Te Vapor Pressure Research Articles

Unexpected interfacial reactions in Co/Sb2Te3 and Co/GeTe couples

BackgroundSb2Te3 and GeTe are important thermoelectric materials. Co/Sb2Te3, Co/GeTe, Ni/Sb2Te3 and Ni/GeTe couples were prepared by electroplating Co and Ni on Sb2Te3 and GeTe substrates, and reacted at 500 °C, 400 °C and 300 °C. Surprisingly, in the couples reacted at 500 °C, in addition to reactions at the Co/Sb2Te3 and Co/GeTe interfaces, reaction phases are also found on the outer surface of the electroplated Co which has no direct contact with the thermoelectric substrate. These unexpected reactions are not found in the couples plated with Ni and in the couples plated with Co reacted at 400 °C and 300 °C. The compositions of the reaction phases on the outer surface of Co are similar to those at the Co/Sb2Te3 and Co/GeTe interfaces. MethodsSystematic investigations were carried out using three different types of reaction couples. Significant findingsThe reaction phases on the outer surface of Co result from Te vaporization of the thermoelectric substrates, which are not completely covered with Co. The Te vapor of Sb2Te3 and GeTe provides the Te reactant source of the reaction phases. However, at temperatures of 400 °C and 300 °C, the Te vapor pressure is not high enough to cause interfacial reactions on the bare Co surface. The formation of the reaction phases on the outer surface of Co can thus be inhibited with a fully-covered Co layer on the Sb2Te3 and GeTe substrates and by reducing the reaction temperatures. In both the Co/Sb2Te3 and Co/GeTe couples, the reaction layers exhibit linear growth rates with aging time.

Phase stability in MoTe2 prepared by low temperature Mo tellurization using close space isothermal Te annealing

In this work, we used isothermal close space (ICS) Te annealings in pure H2 atmosphere for the tellurization of Mo oxide thin films. Differently to previous open tube tellurization annealings, ICS is expected to provide a higher Te vapor pressure because the chamber containing the Te source and the substrate is very small and semi-closed. Then, we used a relatively low temperature of 500 °C at which, according to the phase diagram, 2H phase should be stable. However, we observed that, in all cases, Mo oxide initially transformed to 1T′ before reaching the equilibrium 2H phase. Highly oriented MoTe2 films with the [001] direction perpendicular to the surface were obtained in all samples. On the other hand, according our X-ray photoelectron spectroscopy measurements, we report shifts of 0.38/0.27 eV in the 3d emission of Te/Mo between the two different phases which probably have a chemical origin.

Spontaneous solid-state foaming of nanocrystalline thermoelectric compounds at elevated temperatures

Nanocrystalline thermoelectric materials have improved properties, but are difficult to process to full density. During routine thermal processing operations such as powder consolidation and annealing, such compounds can spontaneously form pores, thus degrading their thermoelectric and mechanical properties. We systematically investigate pore formation during heat treatment of cold-pressed compacts of nanocrystalline Bi2Te3, combining dilatometry and electron microscopy to quantify pore morphology and the rate of pore growth. Pores are found to nucleate on Te-rich precipitates, which are ultimately attributable to a non-equilibrium solubility shift associated with defects in the nanostructured compound, and they grow by diffusional creep under the driving force of Te vapor pressure. This mechanistic insight reveals that, ironically, the same nonequilibrium processing and nanostructure desirable for improved thermoelectric performance also encourage foaming and challenge the formation of stable high density material. With an improved mechanistic understanding, however, we are also able to suggest strategies for improved materials design and processing.

Properties of heavily impurity-doped PbSnTe liquid-phase epitaxial layers grown by the temperature difference method under controlled Te vapor pressure

We propose the use of heavily impurity-doped Pb1-xSnxTe/PbTe epitaxial layers grown via the temperature difference method under controlled vapor pressure (TDM-CVP) liquid-phase epitaxy (LPE) for the preparation of IV–VI compounds for mid- to far-infrared optical device applications. A flat surface morphology and the distribution of a constant Sn concentration for 0.05≤x≤0.33 were observed in the epitaxial layers using electron-probe microanalysis. The segregation coefficient of Sn in Pb1-xSnxTe grown via TDM-CVP LPE (Tg=640°C) was xSSn／xLSn=0.28. The appearance of the Fermi level pinning and persistent photoconductivity effects in In-doped PbSnTe were also proposed; we estimated that the activation energies of these processes were 2.8 and 39.7meV, respectively, based on the In-doped Pb1-xSnxTe carrier profile as a function of ambient temperature. In Hall mobility measurements, Sn was assumed to be a main scattering center in the Pb1-xSnxTe epitaxial crystals. The impurity effect was also observed in Pb1-xSnxTe epitaxial growth, similar to the effects observed for Tl-doped PbTe bulk crystals. We concluded that the heavily doped Pb1-xSnxTe crystals grown via TDM-CVP LPE can be used to fabricate high-performance mid- to far-infrared optical devices.

