-frequency dielectric constant ϱ , the oscillator strength S, and the damping coefficient ␥.…”