Mnx Te Research Articles

Optimization of Mechanical and Thermoelectric Properties of SnTe-Based Semiconductors by Mn Alloying Modulated Precipitation Evolution.

Multiscale defects engineering offers a promising strategy for synergistically enhancing the thermoelectric and mechanical properties of thermoelectric semiconductors. However, the specific impact of individual defects, in particular precipitation, on mechanical properties remains ambiguous. In this work, the mechanical and thermoelectric properties of Sn1.03- xMnxTe (x = 0-0.30) semiconductors are systematically studied. Mn-alloying induces dense dislocations and Mn nano-precipitates, resulting in an enhanced compressive strength with x increased to 0.15. Quantitative calculations are performed to assess the strengthening contributions including grain boundary, solid solution, dislocation, and precipitation strengthening. Due to the dominant contribution of precipitation strengthening, the yield strength of the x = 0.10 sample is improved by ≈74.5% in comparison to the Mn-free Sn1.03Te. For x ≥ 0.15, numerous MnTe precipitates lead to a synergistic enhancement of strength-ductility. In addition, multiscale defects induced by Mn alloying can scatter phonons over a wide frequency spectrum. The peak figure of merit ZT of ≈1.3 and an ultralow lattice thermal conductivity of ≈0.35Wm-1K-1 are obtained at 873 K for x = 0.10 and x = 0.30 samples respectively. This work reveals thaprecipitation evolution optimizes the mechanical and thermoelectric properties of Sn1.03- xMnxTe semiconductors, which may hold potential implications for other thermoelectric systems.

Structural Evolution of High-Performance Mn-Alloyed Thermoelectric Materials: A Case Study of SnTe.

Mn alloying in thermoelectrics is a long-standing strategy for enhancing their figure-of-merit through optimizing electronic transport properties by band convergence, valley perturbation, or spin-orbital coupling. By contrast, mechanisms by which Mn contributes to suppressing thermal transports, namely thermal conductivity, is still ambiguous. A few precedent studies indicate that Mn introduces a series of hierarchical defects from the nano- to meso-scale, leading to effective phonon scattering scoping a wide frequency spectrum. Due to insufficient insights at the atomic level, the theory remains as phenomenological and cannot be used to quantitatively predict the thermal conductivity of Mn-alloyed thermoelectrics. Herein, by choosing the SnTe as a case study, aberration-corrected transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM) to characterize the lattice complexity of Sn1.02- x Mnx Te is employed. Mn as a "dynamic" dopant that plays an important role in SnTe with respect to different alloying levels or post treatments is revealed. The results indicate that Mn precipitates at x= 0.08 prior to reaching solubility (≈10mol%), and then splits into MnSn substitution and γ-MnTe hetero-phases via mechanical alloying. Understanding such unique crystallography evolution, combined with a modified Debye-Callaway model, is critical in explaining the decreased thermal conductivity of Sn1.02- x Mnx Te with rational phonon scattering pathways, which should be applicable for other thermoelectric systems.

Exciton–Mn exchange interactions as a function of translational wavevector in Cd1‐xMnxTe quantum wells

AbstractThe exchange interaction between excitons and manganese ions have been studied in wide quantum wells of Cd1‐x Mnx Te with Cd1‐y Mgy Te barriers. The widths L of the wells are sufficiently large for the excitons to be described by the centre of mass approximation, in which the translational wavevector is quantised according to Kz = Nπ/L. Excitonic transitions with different values of the quantisation index N can be resolved in the optical spectra. The giant Zeeman splittings induced by a magnetic field then enable the separate determination of the exchange energy for each value of the wavevector. We find that the quantity N0 (α – β), which describes the exchange splitting, is constant to within 3% over the range of wavevectors up to 4 × 106 cm–1 (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of the annealing temperature on the structure and optical properties of Cd 1− x Mn xTe thin films

Ternary systems of Cd 1− x Mn x Te were synthesized by the direct fusion technique. Thin films of these compounds were prepared by an electron beam gun under a vacuum of 10 −5 Torr. Structural properties of these compounds in thin film forms were investigated by X-ray diffraction. It was found that these materials crystallize in the zinc-blende form. The optical constants (absorption coefficient and band gap) of the thin films were determined by measuring their reflectance and transmittance in the wavelength region from 500 to 2500 nm. Analysis of the optical absorption spectra revealed the existence of one direct energy gap. The energy gap increases as the Mn concentration increases and with decreasing annealing temperatures. The electrical conductivity σ was measured at room temperature and was found to decrease with increasing Mn content.

