Optical and structural properties of lead iodide thin films prepared by vacuum evaporation method

Thermally processed lead iodide (PbI₂) thin films were prepared by the vacuum evaporation method in a constant ambient. Measured thickness of the film was verified analytically from the optical transmittance data in a wavelength range between 300 and 1600 nm. From the Tauc relation for the non‐direct inter band transition, the optical band gap of the film was found to be 2.58 eV for film thickness 300 nm. X‐ray diffraction analysis confirmed that PbI₂ films are polycrystalline, having hexagonal structure. The low fluctuation in Urbach energy indicates that the grain size is quite small. The present findings are in agreement with the other results. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Luminescent properties of CsI single crystals grown from the melt treated with EuI2

CsI single crystals treated with EuI₂ as a scavenger are grown and their radioluminescence spectra and scintillation light decay curves are obtained. Addition of the quantities of the scavenger comparable with the total concentration of the oxygen‐containing admixtures in the melt results in complete destruction of the latter. In its turn, this causes the disappearance of the band with a maximum at 2.8 eV in the radioluminescence spectrum and decreases the fraction of the slow 2 µs‐component to 0.01. The addition of larger quantities of EuI₂ leads to the appearance of a wide band with the maximum at 2.8 eV characterized by a decay constant of 2 µs; its intensity increases with the EuI₂ concentration. The maximum ratio of two faster components with the decay constants equal to 7 and 30 ns approaches 0.58:0.41 at EuI₂ concentration in CsI melt equal to 0.01 mol·kg⁻¹.     

Transport property measurements in tungsten sulphoselenide single crystals grown by a CVT technique

The transport properties of ternary mixed WS_xSe_2‐x single crystals have been studied by measuring the thermo power, electrical conductivities and Hall parameters in a small temperature range 303‐423 K. The electrical conductivity was highest for selenium rich WSe₂ and lowest for sulphur rich WS₂ crystals. All the crystals showed semiconducting behaviour from the temperature dependence of ‘ρ’, ‘R_H’ and ‘S’. The Hall coefficients showed that the samples are p‐type conducting. The temperature dependence of resistivity, Hall coefficients, carrier concentration showed that all of them are thermally activated. The values of activation energies, pre‐exponential factors and the scattering parameters have been determined. The dominant scattering mechanism for the charge carriers has been explained. The relation between the TEP and the concentration of charge carriers and electrical conductivity was studied. The effective masses of holes and the effective density of states have been determined. These parameters show an increase with increase in sulphur content. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of ion implantation on structural and photoconductive properties of Bridgman grown GaSe single crystals

The structure and temperature dependent spectral photoconductivity of as‐grown and N‐and Si‐implanted GaSe single crystals have been studied. It was observed that post‐annealing results in a complete recovery of the crystalline nature that was moderately reduced upon implantation. The band edge is shifted in the implanted sample which is attributed to the structural modifications and continuous shallow levels introduced upon implantation and annealing. Our calculations showed that the trap density is increased upon implantation and annealing which confirms a possibility of explanation the phenomenon within a framework of continuous trap levels. Photocurrent measurements as a function of photo‐excitation intensity also support continuous distribution of localized states in the band gap. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Synthesis,characterization and structural phase transition studies in trismethylammonium pentachloro barium dihydrate single crystals

Single crystals of trismethylammonium pentachlorobarium dihydrate were grown by slow evaporation method at ambient temperature. The crystals were characterized through powder XRD, thermal, infrared and NMR spectral studies. While the powder XRD pattern confirms the crystallinity of the title compound, the TG indicates the removal of occluded and adsorbed water molecules from the crystal when it is heated up to 86 °C. The TG study also confirms the presence of two water molecules of crystallization which are dehydrated on heating the crystal between 107 °C and 150 °C. The anhydrous compound is found to be stable at least up to 900 °C. The DTA curve shows two endothermic dips corresponding to weight losses observed in the TG curve. The low temperature DSC study shows thermal anomalies during the heating and cooling cycles indicating both first and second order phase transitions. The high temperature DSC shows the stepwise dehydration indicating phase transitions at temperatures 103 °C and 145 °C. The characteristic vibrational frequencies due to methylammonium ion, BaCl₅ and other groups are assigned based on FTIR spectra. The NMR spectrum confirms the presence of protons of the methyl group and water of crystallization in the compound. In this paper, an attempt is also made to understand the effect of methyl group on the phase transitions of the compound in comparison with a closely related compound, trisammoniumpentachloro barium dihydrate. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of lead(II)chloride single crystals grown in silica gel

