Processes of vapor phase growth of AIIBVI single crystals and structures based there upon

Results of investigations of semiconductor A^IIB^VI compounds (for example of CdTe and ZnTe) grown by the chemical vapor transport (CVT) method in a closed volume using three transfer agents containing a halogen, compound NH₄X (X = Cl, Br, I) are presented. The processes of vapor phase growth (composition of the vapor phase and mass transfer) in Me(Cd, Zn)Te–NH₄X (X = Cl, Br, I) systems have been calculated theoretically and the results are have been verified in growth experiments. Optoelectronic properties of the grown materials and barrier structures based there upon are discussed.     

Effect of tellurium deposition rate on the properties of Cu–In–Te based thin films and solar cells

To investigate the effects of tellurium (Te) deposition rate on the properties of Cu–In–Te based thin films (Cu/In=0.30–0.31), the films were grown on both bare and Mo-coated soda-lime glass substrates at 200 °C by co-evaporation using a molecular beam epitaxy system. The microstructural properties were examined by means of scanning electron microscopy and X-ray diffraction. The crystalline quality of the films was improved with increase in the deposition rate of Te, and exhibited a single CuIn₃Te₅ phase with a highly preferred (1 1 2) orientation. Te-deficient film (Te/(Cu+In)=1.07) grown with a low Te deposition rate showed a narrow bandgap of 0.99 eV at room temperature. The solar cell performance was affected by the deposition rate of Te. The best solar cell fabricated using CuIn₃Te₅ thin films grown with the highest deposition rate of Te (2.6 nm/s) yielded a total area (0.50 cm²) efficiency of 4.4% (V_oc=309 mV, J_sc=28.0 mA/cm², and FF=0.509) without light soaking.     

High nonlinear optical properties in TeO2-based glasses: A modifier’s influence study from the localized hyperpolarizability approach

Ab initio localized molecular orbital theory (GAMESS program) has been used to evaluate the effects of a second cation on nonlinear optical properties of tellurium oxide based glasses. Calculations were performed on [H_nTeO₃A_3?n]⁺ (n = 0–3) molecules with alkali cations (A = Li, Na). Results of the calculations on optimized structures show that the effects of the second cation on the nonlinear optical properties are significant and related to the geometric modification of the structural unit. However, the introduction of the second cation induces an increase of both linear polarizability and second hyperpolarizabilty whereas experiments show the opposite, second hyperpolarizabilty is much more affected by the introduction of the second cation. Such calculations have to be performed on bigger size clusters in which the introduction of a second cation implies breaking a Te–O–Te bridge.     

Experimental studies of phase equilibria in high-temperature ternary immiscible metallic melts

Shear viscosity measurements were carried out for a liquid immiscible ternary system In–Se–Te in a temperature range from the monotectic to about 1200 K using an oscillating-cup viscometer. It was revealed that the ternary In₈₀Se_xTe_20−x (0 ⩽ x ⩽ 20 at.%) system may be considered as a set of quasibinary In–(Se/Te) alloys of almost critical concentrations. A variation of the Se to Te ratio at constant content of In changes the properties of coexisting liquids and affects the binodal temperature. The critical parameters describing the peculiarities of viscosity behavior in the vicinity of the critical point are evaluated. The analysis based on the dynamic theory of phase transitions for viscosity in the phase separation region is proposed. The results are compared with available data for immiscible dielectric solutions and metallic melts.     

IR absorption spectra of lithium and silver vanadium–tellurite based glasses

The aim of the present paper is to give structural information in order to set a correlation between the electrical conductivity behavior and structures of lithium and silver vanadium–tellurite based glasses. We report our structural studies and compare the effect of the nature of the metallic cation on glasses of the form XM₂O · (1 − X)V₂O₅ · 2TeO₂ (where 0 ⩽ X < 1 and M = Li or Ag). Fourier transform infra-red (FTIR) spectra were recorded for all compositions and complementary differential scanning calorimetry (DSC) measurements and X-ray diffraction (XRD) measurements were also carried out. This paper should be considered as complementary to a previous article reporting the conductive behavior of theses glasses. In the latter we reported the obtained results on electrical conductivity studies. The results confirm the existence of a transition from a typically electronic (polaronic) conductive regimen when the molar fraction (X) of M₂O is equal to 0, to an ionic conductive regimen when X tends to 1. The evidence for the independent migration path for both electrons and ions was put into evidence by studying the electrical conductivity behavior in a complementary system of the form X M₂O · (1 − X)[0.5V₂O₅–0.5MoO₃] · 2TeO₂. In this system vanadium was partially replaced by molybdenum which acts as a ‘diluting’ agent of the active centers involved in the electronic transport.     

