Traveling Heater Method Research Articles

Thermoelectric properties of p-type (Bi 2Te 3) x(Sb 2Te 3) 1− x prepared via bulk mechanical alloying and hot pressing

In the present paper, starting from elemental Bi, Sb and Te granules with 5 N purity and 1–5 mm diameter, p-type single phase (Bi 2Te 3) x (Sb 2Te 3) 1− x ( x=0.20, 0.225, 0.25, 0.275 and 0.30) thermoelectric materials with high densification (>99% theoretical density) were prepared via bulk mechanical alloying (BMA) and hot pressing (HP). Their thermoelectric properties, are less composition sensitive in the composition range of x≤0.25 and are comparable to that of those materials prepared by travelling heater method (THM); the figures of merit of (Bi 2Te 3) x (Sb 2Te 3) 1− x are about 3×10 −3/K. When x>0.25, the figures of merit of (Bi 2Te 3) x (Sb 2Te 3) 1− x decreased rapidly with increasing x.

Optical properties of CuAlSe2

We have determined the complex dielectric tensor components of the chalcopyrite semiconductor CuAlSe2 in the energy range between 1.4 and 5.2 eV, at room temperature, using spectroscopic ellipsometry. We present results obtained on two single crystals grown by the traveling-heater method using In solvent. Values of refractive indices n, extinction coefficients k and normal-incidence reflectivity R in the two independent polarizations are reported. The structures observed in the energy region studied are analyzed by fitting the second-derivative spectra d2ε(ω)/dω2 to analytic critical-point line shapes. The obtained energies are assigned to certain electronic interband transitions by comparison with existing band structure calculations.

Anisotropy of the selenium diffusion coefficient in bismuth telluride

We present results for the anisotropic diffusion coefficient of selenium, isoelectronic impurity, in Bi2Te3 along the solidus. The single crystals were prepared by travelling heater method and they have a high crystalline quality. The experimental conditions permitted us to obtain a definite stoichiometric deviation, and diffusion profiles were obtained by SIMS. We propose a mechanism of atomic diffusion by antisite defects and thermal vacancies taking into account of the anisotropy of the diffusion coefficients.

Optical properties of CuGaSe2 and CuAlSe2 layers epitaxially grown on Cu(In0.04Ga0.96)Se2 substrates

Photoluminescence (PL) study has been performed on epitaxial layers of CuGaSe2 and CuAlSe2 grown by metalorganic chemical vapor epitaxy on CuGa0.96In0.04Se2 substrates prepared by the traveling heater method. PL properties of epilayers are compared with each other for those grown on GaAs (100), CuGa0.96In0.04Se2 (100), CuGa0.96In0.04Se2 (112), and randomly oriented CuGa0.96In0.04Se2 substrates. PL results are discussed in terms of the lattice mismatches and stress in the epilayers.

A NUMERICAL SIMULATION MODEL FOR THE GROWTH OF GaxIn1-xSb BY THE TRAVELLING HEATER METHOD

A numerical simulation model for the growth of Ga-xIn1-xSb by the traveling heater method (THM) is presented. The cell configuration and the furnace thermal profile are adopted from the experimental setup of Amistar Research inc. The field equations are solved numerically by an adaptive finite element procedure as the interfaces between the solid and liquid phases change in time. Numerical results show that the furnace thermal profile, the thermal as well as solutal convection in the liquid solution have significant effects on the growth process. Results are only presented for small growth times. However, it is asserted that the present simulation model could be used to simulate the entire THM growth process of ternary alloys if the required computational power is available.

CdTe and CdZnTe detectors behavior in X-ray computed tomography conditions

The application of CdTe and CdZnTe 2D array detectors for medical X-ray Computed Tomography (XCT) is investigated. Different metallic electrodes have been deposited on High-Pressure Bridgman Method CdZnTe and on Traveling Heater Method CdTe:Cl. These detectors are exposed to X-rays in the CT irradiation conditions and are characterized experimentally in current mode. Detectors performances such as sensitivity and response speed are studied. They are correlated with charge trapping and de-trapping. The trapped carrier space charges may influence the injection from the electrodes. This enables one to get information on the nature of the predominant levels involved. The performances achieved are encouraging: dynamic ranges higher than 4 decades and current decreases of 3 decades in 4ms after X-ray beam cut-off are obtained. Nevertheless, these detectors are still limited by high trap densities responsible for the memory effect that makes them unsuitable for XCT.

