Se Crystals Research Articles

Zur kristallstruktur von strontiumperrhenat-sesquihydrat Sr(ReO 4) 2·1,5H 2O

Single crystals of Se(ReO 4) 2·1.5H 2O were prepared at room temperature from an aqueous solution of Sr(ReO 4) 2. The structure has been studied by X-ray diffractometry and was refined to the R and R w values of 0.100 and 0.118 respectively. The salt crystallizes in monoclinic symmetry belonging to the space group P2 1/ n (the lattice constants constants a = 13.207 A ̊ , b = 10.028 A ̊ , c = 13.538 A ̊ and β = 92.3°; Z = 8 ). The three-dimensional network is formed by ReO 4 tetrahedra and SrO 9 polyhedra connected by their corners. The water molecules are coordinated exclusively with Sr 2+. Structure relations with other perrhenates are discussed.

Far-infrared magnetoabsorption studies of zero-gap Hg1-xMnxSe crystals.

The results of magnetotransmission measurements performed on ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se crystals (0.021\ensuremath{\le}x\ensuremath{\le}0.05) at different temperatures are presented. The investigated materials have an inverted band structure (like HgSe). Most of the observed features are explained by transitions between conduction-band Landau levels, described by the Pidgeon-Brown model extended to include exchange interaction. The obtained values of the interaction gap ${\mathit{E}}_{0}$ and matrix element P complement results obtained from transport and interband magnetotransmission measurements, and show a nonlinear behavior of ${\mathit{E}}_{0}$ vs Mn composition x and a strong nonmonotonic dependence of P on x. The temperature dependence of Landau-level energies at low temperatures (between 2 and 8 K) is due to the changes of magnetization with temperature. In addition to spin and combined resonances, transitions allowed by warping and/or inversion asymmetry such as the second and third cyclotron harmonics are observed. The transitions, which could not be explained by transitions between Landau levels, are attributed to transitions from an impurity level (probably an ionized acceptor state) resonant with the conduction band. The zero-field energy of the level is estimated to be 3.8 meV above the top of the valence band.

Crystallization kinetics in glassy Se 80−xTe 20In x alloys

Conductivity measurements have been performed to study the kinetics of crystallization in the glassy Se 80− x Te 20In x system. Amorphous to crystalline (A-C) transformations are studied during isothermal annealing at various temperatures between the glass transition and melting temperature. Avrami's equation of isothermal transformation is used to calculate the activation energy of crystallization (Δ E) and the order parameter ( n). The values of Δ E thus calculated match well with the values reported for the same alloys using the nonisothermal DSC technique.

Influence of thermal processing of Mn x Hg1?x Se crystals in mercury and selenium vapor on their magnetic susceptibility

We report a study of the influence of thermal processing of Mn x Hg1−xSe crystals in mercury and selenium vapor on the temperature dependence of their magnetic susceptibility. It is shown that thermal processing of Mn x Hg1−xSe in mercury vapor leads to diffusion of Hg atoms into the crystal, with a resulting increase in the number of interstitial mercury atoms, which act as donors and increase the electron concentration in the samples. As a consequence, the diamagnetic contribution of the interstitial mercury increases, along with the dia- or paramagnetic contribution of the increasing electron concentration, and there is a change in the contribution of the Mn atoms, which migrate during the anneal, leading to a change in the magnetic susceptibility of the Mn x Hg1−xSe crystals. We also show that thermal processing of Mn x Hg1−xSe in Se vapor leads to the formation of new clusters in the crystal, or a change in size for ones which preexist (either by increasing or decreasing the size) due to the thermodynamic equilibrium conditions in the Mn x Hg1−xSe system, i.e. the relationship between the cluster size and the concentration.

121-Sb Mössbauer study of insulating and ion-conducting antimony chalcogenide-based glasses

The local environment of antimony in As 2S 3Sb 2S 3, AgISb 2S 3, AsSbSe and A AgAsSbSe glasses, glassy/crystalline alloys and crystals is investigated using 121-Sb Mössbauer spectroscopy. The isomer shift both for sulfide and selenide vitreous alloys is 1.2–2.0 mm/s more positive than that for corresponding crystalline parents. The smaller s-electron density at the 121Sb nucleus for glasses indicates a decrease of the antimony-chalcogen interatomic distances in comparison with those for crystalline materials. The isomer shift systematics and composition dependencies of the electric-quadrupole coupling constant, eQV zz , and the asymmetry parameter, ν, shows that in As 2S 3Sb 2S 3 and AsSbSe insulating glasses an isomorphous substitution of arsenic by antimony occurs accompanied by an increase of interatomic distances. In the case of Ag + ion-conducting vitreous alloys, however, the antimony-chalcogen distances are governed by the silver content and do not depend on the Sb fraction of the glass. Geometric and topological disorder in the glass network is correlated with an increase of the electric-quadrupole coupling constants and values of the asymmetry parameter ν between 0.4 and 1.0.

