INFRARED-SPECTROSCOPY ON ARTIFICIALLY STRUCTURED SEMICONDUCTORS

The optical properties of sandwiched semiconductor structures consisti ng of a sequence A-B-A-B... of atomic smooth layers can be modelled in tentionally by adjusting the proper layer thicknesses in an epitaxial growth process. For period lengths of a few hundred angstroms quantum confinement effects of the wavefunctions of carriers (electrons and ho les) lead to new phenomena like formation of discrete energy levels in quantum wells, or of new energy bands (''minibands'') in superlattice s. Absolute (transmittance, reflectance, absorbance) and modulation in frared spectroscopy (photo-, electro-, piezoreflectance, differential photoreflectance) have been developed for measuring the change of opti cal absorption as a function of the incident light, and optical or d.c . electrical fields. In our work wide-gap (GaAs/GaAlAs, ZnTe/CdTe) and narrow-gap (PbTe/PbEuTe) semiconductor superlattices are investigated with various layer thicknesses. It has been proved that the optical r esponse (located in the near- and midinfrared for those systems) depen ds strongly on the two-dimensional behaviour of a confined electron ga s (2DEG). FT-IR transmittance spectra are presented for the 2DEG absor ption in a PbTe/PbEuTe multi-quantum well (MQW), photo- and piezorefle ctance spectra for GaAs/GaAlAs-MQWs, and far infrared reflectance spec tra of confined lattice vibrations in a short period CdTe/ZnTe superla ttice. The transmittance spectra are also calculated and compared to t he measurements.