Molecular beam epitaxy of PbTe on BaF 2 ͑111͒ is studied using UHV-scanning tunneling microscopy and atomic force microscopy. It is shown that PbTe growth is totally dominated by growth spirals formed around threading dislocations ͑TD͒ that originate from the growth on the 4.2% lattice-mismatched substrate. Due to dislocation annihilation, the TD density rapidly decreases with layer thickness, which results in a dramatic increase of the electron mobilities in the layers.

The absorption and refractive indices of IV-VI semiconductor PbSe and Pb 1Ϫx Mn x Se (xр0.013) epitaxial layers are strongly influenced by band nonparabolicity in the absorbing regime above the fundamental energy gap E g , as well as below in the transparent regime. This fact follows simply from causality since interband transitions far above the energy gap contribute remarkably to the optical constants near and below E g . The observation of multiple reflection and interference phenomena in the transmission spectrum of the epitaxial layers grown on BaF 2 substrates, which were taken with high photometric accuracy ͑0.1%͒, allowed us to probe the dispersion of the refractive index very precisely as well as to determine the energy gap of PbSe and Pb 1Ϫx Mn x Se with an uncertainty of Ϯ2 meV. A dielectric function is derived which contains the contributions from the interband transitions at the L point of the Brillouin zone and the residual dispersion from higher critical point oscillators at E 1 ϭ1.6 eV, E 2 ϭ2.73 eV, and E 3 ϭ4.2 eV. In contrast to previous studies, the dissipative and dispersive parts of the dielectric function strictly obey the Kramers-Kronig causality relations. Also, in this sense all level broadenings do not only appear in the imaginary part of the dielectric function, but cause corresponding energy shifts in its real part. An analytic formula of the complex dielectric function for the nonparabolic energy-momentum dispersion was derived within the framework of the six-band many-valley k• p model, and compared with the results of a corresponding parabolic model, keeping the oscillator strength and energy-level broadening identical. Only the dielectric function based on the nonparabolic band model correctly explains the observed transmission in the transparent regime below E g and its steep decrease above the gap. It turns out that both the extinction coefficient as well as the refractive index of PbSe have considerably higher values than previously published.

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Magnetoresistance measurements of photolithographically patterned PbSe and Pb1y Mnx Se microstructures were performed. Reproducible magnetoconductance fluctuations with the amplitude increasing with decreasing temperature were observed. Unexpectedly, these fluctuations contain a component periodic in the magnetic field, and their magnitude is greater than that expected from the current theory of the universal conductance fluctuations. Possible explanations are discussed.

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