INFRARED OBSERVATION OF TRANSVERSE AND LONGITUDINAL POLAR OPTICAL MODES OF SEMICONDUCTOR-FILMS - NORMAL AND OBLIQUE-INCIDENCE

The observation of the zone-center transverse optic (TO) and longitudinal optic (LO) phonons of a polar semiconductor crystal as transmission minima in its infrared spectrum, when measured with radiation obliquely incident on a thin film of the crystal, i.e., the Berreman effect, is applied to a diverse group of II-VI and III-V epilayers and superlattices grown by molecular-beam epitaxy (MBE) as well as epilayers of SiC on a Si substrate and free-standing CdS films grown by chemical-vapor deposition (CVD). Whereas the TO phonon appears irrespective of the polarization of the incident radiation for both normal and oblique incidence, the LO phonon can be detected only in the latter with light polarized in the plane of incidence. The technique, successfully applied to tetrahedrally coordinated zinc-blende MnTe, accessible only with MBE growth on GaAs, has resulted in a direct observation of its TO and LO modes. Zinc-blende Cd1-xMnxSe grown on GaAs by MBE, similarly investigated, yields the frequencies of the CdSe-like and MnSe-like TO and LO modes as a function of x, including those for cubic CdSe (x=0) and, by extrapolation to x=1, for zinc-blende MnSe. In the II-VI/II-VI superlattices, confinement of the optical phonons has been observed. In free-standing CdS films with the wurtzite structure, their optic axis being in the plane of the film, the A1(TO), E1(TO), and E1(LO) zone-center phonons have been directly observed. In a GaAs/AlAs/GaAs structure grown by MBE, the LO and TO modes of the submicrometer AlAs epilayer and the GaAs cap layer were observed with the Berreman technique, before and after removing the GaAs substrate, respectively. Finally, the Berreman effect of cubic SiC, grown on a Si substrate by CVD, illustrates the power of the technique in the context of a material of increasing technological importance. © 1995 The American Physical Society.