Liquid Te Research Articles

Phase-field simulations of Te-precipitate morphology and evolution kinetics in Te-rich CdTe crystals

Te precipitates are one of principal defects that form during cooling of melt-grown CdTe or CZT crystals when grown Te-rich. Many factors such as the kinetic properties of intrinsic point defects (vacancy, interstitial, and antisite defects); stresses associated with the lattice mismatch between precipitate and matrix; temperature gradients and extended defects (dislocations, twin and grain boundaries); non-stoichiometric composition; thermal treatment history all affect the formation and growth/dissolution of Te precipitates in CdTe. A good understanding of these effects on Te precipitate evolution kinetics is technically important in order to optimize material processing and obtain high-quality crystals. This research develops a phase-field model capable of investigating the evolution of coherent Te precipitates in a Te-rich CdTe crystal undergoing cooling from the melt. Cd vacancies and Te interstitials are assumed to be the dominant diffusing species in the system, which is in two-phase equilibrium (matrix CdTe and liquid Te inclusion) at high temperatures and three-phase equilibrium (matrix CdTe, Te precipitate, and void) at low temperatures. Using available thermodynamic and kinetic data from experimental phase diagrams and thermodynamic calculations, the effects of Te interstitial and Cd vacancy mobility, cooling rates and stresses on Te precipitate, and void evolution kinetics are investigated.

Dynamical inhomogeneity of liquid Te near the melting temperature proved by inelasticx-ray scattering measurements

The high frequency dynamics of liquid Te near the melting temperature has been investigatedby means of inelastic x-ray scattering measurements. Dynamic structure factorsS(Q,E) are obtainedat 500, 650 and 800 °C. Acoustic excitation is ascertained at each temperature: itsQ-dependentphase velocity v(Q) switches from the high frequency valuevIXS to the ultrasonicvalue vs at a certainmomentum transfer (Q = Qtrans). With decreasing temperature, both the ‘positiveness’ () and the effective length () increase, which means that the structural relaxation in liquid Te changes greatly in thistemperature region. With the aid of a thermodynamic formulation, we show that thisshould be seen as evidence of dynamical inhomogeneity: it can be greatly enhanced by themetal–nonmetal transition which occurs in the supercooled region. The dynamicalinhomogeneity may be induced by a second critical phenomenon, which is similar to thecase for liquid water.

Ab initio molecular dynamics simulations on structure change of liquid Te from normal- tosupercooled-state

Using ab initio molecular dynamics simulations and inherent structure formalism, the localatomic structures and electronic properties of liquid Te from the normal- to supercooled-statewere studied. The calculated structure factors agree acceptably with available experimentaldata. There is a noticeable maximum at around 723 K in the curve of first-peak-height ofS(Q) versus temperature. With increasing temperature, in the low-temperature region from 573to 673 K, the average coordination number increases gradually in contrast to the behaviorof a classical isotropic fluid; but in the high-temperature region from 843 to 1073 K, itremains nearly constant. Our calculated angular limited bond–bond correlationfunctions first show that there are two types of Peierls distorted local atomicstructures in liquid Te: the lengths of short and long bonds in the first type aresimilar to those in crystalline Te, while the lengths of short and long bonds inthe second one are very different from those in crystalline Te. With decreasingtemperature, the lengths of the two short intrachain bonds are unchanged, but thoseof the two long interchain bonds become longer, indicating that the interchaincorrelation reduces with decreasing temperature. Our results suggest that the changeof the Peierls-type distorted local atomic structures with temperature may beresponsible for the striking anomalies of many thermophysical properties of liquid Te.The calculated DOS and LDOS shows that there is an obvious dip in DOS atEF, but the dip at 1073 K is shallower than that at 573 K. The variation of the dip in DOSmainly results from the change of Te p orbital.

Local atomic structure and chemical bonding in liquid Te: An ab initio molecular-dynamics simulation

Local atomic structure and chemical bonding in liquid Te were investigated by ab initio molecular-dynamics simulations and inherent structure formalism. Our results first present two types of Peierls distorted local structures in liquid Te. Accompanying the shoaling of the dip at EF in DOS from 573 to 1073K, the interchain distances are shortened and the tendency of short-long alternation of bonds within the chains of atoms becomes stronger. Our results suggest stronger interchain correlation and short-long alternation of bonds within chains may both play important roles on the metallic nature of liquid Te.

