Mn Te Research Articles

Strong sp–d exchange coupling in ZnMnTe/ZnMgTe core/shell nanowires

AbstractIn this work, our recent progress in the growth and optical studies of telluride nanowire heterostructures containing a small molar fraction of magnetic Mn‐ions of only a few percent is overviewed. ZnMnTe/ZnMgTe core/shell nanowires (NWs) are grown by molecular beam epitaxy by employing the vapor‐liquid‐solid growth mechanism assisted with gold catalyst. The structures are studied by means of photoluminescence and microphotoluminescence in an external magnetic field. In the first step, however, an activation of the near band edge emission from ZnTe and ZnMnTe nanowires is described, which is achieved by coating the nanowires with shells made of ZnMgTe. The role of these shells is to passivate Zn(Mn)Te surface states. The incorporation of Mn ions into the crystalline lattice of ZnMnTe nanowires is manifested as a considerable blue shift of near band edge emission with increasing Mn concentration inside the nanowire cores, which reflects directly the increase of their energy gap. In an external magnetic field the near band edge emission exhibits a giant spectral redshift accompanied by an increase of the circular polarization of the emitted light. Both effect are fingerprints of giant Zeeman splitting of the band edges due to sp‐d exchange interaction between the band carriers and magnetic Mn‐ions. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Heating of the Mn spin system by photoexcited holes in type‐II (Zn,Mn)Se/(Be,Mn)Te quantum wells

The efficiency of the Mn‐spin system heating under pulsed laser excitation is studied in diluted magnetic semiconductor heterostructures Zn0.99 Mn0.01 Se/Be0.93 Mn0.07 Te with type‐II band alignment by means of time‐resolved photoluminescence and pump‐probe reflectivity. An essential role in the heating is played by multiple spin‐flip scatterings of a hole with localized spins of Mn2+ ions. The efficiency of the spin and energy transfer from photoexcited holes to Mn ions of the Zn0.99 Mn0.01 Se layer considerably depends on the hole lifetime in this layer. This lifetime can be limited by the hole relaxation into the Be0.93 Mn0.07 Te layers and is strongly sensitive to the excitation power and Zn0.99 Mn0.01 Se layer thickness. These dependences allow us to determine a characteristic time of about 20ps for the spin and energy transfer from photoexcited holes to the Mn spin system.

The syntheses and structures of mixed-metal dichalcogen complexes [CpMn(CO)2]2(E2)Pt(PPh3)2

The oxidative substitution of diphenylacetylene from the Pt(0) complex (PPh3)2Pt(PhCCPh) by the dimanganese-dichalcogen fragment [CpMn(CO)2]2(E)2 (E=S, Cp=C5H5, C5H4Et (1); E=Se, Cp=C5H5) occurs without E–E bond rupture and allows the formation of the new mixed-metal complexes [CpMn(CO)2]2(E2)Pt(PPh3)2 (2–4 respectively). The analogous Te-containing cluster [CpMn(CO)2]2(Te2)Pt(PPh3)2 (5) was prepared in the transmetallation of [CpMn(CO)2]3(Te2) by (PPh3)2Pt(PhCCPh). The Mn–E and E–E distances in the structures of 1–5 are considerably shortened (Mn–S 2.25, Mn–Se 2.37, Mn–Te 2.53Å). The IR spectra, cyclic voltammetry data and DFT calculations indicate increasing of electron density on the Mn atoms in 2–5.

Growth and Characterization of (Cd, Mn)Te

The present article presents technology, required for repeatable manufacturing of the 1.5 and 2 inch crystal rods of (Cd, Mn)Te is discussed. In order to obtain semiinsultaing crystals, compensation and annealing in Cd-vapours was used. Vanadium was mainly used as the compensating dopant. The resistivity of the obtained monocrystalline plates was around 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> Ω cm, and the μτ-product was about 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V. Mapping of the resistivity was performed on both as-grown crystals and the crystals annealed in the saturated Cd vapour. As-grown (undoped) crystals were inhomogeneous and their resistivities were in the range 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> Ω cm. After annealing resistivity increased up to 1 -2 ·10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> Ω cm and better homogeneity could be seen. Annealing in saturated Cd-vapours had influence on tellurium inclusions/precipitates. It was studied by IR transmission microscopy. For as-grown samples the density of Te inclusions (> 1 μm) was ≈ 3 ·10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> but for annealed samples was ≈ 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . The concentrations (measured by SIMS) of the unintentional impurities Na, Ca, and Ga were in the region 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> - 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , corresponding well with the purity (6N) of the elements used for crystallization of our (Cd, Mn)Te. The low temperature photoluminescence (PL) measurements indicated significantly lowered concentrations of acceptors after annealing. Nearly ohmic contacts to the high resistivity (Cd, Mn)Te plates were obtained by deposition of the amorphous layers of heavily doped semiconductor (ZnTe:Sb). We undertook the present work to show that low vanadium doping level and proper annealing conditions are sufficient to obtain detector grade material.