Giant Faraday rotation in Cd1–xMnx Te (0 < x < 0.82) crystals

AbstractWe investigated the magneto‐optical properties of diluted magnetic semiconductors Cd1–x Mnx Te crystals grown using a vertical Bridgman method. This material crystallizes in the zinc‐blende structure for values of x < 0.82. The band‐gap energy was depended on Mn mole fraction x linearly and increased with decreasing temperature. The increase of the energy gap with increasing x may be due to a decrease in the lattice constant. Faraday rotation as a function of photon energy with various Mn compositions for CdMnTe in the temperature range of 10–300 K has been measured. The Faraday rotation was increased as the photon energy increased near to that of the fundamental band gap. Verdet constant becomes larger since the effect of the sp ‐d exchange interaction is enhanced as the temperature is lowered. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Sb‐treatment effect of GaAs substrate on Sb‐doped Cd1–xMnxTe growth by MOVPE

AbstractSb‐doped Cd1–x Mnx Te was successfully grown on (100)GaAs substrate by metal‐organic vapor phase epitaxy under atmospheric pressure by Sb‐treatment of the GaAs substrate prior to the growth. The Sb‐treatment of GaAs substrate creates a GaSb layer on the GaAs substrate, which relaxes the lattice misfit between the GaAs substrate and the Sb‐doped Cd1–x Mnx Te. The best FWHM of (400) diffraction of Sb‐doped Cd1–x Mnx Te was 580 seconds, where the TESb supply rate of the Sb‐treatment was 2.2 μmol/min. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Laser controlled magnetization in bulk Zn1–x Mnx Te

AbstractZn1–x Mnx Te crystals (0 ≤ x ≤ 0.30) have been grown by the high pressure Bridgman technique. The free exciton energy of Zn1–x Mnx Te was determined from the photoluminescence and reflectance measurements in the temperature range from 1.7 to 270 K. The magneto‐photoluminescence and magneto‐reflectance were studied at T = 1.7 K and in magnetic fields up to 6 T. The magnetization of Zn1–x Mnx Te was calculated from the giant Zeeman effect. It was found that there is an efficient energy transfer channel from the photo‐excited carriers into the Mn2+ ions by spin‐flip scattering processes. It allows one to change the spin temperature of the Mn2+ subsystem and consequently to control its magnetization by tuning laser excitation density. The magnetization of the Zn1–x Mnx Te can be changed by 60% of its maximum value. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of treatment on the CdTe, Cd1–xMnxTe and Cd1–xZnxTe surface stoichiometry

AbstractThe effects of treatment in several etchants on CdTe, Cd1–x Mnx Te, Cd1–x Znx Te surface stoichiometry are studied. Simple chemical etching of CdTe, Cd1–x Mnx Te, Cd1–x Znx Te causes a relief and contamination of near‐surface layer with etchant components. It was determined that Cd, Mn, Zn are washed out into the solution as a result of material treatment in bromine‐methanol etchant, and, consequently, the surface is enriched with elemental tellurium and its oxides. Treatment in alkaline solutions allows minimizing the violation of surface stoichiometry. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pb1-x Mnx Te and PbTe1-xSx compounds and their optical properties

