Lattice dynamics of II-VI mixed semiconductor ZnS1−xSex

Lattice dynamical properties of II-VI compounds having zinc-blende structure have been calculated by three-body shell model. This model incorporates the effect of the short-range repulsive interactions up to and including the second nearest neighbours, in addition to the long-range Coulombic interactions in the frame work of the rigid-shell model with both the ions are polarizable. The model involves in total eleven disposable parameters. Using the above proposed model the phonon dispersion relations for mixed II-VI semiconductor ZnS_1−xSe_x are plotted. We find an overall good agreement with the experimental results. The application of the present model has been made to calculate the phonon dispersion relations of ZnS, ZnSe and mixed semiconductor ZnS_1−xSe_x. The comparison of the theoretical results with the available experimental has been made along high symmetry directions. A reasonably good agreement is observed between theory and experiments.     

The electron trap associated with an anion vacancy in ZnSe and ZnSxSe1−x

A dominant electron trap in ZnSe and ZnS_xSe_1?x has been studied by DLTS. The trap depth of about 0.3 eV is independent of the crystal growth, doped impurities and S mole fraction x in ZnS_xSe_1?x, while the electron-capture cross-section decreases with x. These properties indicate that the electron trap is due to the anion vacancy in ZnSe and ZnS_xSe_1?x.     

The self-activated luminescence of ZnS1−xSex

The self-activated luminescence (SA) of ZnS : I, ZnSe : Cl, ZnSe : A1, and some ZnS_1?xSe_x : Cl single crystals has been investigated at LHeT under excitation with light. The SA-luminescence band has been identified in each case by the temperature dependence and by a direct excitation band resulting in polarized emission. Symmetry C_3v in case of ZnS : I and ZnSe : Cl and symmetry C_s in case of ZnSe : A1 for the SA-centre have been unambiguously revealed from angular dependences of polarization. Thus the nearest neigbour-compensated cation-vacancy has been identified as complex responsible for SA-luminescence.     

Activation energies of the acceptor-bound excitons and the donor-acceptor pairs in nitrogen-doped p-type ZnSe, p-type ZnSySe1−y, and p-type Zn1−xMgxSySe1−y epitaxial films grown on GaAs (1 0 0) substrates

Nitrogen-doped p-type ZnSe, p-type ZnS_ySe_1−y, and p-type Zn_1−xMg_xS_ySe_1−y epilayers were grown on n-type GaAs (1 0 0) substrates by molecular beam epitaxy. Photoluminescence (PL) spectra for the p-type ZnSe and the lattice-matched p-type ZnS_0.06Se_0.94, and p-type Zn_0.92Mg_0.08S_0.12Se_0.88 epilayers showed a deep acceptor bound exciton emission and a donor-acceptor pair emission. Temperature-dependent PL measurements were carried out to determine the activation energies of these states. The activation energies of the acceptor-bound excitons and the donor-acceptor pairs were determined to be 40 and 65 meV in the p-type ZnSe epilayer, 20 and 45 meV in the p-type ZnS_0.06Se_0.94, and 45 and 43 meV in the p-type Zn_0.92Mg_0.08S_0.12Se_0.88 epilayers.     

Superconducting properties of PbxSn1−xTe

Superconducting transition temperatures and critical magnetic field curves on low carrier concentration Pb_xSn_1?xTe samples where 0.25 ?x?0.55 are reported. These data are interpreted in terms of a changing band structure from that of SnTe to one which is similar to that of GeTe. The importance of carriers (holes) in secondary maxima of the valence band is emphasized.     

Phase sensitive spectroscopy of the polariton modes in a ZnSe-ZnSxSe1−x heterostructure

Applying the method of spectral interferometry we investigate the phase of reflected light at a ZnSe-ZnS_xSe_1−x heterostructure. We find a series of polariton modes propagating through the ZnSe layer. They can be related to the different polariton branches split of at the heavy- and light-hole excitons. The phase shows pronounced changes around these modes. The strongest changes by 2π appear at the modes of lowest order located weakly above the exciton resonances, while they are smaller for higher modes. Our experimental findings can be explained considering spatial dispersion, Pekar's additional boundary conditions and a weak extension of the excitonic polarization into the ZnS_xSe_1−x cladding layers.     