Structural and magnetic investigations of Fe2O3–TeO2 glasses

A series of tellurite glasses containing Fe₂O₃ with the nominal composition x(Fe₂O₃)–(1−x)(TeO₂), where x = 0.05, 0.10, 0.15, and 0.20, have been synthesized and investigated using X-ray photoelectron spectroscopy (XPS) and magnetization techniques. The Te 3d core level spectra for all glass samples show symmetrical peaks at essentially the same binding energies as measured for TeO₂ indicating that the chemical environment of the Te atoms in these glasses does not vary significantly with the addition of Fe₂O₃. Furthermore, the full-width at half-maximum (FWHM) of each peak does not vary with increasing Fe₂O₃ content which suggests that the Te ions exist in a single configuration, namely TeO₄ trigonal bipyramid (tbp). The O 1s spectra are narrow and symmetric for all compositions such that oxygen atoms in the Te–O–Te, Fe–O–Fe and Te–O–Fe configurations must have similar binding energies. The analysis of the Fe 3p spectra indicates the presence of Fe³⁺ ions only, which is consistent with the valence state of the Fe ions determined from magnetic susceptibility measurements.     

Study of As―Se―Te glasses by neutron-, X-ray diffraction and optical spectroscopic methods

The atomic structures of amorphous As₄₀Se_(60?x)Te_x (x = 10 and 15) and As₄₀Se₆₀ glasses have been investigated by neutron and high energy X-ray diffraction methods. The two datasets were modeled simultaneously by reverse Monte Carlo (RMC) simulation technique. The RMC simulations revealed a glassy network built-up from As(Se, Te)₃ pyramids in which Te atoms substitute Se atoms. The As―Se correlation function shows a strong and sharp first peak at 2.4 Å and two broad and much less intense peaks at 3.7 and 5.6 Å, related to 1st, 2nd and 3rd neighbor distances of the As―Se bonds, respectively. They are an evidence for existence of short and medium ordering in the studied glasses. The similarity of Θ_Te―As―Te and Θ_Se―As―Se bond distributions suggests that Te atoms have a similar role in the structure formation as Se atoms. The FTIR spectra analysis revealed impurity bonds of Se―H, As―O, Se―O, and Te―O in the glasses which contributed to enhanced absorption in visible spectral range. From the ellipsometric data analysis the optical constants and the energetic parameters of the studied glasses were established. The compositional variation of these parameters is explained in terms of chemical bonds formation and change in the density of charged defects.     

Atomic structure and chemical order in binary Ge–Te and As–Te glasses: A Te K-edge X-ray absorption fine structure spectroscopic study

The nearest-neighbor coordination environments of Te atoms in Ge_xTe_100?x glasses with x = 15 and 20 and in As_xTe_100?x glasses with 40 ? x ? 65 have been studied with Te K-edge EXAFS spectroscopy. The average coordination number of Te atoms in all glasses is found to be ～2.0 and no violation of the 8-N rule is observed. The compositional makeup of the first coordination shell of Te atoms indicates that chemical order is largely preserved in both glass-forming binary systems. Sudden changes in the Te coordination environment and violation of chemical order are observed at the stoichiometric As₄₀Te₆₀ glass implying formation of a constrained network. The compositional dependence of the physical properties in both systems can be correlated to short-range chemical order.     