Liquid-phase epitaxy of In-doped PbTe for the application of mid-infrared semiconductor laser

Indium-doped PbTe epitaxial layers and pn homojunction diodes were grown on PbTe substrate using the temperature-difference method under controlled vapor pressure (TDM-CVP) liquid-phase epitaxy (LPE). The activation ratio of indium donor in the PbTe epitaxial layer was as high as 80%, and we achieved high electron concentrations up to n D=5×10 19 cm −3. In view of the enhanced electron mobility, it is shown that the optimum Te vapor pressure for an In-doped PbTe epitaxial layer is about 2×10 −5 Torr at 470 °C, which agrees well with the results for undoped and heavily Bi-doped PbTe epitaxial layers. In-doped n +-PbTe layer was successfully applied for the fabrication of broad contact pn junction lasers and excellent lasing emissions were achieved, characteristics as compared to Bi-doped and -undoped cladding layers. The threshold current for a broad area diode is 200 A/cm 2 at 15 K and 2.7 kA/cm 2 at 77 K.

High temperature electrical properties of CdTe〈Pb〉 crystals under Te saturation

AbstractThe point defect equilibrium of CdTe〈Pb〉 single crystals under well‐defined Te vapor pressure was investigated up to 1070 K. At 630–900 K these crystals showed p‐type conductivity and at higher temperatures – native n‐type one. During measurements the hole density reached up to ∼2 × 1017 cm–3 at 800 K. The main acceptor dominant species, which determined the electrical properties of crystals, was supposed to be the (Pb$ ^+_{\rm Cd} $V$ ^{2-}_{\rm Cd} $)– associate with its level in the gap located at EV + 0.42–0.45 eV. Above 900 K native electrons began to influence the conductivity type. Three models of point defect structure were used to describe the galvanomagnetic data – (i) frozen defect structure, (ii) defect structure with shallow or deep acceptor levels fixed during thermal cycles and native defects being in three‐phase solid–liquid–gas (SLG) equilibrium, and (iii) defect structure without any fixed energy level and defect densities being determined by the SLG equilibrium. The FWM technique confirmed p‐type photoconductivity at 300 K, but also revealed bipolar carrier generation at high photoexcitation levels with very fast electron trapping. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The nature of point defects in CdTe

High temperature in-situ Hall effect measurements in CdTe single crystals grown by different techniques at 500–1,200 K under well-defined Cd and Te vapor pressure were made. It was established that native point defects (PD) Cd i 2+ and V Te 2+ are dominant at T>770 K in Cd-rich CdTe. Their formation enthalpies were determined (ΔH VTe 2+ =1.3 eV; ΔH Cdi 2+ =2.5 eV). It was shown that V Te 2+ dominate at low temperatures and Cd i 2+ begin to prevail at T>930 K. In Te-saturated CdTe at heating up to ∼800 K, the hole density was Te vapor pressure independent. At higher temperatures (HT), the conductivity changed to intrinsic type, turning then into n-type. The results were explained in the framework of Kroger’s quasichemical formalism assuming the presence of an electrically active foreign point defect, the oxygen acceptor. The PD structure modeling demonstrated satisfactory agreement with experimental results both for temperature and component vapor pressure dependencies.

Predominant point defects in tellurium saturated CdTe

AbstractHigh temperature Hall effect measurements at 570–1070 K under well defined Te vapor pressure in CdTe single crystals grown by THM and Bridgman techniques were made. Both the free carrier density versus Te vapour pressure value and temperature dependencies were studied. At heating up till ∼870 K the hole density was Te vapor pressure independent, but it varied in different samples from 1 × 1016 to 1 × 1017 cm–3. At higher temperatures the conductivity becomes of intrinsic type, turning then into n‐type one. A theoretical analysis of native point defects contents at different conditions in the framework of Krögers quasichemical formalism was performed. It resulted in the impossibility of mutual compensation of native donors and acceptors proposed by different authors. The results were explained assuming the presence of an electrically active foreign point defect – the oxygen interstitial acceptor. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Point defect structure of CdTe〈Cl〉 crystals at high temperatures