Annealing Effects of CuInS2 Single Crystals Grown by Traveling Heater Method

Photoluminescence (PL) and piezoelectric photoacoustic (PPA) measurements are carried out at liquid nitrogen temperature for as-grown and vacuum annealed CuInS2 single crystals grown by a traveling heater method (THM). A new peak, due to S vacancy (VS) defect, is observed in the PPA spectrum after the vacuum annealing. A donor-acceptor pair (DAP) emission band which is due to a recombination between the V donor and In vacancy (VIn) acceptor appears while a DAP (VS donor and Cu vacancy (VCu) acceptor) emission band disappears in the PL spectrum after the vacuum annealing.

Electron Spin Resonance Studies of Fe in CuGaSe2

ESR spectra (X band) were measured in single crystals of CuGaSe2 prepared by traveling heater method (THM) and iodine transport (IT) technique. In the THM-crystal an anisotropic signal around 200-300 mT, which is completely quenched by H2-annealing, is assigned to Fe3+. On the other hand, two types of ESR spectra were observed in IT-crystals. In the type-1 spectrum, nearly isotropic lines were found at 110-130, 400 and 890-950 mT. The spectral features are similar to that of Fe3+ in CuInSe2. On the contrary, in the type-2 spectrum, strongly anisotropic resonance lines were observed and were assigned to Fe2+ states. Annealing in Se vapor completely quenches the type-2 signal but results in appearance of the type-1 spectrum. This finding may be explained by an assumption that the Fermi level is lowered below the Fe2+/Fe3+ demarcation level by reduction of Se-vacancy brought about by the Se- vapor anneal.

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.

A numerical simulation model for the growth of {\rm Ga}_x{\rm In}_{1-x}{\rm Sb} by the travelling heater method

A numerical simulation model for the growth of {\rm Ga}_x{\rm In}_{1-x}{\rm Sb} by the travelling heater method

Growth and characterization of CdTe single crystals for radiation detectors

To improve the productivity of CdTe radiation detectors, the crystal growth by traveling heater method (THM) as well as the quality of the fabricated detectors were investigated. In the THM growth, optimization of the solvent volume was found to be essential because it affects the shape of the growth interface. The use of the slightly tilted seed from 〈1 1 1〉B was also effective to limit the generation of twins having different directions. Single-crystal (1 1 1) wafers, larger than 30×30 mm 2 were successfully obtained from a grown crystal of 50 mm diameter. Pt/CdTe/Pt detectors of dimensions 4×4×2 mm 3, fabricated from the whole crystal ingot, showed an energy resolution (FWHM of 122 keV peak from a 57 Co source) between 6% and 8%. Similarly, Pt/CdTe/In detectors of dimensions 2×2×0.5 mm 3 showed a resolution better than 3%. These characteristics encourage the practical applications of various types of CdTe detectors.

Photoluminescence Related to the Interaction between Carriers and Structural Defects in ZnTe Crystals

High-quality bulk ZnTe single and polycrystals grown by the cold travelling heater method (CTHM) have been studied by photoluminescence (PL). Single crystals have been deformed in order to create structural defects and dislocations. The PL spectra of these deformed samples present the so-called Y1 and Y2 bands and an emission at 598–604 nm which has not been observed in nondeformed single crystals. This last band presents variations of the intensity as a function of the illumination time. The behaviour and analysis of the bands in the deep-level region indicate the existence of centres that likely produce a reduction in the carrier mobility. These centres can drastically degrade the properties of the material, hampering its use in optoelectronic devices or as a substrate for epitaxy.

Numerical simulation of THM growth of CdTe in presence of rotating magnetic fields (RMF)

The influence of rotating magnetic fields (RMF) on flow pattern and compositional uniformity in the solution zone of a traveling heater method (THM) system for growth of CdTe is numerically investigated. The analysis is conducted at the 10 −6 and 10 −1 g 0 as representative of space and ground processing conditions. It is shown that under microgravity conditions application of RMF can be used to overwhelm residual buoyancy-induced convection and to control the uniformity of solution-zone composition at the growth front without appreciable modification of the growth interface shape. At high-gravity levels, RMF is found not to be able to completely dominate buoyancy-induced convection. In this regime, for the range of field strengths studied, RMF is found to result in (a) complex flow structures in the solution zone, (b) enhancement of compositional nonuniformities at the growth front, and (c) increased convexity of the growth interface. A scaling analysis of convection in the solution zone is used to generate a nondimensional map delineating the RMF- and gravity-dominated flow regimes.

ZnSe single-crystal growth with SnSe as solvent

Bulk single crystals of ZnSe have been grown by the travelling heater method (THM) with SnSe as a new and novel solvent. This solvent is favoured for the growth of ZnSe because of the nearly ideal phase diagram for SnSe–ZnSe. ZnSe has a suitable solubility in liquid SnSe in the temperature range of 950–1050°C and there is practically no solubility of SnSe in solid ZnSe. Ingots were grown with dimensions of 10 mm in diameter and about 30 mm in length. The crystals are characterized by greatly reduced density of inclusions, mostly of small sizes. Moreover, their high degree of structural perfection was confirmed by X-ray double-crystal topography and rocking-curve measurements.