Pb1-xMnxTe crystals were obtained by the Bridgman method and PbTe1-xSx crystals were grown by the vapour-liquid-solid technique. The structural properties of Pb1-xMnxTe (x?0.10) and PbTe 1-xSx(x?0.05) were observed by X-ray powder diffraction analysis. The optical properties were studied by Raman spectroscopy as a function of temperature. Measurements on these samples of different composition gave information about the Mn and S position in the lattice (off-centering), their clustering and ordering, as well as of the influence of these processes on the crystal structure and properties. The model of phonon behavior based on the Random Element Isodisplacement model was applied, and it was found that the phonons in PbTe 1-xSx show a two-mode behavior (each TO-LO mode pair of the end members degenerates to an impurity mode), while the Pb1-xMnxTe optical phonons have a intermediate one-two-mode behavior (the LO-mode frequency shifts continuously from PbTe to MnTe, while the other modes resemble the two-mode case). .

High‐pressure X‐ray diffraction study of ternary and non‐stoichiometric PbTe and PbSe crystals

AbstractThe paper reports the pressure‐induced phase transitions observed in ternary PbTe and PbSe compounds (Pb1–x Snx Te, Pb1–x Mnx Te, Pb1–x Snx Se) and non‐stoichiometric PbTe ones (Pb0.55Te0.45, Pb0.45Te0.55) at room temperature in the range of 0 to14 GPa by means of angle‐dispersive X‐ray diffraction (ADXRD). The structure of the intermediate high‐pressure phase appearing above ∼6 GPa for all PbTe‐based compounds was established to be Pnma (orthorhombic), like in binary PbTe. The patterns of the intermediate high‐pressure phase of Pb1–x Snx Se (x = 0.125) were found to correspond entirely neither to GeS nor to orthorhombic Pnma, but the calculated reflections for GeS type were closer to the experimental ones. A model of the phase transformations is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical manifestation of ferromagnetic ordering in phosphorus‐doped semimagnetic semiconductor Zn1–xMnxTe

AbstractZn1–x Mnx Te semimagnetic semiconductor doped with phosphorus has been investigated by means of the resistivity, Hall effect, andmagnetic susceptibilitymeasurements. Itwas demonstratedthat in Zn1–x Mnx Te:P the Mn‐Mn antiferromagetic superexchange interaction can be controlled by changing the hole concentration and that the Mn‐Mn ferromagnetic interaction was observed at the hole concentration level of 5 × 1018 cm–3. This Mn‐Mn ferromagnetic interaction has resulted in a strong temperature dependence of the resistivity, which follows log ρ ∼ 1/T 2. A phenomenological calculation has been performed taking into account the magnetic contribution to the carrier binding energy. The calculation well describes the temperature dependence of the resistivity of the highly P‐doped Zn1–x Mnx Te crystals. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Exchange interaction in magnetic HgMnTe quantum well structure

AbstractLow temperature magnetotransport measurements have been used to study the exchange interactions in Hg1–x Mnx Te magnetic quantum wells (QWs). An analysis of Shubnikov–de Haas oscillations in conjunction with 8 × 8 k · p bandstructure calculations allow the exchange constants N 0α and N 0β , the antiferromagnetic temperature T 0, and the effective spin S 0 to be determined. The latter displays a distinct temperature dependence and appears to be enhanced compared with bulk materials of equivalent Mn concentrations. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optically detected electron spin‐flip resonance in CdMnTe

AbstractWe show that the spin‐flip of electrons at neutral donors in a dilute magnetic semiconductor can be observed directly by means of optically‐detected magnetic resonance (ODMR). Spectra obtained at 105 GHz for a bulk crystal of Cd1–x Mnx Te with x = 0.005 showed strong signals with g ‐values ranging between 12 (at 4.2 K) and 35 (at 1.7 K), with magnetic resonance linewidths ranging from 0.3 Tesla to 0.1 Tesla at the lowest temperature. In energy terms, these linewidths are independent of temperature and agree with those in spin‐flip Raman spectra from the same specimen. The line broadening is caused by fluctuations in the number of manganese ions that interact with a particular donor and an analysis of this leads to a value for the donor Bohr radius of 4.5 nm. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier-induced ferromagnetic interactions in p-doped Zn (1− x) Mn xTe epilayers