Chain Geometries around Voids in Liquid Te

The reverse Monte Carlo (RMC) and Voronoi–Delaunay analyses have been applied to derive the void-based partial pair distribution functions g ' Te–Te ( r ), g ' V–Te ( r ), and g ' V–V ( r ), the partial coordination number distributions P ( N TeTe ), P ( N VTe ), and P ( N VV ), and the bond angle and dihedral angle in l-Te to clarify the connectivity of the covalently bonded Te chains supporting voids. The P ( N TeTe ) reveals that shorter and longer interchain bonds coexist. Large fluctuations in the interchain distance create the small voids (2.5 A in radius) supported by stacked chains and the large voids (3.1 A in radius) supported by rings of ∼8 Te atoms. The number of Te rings increases with increasing temperature. It is found that the inter-Te ring correlation dominates the intermediate-range chemical ordering of voids–Te atoms and voids–voids in l-Te, which is responsible for a small shoulder on the leading edge of the principal peak of S ( Q ).

Dynamical structure in liquid Te using inelastic x-ray scattering: from semiconductor-metal transition to metallic regime

We have carried out inelastic x-ray scattering measurements of liquid Te at SPring-8 in Japan and obtained the dynamic structure factors S(Q,W) at 500, 650 and 800°C in Q-range from 1.3 to 43 nm1 and W-range from -40 to 40 meV, respectively. Collective excitation is ascertained in each S(Q,W). Dynamical velocity vQ of the excitation mode decreases with increasing temperature, which is normal but contradicts to the behaviour of the ultrasonic sound velocity vs that anomalously increases against temperature. This contradiction means that the ultrasonic anomaly is not derived from the dynamics in this Q-region and larger-size (> a few nm) structural relaxation process exists. The value of "positive dispersion" (≡ (vQ - vs)/vs) decreases from 70% at 500°C to 15% at 800°C. Large positive dispersion is also observed at the nonmetal-metal transition regime in other liquids such as supercritical Hg, which imply that this is a common phenomenon concerning with nonmetal-metal transition.

Microscopic collective dynamics in liquid tellurium

The spectra of collective excitations of liquid tellurium have been studied by means of inelastic neutron scattering. Here we report on the dynamics of liquid Te as measured at two different temperatures, just above melting (Tm=723K) and at ∼1000K. Estimates for the velocity of propagating excitations for both temperatures have been obtained from the experimental data and a contrasting behavior is found with respect to anomalies shown by the adiabatic sound velocity measured by ultrasound methods.

Experimental evidence of the transformation from microheterogeneous to microhomogeneous states in Ga–Sn melts

The electrical resistivities of liquid Ga–Sn system have been carefully measured as a function of temperature for different compositions employing direct-current four-probe method. It was well known that the electrical resistivity varies linearly with temperature for typical liquid metals. However, an abnormal change on the resistivity-temperature curve in the intermediate temperature region Ts –Te (385–422 ◦ Cf or Ga20Sn80 melt and 395–449 ◦ C for liquid Ga10Sn90 melt, here Ts and Te are denoted as the start-temperature and the end-temperature of the abnormal change) is observed in the initial Ga20Sn80 and Ga10Sn90 melts during first heating, but this abnormal behavior disappears during subsequent cooling as well as reheating process. This result indicates that in the initial Ga20Sn80 and Ga10Sn90 melts there may exist the microheterogeneities, during heating in the temperature range from Ts to Te the irreversible structure transition possibly takes place from microheterogeneous melt to microhomogeneous melt. It suggests that the necessary overheating above liquidus up to Te is a guarantee of the molten alloy getting the microhomogeneous sate or the true solution and conserving it during cooling down to liquidus at any cooling rates.

Collective dynamics of liquid Te: The most non-simple liquid metal

Dynamic structure factor, S(Q,ω), of liquid Te at 470°C was measured by inelastic X-ray scattering. In the low Q region, inelastic excitations were found, which are dominated by propagating modes. Results from a damped harmonic oscillator analysis showed that the energies of the inelastic excitations positively deviate from the hydrodynamic value by about 50%, which is much larger than those in simple liquid metals and several non-simple liquid metals, but similar to those in a van der Waals liquid Ar and in the other liquid chalcogens. The width of the quasielastic line does not show the so-called de Gennes narrowing, which may reflect the strong covalent nature of the nearest-neighbor correlation.

High-frequency collective excitations in molten and glassy Te studied by inelastic neutron scattering

The spectra of collective excitations of liquid and glassy tellurium have been studied by means of inelastic neutron scattering. Here we report on the dynamics of liquid Te as measured at two different temperatures, just above melting $({T}_{m}=723\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and at $\ensuremath{\sim}1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ as well as the glass that is studied at room temperature. Estimates for the velocity of propagating excitations for both temperatures have been obtained from the experimental data, and a contrasting behavior is found with respect to anomalies shown by the adiabatic sound velocity measured by ultrasound methods. The origin of such differences is finally discussed.