Preparation and properties of bulk ZnSe : Cr single crystals

Cr-doped zinc selenide single crystals have been prepared by a two stage process. In the first stage, undoped single crystals were grown by a sublimation traveling heater method (STHM) without seeding. The high quality of the as-grown crystals was confirmed by double crystal X-ray diffraction, which showed the full-width half-maximum (FWHM) of the rocking curves to be less than 20 arcsec on the (2 2 0) diffraction planes of the crystals. Selective chemical etching showed dislocation density of these faces to be less than 5×10 4 cm −2. In the second stage, the ZnSe crystals were Cr-doped by the method of the thermal diffusion from a solid metal source. Cr concentration of 3.3×10 19 cm −3, absorption coefficient of more than 30 cm −1 at 1750 nm, and passive losses of less than 0.3 cm −1 at 2500 nm were obtained under optimal technological conditions.

Vapour growth of bulk ZnSe single crystals

Large, partly faceted ZnSe single crystals have been grown in the closed tube system by the unseeded sublimation travelling heater method (STHM) at 1100°C and undercooling Δ T less than 10°C. As-grown crystals are highly transparent and have very good optical homogeneity by sight. High quality of the crystals was confirmed by double crystal X-ray diffraction which showed the full-width at half-maximum (FWHM) of the rocking curves to be less than 20 arcsec on the (2 2 0) ZnSe diffraction planes of the crystals. Selective chemical etching showed a dislocation density of these faces to be less than 5×10 4 cm −2. The room-temperature photoluminescence (PL) spectrum under pulse excitation consists of only one near-band-edge line. Free and bound exciton as well as donor–acceptor pair recombination are observable at 13 K.

Resonant Raman Effect on a CuGaSe2 Crystal Grown by the Traveling Heater Method

Resonant Raman scattering phenomena in the case of CuGaSe2 crystals grown by the traveling heater method have been investigated. Seven resonant Raman peaks which are assigned to one A1, one B2(L) and five E(L) modes, have been observed over a wide excitation energy range (1.54–1.66 eV). The enhancement of these Raman peaks is due to incoming resonance mediated by the donor–acceptor pair transition states which are observed as a luminescence peak. The breakdown of the Raman selection rule is attributed to the resonance effect. Temperature dependence of Raman intensity also suggests that the transition states of the donor–acceptor pair act as intermediate states of the resonant Raman scattering.

Seedless THM growth of Cd xHg 1− xTe ( x≈0.2) single crystals within rotating magnetic field

Single CdHgTe crystals with diameter of 25 mm have been grown by the traveling heater method within a rotating magnetic field (RMF) of 2–6 mT. The experiments have been conducted without seed to minimize the free volume in the ampoule. Some results of the research are described with emphasis on the effect of RMF on radial composition homogeneity. Investigations of optical transmission and electric characteristics of the samples have been carried out as well.

Low-pressure synthesis and Bridgman growth of Hg 1− xMn xTe

To reduce Hg high pressure related to the high-temperature synthesis reaction between the components in elemental form, Hg 1− x Mn x Te bulk crystals were produced by a two-step procedure including (I) the alloy synthesis using HgTe crystals grown by the cold travelling heater method and elemental Mn and Te to complete the desired composition, followed by (II) the Bridgman growth. The growth was carried out at temperature in the range of 700–850°C and rate of 1 mm/h. The Hg 1− x Mn x Te crystals have been characterised by X-ray diffractometry, energy dispersive X-ray analysis, Fourier transformed infrared spectroscopy and Hall effect techniques. Although the distribution coefficient of Mn was found to be in the range of 3–3.5 in the first-to-freeze region, the crystals show a good axial composition uniformity throughout more than a half of the total length. The lattice parameter of Hg 1− x Mn x Te is reported for the studied composition range x=0–0.15.

Growth of single CuInSe 2 crystals by the traveling heater method and their characterization

Single crystals of CuInSe 2 were prepared under conditions to suppress the occurrence of twinning. This was accomplished by growth from an In melt by the traveling heater method to keep the growth temperature below that of the sphalerite–chalcopyrite phase transition. The resulting crystals were n-type and could be converted to p-type by a Se anneal. Both n and p-type crystals were characterized structurally by X-ray diffraction, compositionally by microprobe analyses, morphologically by atomic force and scanning electron microscopy, and electrically. They were found to be mostly homogeneous with mirror-like cleavage and without twins.