INFRARED OBSERVATION OF LONGITUDINAL OPTICAL MODES IN MBE-GROWN POLARSEMICONDUCTOR-FILMS

The feasibility of observing the zone-center longitudinal optical phon on (LO) of a crystal as a minimum in its infrared transmission spectru m, when measured with radiation obliquely incident on a thin film of t he crystal, i.e., the Berreman effect, is demonstrated with an MBE-gro wn zinc-blende MnTe epilayer. Whereas the zone-center transverse optic phonon (TO) appears irrespective of the polarization of the incident radiation for both normal and oblique incidence, the LO can be detecte d only in the latter with light polarized in the plane of incidence. W e have verified the Berreman effect in a variety of MBE-grown II-VI ep ilayers and heterostructures.