Temperature dependence under pressure of the kinetics of crystallization of bulk amorphous selenium

Abstract The pressure (0·28–0·69 GPa)–temperature (382–409 K) dependence of the kinetics of the crystallization of amorphous Se to the trigonal phase has been investigated. The crystallized fraction-time data under different isobaric-isothermal conditions have been obtained, by monitoring the time dependence of the volume change, using the piston-displacement technique. The temperature dependence of the spherulitic growth rate g at different pressures has also been measured by optical microscopy. The isothermal rate of crystallization is found to increase with increase in temperature and to decrease with increase in pressure. The temperature dependences of the incubation period, the time t0·5 for 50% crystallized fraction and g at each pressure have been used to determine the activation energy barriers for nucleation, for overall crystallization and for the growth processes. Ali these energies correspond to the activation energy ΔG∗d for the diffusion process involved in the nucleation of spherulites. ΔG∗...

Photostimulated gettering of deep band-gap impurities from semiconductors by resonance excitation: Fe from Cd0.98Fe0.02Se.

Impurities that lie deep in the band gap of a semiconductor (deep impurities) behave as a strong recombination center as shown by the Schockley-Read mechanism for surface recombination [A. Rothwarf and K. W. Boer, Prog. Solid State Chem. 10, 71 (1975)]. Thus, removing such impurities from the semiconductor is imperative for the optimal operation of photonic devices and solar cells, in particular. In the following work a method is proposed to remove such impurities. In this process resonance excitation of electrons (holes) from the impurity level into the conduction (valence) band leads to ionization of the impurity, which in turn leads to fast outdiffusion of the impurity and its dissolution by a chemical (electrochemical) reaction at the semiconductor surface. As a demonstration of this principle Fe, which is a deep donor in CdSe, is selectively removed from a ${\mathrm{Cd}}_{0.98}$${\mathrm{Fe}}_{0.02}$Se crystal. A giant diffusion constant (D=${10}^{\mathrm{\ensuremath{-}}9}$ ${\mathrm{cm}}^{2}$/sec) is measured for Fe in this experiment. The application of this process for the control of impurity profiles in various semiconductor devices is briefly discussed.

Raman spectra of tetrahedral vibrations in crystalline germanium dichalcogenides, GeS 2 and GeSe 2, in high and low temperature forms

Raman spectra of GeX 2 (X=S and Se) crystals in high (HT) and low (LT) temperature forms have been studied on the view point of the connectivity of GeX 4 tetrahedra. The characteristic Raman bands are understood by a simple valence-force-field model combined with a bond polarizability model. The most intense Raman bands in HT-GeX 2 have been assigned to tetrahedral A 1 vibrations extended in the chain structures along the [100] direction. Quasi-localized A 1 vibrations in the bridge structures in HT-GeX 2 are responsible for a higher side-band of the main band. The main bands in the spectra of LT-GeX 2 have been assigned to nearly degenerated A 1 vibrations linearly extended in the two chains along the [001] direction. The difference between the spectra of GeS 2 and GeSe 2 has been roughly explained to be due to the difference between atomic masses.

Magnetic-field affected luminescence of Mn 2+ ions in Zn 1− xMn xSe compounds under resonance excitation of excitons

Under the resonance excitation of free excitons in cubic Zn 0.99Mn 0.01Se crystals we observed an unexpectedly strong effect of applied magnetic field on the intra-centre luminescence of the Mn 2+ ions. Strong enough difference is revealed between the manifestation of this effect in σ + and σ - Mn 2+ luminescence under σ + and σ - exciton pumping. The observed effect is attributed to a specific influence, left out of the consideration in earlier studies, of the carrier-ion exchange interaction on the energy transfer from excitons to the subsystem of the Mn 2+ ions.