Mode dispersion in the optical-phonon spectra of mixed crystals ZnS1−x Sex

The IR reflection and the Raman spectra of ZnS_1?x Se_x crystals (0≤ x ≤ 1) are measured. The mode dispersion of the solid solutions is found to deviate from that calculated using an isodisplacement model. The reasons behind this deviation are discussed. Two additional modes are found in the frequency range between the ZnS-like TO and LO modes. It is assumed that one of them is a line of the second-order spectrum amplified by the Fermi resonance and the other is linked to the resonance (additional local) mode of Se impurity atoms. The latter conclusion is confirmed by calculating the spectrum of a Se impurity in a ZnS crystal in terms of the microscopic lattice dynamics theory in the low impurity concentration approximation. The oscillator strengths of the main and additional optical phonons in ZnS_1?x Se_x solid solutions are discussed.     

Stoichiometry,electrical properties,and electronic structure of Cr1.70Nb2.30Se10

Both powder and single-crystal X-ray investigations show that Cr_{1 + x}Nb_3?xSe₁₀ which belongs to the FeNb₃Se structure type, exists only in a narrow range of stoichiometry close to x=0.70. In contrast to the Fe analog, Cr_1.70Nb_2.30Se₁₀ undergoes no disticnt metal-insulator transition; the resistivity and the thermoelectric power remain of the same order as the temperature is lowered from 300 K to 4.2 K. The thermoelectric powers S of Fe_{1 + x}Nb_3?xSe₁₀ (0.25<x<0.40) and FeVNb₂Se₁₀, however, follow such a temperature T dependence as $\text{S∝}\text{1}\text{T}$ even at room temperature. On the basis of light-binding band calculations, these observations are interpreted in terms of the contribution of Cr or Fe d-orbitals to electron conduction.     

Production and Laser Characteristics of Fe<Superscript>2+</Superscript>:ZnS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> Polycrystals

A method is developed for producing active laser elements (spectrum range 4 to 5 μm) based on polycrystalline solid solutions ZnS_xSe_x-1 doped with iron ions. Bilateral diffusion doping of the elements by Fe²⁺ ions is performed during hot isostatic pressing. Spectral and energy characteristics of the laser are investigated with the Fe²⁺:ZnS_0.1Se_0.9 active element kept at room temperature. It is found that the absorption band of the Fe²⁺:ZnS_0.1Se_0.9 crystal is blueshifted with respect to the Fe²⁺:ZnSe absorption band, while the lasing spectra of the Fe²⁺:ZnSe and Fe²⁺:ZnS_0.1Se_0.9 lasers and their energy parameters are almost identical. The lasing energy of 580 mJ is obtained at the slope efficiency with respect to the absorbed energy of 46%. Further increase in the lasing energy is limited by development of transversal parasitic oscillation at a large size of the pump beam spot.     

New ferrimagnetic spinel compositions in the system MCr2S4−xSex where M=Fe,Co, Mn

Phase relationship between ferrimagnetic semiconducting spinels of the type MCr₂S₄ and defect NiAs compositions of the type MCr₂Se₄ (where M=Mn, Fe, Co) are established. It was found that the amount of Se which can be substituted, with retention of the spinel structure, decreases from MnCr₂S_4?xSe_x (x=0 to x=2) to FeCr₂S_4?xSe_x (x=0 to x=1·25) to CoCr₂S_4?xSe_x (x=0 to x=1). This phenomena is accounted for on the basis of sulfide spinel stability and unit cell size. The Curie temperatures of the spinel compositions decrease slowly with increasing Se content. This is presumably caused by weakening of the spinel A-B superexchange interaction.     