High temperature Hall effect measurements in CdTe〈Cl〉 single crystals grown by THM and Bridgman techniques at 200–900 °C under well defined Cd and Te vapor pressure were made. Associates decay at heating from 500 to 700 °C in THM grown samples was observed. Cd vapour pressure influence on electron concentration [e–] in all samples was established. The main point defects present in crystals were: at T 700 °C - chlorine donors, Cd vacancies and Cd interstitials. Bridgman grown CdTe〈Cl〉 crystals had [e–] similar to undoped CdTe and differed in Cd vapor pressure dependencies by line slopes. Under Te vapor pressure the samples demonstrated p-type conductivity up to 550 °C. In this case the hole density changed from [h+] = 1016 cm–3 to 3 × 1017 cm–3 and did not depend on Te vapor pressure. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties of Bi-doped PbTe layers grown by liquid phase epitaxy under controlled Te vapor pressure

Effects of Bi doping in PbTe liquid-phase epitaxial layers grown by the temperature difference method under controlled vapor pressure (TDM-CVP) are investigated. For Bi concentrations in the solution, xBi, lower than 0.2 at.%, an excess deep-donor level (activation energy Ed≈0.03–0.04 eV) appears, and Hall mobility is low. In contrast, for xBi>0.2 at.%, Hall mobility becomes very high, while carrier concentration is in the range of 1017 cm−3. Inductive coupled plasma (ICP) emission analysis shows that, for xBi=1 at.%, Bi concentration in the epitaxial layer is as high as NBi=2.3–2.7 × 1019 cm−3. These results indicate that Bi behaves not only as a donor but also as an acceptor, and the nearest neighbor or very near donor-acceptor (D-A) pairs are formed, so that strong self-compensation of Bi takes place. Carrier concentration for highly Bi-doped layers shows a minimum at a Te vapor pressure of 2.2 × 10−5 torr for growth temperature 470°C, which is coincident with that of the undoped PbTe.

Calculation of the optimum vapor pressure for stoichiometry in PbTe and PbSnTe

Optimum vapor pressure for stoichiometry in growth of PbTe and PbSnTe has been calculated. The result is in a good agreement with the experimentally obtained ones for vapor pressure controlled Bridgman growth and liquid phase epitaxy. The procedure of calculation follows that performed earlier for GaAs, which assumed the equality of the chemical potentials of the volatile element in gas, liquid and solid phases. Also it is pointed out that the nonequilibrium reactions of interstitial Te atoms with Pb vacancies forming anti-site Te, as well as Te precipitations, take place at the highest applied Te vapor pressure region.

Properties of Bi-doped PbTe epitaxial layers and pn junction diodes grown by TDM-CVP

Effects of Bi-doping in PbTe liquid-phase epitaxial layers grown by the TDM-CVP have been investigated. For Bi concentration in the solution, x Bi, lower than 0.2 at.%, Hall mobility is low. In contrast, for x Bi>0.2 at.%, Hall mobility is high, while carrier concentration is in the range 10 17 cm −3. However, ICP emission analysis shows that, for x Bi=1.0 at.%, Bi concentration in epitaxial layer is N Bi=2.3–2.7×10 19 cm −3. These results indicate that Bi behaves not only as a donor but also as an acceptor; the nearest neighbor or very near DA pairs are formed. Carrier concentration for Bi-doped layers takes a minimum value at a Te vapor pressure of 2.2×10 −5 Torr for growth temperature 470°C, which is coincident with that of the undoped PbTe. And broad contact pn junctions with highly Bi-doped layers easily cause laser emission compared to undoped junctions. The result suggests that the nearest lattice site Bi–Bi DA pairs behave as strong radiative centers in PbTe.

Growth and crystal properties of Tl-doped PbTe crystals grown by Bridgman method under Pb and Te vapor pressure

Growth of high-quality Tl-doped p-type PbTe single crystals by Bridgman method under controlled Te or Pb vapor pressure has been investigated. For high concentration of Tl in the melt (0.65 at%), the hole concentrations for crystals grown under applied Pb vapor pressures (1.5×10 19–2×10 19 cm −3) are higher than those for crystals grown under applied Te vapor pressures (5×10 18–8×10 18 cm −3). For low concentration of Tl (0.082 at%), the hole concentration is decreased with increasing Te vapor pressure. The PbTe crystals grown with high concentration of Tl under applied Pb vapor pressures show small FWHM of X-ray rocking curve ranging from 20 to 50 arcsec. Their etch pit densities are in the range of 8×10 4–6×10 5 cm −2, which are an order of magnitude lower than those of undoped PbTe crystals. These results indicate that highly Tl-doped (0.65 at%) PbTe crystals, in particular, those grown under applied Pb vapor pressure, have an excellent crystal quality.