Electrical properties of a-Se85−xTe15Snx thin films

Se–Te alloys are an important system of chalcogenide glasses from application point of view. The incorporation of Sn additive alters the electrical properties of these alloys. The conductivity measurements have been done on the thin films of a-Se_85−xTe₁₅Sn_x (x = 0, 2, 4, 6 and 10 at.%) deposited using vacuum evaporation technique. Both dark (σ_d) and photoconductivity (σ_ph) show a maximum for x = 6 at.% of Sn, which, decreases on further Sn addition to the binary Se–Te alloy. The dark activation energy (ΔE_d) shows a minimum for x = 2 at.% of Sn, but increases on further Sn addition. There is a sharp decrease in photosensitivity (σ_ph/σ_d) on Sn addition to Se₈₅Te₁₅ alloy. The charge carrier concentration (n_σ) calculated with the help of dc conductivity measurements also show a maximum at x = 6 at.% of Sn. The results are explained on the basis of increase in the density of localized states present in the mobility gap on Sn incorporation.     

Optical band gap and Raman spectra in some (Bi2O3)x(WO3)y(TeO2)100−x−y and (PbO)x(WO3)y(TeO2)100−x−y glasses

The glasses representing (Bi₂O₃)_x(WO₃)_y(TeO₂)_100?x?y and (PbO)_x(WO₃)_y(TeO₂)_100?x?y systems were prepared. The dilatometric glass-transition temperatures of examined glass samples were found in the region 383–434 °C, the coefficient of thermal expansion varied from 12 to 16 ppm/°C and the density ranged from 6.30₂ to 6.80₈ g/cm³. From the optical transmission measurements of thin glassy bulk samples prepared by a glass blowing, the optical gap values were found in the narrow region 3.21–3.36 eV. For the temperature interval 300–480 K, the values of the temperature coefficient of the optical band gap varied from 3.7 × 10^?4 to 5.24 × 10^?4 eV/K. It is suggested that Raman feature observed at around 350 cm^?1 can be assigned to an overlap of Raman bands attributed to WO₆ corner shared octahedra and to the following three atomic linkages: Bi–O–Te, Pb–O–Te and W–O–Te.     

Mechanisms controlling primary crystallisation of Ga20Te80 glasses

The kinetic study of the crystallisation process of Ga₂₀Te₈₀ glass from isothermal and continuous heating calorimetric data have been performed applying a recently developed procedure. The kinetic information was complemented with X-ray diffraction measurements. With this scope, three crystallisation patterns, with three-dimensional isotropic growth have been analysed: (i) site saturation and interface controlled growth. (ii) homogeneous nucleation with interface controlled growth and (iii) homogeneous nucleation with two simultaneous modes of crystal growth (interface- and diffusion-controlled). A complex model with two simultaneous modes of three-dimensional isotropic crystal growth with decreasing homogeneous nucleation and soft impingement has been applied for modelling primary crystallisation of the Ga₂₀Te₈₀ glass. The model goes beyond the isokinetic hypothesis when coupling isothermal and continuous heating kinetic data. The apparent activation energy E_a = (2.06 ± 0.03) eV/at obtained for the primary crystallisation of the phase Te is shown to correspond to an activation energy for nucleation E_I = (2.85 ± 0.03) eV/at and an interface controlled activation energy for growth E_u = (1.90 ± 0.03) eV/at at the crystallisation onset.     

Spectral investigations on VO2+ ion doped in CaO–SrO–Na2O–B2O3 glass systems

Glasses of the (20 ? x)CaO–xSrO–(20 ? y)Na₂O–60B₂O₃ ? y (CSNB) system with (5 ≤ x ≤ 15) mol% and y = 0.1 mol% of V₂O₅ were characterized by X-ray diffraction (XRD), EPR (Electron Paramagnetic Resonance), Optical absorption Spectra and FT-IR (Fourier transform Infrared Spectroscopy) studies. EPR spectra of all the glass samples exhibit resonance signals characterstic of VO²⁺ ions. The values of spin-Hamiltonian parameters indicate that the VO²⁺ ions in CSNB glasses were present in octahedral sites with tetragonal compression and belong to C_4v symmetry. Spin-Hamiltonian parameters ‘g’ and ‘A’ were evaluated. The Optical band energy (E_opt) and Urbach energy (ΔE) were calculated from their ultra violet edges. By correlating EPR and Optical data the molecular orbital coefficients have been evaluated. IR spectra of these glasses were analyzed in order to identify the contribution of each component to the local structure that determines the physical properties of these glasses.     