Low temperature conversion electron Mössbauer spectroscopy on FexGe1−x amorphous thin films

Conversion Electron Mössbauer Spectroscopy has been performed between room temperature and 4.2 K on a series of thin films 1000 Å thick of amorphous Fe_xGe_1?x alloys, 0.4 ? x ? 0.60. It shows that the onset of ferromagnetism for x > 0.4 is associated with the apparition of a small magnetic moment in Fe atoms, increasing rapidly with x. For a given composition, the distribution in the values of moments born by the Fe atoms is large, reflecting the statistical fluctuations of the nearest neighbour environment of the iron atoms.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Structural,optical, and electrical properties of boron-doped ZnO1-xSx thin films deposited by MOCVD

Boron-doped ZnO_1-xS_x (ZnO_1-xS_x:B) thin films were fabricated by metalorganic chemical vapor deposition (MOCVD). We investigated the structural, optical, and electrical properties of the ZnO_1-xS_x:B thin films. X-ray diffraction patterns showed that, except for the ZnO:B (x?=?0) and ZnS:B (x?=?1) thin films, the ZnO_1-xS_x:B thin films exhibit amorphous characters. Optical transmittance spectra were analyzed to estimate the band gaps of the thin films with different S content. All thin films showed direct band gaps ranging from 3.34?eV (ZnO:B) to 3.49?eV (ZnS:B). The influence of sulfur content on carrier concentration, electrical resistivity, and Hall mobility of the ZnO_1-xS_x:B thin films were analyzed from Hall effect measurements measured at temperatures ranging from liquid nitrogen temperature to room temperature. The ZnO_1-xS_x:B thin films exhibited n-type electrical conductivity except for ZnS:B, which was not measurable in this study due to its high resistivity (>100?Ω?cm).     

Optical absorption in Hg1−xMnxTe, x < 0.2 mixed crystals

Optical absorption in Hg_1?xMn_xTe, x < 0.2 mixed crystals was measured in the photon energy range from 0.03 to 0.6 eV at the temperatures 295, 80, 15 K. Experimental results were interpreted on the basis of the Kane model for Δ ? E_G. A transition from the inverted band structure (α-Sn-type) to the simple (InSb-type) structure was observed and was found to be composition and temperature dependent.     

Competing anisotropy and random field effects in Mn1−xFexCO3

Previously published ⁵⁷Fe Mössbauer data for MnCO₃:Fe have been interpreted as arising from a competing anisotropy system. By considering the variation of the magnitude H_hf and direction θ_hf of the magnetic hyperfine field with temperature, the phase diagram for Mn_1?xFe_xCO₃ is deduced. The boundaries of the mixed phase are clear in the Mössbauer data. However, these boundaries are not sharp but are broadened by the effect of internal random magnetic fields.     

Studies on growth and characterization of ternary CdS1−xSex alloy thin films deposited by chemical bath deposition technique

Ternary alloyed CdS_1−xSe_x thin films of variable composition ‘x’ were grown by the simple and economical chemical bath deposition technique. The as-grown thin films were characterized for structural, compositional, surface morphological, optical and electrical studies. The X-ray diffraction (XRD) patterns of the sample indicated that all the samples were polycrystalline in nature with hexagonal structure. Scanning electron microscopy (SEM) micrographs showed uniform morphology with spherical shaped grains distributed over entire glass substrate. EDAX studies confirmed that the CdS_1−xSe_x films were having approximately same stoichiometry initially as well as finally. Room temperature optical measurements showed that band gap engineering could be realized in CdS_1−xSe_x thin films via modulation in composition ‘x’. Electrical resistivity of CdS_1−xSe_x thin films for various compositions was found to be low. The broad and fine tunable band gap properties of ternary CdS_1−xSe_x thin films have potential applications in opto-electronic devices.     