Characterization of CdTe, (Cd,Zn)Te, and Cd(Te,Se) single crystals by transmission electron microscopy

CdTe, (CD,Zn)Te, and Cd(Te,Se) crystals grown by the Bridgman technique have been characterized by transmission electron microscopy. Results indicate that the Te precipitates are seen in all the crystals, but their density and size are lowest and largest in the case of Cd(Te,Se) crystals. In addition, dislocations, stacking faults, and microtwins are observed in as-grown CdTe, (Cd,Zn)Te, and Cd(Te,Se) crystals. Arguments have been developed to rationalize these observations and their ramifications on crystal perfection are discussed.

Kinetics of CW laser-induced crystallization and oxidation of thin, Sb, Se, and Sb2Se3 films in air

Thin films of Sb, Se and Sb2Se3 are deposited onto glass and irradiated by a cw-Ar+ laser beam. The kinetics of crystallization and oxidation are traced via the time dependence of optical reflectivity and temperature, T, of the irradiated zone. For Sb2Se3, transformations start abruptly when T attains a critical value, Tc, independently of the laser beam power. These Tc values are comparable to the ones observed under furnace annealing conditions.

Structures and crystallization of vacuum-deposited amorphous Se and Sb films

In hexagonal Se crystal each atom is covalently bound to two others to form an endless spiral chain, and in Sb crystal each atom to three others to form an extended puckered sheet. Such chains and sheets may be regarded as one- and two- dimensional molecules, respectively. In this paper we investigate the structures in amorphous state of these elements and the crystallization.HRTEM and ED images of vacuum-deposited amorphous Se and Sb films were taken with a JEM-200CX electron microscope (Cs=1.2 mm). The structure models of amorphous films were constructed on a computer by Monte Carlo method. Generated atoms were subsequently deposited on a space of 2 nm×2 nm as they fulfiled the binding condition, to form a film 5 nm thick (Fig. 1a-1c). An improvement on a previous computer program has been made as to realize the actual film formation. Radial distribution fuction (RDF) curves, ED intensities and HRTEM images for the constructed structure models were calculated, and compared with the observed ones.

Free and bound exciton in II-VI semimagnetic compounds

The extremely large exchange interaction typical of semimagnetic semiconductors strongly affects the exciton energies. In hexagonal Cd 1- x Mn x Se crystals, the existence of a uniaxial crystal field leads to a strong dependence of exciton splittings not only on the value, but also on the direction of the magnetic field with respect to the crystal axis. In ordinary wurtzite type crystals such as CdSe, the transverse component g h⊥ of the g-factor at the top of the valence band (A-band) is equal to zero and these states are thus not split in the transverse magnetic field ( H⊥ C 6). By contrast, a large exchange interaction in the Cd 1- x Mn x Se crystals results in a non-zero g-factor and spin splitting of the top valence states in the H⊥ C 6 magnetic field is observed. This splitting arises as a result of a different valence subbands mixing in the exchange field and its proportional to the cube of the exchange field ∝ G 3 h . The subban mixing leads also to new selection rules for optical exciton transitions. The valence and conduction band spin splittings result in ionization of the neutral donor (acceptor) bound excitons in semimagnetic semiconductors. The influence of spin-polaron effects on the energies and stability of the bound excitons is discussed.

Electrical properties of Cd1-xFexSe.

We present results of resistivity and Hall-effect measurements performed on n-type as-grown ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$Se crystals with 0x\ensuremath{\le}0.07 in the temperature range 1.7--300 K. For T80 K, activated behavior in conductivity is observed. We found that ${\mathit{E}}_{1}$ (the energy required to raise electrons from the donor center to the conduction band) and ${\mathit{E}}_{3}$ (the activation energy in hopping conduction) vary strongly with composition x. These results are discussed in the framework of both Anderson localization (produced by substitutional disorder) and random fluctuations in composition and central-cell potentials.

Growth of Zn(1−x)Mn(x)Se and Zn(1−x)Fe(x)Se mixed crystals

Single crystals of (Zn,Mn)Se and (Zn,Fe)Se were grown by the modified Bridgman method under argon pressure. The value of x in Zn(1−x)Mn(x)Se was changed from 0 up to 0.53. Zn(1−1)Fe(x)Se crystals were grown with a concentration 0.01<x<0.22. The pressure during the growth process was 5.3±0.1 MPa. The mass losses during the crystallization process were less than 1 per cent.

