M. Baleva

On the temperature dependence of the energy gap in PbSe and PbTe

A detailed temperature dependence of the energy gap E0 in the PbSe and PbTe was obtained from the transmittance measurements. A non-linear change of E0 in the vicinity of the Debye temperature was noticed. An attempt to find an empirical formula describing the experimental dependence E0(T) led to the conclusion that the non-linear change can be attributed to the electron interaction with the optical phonons.

Structural and optical characterization of laser-deposited PbTe films on silicon substrates

Abstract The structural and optical properties of PbTe films, fabricated using pulsed laser evaporation, are studied. The grazing incident asymmetric X-ray diffraction patterns, obtained at different grazing angles, unambiguously demonstrate that the films represent three-layered structures. Each of the layers is of one of the competitive PbTe phases—the metastable CsCl-type, the metastable GeS-type and the stable NaCl-type crystal modifications. The CsCl-type phase lattice parameter and layer thickness in the films grown on Si substrates are compared with those in the films grown on substrates with the NaCl-type structure. The energy dependence of the refractive index was obtained from the extrema of the interference fringes in the transmittance and reflectance spectra. The dependence can be interpreted by treating the films as three-layered structures with a different crystal modification of each of the layers.

Influence of grain size on the optical conductivity of β-FeSi2 layers

Abstract Monophase β-FeSi2 layers, fabricated using ion beam synthesis with high implantation doses followed by rapid thermal annealing are studied. The structure of the films was analyzed by Rutherford backscattering, X-ray diffraction and atomic force microscopy. The average grain sizes of the layers, formed under different annealing regimes were determined. The room temperature reflectance and transmittance spectra were measured in the range from 0.2 to 1.3 eV . The spectra were analyzed in terms of a multilayered structure. The dispersion of the optical constants was calculated using a generalized matrix method, accounting for the surface and interface roughness. From the optical constants dispersions the effective optical conductivity and that in the grains were estimated. The grain sizes were found to influence the optical conductivity.

Distribution of the crystal modifications in polymorphous PbSe films revealed by microhardness measurements

Microhardness measurements of homogeneous and layered polymorphous PbSe films as well of the KCl and BaF2 substrates on which the films were deposited were undertaken. The interpretation of the experimental results lead to a formula describing more adequately the indentation size effect. It is also shown that an extremum in the microhardness depth profile appears whenever the indentor crosses an interface.

Structural and optical characterization of the formation of β-FeSi2 nanocrystallites in an n-type (100) Si matrix

Ion-beam synthesized (IBS) samples, prepared with a low dose of iron ions and subjected to rapid thermal annealing (RTA) were studied. The samples were characterized with cross-sectional transmission electron microscopy (XTEM), including high-resolution electron microscopy (HREM) and far infrared transmittance (FIRT) spectroscopy. The formation of ?-FeSi2 nanocrystallites, with various shapes and sizes, in the Si matrix was revealed. The optical constants dispersions of the samples were obtained from the reflectance (R) and transmittance (T) spectra, taken between 0.38 and 1.2?eV. From these dispersions, the energy band diagram of the interface ?-FeSi2/Si was determined, and compared to those reported by other authors.

Optical absorption in PbTe doped with Cr

The absorption spectrum of PbTe doped with Cr is measured in the photon energy range between 0.1 eV and 0.34 eV at room temperature. The observed impurity absorption band is assigned to optical transitions from the bottom of the conduction band to a widened term of the Cr3+ ion. The density of states in the Cr3+ impurity band is described by a Lorentzian dispersion curve.

Influence of doping and substrate type on the metastable phase growth in pulsed-laser evaporated PbTe films

PbTe films, undoped and doped with 0.02, 0.3 mol% Cr and 0.02 mol% Co, grown by pulsed-laser evaporation (PLE) on different types of substrates have been investigated. The crystal structure of the films was studied by X-ray diffraction. Along with the stable fcc phase, metastable GeS- and CsCl-type phases of PbTe were detected in the films. The growth of the metastable phases as well as the influence of the substrate type and temperature and the doping on the growth have been analyzed in the framework of the classical thermodynamic theory of crystallization. It was found that the influence of doping consists in the change of the surface free energy of the deposited material, while the substrate-type influence depends on its linear thermal expansion coefficient-when it is equal to that of the deposited compound the substrate stimulates the growth of the metastable phases. It is concluded that the growth mechanism is graphoepitaxy.

Energy gap and optical dielectric constant of Pb1−xCdxSe films

Abstract The transmission spectra of laser-deposited Pb 1− x Cd x Se films ( x = 0, 0.02, 0.05, 0.08 and 0.12) have been measured over the energy range 0.1–0.65 eV at two different temperatures 90 and 300 K. From these spectra the dispersion of the refractive index and the absorption coefficient-energy dependence have been obtained. Thus the dependences of the gap and the optical dielectric constant on the Cd content x have been investigated. The experimental results have been discussed in the framework of the recently developed theoretical model by Volkov and Pankratov et al 1,2 .

The PbSe metastable phase. II. The energy gap of the CsCl-type phase of PbSe and PbSe doped with Cd

The transmittance and reflectance spectra of films of PbSe and PbSe doped with 2 mol% Cd grown by two different non-equilibrium growth techniques, laser-assisted deposition and electron-beam evaporation, on KCl and BaF2 substrates are measured from 400 to 4000 cm-1 in the temperature range 80-300 K. The energy gaps of the metastable CsCl-type phases of PbSe and PbSe:Cd and their temperature coefficients are determined from the interference spectra. The energy profiles of the films are also obtained.

Polarized micro-Raman scattering characterization of Mg2Si nanolayers in (001) Si matrix

An orientational growth of the Mg2Si lattice relative to the Si lattice is considered assuming minimum mismatch of their lattice parameters. The Raman scattering cross-sections are calculated for the four possible orientations of the Mg2Si lattice positioned in this way. The integral intensity ratios for the F2g mode of Mg2Si in different polarization configurations, obtained from the experimental spectra, are compared with the calculated ratios. It is found that the Mg2Si nanolayers morphology is sensitive to the implantation energy, which determines both the peak Mg concentration in the initial implantation profile and its position in the sample depth. At a peak concentration of the order of the stoichiometric concentration, the layers are highly oriented. When the peak concentration is higher and the peak is placed closer to the surface, the layers are polycrystalline.