Photoemission study of amorphous and crystalline GeTe and (Ge,Mn)Te semiconductors

Resonant photoemission spectroscopy was applied to compare the valence band structure of Ge0.9Mn0.1Te and GeTe semiconductor layers deposited on BaF2 substrate in monocrystalline and amorphous forms. In (Ge,Mn)Te the contribution of Mn 3d5 electronic orbitals to density of states was found in three binding energy regions: below the top of the valence band (Eb<4.2eV), at the binding energy range 4.2–4.4eV, and in many-body satellite at 9–13eV. The comparative analysis of the photoemission spectra based on configuration interaction model showed that p–d hybridization effects, important for magnetic and optical properties of (Ge,Mn)Te, are stronger in monocrystalline than in amorphous (Ge,Mn)Te layers.

Subnanosecond magnetization dynamics induced by a pulsed magnetic field in diluted magnetic semiconductor quantum wells

The magnetization dynamics induced by a pulsed magnetic field is investigated by time- and polarization-resolved photoluminescene measurements in (Cd,Mn)Te/(Cd,Mg)Te quantum wells. The magnetization dynamics of Mn${}^{2+}$ ions is found to be strongly dependent on the external static magnetic field. A dynamical response of the magnetization on a subnanosecond time scale is observed at zero static magnetic field, while it drastically slows down and approaches the spin-lattice relaxation time constant for a nonzero static field. Theoretical calculations emphasize the importance of local spin interactions that interplay with the Zeeman interaction for the observed magnetization dynamics.

Optical pump-probe detection of manganese hyperfine beats in (Cd,Mn)Te crystals.

Optical pump-probe experiments reveal spin beats of manganese ions in (Cd,Mn)Te, due to hyperfine and crystal fields. At "magic" orientations of the magnetic field, the effect of local crystal field is strongly suppressed. In this case, the spin precession of Mn(2+) embedded in the lattice approaches the precession expected for the free ion. Following optical excitation, regular spin pulses show up, revealing the one-to-one correspondence between precession frequency and Mn(2+) nuclear spin state. The period of the spin pulses accurately determines the hyperfine constant |A|=705 neV. The manganese spin coherence time up to T(2)(Mn)≃15 ns is measured for a manganese concentration x=0.0011.

Magneto-optical effects enhancement in DMS layers utilizing 1-D photonic crystal

We propose a theoretical design of a structure combining a diluted magnetic semiconductor (DMS) layer with one-dimensional photonic crystal and show that it allows for a sizable (a few-fold) enhancement of the magneto-optical Kerr effect (MOKE) magnitude. We base our calculations on the transfer matrix method assuming realistic, and optimal with respect to a sample growth, parameters. We investigate representative cases of (Ga, Mn)N or (Cd, Mn)Te DMS layers deposited on the top of a respectively GaN- or CdTe-based distributed Bragg reflector, and highlight the applicability of the proposed design for both: III–V wurtzite and II–VI zinc-blende DMS.

MBE Growth of CdTe/ZnTe Quantum Dots with Single Mn Ions

We report MBE growth and properties of samples with self assembled quantum dots with single manganese ions and low density of quantum dots. Manganese concentration was calibrated using magneto-re ectivity measurements and the giant Zeeman e ect in (Cd,Mn)Te and (Zn,Mn)Te layers. Successful incorporation of Mn in the CdTe/ZnTe quantum dots was con rmed using micro-photoluminescence measurements: single manganese ion in quantum dot manifests in sixfold splitting of exciton emission lines due to s, p d exchange interaction.

MBE Growth and Magnetooptical Properties of (Zn,Co)Te Layers

The (Zn,Co)Te is a one of diluted magnetic semiconductor (DMS) with particularly strong interaction of magnetic ions and free holes. The interaction can be described by p d exchange integral N0β = −3.03 ± 0.15 eV [1], so the absolute value is two or three times larger than for the other Co or Mn based DMSs (e.g. for (Cd,Mn)Te N0β = −0.88 eV [2]). High optical quality bulk (Zn,Co)Te samples studied in Ref. [1] were grown using the Bridgman method, with cobalt concentration between 0.05 and 1.06%. Other technique to introduce Co into ZnTe was thermal di usion [3], which allowed infrared spectroscopy of intraionic Co transitions [3, 4]. Main di culty in doping of the ZnTe crystals during growth is high evaporation temperatures of Co as compared with the evaporation temperatures of Zn and Te [3]. For molecular beam epitaxy (MBE) this di culty should be not as important as for other crystal growth techniques, but for our best knowledge, there was no reports on MBE (Zn,Co)Te. It would be very interesting to enhance magnetooptical e ects using higher Co concentrations and epitaxial nanostructures such as quantum wells, quantum dots, or distributed Bragg reectors. This is the reason why we equipped our new MBE machine (SVT model 26-V-3) in high temperature e usion cell.

