V. I. Shtanov

X-ray photoelectron studies of clean and oxidized α-GeTe(111) surfaces

Clean and oxidized (104–1015 L of O2) surfaces of α-GeTe have been investigated with x-ray photoelectron spectroscopy by using the synchrotron radiation facility BESSY II as well as an Al Kα source. To understand the first steps of oxidation, complementary quantum chemical calculations were performed. The cleaved surfaces of α-GeTe were found to be rumpled with (111) domains that can be related to the domain (twin) structure of the bulk. Both the Ge 3d and the Te 4d spectra of freshly cleaved surfaces exhibit at least three components, which are explained by a Ge or Te termination of the surface domains with possible contributions of a surface reconstruction. The surface oxidation starts at exposures of 104 L and proceeds via several steps. At low exposures, only changes in the Ge spectra are observed. Consequently, the first step of the reaction is associated with the formation of intermediate peroxidelike structures, wherein both oxygen atoms are bonded to germanium atoms. In the range of exposures between 1010 and 1015 L, a layer of a relatively stable oxidation product with the approximate stoichiometry Ge1+δ+4Te1−δ0O2(1+δ)2− is formed, which shows growth kinetics that obey a time-logarithmic law. At this stage, the peroxidelike structures are still present at the oxide/crystal interface. Once the oxidized layer exceeds a thickness of ≈2.5 nm at ∼1013 L, a transformation of the Te0 state into the Te+4 state is observed at the surface of the oxide layer. The final oxidation product can be described as mGeO2×nTeO2.Clean and oxidized (104–1015 L of O2) surfaces of α-GeTe have been investigated with x-ray photoelectron spectroscopy by using the synchrotron radiation facility BESSY II as well as an Al Kα source. To understand the first steps of oxidation, complementary quantum chemical calculations were performed. The cleaved surfaces of α-GeTe were found to be rumpled with (111) domains that can be related to the domain (twin) structure of the bulk. Both the Ge 3d and the Te 4d spectra of freshly cleaved surfaces exhibit at least three components, which are explained by a Ge or Te termination of the surface domains with possible contributions of a surface reconstruction. The surface oxidation starts at exposures of 104 L and proceeds via several steps. At low exposures, only changes in the Ge spectra are observed. Consequently, the first step of the reaction is associated with the formation of intermediate peroxidelike structures, wherein both oxygen atoms are bonded to germanium atoms. In the range of exposures betw...

Phase relations in pseudobinary systems of germanium, tin, and lead chalcogenides

Available experimental data for the GeTe-SnTe, GeTe-PbTe, SnTe-PbTe, PbS-PbSe, PbS-PbTe, and PbSe-PbTe systems are critically evaluated, and their T-x phase diagrams are calculated. The results indicate that the phase diagrams of these systems can be described in terms of a four-parameter model for the excess Gibbs energy. The effects of cation and anion substitutions on the topology of phase diagrams and excess Gibbs energy are analyzed in relation to the character of chemical bonding in constituent chalcogenides.

Phase Relations between Germanium, Tin, and Lead Chalcogenides in Pseudobinary Systems Containing Orthorhombic Phases

As an extension of our earlier studies of phase relations in pseudobinary systems of germanium, tin, and lead chalcogenides, we have calculated the T-x phase diagrams of all the pseudobinary systems formed by IV–VI compounds and containing orthorhombic phases: SnS-SnSe, SnS-SnTe, SnS-PbS, SnSe-SnTe, and SnSe-PbSe. In the calculations, we used a unified thermodynamic model and the same thermodynamic data for the end members of the pseudobinary systems. The interaction parameters were determined using the relationship, derived earlier for IV–VI compounds with cubic structures, between the excess Gibbs energy and the difference in chemical bonding between the end members of the systems, quantified by a combination of Simons-Bloch orbital radii.

Phase equilibria in pseudoternary systems of IV-VI compounds

Using literature data, experimental data obtained in this study, and a unified thermodynamic model, we have calculated the T-x-y phase diagrams of all the pseudoternary systems formed by the Group 14 chalcogenides: (Ge, Sn, Pb)Te, Pb(S, Se, Te), and Sn(S, Se, Te). The phase diagrams have been constructed using the known interaction parameters of the pseudobinary systems and the ternary interaction parameters derived from experimental data. The systems have small negative ternary interaction parameters, which decrease in the order β-(Ge, Sn, Pb)Te < β-Pb(S, Se, Te) < δ-Sn(S, Se, Te).

Oxygen chemisorption on the PbS(001) surface: Quantum-chemical modeling

The structure and stability of local centers involving a different number of oxygen atoms on the surface of crystalline lead sulfide (001) were calculated in the framework of the cluster approach by the hybrid density functional theory B3LYP method. The trends of the formation of such centers and changes in the core electron binding energies for the sulfur and lead atoms constituting these centers were considered.

The In/PbTe barrier structures with a thin intermediate insulating layer

Epitaxial n-PbTe layers were grown on BaF2 {111} single-crystal substrates by hot-wall epitaxy from the gaseous phase. These layers were kept in atmospheric air for 15–30 days, after which In and protective BaF2 layers were deposited. Current-voltage characteristics and photoelectric sensitivity spectra of the In/n-PbTe barrier structures were measured in the temperature range T=80–300 K. Based on the experimental results, a model of charge transport is suggested and the effective barrier height ϕbeff, the insulator layer thickness δ, and the surface-state density DS are determined.

Comparative reactivity of A IV B VI compounds in their reactions with dioxygen

The interaction of AIVBVI semiconductors with molecular oxygen has been investigated theoretically (by first-principles calculations using density functional theory) and experimentally (by X-ray photoelectron spectroscopy). It is demonstrated by correlating calculated and experimental data that the comparative reactivity of the solid phases can be quickly estimated using the small-cluster model.

Theoretical modeling of oxygen adsorption on the PbTe(001) surface

For the semiconducting compound PbTe, the initial stages of oxidation, which are important for technology of IR-and thermoelectric devices, have been theoretically studied. The structure, stability, and changes in the electrostatic potentials at the oxidized sites in lead telluride have been calculated in the framework of the cluster approach by the hybrid density functional theory B3LYP method. Different variants of attachment of one to six oxygen atoms to the atoms of the surface and subsurface layers have been considered. The most stable oxidation products have been found. The calculation results are quantitatively consistent with experimental XPS data on chemical shifts.

Effects of Paste Composition and Heat-Treatment Conditions on the Microstructure of Polycrystalline CdS 1 - x Se x Films

We have studied the effects of flux content, synthesis procedure, and grinding conditions of CdS1−xSex powders for paste preparation on the composition and microstructure of films produced by screen printing. The results demonstrate that, to obtain single-phase polycrystalline CdS1−xSex films with well-formed grain boundaries, the flux content of the paste must lie in the range 10–12 wt %, and the solid solution with necessary doping impurities should be presynthesized. The powder must be ground to a particle size no greater than 6 μm.

Simulation of core-level binding energy shifts in germanium-doped lead telluride crystals

To simulate the changes in core-level binding energies in germanium-doped lead telluride, cluster calculations of the changes in the electrostatic potential at the corresponding centers have been performed. Different locations of the Ge atom in the crystal bulk have been considered: near vacancies, near another dopant site, and near the surface. For calculating the potential in the clusters that model the bulk and the surface of the lead telluride crystal (c-PbTe), the electron density obtained in the framework of the Hartree-Fock and hybrid density functional theory (DFT) methods has been used.