Structural study of amorphous Te 2X (X = Br, I): X-ray diffraction, neutron diffraction and reverse Monte Carlo simulations

Reverse Monte Carlo simulations of the Te 2Br 0.75I 0.25 chalcohalide glass structure using high-energy X-ray and thermal neutron diffraction data show that the glass structure consists of finite –Te–Te–Te– and –Te–Br–Te– chain fragments interconnected by 3-coordinated I atoms. The average Te–Te nearest-neighbor coordination is equal to 2.2 ± 1.4, the Te–Te next nearest-neighbor coordination is equal 7.3 ± 2.6 and the Br–Br coordination is equal to 5.6 ± 0.6. Detailed analysis of the coordination numbers, bond-angle distributions, ring statistics and Warren–Cowley parameters of the different models demonstrates that the short-range order of the glass differs from that of crystalline Te 2X (X = Br, I). The presence of long Te–Te bonds (>3.0 Å) typical for liquid Te and crystalline tellurium-rich polytellurides is also established.

Thermophysical properties of liquid Te: Density, electrical conductivity, and viscosity

The thermophysical properties of liquid Te, namely, density, electrical conductivity, and viscosity, were determined using the pycnometric and transient torque methods from the melting point of Te (723 K) to approximately 1150 K. A maximum was observed in the density of liquid Te as the temperature was increased. The electrical conductivity of liquid Te increased to a constant value of 2.9×105Ω−1m−1 as the temperature was raised above 1000 K. The viscosity decreased rapidly upon heating the liquid to elevated temperatures. The anomalous behaviors of the measured properties are explained as caused by the structural transitions in the liquid and discussed in terms of Eyring’s [A. I. Gubanov, Quantum Electron Theory of Amorphous Conductors (Consultants Bureau, New York, 1965)] and Bachinskii’s [Zh. Fiz.-Khim. O-va. 33, 192 (1901)] predicted behaviors for homogeneous liquids. The properties were also measured as a function of time after the liquid was cooled from approximately 1173 or 1123 to 823 K. No relaxation phenomena were observed in the properties after the temperature of liquid Te was decreased to 823 K, in contrast to the relaxation behavior observed for some of the Te compounds.

Neutron Diffraction Studies on the Liquid Te and As–Te Mixtures

Neutron diffraction measurements for liquid (l-) Te and l-As–Te mixtures (10, 20, 30 and 40 at.% As) have been performed. The first minimum after the first peak of g ( r ) for l-Te and l-As–Te mixtures is filled up with temperature, and the second peak diminishes substantially and flattens out at high temperature. The temperature variations of g ( r ) are discussed combining the structural information available from EXAFS and computer simulation results. We propose that the volume contraction near the semiconductor to metal (S–M) transition causes the shrinkage of the inter-chain distance, and the short- and long-distance inter-chain correlations may coexist at high temperature. In the metallic region the –As–Te– chains are inferred to have nearly the same configuration as in l-Te. The prepeak near Q =1.1 – 1.5 A -1 is observed for l-As 20 Te 80 and l-As 40 Te 60 , which diminishes with increasing temperature. The prepeak in l-As 40 Te 60 lies at lower Q region compared with l-As 20 Te 80 . It is conclude...

Optical properties of tellurium-dopedInxGa1−xAsySb1−y epitaxial layers studied by photoluminescence spectroscopy

Controlled doping of quaternary alloys ofInxGa1−xAsySb1−y with tellurium is fundamental to obtain the n-type layers needed for the development ofoptoelectronic devices based on p–n heterojunctions. InGaAsSb epitaxial layers weregrown by liquid phase epitaxy and Te doping was obtained by incorporating smallSb3Te2 pellets in the growth melt. The tellurium doping levels were in the range1016–1017 cm−3. We have used low-temperature photoluminescence (PL) spectroscopy to study theinfluence of the Te donor levels on the radiative transitions shown in the PL spectra. ThePL measurements were done by exciting the samples with the 448 nm line of an Ar ion laserwith varying excitation powers in the range from 10 to 200 mW. For the low-doped samplethe PL spectrum showed a narrow exciton-related peak centred at around 610 meV with afull width at half maximum (FWHM) of about 7 meV which is evidence of the goodcrystalline quality of the layers. For higher Te doping, the PL spectra show thepresence of band-to-band and donor-to-acceptor transitions which overlap as the Teconcentration increases. The peak of the PL band shifts to higher energies as Tedoping increases due to a band-filling effect as the Fermi level enters into theconduction band. From the peak energy of the PL spectra, and using a model thatincludes the band-filling and band-shrinkage effects due to the carriers, we haveestimated the effective carrier concentration due to doping with Te in the epilayers.