A concerted rational crystallization/amorphization mechanism of Ge2Sb2Te5

Reverse Monte Carlo refinements using electron diffraction data and density functional theory calculations of the local atomic structure of amorphous Ge₂Sb₂Te₅ confirm presence of a noticeable number of four-membered rings with the general Ge(Sb)TeGe(Sb)Te composition similar to the building blocks of its cubic crystalline phase. The persistence of these rings, as well as the presence of the medium range order at the scale of about 1 nm, suggests that the amorphization/crystallization transition in Ge₂Sb₂Te₅ can be modelled with a concerted rotation of the sheets of atom-squares in {1 0 0} faces of cubic subcells of the cubic crystalline phase, similar to Rubik’s cube rotation. This mechanism can produce large models of material that agree with a range of the previous experimental and theoretical studies and also with the experimental electron diffraction data.     

A composition dependent thermal behavior of GexSe35−xTe65 glasses

Alternating differential scanning calorimetric (ADSC) studies have been performed to understand the thermal behavior of bulk Ge_xSe_35?xTe₆₅ glasses (17 ? x ? 25); it is found that the glasses with x ? 20 exhibit two crystallization exotherms (T_c1 & T_c2). On the other hand, those with x ? 20.5, show a single crystallization reaction upon heating. The exothermic reaction at T_c1 has been found to correspond to the partial crystallization of the glass into hexagonal Te and the reaction at T_c2 is associated with the additional crystallization of rhombohedral GeTe phase. The glass transition temperature of Ge_xSe_35?xTe₆₅ glasses is found to show a linear but not-steep increase, indicating a progressive, but a gradual increase in network connectivity with Ge addition. It is also found that T_c1 of Ge_xSe_35?xTe₆₅ glasses with x ? 20, increases progressively with Ge content and eventually merges with T_c2 at x ≈ 20.5 (〈r〉 = 2.41); this behavior has been understood on the basis of the reduction in TeTe bonds of lower energy and increase in GeTe bonds of higher energy, with increasing Ge content. Apart from the interesting composition dependent crystallization, an anomalous melting behavior is also exhibited by the Ge_xSe_35?xTe₆₅ glasses.     

Kinetics of the exchanges at the solid–liquid interface during the dissolution process of gallium orthophosphate

Gallium orthophosphate is a piezoelectric material with an α-quartz structure. In order to manufacture bulk acoustic wave devices (BAW), controlled chemical dissolution is often used to reduce the thickness of the piezoelectric membranes. This paper presents the kinetics of the chemical exchanges, which occur at the solid–liquid interface during the chemical dissolution of GaPO₄ in phosphoric acid. Based on chemical composition of phosphoric acid solvent, the pure dissolution rate is determined. A strong anisotropy of chemical reactivity is formed. The dissolution rate is the lowest for the crystallographic z-plane (0 0 0 1) but this orientation is the most sensitive with respect to the proton concentration and the temperature. In accordance with the crystal growth rates, the nucleation at the interface for the (1 0 2 0) plane, named X-plane, is also the most rapidly dissolved. Assuming the activation energies corresponding to dissolution and to nucleation are like standard activation energies, the different values of the standard enthalpy variation are calculated. The most important variation is obtained for the z-plane (Δ_rH=−14.3 kJ/mol) and the lowest for the X-plane (Δ_rH=−5.4 kJ/mol).     

Formation of icosahedral quasicrystals in (Ag,Au)-based ternary systems

We investigated the phase constitution of rapidly solidified (Ag,Au)-based ternary alloys at various compositions of (M,X)_85.7RE_14.3 (M = Ag, Au, X = Al,Ga,In, RE = Er,Lu). It is found that an icosahedral phase is formed at an average electrons per atom ratio close to 2.0 as well as at the composition ratio of (M,X)₆RE, suggesting that these factors have crucial roles in the stability of the icosahedral phases. It is also found that the trend of the icosahedral phase formation in the Au-based alloys is quite different from that in the Ag-based alloys.     

