Optical phonons inPb1−xEuxTeepilayers andPbTe/EuTesuperlattices: Berreman effect

Abstract

The frequencies of the zone-center optical phonons in ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Te}$ $(0<~x<~1)$ epilayers both grown by molecular beam epitaxy and of the confined optical phonons in PbTe/EuTe superlattices have been determined from the transmission minima in the far infrared. With radiation incident on the film normally, only the transverse optical (TO) phonons can be observed, whereas oblique incidence allows the appearance of the longitudinal optical (LO) phonons as well, due to the Berreman effect. The first-order infrared spectrum of ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Te}$ exhibits a pair of ``EuTe-like'' zone-center optical phonons originating in the localized mode of Eu in PbTe and evolving into the LO-TO pair of EuTe; the LO modes appear only in the oblique Berreman geometry, the radiation being polarized in the plane of incidence. From a high-quality EuTe epilayer, the frequencies of the TO and the LO phonons at 5 K have been determined to be $109.5 {\mathrm{cm}}^{\ensuremath{-}1}$ and $147.2 {\mathrm{cm}}^{\ensuremath{-}1},$ respectively. The phonon dispersion for EuTe with finite wave vectors along $〈111〉$ has been deduced from the confined TO and LO modes observed in $(\mathrm{PbTe}{)}_{m}/(\mathrm{EuTe}{)}_{n}$ superlattices.