Iodine Vapor Transport Method Research Articles

Superconductivity in single crystals of ZrP1.27Se0.73

Results are reported for single crystals of the PbFCl-type layered compound ${\mathrm{ZrP}}_{1.27}{\mathrm{Se}}_{0.73}$ that were produced using the iodine vapor transport method. Electrical transport, magnetization, and heat capacity measurements reveal disordered metallic behavior and the occurrence of bulk superconductivity, with a transition temperature $({T}_{c})$ of 7.1 K and an anisotropic orbitally limited upper critical field. $^{31}\mathrm{P}$ nuclear magnetic resonance measurements provide additional microscopic information, where the line shape, Knight shift, and spin-lattice relaxation data are consistent with the superconductivity originating from the corrugated Zr-P(2c)/Se plane. This suggests that the superconductivity first forms in the corrugated plane and the bulk superconductivity eventually occurs via coupling between the square planar layers. These data also show that either there are multiple superconducting gaps or there is a single gap that does not fully open across the entire Fermi surface. These results clarify the superconducting state in this material and will enable further measurements that require single-crystal specimens.

Growth and Electronic Structure Studies of Metal Intercalated Transition Metal Dichalcogenides MxNbSe2 (M: Fe and Cu)

There has been renewed interest in transition metal dichalcogenides (TMDs) as electronic materials. The modification of the electronic structures of TMDs by means of metal intercalation would be of interest in that respect. With this motivation, we have synthesized and performed angle-resolved photoemission studies on Fe- and Cu-intercalated TMD FexNbSe2 and CuxNbSe2 with various x values. FexNbSe2 (x = 0, 0.25) and CuxNbSe2 (x = 0, 0.06) were grown by the iodine vapor transport method. Intercalation and doping were confirmed by changes in c-axis lattice constant and electronic structure, respectively. A significant amount of electron doping from Fe and Cu was observed, as indicated in the ARPES data measured as the rigid downshift of bands. In addition, band folding was observed in FexNbSe2, for which the unit cell is doubled owing to the Fe order. However, the extra band folding expected from the antiferromagnetic order of Fe was not observed.

Magnetic Properties of FeAs Single Crystal

Magnetic measurements have been performed on FeAs single crystals and polycrystalline samples. Magnetic measurements revealed a very sharp spin-glass magnetic transition in self flux grown single crystal. The bifurcation point between zero field cooled (ZFC) and field cooled (FC) magnetization occurs at 130 K. Susceptibility of flux grown single crystals revealed ferromagnetic-type behavior in the temperature range 4–300 K, while single crystals grown by iodine transport vapor were non-magnetic revealed features associated with antiferromagnetic and spin wave characteristics. The shift in Raman lines reveled sharper peaks for the crystals grown by vapor transport method than that obtained for the flux grown crystals. The difference in the spectrum may indicate sharper distribution in the bonding strength between Fe and As ions in crystals grown by the iodine vapor transport method.

Photoemission studies of Cu intercalated NbSe2

We report growth and angle resolved photoemission studies of Cu intercalated NbSe2. CuxNbSe2 with x=0, 0.04 and 0.08 were grown by iodine vapor transport method. Intercalation and doping was confirmed by the change in the c-axis lattice constant and the change in the electronic structure, respectively. To investigate the nesting condition, we performed auto-correlation analysis on the data. We find that the magnitude of the nesting vector decreases with doping but the direction remains unchanged. Our observation is consistent with the nesting coming from intra-pocket transition.

Electrical and optical characterisation of CuInS 2 crystals and polycrystalline coevaporated thin films

Single crystals CuInS 2 were grown by iodine vapour transport method, whereas polycrystalline thin films were obtained by coevaporation technique from three sources. The temperature dependence of the hole mobility in valence band is analysed by taking into account contributions from several scattering mechanisms of the charge carriers. To account for the temperature dependant conductivity of polycrystalline CuInS 2 thin films, grainboundary conduction process was suggested. In the low temperature region, we interpret the data in terms of the Mott law and the analysis is very consistent with the variable range hopping. However, thermionic emission is predominant at high temperatures. Photoluminescence measurements have been performed on CuInS 2 crystals and the analysis has revealed that the emission is mainly due to free-to-bound and donor–acceptor pair transitions. The band gap of that compound is derived from the excitonic emission line at 1.53 eV.

