Search

In this paper we used far-infrared spectroscopy, reflection high energy electron diffraction (RHEED), X-ray diffraction and atomic force microscopy (AFM) to investigate structural and optical properties of Pb1−xMnxTe layers grown by molecular beam epitaxy (MBE). A numerical model for calculating the reflectivity coefficient for complex systems which include films, buffer layer and substrate has been applied. The infrared reflectivity spectra consist of Pb1−xMnxTe phonons, which exhibit intermediate one–two mode behavior, and MnTe phonons. A good agreement between calculated and experimental spectra is achieved. We registered the local distribution of Mn impurities depending on substrate type. For films growth on BaF2 substrate we registered the orthorhombic local structure of MnTe clusters, while in the case of KCl substrate this structure is cubic. The Pb1−xMnxTe long wavelength optical phonons were described by the modified Genzel's model.

The substitution of Gd 3+ for Eu 2+ ions introduces conducting electrons and qualitatively changes the model of description of magnetic properties of these semiconductor alloys: antiferromagnetic order observed in insulating layers of EuTe is replaced by ferromagnetic (RKKY interaction driven) state in metallic n-type (Eu, Gd)Te. The high-energy photoemission spectroscopy ( hν = 3000 eV) was used to study the electronic structure and valence of magnetic ions in about one micron thick monocrystalline, ferromagnetic n-type (Eu, Gd)Te layers grown by molecular beam epitaxy on BaF 2 (1 1 1) substrate with EuTe buffer layer. The photoemission spectra of Eu (3d, 4d, 4f), Gd (3d, 4d, 4f), Te (3d, 4d) electrons as well as valence band electrons were measured after sequential Ar ion etching. For surface of the sample exposed to the air the oxidation of surface takes place and measured spectra showed the dominant Eu 3+ contribution in comparison to Eu 2+ . The chemical shift of Te 3d and Te 4d peaks was observed for Te atoms oxidized on the surface. Surface sputtering by Ar ions removes the top layer with oxygen adsorbed at the surface and oxygen diffused in the surface region and clean (Eu, Gd)Te surface of the layer was measured. For clean surface the Eu 2+ dominate in the spectra and the peaks of oxidized Te atoms disappear. The contribution of Eu 2+ 4f electrons to the electronic structure of the (Eu, Gd)Te was found at the top of the valence band.

Monocrystalline thin layers of (Eu,Gd)Te, n-type ferromagnetic semiconductor, were grown by molecular beam epitaxy technique on BaF2 (111) substrates. Reflection high-energy electron diffraction, X-ray diffraction, and atomic force microscopy characterization proved epitaxial mode of growth and high crystal quality of the layers. Magnetic susceptibility and magnetic resonance measurements showed that in (Eu,Gd)Te layers ferromagnetic transition takes place at about 13 K. Electrical characterization carried out by the Hall effect and resistivity measurements revealed very high electron concentration of 10 cm−3 and sharp maximum of resistivity at transition temperature.

Thin layers of semimagnetic (diluted magnetic) Sn1−xMnxTe semiconductor with Mn content x⩽0.04 and layer thickness 0.2 – 2 μm were grown by molecular beam epitaxy (MBE) on BaF2 (1 1 1) substrates with SnTe buffer layer. Apart from SnTe and Mn fluxes, we employed an additional Te flux to provide the efficient way of controlling the deviation from stoichiometry in the layers. By manipulating the MBE growth conditions we obtained SnMnTe crystalline layers with the conducting hole concentration in the range from p=5×10 19 cm −3 (no extra Te flux) to p=2×10 21 cm −3 (additional Te flux present). In the layers grown under extra Te flux conditions we observe ferromagnetic transition with the Curie temperature T C ⩽6 K (depending on both x and p). Layers with the same Mn content but grown with no extra Te flux are paramagnetic. This dramatic change of magnetic properties, referred to as a carrier concentration-induced ferromagnetic transition, can be understood in the framework of a model attributing the ferromagnetic interactions in SnMnTe to the mechanism of indirect exchange via conducting holes (Ruderman–Kittel–Kasuya–Yosida interaction).

X-ray photoelectron and ultraviolet resonant photoemission spectroscopy was used to study the Sn0:96Mn0:04Te MBE layers. The surface chemical composition of the samples after different treatment conditions was studied by means of XPS. Resonant photoemission spectroscopy with application of the synchrotron radiation was applied to investigate the electronic structure and the contribution of Mn 3d electrons to the valence band of Sn0:96Mn0:04Te layers. The contribution of the Mn 3d electrons to the valence band electronic structure appears at the valence band with the maximum located at 4.0 eV below the valence band edge. 2002 Published by Elsevier Science B.V.

This work presents the results of X-ray diffractometric measurements performed on Sn 1− x Mn x Te heteroepitaxial layers grown using the molecular beam epitaxy (MBE) technique. The examinations included measurements of lattice constants, orientation, width of peak profiles and trace phases versus Mn content.

Carbon is the main impurity in boron, but no “carbon centre” has been identified until now. However, high densities of two kinds of trapping centres for electrons have been found in this material. Therefore we attempted to answer the question whether there is a relation between the carbon atoms and these trapping centres. Photoinduced electron spin resonance excited under special conditions that were chosen so as to obtain the occupation by electrons of nearly all trapping centres existing in the sample was investigated in samples with different carbon contents. The preliminary results indicate that one type of trapping centre may be related to carbon atoms.

A.c. conductivity measurements over a frequency range 100 Hz -60 MHz and over a temperature range 150–300 K are reported. Measurements were carried out either in thermal equilibrium or after previous photoexcitation of a boron sample. The results are interpreted as confirming the existence of two different kinds of localized states responsible for two types of hopping conductivity in β rhombohedral boron and as supporting the hypothesis that the states of one kind originate from special boron atoms in the crystal structure.