Acousto‐optical materials play an important role in acousto‐optic devices as acousto‐optic modulators. Lead(II) chloride is an acousto‐optical material, having high figure of merit approximately ten times greater than that of lead molybdate, which is an efficient acousto‐optical material. It also possesses high birefringence, low attenuation coefficient, wide transparency range and good mechanical properties. This paper discusses in detail, the growth of single crystals of lead(II) chloride in silica gel by the process of diffusion in a highly acidic medium. Needle‐type lead(II) chloride crystals have been obtained. The crystal system is confirmed to be orthorhombic by powder X‐ray diffraction analysis. To study the optical transparency of the grown crystal, the transmission spectrum has been recorded in the range 190 – 1100 nm. Thermal stability of the crystal is also studied. Results are discussed in detail. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature dependence of lattice parameters of SBN single crystals in the vicinity of their structural phase transitions

Changes of lattice parameters of Sr_xBa_1‒xNb₂O₆ (0.35 < x < 0.72) solid solution single crystals were measured as a function of temperature. The X‐ray Bond’ method was used to carry out very precise measurements of the lattice parameters. Fine correlations between values of the lattice parameter and the Sr concentration in the solid solution of Sr_xBa_1‒xNb₂O₆ single crystals are found. A conventional analysis of lattice parameter data in terms of spontaneous strain and strain/order parameter coupling shows that a normal structural phase transition does occur. While the ferroelectric system (SBN26) displays a nearly tricritical behavior, β ≈ 0.20, the relaxor one (SBN61) complies with the two‐dimensional Ising‐model‐like criticality, β ≈ 0.17.     

Growth and characterization of CdSe single crystals by modified vertical vapor phase method

Good quality, large single crystals of CdSe were grown by the modified growth method (i.e., vertical unseeded vapor phase growth with multi-step purification of the starting material in the same quartz ampoule without any manual transfer between the steps). Lower temperature gradients (8–9°C/cm) at the growth interface were used for the crystal growth. As-grown CdSe crystals was characterized by X-ray diffraction, scanning electron microscopy, energy dispersive analyzer of X-rays, high-resistance instrument measurement, and etch-pit observation. It is found that there are two cleavage faces of (1 0 0) and (1 1 0) orientations on the crystal, the resistivity is about 10⁸ Ω cm, and the density of etch pits is about 10^3–4/cm². The crystal was cut into wafers and was fabricated into detectors. The detectors were tested using an ²⁴¹Am radiation source. γ-ray spectra at 59.5 keV were obtained. The results demonstrated that the quality of the as-grown crystals was good. The crystals were useful for fabrication of room-temperature-operating nuclear radiation detectors. Therefore, the modified growth technique is a promising, convenient, new method for the growth of high-quality CdSe single crystals.     

Study of crystalline perfection and thermal analysis of zinc cadmium thiocyanate single crystals grown in silica gel

Zinc cadmium thiocyanate ZnCd(SCN)₄ abbreviated as ZCTC is a bimetallic thiocyanate complex that exhibits excellent nonlinear optical property. Single crystals of ZCTC have been grown in silica gel by the process of diffusion. Colorless transparent crystals of size 12 mm x 2 mm x 1.3 mm have been obtained. High resolution X‐ray diffraction study was carried out to investigate the crystalline perfection of the grown crystal and the quality of the crystal was found to be quite good. Thermal stability of the grown crystal was investigated by thermogravimetric and differential thermal analyses. Fourier Transform Infrared spectrum was recorded to confirm the functional groups. Microhardness of the crystal is also studied. Being a nonlinear optical material, a comparative study of its second harmonic generation efficiency with urea has been made. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Precipitation of intermetallic phase in Zn‐Ti alloy single crystals

Pure and 0.14 at.%Ti alloyed Zn single crystals have been grown by the Bridgman technique. The alloyed crystal exhibits a sheet‐like precipitation structure parallel to the basal plane of the hexagonal solid solution Zn‐Ti matrix. The tetragonal precipitates are intermetallic compounds of Zn₁₆Ti composition. They are needle‐shaped with the needle axis peferentially aligned along the <a> axis. The approximate orientation relationship between precipitate and matrix is (0001)<11$\bar 2$ 0>_Matrix||(010)[001]_Zn16Ti. To study the influence of alloying on the mechanical properties, the critical resolved shear stress for basal slip has been measured in compression at different temperatures and strain rates. It is shown that the critical resolved shear stress can be very well explained by theories of solid solution and precipitation hardening. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structure and phase transition of single crystalline CsNH2SO3