Electrical properties of CuGaSe2 single crystals and polycrystalline coevaporated thin films

Abstract CuGaSe2 single crystals were grown by iodine vapour transport method, whereas polycrystalline thin films were obtained by coevaporation technique from three sources. The variation of the hole concentration with temperature of CuGaSe2 single crystal in the high temperature region is interpreted assuming thermal activation in the valence band. The density of states effective mass of the holes is deduced. Acceptor and donor concentrations as well as their compensation ratio are reported. The temperature dependence of the hole mobility in valence band is analysed by taking into account several scattering mechanisms of the charge carriers. To account for the temperature dependant conductivity of polycrystalline CuGaSe2 thin film, grain boundary conduction process was suggested. Quantitative values of the mean potential barrier height at grain boundaries and the standard deviation are reported. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermoelectric Properties of TiS2 type materials

ABSTRACTTiS2 belongs to a family of layered compounds that displays promise as a thermoelectric material. At room temperature the thermopower (a) of TiS2 displays an n-type behavior, with a magnitude of ≈ -200 μV/K. The electrical resistivity (ρ), is on the order of 1 mΩ-cm at room temperature and displays a “metallic-like” behavior with dR/dT > 0 from 300 K to 10 K. Thus, these compounds exhibit relatively large power factors (PF = α2/ρ) with a PF ∼30 μW/K2 cm at T = 300 K, which are comparable to the state-of-the art Bi2Te3 type materials, which have a PF ∼40 μW/K2, at T = 330K. These values suggest that further investigations of these systems could be profitable. Thin plate-like crystals of TiS2 are grown by the iodine vapor transport method with planar dimensions of 1 cm and thicknesses of 20 μm or more. In this synthetic approach some dopants can be integrated into the parent compound, effectively providing a route for the tuning of electronic properties. We present here some effects of elemental doping on the electronic properties in these TiS2 based materials.

Photoluminescence excitation spectra of CuInS2 crystals

The photoluminescence excitation (PLE) spectra of CuInS2 crystals grown by the traveling heater method (THM) and the iodine vapor transport method (IT) have been examined. THM crystals show significant structure with many peaks and dips in the PLE spectra, whereas IT crystals exhibits two clear peaks at two free exciton energies in the spectrum. The peak and dip structures of PLE spectra at the free exciton energy depend on the monitoring PL peak, the kind of crystals, and temperatures. A proposed relaxation process of excited electron–hole pairs explains peak and dip structures of PLE spectra at the free exciton energies.

Influence of CDW stacking disorder on metal–insulator transition in 1 T-TaS 2

We have systematically studied the metal–insulator (MI) transition in 1 T-Ta 1− x S 2 single crystals grown by the iodine vapor transport method under various sulfur pressures. From the chemical analyses and X-ray diffraction measurements, it is concluded that the obtained crystals contain Ta vacancies up to x∼5%. The MI transition temperature T MI is found to decrease with decreasing x. However, this can hardly be interpreted by the carrier doping effect on the Mott transition, in which T MI should be very sensitive to band filling, suggesting that the formation of Ta disorders in the nearly stoichiometric compounds may play an important role. STM images of a nearly stoichiometric crystal which shows no MI transition reveal characteristic mesh-like microstructures and the MI transition induced by the electric field of the STM tip. This also indicates that CDW stacking faults along the c-axis, possibly induced by Ta disorders, is important in determining the transition temperature.

Photoluminescence Excitation Spectra of CuGaSe2

Photoluminescence excitation (PLE) spectra have been examined for the first time on CuGaSe2 crystals using a tunable laser as an excitation source. Free exciton peaks of PLE spectra are clearly observed on crystals grown by the iodine vapor transport method (IT) and the traveling heater method (THM). For IT crystals the activation energies for donor levels and acceptor levels are estimated to 100 meV and 65 meV, respectively, whereas for THM crystals containing 1 at% of In atoms, 80 meV and 50 meV, respectively.

Resonant Raman scattering and luminescence in CuInS2 crystals

Resonant Raman scattering and luminescence have been examined for CuInS2 crystals grown by the traveling heater method (THM) and the iodine vapor transport method (IT). Resonant Raman spectra of CuInS2 have been observed, and the spectra show seven single-phonon peaks and one two-phonon peak. Among them, three single-phonon modes have been found in the low-Raman-shift region because of resonant enhancement of phonon modes. The enhancement of these phonon modes is caused by incoming resonance mediated by bound excitons on the THM crystal, while it is attributed to outgoing resonance due to intermediate states of free excitons on the IT crystal.

Near-blue photoluminescence of Zn-doped GaS single crystals

The photoluminescence and optical absorption properties of Zn-doped GaS crystals prepared by the iodine vapor transport method are reported. Undoped GaS crystals are also used for comparison. Zn substituted for a Ga site acts as an acceptor having a deep energy level and forms a complex with the iodine coactivator, which can be the luminescence center. When increasing the charged amount of Zn, the near-blue emission band of 2.47 eV at 97 K becomes dominant, while the 2.17-eV emission band due to the Ga vacancies, which is dominant in the undoped crystals, gradually vanishes because of the reduction of the Ga vacancies. It is thus described that Zn is a promising dopant in GaS for near-blue-light-emitting devices.

Strain-free, ultra-high purity ZnSe layers grown by molecular beam epitaxy

ZnSe layers have been grown by molecular beam epitaxy on high-purity, high-quality ZnSe wafers [(100) oriented] cut from ingots grown by the iodine vapor transport method. Photoluminescence (PL) analysis indicates the homoepitaxial ZnSe material to be of ultra-high purity as well as being strain-free relative to ZnSe/(100) GaAs layers which exhibit in-plane biaxial tension. The 10 K PL spectra recorded from homoepitaxial layers exhibit unsplit free- and donor-bound exciton transitions of comparable intensity together with a strong peak at 2.7768 eV believed to be the so-called Iv transition in relaxed ZnSc. The ultra-high purity nature of the homoepitaxial layers is attributed to the high purity of the substrate material in addition to the use of high purity Zn and Se source materials in this work.