Growth of Pb 1− xSn xTe ( x≈0.12) epitaxial layers by temperature difference under controlled vapor pressure liquid-phase epitaxy

Pb 1− x Sn x Te ( x≈0.12) liquid-phase epitaxial layers have been grown by the temperature difference method under controlled vapor pressure (TDM–CVP) using Pb solvents on PbTe substrates at temperatures 490°C and 600°C. The grown layers are usually n-type, and the optimum Te vapor pressure where the carrier concentration becomes minimum is 3.1×10 −3> P Te>1.4×10 −3 Torr for T g=600°C, and 3.1×10 −5 >P Te >1.4×10 −5 Torr for T g=490°C. The activation energy of the optimum Te vapor pressure for Pb 1− x Sn x Te ( x≈0.12) is 2.38 eV, which is larger than the activation energy for PbTe, 2.12 eV. Two different types of etch pits are observed on Pb 1− x Sn x Te ( x≈0.12) layers. The density of the smaller size etch pits tends to become minimum at the optimum Te vapor pressure, while the larger size etch pits are dominantly observed at the vapor pressure region above the optimum vapor pressure. This tendency and lower electron mobility suggest defect aggregation taking place above the optimum vapor pressure.

Te vapor pressure dependence of the pn junction properties of PbTe liquid phase epitaxial layers

The pn junction characteristics of the PbTe epitaxial layers grown on PbTe substrates are investigated. Both the substrate crystals and the epitaxial layers are grown under controll Te vapor pressures. The ideality factors n of pn junctions decrease close to 1 in the temperature region from 40K to about 120K, but then increase with increasing temperature. The increase of n is more pronounced for higher Te vapor pressure. As a possible origin of the recombination current, deep levels with Ed=0.09 eV are found from Hall measurements for pn junctions grown under higher Te vapor pressure.

Growth and electrical properties of PbTe epitaxial layers grown by temperature difference method under controlled vapor pressure liquid phase epitaxy

Temperature difference method under controlled vapor pressure (TDM-CVP) liquid phase epitaxy has been applied to the growth of PbTe using Pb solvent and with applied Te vapor pressure. Epitaxial layers with a good morphology were obtained with a growth temperature ( T g) ranging from 410 to 700°C. Growth thickness as a function of growth time, subsidiary heater power, and growth temperature are measured and analyzed. All the unintentionally doped epitaxial layers are n-type. The carrier concentrations of the epitaxial layers become minimum at a Te pressure of 4 × 10 −4−5 × 10 −4 Torr for T g = 560°C and 0.9 × 10 −3−1 × 10 −3 Torr for T g = 600°C.

Behavior of Excess Te Atoms in Undoped p-type CdTe Annealed under Te Vapor Pressures

The number of Te atoms and hole concentrations in undoped p-type CdTe quenched into water after annealing were investigated as functions of annealing temperature and Te2 vapor pressure. The number of Te atoms diffused from the annealing atmosphere into CdTe increased with increasing annealing temperature and Te2 vapor pressure, but decreased with additional increasing Cd vapor pressure. Te precipitates were observed by a scanning electron microscope in CdTe with diffused Te atoms more than approximately 1×1019 cm-3. The number of Te precipitates was increased with increasing Te vapor pressure, but was decreased by adding Cd vapor pressure. It was found that the precipitates resulted in a decrease in the concentration of the holes in p-type CdTe.

Pb1-xSnxTe epitaxial layers prepared by the hot-wall technique

Epitaxial layers of Pb1-xSnxTe were grown on polished-etched KCl substrates using the modified hot-wall technique. Four layers, each 5-30 μm thick, of area 5.5 × 5.5 mm2 were simultaneously grown at growth rates of 0.3–4.8 µm/hr. A typical substrate temperature of 325°C , and baffle and source temperatures of 450-560°C were used. P-and n-type layers were obtained from stoichio-metric and metal-rich sources of (Pb1-xSnx)1+δ Te1-δ, 0.01≤δ ≤0.02, respectively. From metal-rich sources with 0<δ<0.01, the layers obtained were n-type at 300K but p-type at 77K. The layers became intrinsic between approximately 250K and 295K. Carrier concentrations of as-grown layers with 0≤x ≤0.26 were in the range of 1016-1017 /cm3 and mobilities were of the order of 104 cm2 /V-sec at 77K. Both n-and p-type layers were also obtained from metal-rich sources of δ = 0.01 by controlling the Te vapor pressure from a separate reservoir. With Te temperatures higher than the n-p turnover point of 240°C , p-type characteristics were obtained. Layers that were p-type at 77K grown with the Te temperatures near the n-p turnover point had the best mirror-like surfaces.