High Curie Temperature Bi1.85Mn0.15Te3 Nanoplates

Bi(1.85)Mn(0.15)Te(3) hexagonal nanoplates with a width of ~200 nm and a thickness of ~20 nm were synthesized using a solvothermal method. According to the structural characterization and compositional analysis, the Mn(2+) and Mn(3+) ions were found to substitute Bi(3+) ions in the lattice. High-level Mn doping induces significant lattice distortion and decreases the crystal lattice by 1.07% in the a axis and 3.18% in the c axis. A high ferromagnetic state with a Curie temperature of ~45 K is observed in these nanoplates due to Mn(2+) and Mn(3+) ion doping, which is a significant progress in the field of electronics and spintronics.

Optically detected magnetic resonance of Mn-related excitations in (Cd,Mn)Te quantum wells

Optically detected magnetic resonance (ODMR) was applied to reveal the exchange interaction effects between Mn2+ ions and confined holes in (Cd,Mn)Te quantum wells with 2D hole gas. Two anisotropic ODMR signals with different angular variations were found and ascribed to isolated manganese ions and to exchange-coupled complexes consisting of manganese and holes. It is shown that calculations on the basis of spin Hamiltonian for these systems are in agreement with the experimental data.

Degradation of quantum dots under nuclear irradiation and its influence on exciton spectra in semimagnetic semiconductors

AbstractThe paper studies radiation‐induced modification of energetic spectra of excitons in CdTe quantum dots (QDs) in a (Cd, Mn)Te matrix. Exciton levels in such system are very sensitive to the changes in Mn concentration due to the exchange interaction between magnetic Mn ions and charge carriers (the giant splitting effect). Radiation‐enhanced diffusion causes Mn penetration into the QD that leads to the changes in the exciton level energy. We found the spatial distribution of Mn ions for different durations of the irradiation and performed quantum calculations of the energy of exciton states with the obtained Mn concentration profiles. The dependences of the position of the exciton levels and the splitting between the energy of exciton states with σ+ and σ– polarizations on the irradiation parameters and the QD's radius have been investigated. The calculations show that the irradiation causes a large shift of exciton levels. The splitting between levels with σ+ and σ– polarizations may be increased in the order of value due to the irradiation. The radiation effects are stronger for QD with smaller radius.

Spin-polarized electric currents in diluted magnetic semiconductor heterostructures induced by terahertz and microwave radiation

We report on the study of spin-polarized electric currents in diluted magnetic semiconductor (DMS) quantum wells subjected to an in-plane external magnetic field and illuminated by microwave or terahertz radiation. The effect is studied in (Cd,Mn)Te/(Cd,Mg)Te quantum-wells (QWs) and (In,Ga)As/InAlAs:Mn QWs belonging to the well-known II-VI and III-V DMS material systems, as well as in heterovalent AlSb/InAs/(Zn,Mn)Te QWs, which represent a promising combination of II-VI and III-V semiconductors. Experimental data and developed theory demonstrate that the photocurrent originates from a spin-dependent scattering of free carriers by static defects or phonons in the Drude absorption of radiation and subsequent relaxation of carriers. We show that in DMS structures, the efficiency of the current generation is drastically enhanced compared to nonmagnetic semiconductors. The enhancement is caused by the exchange interaction of carrier spins with localized spins of magnetic ions resulting, on the one hand, in the giant Zeeman spin splitting, and, on the other hand, in the spin-dependent carrier scattering by localized Mn${}^{2+}$ ions polarized by an external magnetic field.

Reprint of: Amorphous contact layers on (Cd,Mn)Te crystals

The communication describes our research on a technique of making electrical contacts to semi-insulating (Cd,Mn)Te crystals for radiation detector applications. Since the electrical contacts made by deposition of Au directly onto the (Cd,Mn)Te surface turned out to be unreproducible, and the contacts made by deposition of monocrystalline layers of heavily doped semiconductors were unpractical for applications, our investigations were focused on the amorphous contact layers. The amorphous layers of heavily doped ZnTe:Sb or CdTe:In were evaporated on the “epi-ready” surfaces of good-quality, high-resistivity (108–1010Ωcm) single crystals of (Cd,Mn)Te. The evaporation was carried out in the medium–high vacuum of about 10−7Torr. For electrical connections the amorphous contact layers were covered by an evaporated Au top layer. We investigated, the effect of thickness of the amorphous layer on the functioning of the contact. At the moment 1μm thickness seems to be satisfactory.