Near band-edge optical investigation of α-In 2Se 3 and α-In 2S 3− xSe x mixed crystals

α-In 2Se 3 compound and α-In 2S 3− x Se x mixed crystals are grown by direct melting of stoichiometric mixtures of their components. Optical absorption measurements were made in the temperature range 15–330 K and the absorption coefficients were measured as a function of the wavelength of the incident light between 750 and 920 nm in the composition range 2.4 < x < 3. These crystals are layered and keep the α-In 2Se 3 defect hexagonal wurtzite-like structure, with cationic lattice vacancies. For all compounds the energy gap is temperature dependent and the absorption edges shift to lower energy values with increasing temperature. An almost linear variation of the absorption edges with chemical concentration and shift to lower energy values is observed when the Se content is increased. The relation of the absorption coefficient to some power of ( hv − E g ) is not simple, possible mechanisms of the optical transitions are discussed, and an attempt is made to correlate them with the peculiarities of the defect structures of this system. Finally, no phase transitions were found down to 15 K for all samples.

Preparation and x-ray crystal structure of 1,4-diiodo-cyclo-hexaselenium bis(hexafluoroarsenate)-bis(sulfur dioxide), Se6I2(AsF6)2.cntdot.2SO2, and (Se6I)n.cntdot.nMF6 (M = As, Sb) containing the catena-poly[cyclohexaselenium(1+)-4:1-.mu.-iodo] cation

Highly crystalline Se 6 I 2 (AsF 6 ) 2 •2SO 2 was obtained from an SO 2 solution of a very complex equilibrium mixture of average composition Se 6 I 2 (AsF 6 ) 2 . (Se 6 I) n •nAsF 6 was prepared quantitatively by reacting stoichiometric amounts of Se, I 2 , and AsF 5 in SO 2 . Attempts to prepare related selenium-iodine cations led to mixtures of (Se 6 I) n •nAsF 6 and Se 6 I 2 (AsF 6 ) 2 •2SO 2 . Single crystals of (Se 6 I) n •nSbF 6 were obtained from a reaction of elemental selenium and I 2 Sb 2 F 11 in liquid SO 2

Kinetic study of isothermal and continuous heating crystallization in Se 100− xBi x ( x ⩽ 8) glasses

Calorimetric studies of chalcogenide glasses Se 100- x Bi x obtained by vacuum co-evaporation have been performed by isothermal and continuous heating experiments in a Perkin Elmer DSC-2C calorimeter. The Se-Bi system displays stable immiscibility and this could be an explanation for the small number of papers in the literature. Results obtained in both isothermal and non-isothermal experiments are interpreted with the same set of kinetic parameters. Our data are in agreement with the kinetic Johnson-Mehl-Avrami-Erofe'ev equation. The interpretation of the Se-Bi crystallization can best be understood by reference to phase separation phenomena.

Photoemission yield study of semimagnetic semiconductor Hg 1−xFe xSe crystals

Photoemission yield spectroscopy (PYS) was used to investigate HgSe and Hg 0.88Fe 0.12Se crystal surface freshly cleaved in a UHV chamber at a pressure of about 10 −9 Torr. The spectral photoemission yield curves were measured in the vacuum-ultraviolet range (4 < hv < 11 eV). The experimental results show that the contribution of the Fe3d 6 electrons to the electronic structure of the valence band leads to a shift of the work function from 4.30 to 4.15 eV. This contribution appears as an additional shoulder on the edge of the spectral photoemission yield curve. The interaction of the ambient atmosphere with the surfaces of freshly cleaved Hg 1− x Fe x Se ( x = 0.0, 0.12) crystals causes a decrease of the ionization energy E i and the work function W.

GaAs on Si technology

Resonant photoemission spectra of Fe atoms (3p-3d) were measured for freshly cleaved Cd0.86Fe0.14Se crystal (110) surface and for the same surface with sequentially deposited Fe atoms in the range 0.3–80 ML. The synchrotron radiation (LURE, Orsay, France) was used to measure valence band density of state distributions and Cd 4d core level for the radiation energy range 50–60 eV, which includes the Fe 3p-3d resonant energy equal to 56 eV. At the lowest stages of Fe atom deposition (up to about 3 ML), Fe atoms replace Cd atoms in the crystal, making a Cd1−xFexSe alloy with a higher x and leaving extra Cd as small islands or clusters. For the next stages of Fe deposition (about 3–10 ML) the dissociated Cd atoms create a CdFe alloy. At higher stages of Fe deposition (above about 10 ML) a Fe thin film is formed.