Differential scanning calorimetry (DSC) and X‐ray diffraction measurements have been performed on cesium sulfamate CsNH₂SO₃ single crystal. Two distinct endothermic peaks in the DSC curves are observed at 330 and 436 K. It is pointed out that the peak at 330 K is attributed to the structural phase transition, and the other peak at 436 K is associated with the thermal decomposition of the crystal. The structures in room‐ and high‐temperasture phases are determined, and the space group of the sample crystal is found to change from monoclinic P 2₁/c to orthorhombic Pnma. The structure of the room‐temperature phase consists of two different types of N‐H···O hydrogen bond, but in the high‐temperature phase there is no specific hydrogen bond between the NSO₃ pseudo‐tetrahera. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Luminescence of GaN single crystals prepared by heating a Ga melt in Na–N2 atmosphere

Colorless transparent prismatic crystals (0.5‐2.0 mm long) and hopper crystals (1.0‐2.5 mm long) of GaN were prepared by heating a Ga melt at 800°C in Na vapor under N₂ pressures of 7.0 MPa for 300 h. The photoluminescence (PL) spectrum of a prismatic crystal at 4 K showed the emission peaks of neutral donor‐bound exciton (D⁰‐X) and free exciton (X_A) at 3.472 eV and 3.478 eV, respectively, in the near band edge region. The full‐width at half‐maximum (FWHM) of (D⁰‐X) peak was 1.9 meV. The emission peaks of a donor–acceptor pair transition (D⁰‐A⁰) and its phonon replicas were observed in a lower energy range (2.9‐3.3 eV). The emission peaks of the D⁰‐A⁰ and phonon replicas were also observed in the cathodoluminescence (CL) spectrum at 20 K. The (D⁰‐X) PL peak of a hopper crystal at 4 K was at 3.474 eV (2.1 meV higher), having a FWHM of 6.1 meV which was over 3 times larger than that of the prismatic crystal. A strong broad band with a maximum intensity around 1.96 eV was observed for the hopper crystals in the CL spectrum at room temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Factors affecting the formation of misoriented domains in 6H‐SiC single crystals grown by PVT method

Misoriented domains (MDs) are common defects in 6H‐SiC single crystals. We performed an experimental study on the formation of MDs in 2‐inch 6H‐SiC single crystals. Micro‐Raman spectroscopy revealed that the polytype of MDs was mainly 4H‐SiC. By changing growth conditions, it was found that the MDs' formation was closely related to growth rate and the position of highest temperature relative to growth interface. When the growth rate of ingots was relatively high the MDs were more likely to form. Furthermore, the nearer growth interface the position of highest temperature was, the larger the size of the MDs. Based on our experimental findings we suggested that the MDs' formation and the polytype switching from 6H‐ to 4H‐SiC were due to too large axial and/or radial temperature gradients.The results would be helpful to improve the quality of SiC single crystals grown by PVT technique. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Evolution of the growth interface in liquid phase diffusion growth of bulk SiGe single crystals

The article presents a study for the evolution of growth interface in crystal growth by Liquid Phase Diffusion (LPD). Specific LPD experiments were designed to grow compositionally graded, germanium‐rich Si_xGe_1‐x single crystals of 25 mm in diameter with various thicknesses. Measured interface shapes show the evolution of the growth interface. Silicon compositions were measured by the Energy Dispersive X‐ray analysis (EDX) in the growth and radial directions. The study shows the feasibility of extracting the desired seeds of uniform composition from LPD grown crystals, for subsequent use in other epitaxial growth processes. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Surface growth mechanisms and structural faulting in the growth of large single and spherulitic titanosilicate ETS-4 crystals

Morphological, surface and crystallographic analyses of titanosilicate ETS-4 products, with diverse habits ranging from spherulitic particles composed of submicron crystallites to large single crystals, are presented. Pole figures revealed that crystal surfaces with a-, b- and c- axes corresponded to 110, 010 and 001 directions, respectively. Thus, technologically important 8-membered ring pores and titania chains in ETS-4 run along the b-axis of single crystals and terminate at the smallest crystal face. Height of the spiral growth steps observed on 1 0 0 and 0 0 1 surfaces corresponded to the interplanar spacings associated with their crystallographic orientation, and is equivalent to the thickness of building units that form the ETS-4 framework. Data suggest that the more viscous synthesis mixtures, with a large driving force for growth, increased the two- and three-dimensional nucleation, while limiting the transport of nutrients to the growth surface. These conditions increase the tendency for stacking fault formation on 1 0 0 surfaces and small angle branching, which eventually results in spherulitic growth. The growth of high quality ETS-4 single crystals (from less viscous synthesis mixtures) occurred at lower surface nucleation rates. Data suggest that these high quality, large crystals grew due to one-dimensional nucleation at spiral hillocks, and indicate that under these conditions un-faulted growth is preferred.     

