Nebojša Romčević

Raman spectroscopy of impurity states in gallium-doped PbTe

We present Raman spectra and results of galvanomagnetic measurements of PbTe single crystals, doped with gallium, between 10 and 300 K. The effect of persistent photoconductivity depends on the gallium concentration. In all samples well-resolved peaks were observed at about 104 cm-1 (impurity-induced PbTe LO mode) and 166 cm-1 at all temperatures. Another mode appears at about 117 cm-1 at temperature below 250 K. One additional mode, at about 188 cm-1, is observed in PbTe + 0.4 at% Ga. These modes are discussed in terms of local vibrations of impurities corresponding to different Ga charge states.

Far-infrared study of impurity local modes in gallium-doped PbTe

Abstract We present far-infrared reflection spectra and results of galvanomagnetic measurements of PbTe single crystals doped with gallium between 10 and 300 K. The analysis of the far-infrared reflection spectra was made by a fitting procedure based on the model of coupled oscillators. Together with the strong plasmon–phonon coupling we obtain three local modes of gallium at about 122, 166 and 192 cm −1 . The position of these modes depends of impurity center charge, and their intensity depends of temperature and of gallium concentration. Persistent photoconductivity effect was registered in the sample with 0.4 at.% Ga by galvanomagnetic and far-infrared measurements.

Raman scattering spectra of indium-doped PbTe

The non-polarized Raman scattering spectra of indium-doped PbTe single crystals were measured in the temperature range between 10 and 300 K. Well resolved peaks at about 68, 126 and 143 cm-1 were observed for all temperatures. An additional mode appears at about 115 cm-1 for temperatures below 100 K. The intensity of this mode increases sharply when the temperature is lowered below 25 K, the temperature where a persistent photoconductivity effect in PbTe(In) appears. This mode is assigned as a local In impurity mode and represents a population of metastable states due to the transfer of electrons from two-electron to one-electron metastable impurity states.

Effect of the substrate material on diamond CVD coating properties

Abstract Diamond coatings were deposited onto different substrates (Cu, Si, WC–Co, Mo) by hot-filament chemical vapor deposition (CVD). Characterization of the obtained coatings was performed using scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, Raman spectroscopy and Fourier transform infrared reflection (FTIR) spectroscopy. The results and observed differences are discussed from the aspect of the chemical nature of the substrate and its reactivity with a gaseous medium.

Optical properties of PbTe:Mn

Abstract In this paper, we investigate Mn concentration and crystal growth rate influence on optical properties of PbTe:Mn single crystals. We measured far-infrared reflectivity spectra at room temperature in the 10–250 cm −1 spectral range. The analysis of the far-infrared spectra was made by a fitting procedure based on the model of coupled oscillators. In spite of the strong plasmon-LO phonon interaction, we found that the plasma frequency decreases as Mn concentration increases, while the crystal growth rate has the opposite effect. Also, we obtained the Mn impurity in-band mode at about 50 cm −1 .

Far-infrared spectroscopy of localized states in indium doped PbTe and Pb1-xAxTe (Ax = Mn0.017; Sn0.18) alloys

Far-infrared reflection spectra of indium doped PbTe single crystal as well as of Pb1−xAxTe alloys (Ax = Mn0.017;Sn0.18) in the temperature range between 10 and 300 K are presented. Analysis of the spectra was made by a fitting procedure based on the model of coupled oscillators. At temperatures below T = 200 K, two additional modes appear, along with the modes which describe strong plasmon-LO phonon coupling. The first mode, at about 122 cm−1, is a local impurity mode of indium which represents a population of metastable states due to the transfer of electrons from two-electron to one-electron metastable impurity states. The second mode at about 488 cm−1 we believe to appear as a consequence of the electron transfer from a stable impurity two-electron state to the conduction band.

Structure of DX-like centers in narrow-band IV–VI semiconductors doped with group-III elements

This paper presents a study of the structure of the IR reflectance spectra in the sub-gap region of lead telluride doped with indium and gallium and the Raman spectra in PbTe(In). In the Raman and reflectance spectra of PbTe(In), features are observed at a frequency of ω0⋍120 cm−1, whose amplitude sharply increases at temperatures T below the temperature where delayed photoconductivity appears, Tc⋍25 K. A similar feature at a frequency of ω0⋍155 cm−1 is also observed in PbTe(Ga), with the amplitude of the feature sharply increasing for T>Tc⋍80 K. An analysis of the resulting data makes it possible to conclude that, in contrast with classical DX centers in III–V semiconductors, the microscopic structure of the impurity centers in the two-electron (DX-like) ground state does not correspond to an impurity atom shifted from a lattice site, whereas the impurity atom is shifted from a lattice site for the metastable one-electron impurity state.

Study of NiFe2O4 and ZnFe2O4 Spinel Ferrites Prepared by Soft Mechanochemical Synthesis

Two types of ferrites, NiFe2O4 and ZnFe2O4 were prepared by soft mechanochemical synthesis. XRD and Raman spectroscopy were used to characterize the ferrite samples. On the basis of magnetic measurements was confirmed that the degree of inversion changes after sintering. The conduction activation energy, ΔE was determined by fitting the DC conductivity data with the Arrhenius relation. The effect of temperature on impedance parameters was studied using an impedance analyzer in a wide frequency range (100 Hz - 10 MHz). It was observed that the impedance spectra of NiFe2O4 and ZnFe2O4 ferrites include both grain and grain boundary effects.

Low-temperature far-infrared study of localized states in In-doped Pb0.75Sn0.25Te single crystals

The authors present low-temperature far-infrared reflection spectra of In-doped Pb0.75Sn0.25Te single crystals at various doping concentrations. These spectra at temperatures below 20 K are fitted using a modified plasmon-phonon interaction model with an additional oscillator, which describes the electron transition from two- to one-electron states at the In impurity level. The In-doped Pb0.75Sn0.25Te impurity-state energy structure is explained.

Study of Structure and Properties of Bi4Ti3O12 Prepared by Mechanochemical Syntheses

Bi 4 Ti 3 O 12 powder was synthesized from bismuth oxide and titanium oxide. Mixture of oxides was milled in zirconium oxide jar in the planetary ball-mill during 1, 3 and 6 h. Extended time of milling directed to formation of higher amount of titanates perovskite phase. Bi 4 Ti 3 O 12 was formed between 1 and 3 h of milling time. The phase formation of Bi 4 Ti 3 O 12 , crystal structure and powder particle size were followed by XRD, Raman spectroscopy and SEM analysis. After milling for various times the powders were compacted by pressing and isothermal sintering. Sample milled for 3 h and subsequently sintered at 1000°C for 24 h exhibit a hysteresis loop, confirming that the synthesized material possesses ferroelectric properties. All results affect that the structure Bi 4 Ti 3 O 12 is strongly dependent on the milling time.