The dependence of the structural and optical properties on the Te mole fraction in ZnS1−xTex/GaAs heterostructures

Lattice-mismatched ZnS_1−xTe_x epilayers with various Te mole fractions on GaAs (100) substrates were grown by double well temperature gradient vapor deposition. X-ray diffraction patterns showed that the grown ZnS_1−xTe_x layers were epitaxial films. The photoluminescence spectra showed that the peak position of the acceptor-bound exciton (A⁰, X) varied dramatically with changing the Te mole fraction and that the behavior of the (A⁰, X) peak position of the ZnS_1−xTe_x epilayers with a small amount of the Te mole fraction was attributed to a bowing effect. The reflectivity and ellipsometry spectra showed that the absorption energy peak was significantly affected due to the Stoke's effect. These results provide important information on the structural and optical properties of ZnS_1−xTe_x/GaAs heterostructures for improving optoelectronic device efficiencies operating in the spectral range between near ultraviolet and visible regions.     

Hall effect of FeTe and Fe(Se1–xTex) thin films

The Hall effect is investigated in thin-film samples of iron–chalcogenide superconductors in detail. The Hall coefficient (R_H) of FeTe and Fe(Se_1–xTe_x) exhibits a similar positive value around 300 K, indicating that the high-temperature normal state is dominated by hole-channel transport. FeTe exhibits a sign reversal from positive to negative across the transition to the low-temperature antiferromagnetic state, indicating the occurrence of drastic reconstruction in the band structure. The mobility analysis using the carrier density theoretically calculated reveals that the mobility of holes is strongly suppressed to zero, and hence the electric transport looks to be dominated by electrons. The Se substitution to Te suppresses the antiferromagnetic long-range order and induces superconductivity instead. The similar mobility analysis for Fe(Se_0.4Te_0.6) and Fe(Se_0.5Te_0.5) thin films shows that the mobility of electrons increases with decreasing temperature even in the paramagnetic state, and keeps sufficiently high values down to the superconducting transition temperature. From the comparison between FeTe and Fe(Se_1–xTe_x), it is suggested that the coexistence of ‘itinerant’ carriers both in electron and hole channels is indispensable for the occurrence of superconductivity.     

Synthesis and characterization of new LnxSb2−xSe3 (Ln: Yb, Er) nanoflowers and their physical properties

New Ln_xSb_2−xSe₃ (Ln: Yb³⁺, Er³) based nanomaterials were synthesized by a co-reduction method. Powder XRD patterns indicate that the Ln_xSb_2−xSe₃crystals (Ln=Yb³⁺, Er³⁺, x=0.00-0.12) are isostructural with Sb₂Se₃. The cell parameters b and c decrease for Ln=Er³⁺ and Yb³⁺ upon increasing the dopant content (x), while a increases. SEM images show that doping of the lanthanide ions in the lattice of Sb₂Se₃ generally results in nanoflowers. UV-vis absorption and emission spectroscopy reveals mainly electronic transitions of the Ln³⁺ ions in case of Yb³⁺ doped nanomaterials. Emission spectra of doped materials, in addition to the characteristic red emission peaks of Sb₂Se₃, show additional emission bands centered at 955 nm, originating from the ²F_7/2→²F_5/2 transition (f-f transitions) of the Yb³⁺ ions. DSC curves indicate that Sb₂Se₃ has the highest thermal stability. The temperature dependence of the electrical resistivity of doped-Sb₂Se₃ with Yb³⁺ and Er³⁺ was studied.     

Structural and optical properties of In35Sb45Se20−xTex phase-change thin films

Physical properties of In₃₅Sb₄₅Se_20−xTe_x thin films with different compositions (x=2.5, 5, 7.5, 10, 12.5 and 15 at %) prepared by electron beam evaporation method are studied. X-ray diffraction results indicate that the as-evaporated films depend on the Te content and the crystallized compounds consist mainly of Sb₂Se₃ with small amount of Sb₂SeTe₂. Transmittance and reflectance of the films are found to be thickness dependent. Optical-absorption data indicate that the absorption mechanism is direct transition. Optical band gap values decrease with increase in Te content as well as with increase in film thickness.