Investigation of carrier scattering mechanisms in molybdenum diselenide single crystals by hall effect measurements

Molybdenum diselenide (MoSe₂) belong to the large family of layered transition metal dichalcogenides. It consists of weakly coupled sandwiched layers i.e. Se – Mo – Se in which a Mo atom layer is enclosed within two Se layers. This structure makes MoSe₂ extremely anisotropic in character and leads to unusual structural properties. In addition, MoSe₂ possess flexible nature along with good carrier mobility to make them potential candidate for fabricating flexible high mobility electronic devices such as Schottky barrier devices, FETs, solar cell etc. In context of this authors made an effort to study the low temperature (12 < T < 300 K) electronic transport properties of Molybdenum diselenide (MoSe₂). Through the investigation the temperature dependent Hall mobility study revealed that the grown crystals of MoSe₂possess a mixed scattering mechanism. It has been found that observed temperature dependant mobility has at least two transitions from lattice to impurity scatterings showing an imprint of multicarrier nature of this semiconductor originating from its complex band structure. It has been observed that the studied crystals have at least two group of carriers of differing origins in which transition between dominant scattering mechanisms occur at different temperatures. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of melt stirring intensity on the axial impurity distribution in proustite single crystals grown by the Stockbarger method using ACRT

The influence of modulated crucible rotation on the axial distribution of Cu, Mg and Si impurities in proustite single crystals grown by the Stockbarger method using ACRT is studied in a wide range of Taylor numbers (1.9·10⁵ < Ta < 7.12·10⁷). The impurity content in the upper part and in the tail portion of the grown crystals is measured using X‐ray‐phase analysis. Micro and macrostriations are observed in the grown crystals. The wavelengths of impurity content fluctuations have been determined. The microfluctuations of axial impurity content are caused by modulated crucible rotation. The studies have revealed that the ACRT provides an effective removal of impurities from the main part of the grown crystal at high intensity of melt stirring, and consequently, the ACRT can be applied validly to decrease the impurity content during the growing of high‐quality proustite single crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defect structure and light‐induced scattering of Zr‐doped near‐stoichiometric Mn:Fe:LiNbO3 crystals grown by TSSG method

Near‐stoichiometric Mn:Fe:LiNbO₃ crystals doped with various concentration of ZrO₂ were grown by top seed solution growth (TSSG) method in the air atmosphere. The Zr concentration in the crystal was determined by inductively coupled plasma optical emission spectrometer. The defect structures were analyzed by means of ultraviolet‐visible and infrared transmittance spectra. The appearance of vibration peak at 3466 cm^‐1 in infrared spectra manifested that Li/Nb ratio in crystals approached to stoichiometric proportion. The fundamental absorption edge represented continuous red‐shift which was discrepancy with congruent doped LiNbO₃ crystals showed that doping ions possessed different location mechanism. The light‐induced scattering of the doped stoichiometric LiNbO₃crystals were quantitatively scaled via incident exposure energy. The results demonstrated that Zr(2 mol%):Mn:Fe:LiNbO₃ crystal had the weakest light‐induced scattering and the mechanism related to their defect structures was discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of the intensity of melt stirring on the radial impurity distribution in proustite single crystals grown by the Stockbarger method using ACRT

The influence of the intensity of melt stirring on the radial impurity distribution and optical quality of proustite single crystals grown by the Stockbarger method using ACRT is studied by the example of Cu, Sb and Mg impurities. We report results obtained in a wide range of Taylor numbers (1.9×10⁵<Ta<7.12×10⁷). The studies revealed that the ACRT can be applied validly to decrease the impurity content during the growing of high-quality single crystals.     

Investigation of domain structures in PZNT93/7 crystals by chemical etching

The domain structures in 0.93Pb (Zn_1/3Nb_2/3)O₃‐0.07PbTiO₃ (PZNT93/7) crystals were investigated by chemical etching technique. Original antiparallel 180° domains of size 20–40 μm were observed on the surface of as‐grown PZNT93/7 crystal. It was found the domain states are sensitive to the stress field induced by mechanical processing or impurities. As the composition of PZNT93/7 crystal was located near the morphotropic phase boundary, various domain configurations were observed. On the <001>‐oriented wafer, the etched a ‐ and c ‐domains revealed homogenous and island‐like patterns. The c ‐domains consist of upper and lower height regions corresponding to the tail and head of the polarization, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)