p-type doping of molecular-beam-epitaxy grown layers of the diluted magnetic semiconductor Zn (1− x) Mn x Te is achieved by using an active nitrogen cell. The strong interaction between the localized Mn spins and the holes deeply modifies the transport properties (metal–insulator transition, spin-dependent Hall effect). In spite of the weak localization of the carriers at low temperature, the holes clearly induce a ferromagnetic interaction between the localized spins, which is discussed as a function of Mn content and hole concentration.

Spin-flip scattering contribution to resonant-tunneling current in semimagnetic semiconductor heterostructures

We calculate the characteristic current-voltage curve of a tunneling device based on semimagnetic semiconductor materials. The device is a heterostructure with layers of Cd1- x Mnx Te in which the magnetic ions Mn2+ interact strongly with the conducting electrons via the s-d exchange interaction. Thermal fluctuations of Mn2+ magnetic moments cause spin-dependent electron scattering that modifies the characteristic current-voltage curve. Our calculation shows how this electron-ion scattering is expected to affect the spin dynamics in transport measurements.

Transport in Quantum Structures of Diluted Magnetic Semiconductors

This paper reviews recent millikelvin studies of magnetoconductance and noise in nanostructures of a diluted magnetic semiconductor n-Cd1x Mnx Te. These studies were particularly rewarding for probing the influence of magnetic ions upon mesoscopic phenomena. The accumulated results demonstrated the existence of a new driving mechanism of the universal conductance fluctuations in magnetic systems. Several signatures of spin-glass freezing were observed, such as the appearance of 1/f conductance noise, aging, thermal, and magnetic irreversibilities as well as a strong increase in the amplitude of both conductance fluctuations and noise when temperature and the magnetic field were lowered below the freezing line. A statistical analysis of conductance noise made it possible to investigate the nature of excitations in spin-glass phase, and to discriminate between competing theoretical models.

Population effects in magnetoabsorption of D 0X centers in Cd 1− x Mn xTe

Magnetoabsorption measurements were performed on molecular beam epitaxy-grown semimagnetic CdMnTe layers doped with iodine. Absorption lines caused by the formation of light- and heavy-hole free excitons were observed. A shoulder resulting from the formation of neutral donor bound excitons (D 0X) was detected on the low energy slope of the heavy-hole free exciton line. A careful numerical analysis allowed us to separate the D 0X line and to determine its parameters. The integrated intensity was found to be proportional to the occupation of the initial spin state of the electron of the neutral donor. Giant Zeeman splitting, characteristics for semimagnetic CdMnTe crystals, allowed us to perform this spin sensitive population study at a low magnetic field, without perturbing the orbital part of the donor electron wavefunction.

Improvement of quality of Cd 1− x Mn xTe crystals by special annealing

Electrical, magnetic and optical properties of Cd 1−x Mn x Te (0.1⩽x⩽0.45) single crystals grown by the Bridgman technique and annealed under various conditions were investigated. The nonstabilities of electrical properties of as-grown single crystals were revealed, caused by intrinsic ( V Cd′, V Cd″ and extrinsic defects which were able to create low-binding-energy complexes. Heat treatment followed by slow cooling to 723 K ( v=1 K/h) under conditions preventing sample evaporation stabilizes the electrical properties of the material.