Composition Dependence of Exciton Band Gaps and Edge Emissions of AgGa(S1-xSex)2

Composition dependence of the lattice constants and edge emissions of crystals of AgGa(S1-xSex)2 solid solution grown by the sintering method were examined in comparison with those grown by the iodine vapour transport method using sintered crystals as raw materials. Phase shift difference measurement of reflection showed a linear dependence of the band gap on composition, whereas a slight bowing characteristic was observed in composition dependence of edge emission. The shift of composition, x, during the iodine vapour transport growth and slight bowing of the edge emissions confirmed from these experiments are discussed.

ZnSe homoepitaxial layers grown at very low temperature by atmospheric pressure metalorganic vapor-phase epitaxy

ZnSe homoepitaxial layers with high crystalline quality were grown on (100) ZnSe substrates (grown by the iodine vapor transport method) between 190 and 250 °C. The surface morphologies of homoepitaxial layers (homoepilayers) grown below 250 °C were all mirrorlike. The concentration of self-activated (SA) centers is related to the growth temperature. The band-edge photoluminescence (PL) intensity increases drastically with increasing temperature from 190 up to 210 °C, decreases above 210 °C, and the SA-PL intensity increases monotonically as the growth temperature increases. The mechanism of suppressing the occurrence of SA centers below 250 °C is assumed that the concentration of Zn vacancies diffused from the highly iodine-doped ZnSe substrates into the ZnSe homoepilayers reduces at this temperature region. The SA-PL intensity of the ZnSe homoepilayer with the strongest band-edge emission (grown at 210 °C) is the same or weaker than that of the best ZnSe heteroepitaxial layer (heteroepilayer). Below 200 °C, the FWHM of the ZnSe (400) diffraction is 14 arcsec which is the best value we have ever obtained, though the growth temperature seems to be too low to maintain the crystalline quality. The comparison on the crystalline quality with the ZnSe heteroepilayers is discussed in detail from the viewpoints of the epitaxial growth mechanism.

Electrical Anisotropy of Layered Compound ZrSe2and HfSe2

The electrical resistivity and the Hall coefficient of the layered compound ZrSe 2 and HfSe 2 grown by iodine vapour transport method have been measured. High anisotropy in electrical resistivities has been found for these compounds. The Hallmobility exhibits a temperature dependence of the form µ∝ T -1.6 in ZrSe 2 as well as in HfSe 2 . It is well explained in terms of homo-polar-mode optical scattering.

Electrical conduction in the ZnSiAs2-xPx system

Single crystals of ZnSiP2, ZnSiAs2 and mixed ZnSiAs2-xPx crystals were grown by various methods, including an iodine vapour transport method, direct growth from the melt, and growth from a molten tin solution. For each sample, the electrical conductivity and Hall coefficient were measured from 80 to 350K, and the conductivity type found. Excess P and I impurities are proposed as donors in the n-type photophorus-containing compounds, and the acceptors in p-type ZnSiAs2 are associated with an As deficiency.

Some low temperature transport properties and superconductivity of NbSe 2 grown by direct vapor transport

A procedure for the preparation of 2HNbSe 2 single crystals by a direct vapor transport method is reported. Crystals grown by this method and the iodine vapor transport method have been studied in a comparative fashion. The effect of iodine and other impurities on some of the low temperature transport properties including superconductive parameters is discussed.

Vapour Growth of CuGaS2 and Its Optical Properties

Single crystals of CuGaS2 are prepared by the iodine vapour transport method using either solution grown crystals or two constituent binary compounds, Cu2S and Ga2S3 as the starting materials. In both cases, a slight difference in the growth condition leads to different colouration of the crystals yielded; yellow, red and black varieties. Polarization measurements of transmission, reflection and photoconduction on these different coloured crystals also show different characteristics of these crystals. From the results of both growth experiments and optical measurements, the differences in colouration and optical properties among these crystals are expected to be due to a slightly off-stoichiometric unbalance between Cu2S and Ga2S3, and the yellow crystal is considered to be the most stoichiometric one.

Preparation and crystallisation of ZnSiP 2 by the iodine vapour transport method

ZnSiP 2 has been prepared from the constituent elements by the iodine vapour transport method. The initial synthesis produced ZnSip 2 in microcrystalline form. Modifications in the design of the preparation ampoule and experimentation with the temperature profile permitted the microcrystalline material to be grown into significant sized single crystals. The crystals occurred as truncated pyramids with base and top widths of 5 and 1.5 mm respectively and of height 2.5 mm. It is conjectured that convective flow is the dominant transport mechanism in the temperature and pressure ranges used to grow the ZnSiP 2.