ChemInform Abstract: Ferromagnetism in Semiconductors and Oxides. Prospects from a Ten Years′ Perspective

AbstractReview: 109 refs.

Magnetic Proximity Effect as a Pathway to Spintronic Applications of Topological Insulators

Spin-based electronics in topological insulators (TIs) is favored by the long spin coherence(1,2) and consequently fault-tolerant information storage. Magnetically doped TIs are ferromagnetic up to 13 K,(3) well below any practical operating condition. Here we demonstrate that the long-range ferromagnetism at ambient temperature can be induced in Bi(2-x)Mn(x)Te(3) by the magnetic proximity effect through deposited Fe overlayer. This result opens a new path to interface-controlled ferromagnetism in TI-based spintronic devices.

Nearest‐neighbour antiferromagnetic interaction as a limiting factor for critical temperature in a model DMS system

AbstractIn numerous diluted magnetic semiconductor (DMS) systems, the competition takes place between the short‐range antiferromagnetic (AF) superexchange interactions and the long‐range Ruderman–Kittel–Kasuya–Yosida (RKKY) coupling mediated by the charge carriers. Such a situation strongly influences the critical temperature, the maximization of which constitutes a challenging task in DMS physics and technology. The aim of the paper is to discuss theoretically the limiting effect of AF interactions between nearest‐neighbour (NN) magnetic ions on the stability of inhomogeneous ferromagnetic state in a model diluted magnetic system reflecting some crucial features of DMS. The modified molecular field‐based model is constructed to account for the magnetic inhomogeneity. The behaviour of the system is studied as a function of the ratio of superexchange integral to effective ferromagnetic coupling integral, including the possibility of clustering/anticlustering tendency for the magnetic ions. The ground state of the system is analyzed. The critical temperature is found to change non‐monotonically with the concentration of magnetic ions and decrease severely for larger concentrations. The behaviour of the system significantly differs from the predictions of the usual homogeneous mean‐field model. Brief comparison with selected experimental results for (Zn,Mn)Te is provided.

Magnetic anisotropy of semiconductor (Ge,Mn)Te microstructures produced by laser and electron beam induced crystallization

AbstractFerromagnetic transition and magnetic anisotropy was studied by SQUID magnetometry and ferromagnetic resonance (FMR) methods in polycrystalline Ge1‐xMnxTe (x = 0.085) semiconductor microstructures embedded in an amorphous, insulating, and paramagnetic (Ge,Mn)Te matrix. The microstructures were produced by pulsed laser and electron beam induced local re‐crystallization of amorphous layers deposited on insulating BaF2 substrates. The angular dependence of the FMR resonance field observed below the Curie temperature TC = 70 K of the microstructures is quantitatively described by the analysis of Zeeman and demagnetization contributions to magnetic free energy. Good agreement with experimental results obtained in the case of structures produced by pulsed laser re‐crystallization indicates the formation of ferromagnetic (Ge,Mn)Te thin disks of submicron dimensions.

Contacts for High-Resistivity (Cd,Mn)Te Crystals

Semi-insulating (Cd,Mn)Te crystals offer a material that may compete well with the commonly used (Cd,Zn)Te crystals for manufacturing large-area X- and gamma-ray detectors <citerefgrp xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><citeref refid="ref1"/></citerefgrp> . The Bridgman growth method yields good quality, high-resistivity <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(10^{9}-10^{10}\ \Omega\cdot{\rm cm})$</tex></formula> crystals of (Cd,Mn)Te:V. Doping the as-grown crystals with the compensating agent vanadium <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\approx 10^{16}\ {\rm cm}^{-3})$</tex></formula> ensures their high resistivity; thereafter, annealing them in cadmium vapors reduces the number of cadmium vacancies. Applying the crystals as detectors necessitates having satisfactory electrical contacts. Accordingly, we explored various techniques of ensuring good electrical contacts to these semi-insulating (Cd,Mn)Te crystals, assessing metallic layers, monocrystalline semiconductor layers, and amorphous (or nanocrystalline) semiconductor layers. We found that ZnTe heavily doped <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\approx 10^{18}\ {\rm cm}^{-3})$</tex></formula> with Sb, and CdTe heavily doped <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(\approx 10^{17}\ {\rm cm}^{-3})$</tex></formula> with In, proved satisfactory semiconductor contact layers. They subsequently enabled us to establish good contacts (with only narrow tunneling barriers) to the Au layer that usually constitutes the most external contact layer. We outline our technology of applying electrical contacts to semi-insulating (Cd,Mn)Te, and describe some important properties.