Transition from a single-ion to a collective diffusion mechanism in alkali borate glasses

Two distinct regions of the dc conductivity and its temperature dependence as a function of the mol fraction of alkali oxides, X, are observed in Na- and Li-borate glasses. At low alkali contents the dc conductivity σ_dc increases only moderately with X. However, at higher alkali contents, log σ_dc increases linearly and the activation enthalpy ΔH of σ_dc × T decreases linearly with log X, i.e. the dc conductivity reveals an effective power-law behavior. The transition between low alkali and high alkali behavior takes place at X ≈ 0.08 for Na-borate and at X ≈ 0.09 for Li-borate glasses. This behavior suggests that the diffusion mechanism changes at these alkali contents. The results are discussed in terms of ion separations and the transition from a single-ion jump to a collective diffusion mechanism. The vanishing of the mixed-alkali peak in Na–Rb borate and alumino-germanate glasses at sodium contents similar to that observed for the change in slope of σ_dc(X) in this work suggests that both phenomena share the same origin.     

Incorporation of B2O3 in CaO-SiO2-P2O5 bioactive glass system for improving strength of low-temperature co-fired porous glass ceramics

Bioactive glass ceramics (BGCs) have different rates of biodegradation and mechanical properties depending on their chemical compositions and sintering temperatures. The present study was aimed to develop the boron-rich, phosphorus-low CaO–SiO₂–P₂O₅–B₂O₃ bioactive glasses (BG-B_x, X = 0, 10, 20) potentially for improving the mechanical properties of BGCs via low-temperature co-fired process. The B₂O₃-free BG-B₀ shrunk well at ~ 726 °C and melted at over 1050 °C, while the onset shrinking and melting temperatures of the 20 mol% B₂O₃-doped BG-B₂₀ was lowered to ~ 648 °C and ~ 952 °C, respectively. The BG-B₂₀ thermally treated at 850–950 °C was transformed into wollastonite and calcium borate, and crystallization decreased the kinetics but did not inhibit the development of hydroxyapatite on their powder and disc surface when immersed in simulated body fluid. The in vitro degradation in Tris buffer confirmed that the degradation rate markedly increased with increasing boron content in BG-B_x. The compressive strength and flexural strength of the 10% BG-B₂₀-reinforced 45S5 porous BGC sintered at 850 °C was nearly four times than that of 45S5 porous constructs. These studies suggest that the boron-rich, phosphorus-low CaO–SiO₂–P₂O₅–B₂O₃ system is a promising biomaterial and potential low temperature co-fired aid for improving the mechanical and biological properties of porous BGCs.     

Te-rich Ge–As–Se–Te bulk glasses and films for future IR-integrated optics

Ge₁₅As₁₅Se_70−xTe_x materials with x = 56, 60 and 63, of potential use in IR-integrated optics, were prepared by the classical melt-quenching method. A macroscopic phase separation was observed with a crystalline phase on the top and an amorphous one at the bottom. The glasses from the bottom were transparent from 1.9 to 16 μm without any purification of the elemental precursors Ge, As, Te and Se. The higher the Te/Se content in the glasses the lower their glass transition temperature and thermal stability. Films 7–12 μm thick of the above stated compositions were deposited by thermal evaporation. The higher the tellurium content, the larger the optical band gap shift of the films in the infra-red and the higher the refractive index.     

Static and dynamic structures of liquid GeTe mixtures

Neutron diffraction measurements of the liquid Ge–Te system and quasi- and inelastic neutron scattering experiments of the eutectic liquid Ge₁₅Te₈₅ were performed in the temperature range from 400 to 560 °C. The large temperature dependence of the static structure for liquid Ge₁₅Te₈₅ is reconfirmed; however it is found that the compositional dependence obtained at around 80 °C above the liquidus curve is rather small. The dynamic structure factors of liquid Ge₁₅Te₈₅ in the Q-region from 0.2 to 2.6 Å⁻¹ show a normal temperature dependence. From the vibrational density of states, it is found that the Te–Te covalent bond persists in liquid Ge₁₅Te₈₅; however a bending mode was not assigned clearly.