The optical absorption spectra in the region of the 4f 7 → 4f 6 5d (t 2g ) transition energies of epitaxial layers of of EuTe and Pb 0.1 Eu 0.9 Te, grown by molecular beam epitaxy, were studied using circularly polarized light, in the Faraday configuration. Under σ − polarization a sharp symmetric absorption line (full width at half-maximum 0.041 eV) emerges at the low energy side of the band-edge absorption, for magnetic fields intensities greater than 6 T. The absorption line shows a huge red shift (35 meV/T) with increasing magnetic fields. The peak position of the absorption line as a function of magnetic field is dominated by the d-f exchange interaction of the excited electron and the Eu 2+ spins in the lattice. The d-f exchange interaction energy was estimated to be J df S = 0.15 ± 0.01 eV. In Pb 0.1 Eu 0.9 Te the same absorption line is detected, but it is broader, due to alloy disorder, indicating that the excitation is localized within a finite radius. From a comparison of the absorption spectra in EuTe and Pb 0.1 Eu 0.9 Te the characteristic radius of the excitation is estimated to be ∼ 10Å.

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The near band-edge polarized optical optical absortion spectra of EuTe at low temperatures and high magnetic fields were investigated. The samples were grown by MBE on BaF 2 substrates, and the thickness varied in the 0.18-2.0 μm range. At high magnetic fields, the well-known 4f7→4f65d(t2g) optical transition splits into two well resolved lines at σ+ and two lines for σ-. These lines can be described by localized transitions tunable by the d-f exchange interaction, with a quadratic dependence on the intensity of the external magnetic field. Comparative measurements of the magnetization and the optical absorption as a function temperature provides a further test of the model of a localized excitation extending over a few lattice sites.

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Lattice-matched InP InxGa1,xAs short period superlattices x = 0 :53 -doped with Si in the middle of the InP barriers were studied. The samples had a high carrier concentration which lled two minibands. In addition to a peak associated with the electrons from the second miniband, E2, the Shubnikov-de Haas spectra showe d a w ell resolved doublet structure that is assigned to E1 electrons of superlattice wave v ectors kz = 0 and kz = =d. F rom the lineshape of the Shubnikov-de Haas oscillations, an E1 quantum mobility of 970 cm 2 Vs was deduced, which represents an increase of about 40 over the value for periodically delta-doped semiconductors. The photoluminescence exhibits a band at photon energies higher than the InGaAs bandgap and whose FWHM approximates the Fermi energy of the con ned carriers. Thus the photoluminescence observed is consistent with the recombination of electrons con ned by the superlattice potential and photoexcited holes. I IntroductionIn doped superlattices electrons are con ned by a p eriodic potential in one dimension which splits the continuous conduction band into a set of discrete energy minibands. By changing the thickness of the component l a yers and the doping density, the energy spectrum of carriers can be tuned within a wide range. This characteristic makes doped superlattices a unique system in which to study the physical properties of an electronic system with a dimension between 2 and 3, whereby such interesting e ects as negative di erential resistance 1 and chaotic transport 2, 3 can be observed.In addition to the lineshape of the periodic potential and the density of con ned carriers, the lifetime of the single-particle states is also a key factor in determining the electronic properties of the superlattice. Our previous work on periodically delta-doped superlattices 4, 5 has shown that the lifetime of the electronic state at the Fermi energy increases with the average distance between the electron spatial distribution and the ionised impurity sheet. In this work we attempted to reduce the rate of scattering by increasing the distance between electrons and impurities. Results obtained on doped InP InGaAs superlattices are presented. The doping sheet of Si atoms was placed in the InP barriers. In such a structure electrons are repelled by the InP barriers, and this repulsion favours their spatial separation from the impurity atoms, which leads to longer single-particle lifetimes.