Ab initioMolecular-Dynamics Simulations of Anomalous Structural Change in Liquid Tellurium under Pressure

The pressure dependence of the structural and electronic properties of liquid Te is investigated by means of ab initio molecular-dynamics simulations. The anomalous pressure-induced structural change is successfully reproduced by our simulations being in agreement with the observations by the recent synchrotron X-ray-diffraction experiments. Our analyses including the overlap populations reveal that there are two stages in the compression process of the liquid Te. In the first stage under pressure up to about 6 GPa, the elongation of the nearest-neighbor distance is induced by forming a weak covalent bonding state between two Te atoms. In the second stage for further compression, the anisotropy of atomic configuration around each Te atom is reduced with increasing pressure, and the electronic states have a weak pressure dependence.

Vibrational, single-particle-like, and diffusive dynamics in liquid Se, Te, and Te50Se50

Neutron scattering measurements were carried out for liquid Se, liquid Te, and liquid Te50Se50. The results are discussed in the context of semiconductor-to-metal transition. As to liquid Te50Se50, the vibrational modes show a remarkable change when the semiconductor-to-metal transition occurs by temperature increase. A soft stretching mode was observed for the metallic phase and it shows the structural change; the appearance of long bonds in the metallic phase. The vibrational modes for liquid Se and liquid Te were obtained and agreed well with former works. The crossover from collective to single-particle-like regime was also observed for all liquid samples and the transition ranges (Q=4.5–8 Å−1) are presented. The diffusive features in the low-Q region and single-particle-like properties in the high-Q region are discussed and are shown to be consistent with the results for the vibrational modes.

Bonding Properties of Liquid Tellurium under Pressure: A Maximally Localized Wannier Function Approach with Ultrasoft Pseudopotentials

The bonding properties of liquid Te are investigated by means of ab initio molecular-dynamics simulations as a function of pressure. We discuss the pressure dependence of electronic states around each Te atom in connection with the anomalous structural change of liquid chalcogenides under pressure. It is demonstrated that maximally localized Wannier functions are highly useful for investigating the electronic properties of the liquids consisting of covalently bonded atoms.

Thermodynamic properties of liquid Te–Ga and Te–Tl alloys

We have used a statistical mechanics model based on the assumption of compound formation in binary alloys to study the bulk and surface properties of Te–Ga and Te–Tl liquid alloys. The assumption of appropriate complexes, specifically, equiatomic TeGa for Te–Ga alloys and Tl 4Te for Te–Tl in conjunction with the model enables us to gain a better insight into the nature of order in the two liquid alloys. Using the basic interaction parameters that characterize the model, we have been able to compute the bulk thermodynamic properties of the alloys. Essentially, from our calculations, we are able to show that the Te–Ga liquid alloy is one that undergoes an order–disorder transformation depending on the proportion of Te in the alloy. The Te–Tl system on the other hand exhibits compound formation over the whole composition range. With regards to surface properties, at lower concentration Te segregates more to the surface in Te–Ga alloys than in Te–Tl, however, at higher concentrations the two alloys exhibit quantitatively similar features in the composition dependence of surface concentration.

Local Structure of Liquid Te Studied by Neutron Diffraction and EXAFS

Abstract Neutron diffraction measurements for liquid (l-) Te have been performed. The first minimum after the first peak of g(r) is well above zero and filled up with temperature, while the second peak diminishes substantially and flattens out at high temperature. The temperature variations of g(r) are analyzed combining the structural information available from our EXAFS results. The curve fitting of g(r) for l-Te predicts that two different inter-chain components coexist. The first corresponds to the short-distance inter-chain component around 3.2Å and the second corresponds to the long-distance inter-chain component around 3.8Å. The filling of the first minimum around 3.2Å at high temperature is clearly related to the metallization. EXAFS analysis on the basis of the model g(r) indicates that the short-distance inter-chain component (P2) responsible for the metallic conduction in l-Te as well as the nearest intra-chain component (P1) are required for fitting the oscillatory spectrum in the region of short-range sensitivity of the EXAFS signal. It is concluded that the S-M transition observed for l-Te arises from the transformation to the densely packed configuration composed of shortened chain molecules.

Electrical properties of liquid Te–RE (RE = Ce, Sm, Er) alloys

When Ce, Sm and Er are added to liquid Te, the electrical conductivity, σ, and the thermoelectric power, S, of the liquid alloys are observed to decrease rapidly and to increase gradually, respectively, with increasing RE (RE=Ce, Sm, Er) composition. The composition dependences of σ in liquid Te–Ce, Te–Sm and Te–Er all approach zero at the stoichiometry RE 2Te 3. The similar composition dependences of σ suggest that the 4f electrons of the RE ions weakly interact with the conduction electrons. The σ and S of liquid Te–RE alloys are analyzed by the pseudogap model of liquid semiconductors. According to this model, the density of states, N( E F), at the Fermi level decreases smoothly with increasing RE composition and its gradient value, d N( E F)/d E, is weakly dependent on the RE composition.