Magnetic Characterization of Molecular Beam Epitaxy Grown Cd1xMnxTe Structures

Results of previous studies [1] on magnetic properties of CdMnTe epilayers grown by molecular beam epitaxy (MBE) on InSb substrates [2] revealed that magnetization measurements may serve as an effective tool for investigation of Mn distribution within the epilayer. Namely, we found systematically greater values of the high temperature magnetic susceptibility in MBE layers than in their bulk counterparts. Since spin—spin interactions in Cd 1-x Mn xTe are antiferromagnetic it was concluded in [1] that a smaller clusterization occurred during the non-equilibrium growth. However, this process did not lead to a change of magnetic properties at low temperatures, where transitions to either spin-glass (x < 0.60) or antiferromagnetic (x = 0.70) phases were observed to occur at the same temperatures as in bulk samples. The latter fact suggests that the growth technique does not affect these properties of the distribution function of magnetic atoms which is important for the long range magnetic order. Also, an enhanced paramagnetic contribution to the magnetic susceptibility at low temperatures has been found in CdTe/CdMnTe superlattice. The latter effect was assigned to the presence of isolated paramagnetic ions in the interface regions as a result of an intermixing of Mn and Cd ions and/or a smaller number of magnetic bonds at the interface. In this paper we report results of magnetic measurements performed on both thick (Cd0.60 Mn0.40Τe and MnTe) epilayer and CdTe/Cd 0.50Mn0.50Te superlattices grown by MBE on GaAs substrates [3]. In order to reduce effects of

The synthesis of some alkyltelluride-manganese(I) complexes, and an assessment of their suitability for MOCVD applications

Tellurium does not insert directly into the MnC bond of RMn(CO) 5 (R = Me or PhCH 2). However, reactions between RMn(CO) 5 (R = Me, PhCH 2) and TePR′ 3 (R′ = Me, Et) result in Te insertion accompanied by CO substitution to produce RTeMn(CO) 3(PR′ 3) 2 complexes. These complexes are formed by initial substitution to give RMn(CO) 3(PR′ 3) 2 followed by Te insertion. The crystal structure of the complex with R = PhCH 2 and R′ = Et has been determined from X-ray diffraction data. Crystal data: C 22H 37O 3P 2MnTe; M = 594.0; P 1 , a = 9.263(2), b = 14.650(5), c = 20.265(7)Å, α = 91.27(3), β = 90.67(2), γ = 103.96(2)°, U = 2667.7(14) Å 3; D calc = 1.479 ( Z = 4), D meas = 1.480(5) mg m −3, μ = 1.687 mm −1 for Mo- K α radiation (λ = 0.7107 Å), final R = 3.11, R w = 5.46, from 7788 observed reflections (12974 collected). The geometry about manganese is slightly distorted octahedral with a trans-arrangement of the phosphine groups and a meridional placement of the carbonyls. Pyrolysis of PhCH 2TeMn(CO) 3(PEt 3) 2 at 300°C in a hydrogen stream gives a film of MnTe. Treatment of Mn(CO) 5Br with LiTeR yields the dimeric products [Mn(CO) 4(μ-TeR)] 2 (R = Me, Et, Pr i , CH 2SiMe 3, Si(SiMe 3) 3, and Ph). For most of these complexes, two conformations have been detected by multinuclear NMR spectroscopy. Variable temperature 1H NMR spectroscopy shows that the conformers interconvert for R = Me and CH 2SiMe 3. The mass spectra of [Mn(CO) 4(μ-TeR)] 2 (R = Me, Et, Pr i ) show gas phase fragmentation to Mn xTe y species. Pyrolysis of two of these complexes (R = Me, Et) in a hydrogen atmosphere produced a film containing MnTe. Addition of PEt 3 to a solution of [Mn(CO) 4(μ-TeME)] 2 results in bridge cleavage to yield MeTeMn(CO) 3(PEt 3) 2, but the reaction is slow and other unidentified products are formed. A similar reaction occurs when R = CH 2SiMe 3. The formation of MeTeMn(CO) 3 (PEt 3) 2 was also achieved from the reaction between BrMn(CO) 3(PEt 3) 2 and LiTeMe. The HgMn complex MeHgMn(CO) 5 was obtained by treatment of [Mn(CO) 5] − with MeHgCl. This complex readily disproportionates to Me 2Hg and Hg[Mn(CO) 5] 2. Pyrolysis of MeHgMn(CO) 5 at 300°C gives a film of manganese with no retention of mercury.