We review the physical properties of diluted magnetic semiconductors (DMS) of the type AII1−xMnxBVI (e.g., Cd1−xMnxSe, Hg1−xMnxTe). Crystallographic properties are discussed first, with emphasis on the common structural features which these materials have as a result of tetrahedral bonding. We then describe the band structure of the AII1−xMnxBVI alloys in the absence of an external magnetic field, stressing the close relationship of the sp electron bands in these materials to the band structure of the nonmagnetic AIIBVI ‘‘parent’’ semiconductors. In addition, the characteristics of the narrow (nearly localized) band arising from the half‐filled Mn 3d5 shells are described, along with their profound effect on the optical properties of DMS. We then describe our present understanding of the magnetic properties of the AII1−xMnxBVI alloys. In particular, we discuss the mechanism of the Mn++‐Mn++ exchange, which underlies the magnetism of these materials; we present an analytic formulation for the magnetic susc...

Diluted magnetic semiconductors

This review compiles results of experimental and theoretical studies on thin films and quantum structures of semiconductors with randomly distributed Mn ions, which exhibit spintronic functionalities associated with collective ferromagnetic spin ordering. Properties of p-type Mn-containing III-V as well as II-VI, IV-VI, V-2 -VI3, I-II-V, and elemental group IV semiconductors are described, paying particular attention to the most thoroughly investigated system (Ga, Mn)As that supports the hole-mediated ferromagnetic order up to 190 K for the net concentration of Mn spins below 10%. Multilayer structures showing efficient spin injection and spin-related magnetotransport properties as well as enabling magnetization manipulation by strain, light, electric fields, and spin currents are presented together with their impact on metal spintronics. The challenging interplay between magnetic and electronic properties in topologically trivial and nontrivial systems is described, emphasizing the entangled roles of disorder and correlation at the carrier localization boundary. Finally, the case of dilute magnetic insulators is considered, such as (Ga, Mn)N, where low-temperature spin ordering is driven by short-ranged superexchange that is ferromagnetic for certain charge states of magnetic impurities.

Dilute ferromagnetic semiconductors: Physics and spintronic structures

Polarized detection has been brought into operation for optics applications in the visible band. Meanwhile, an advanced requirement in short-wave near-infrared (SW-NIR) (700–1100 nm) is proposed. Typical IV–VI chalcogenides—2D GeSe with anisotropic layered orthorhombic structure and narrow 1.1–1.2 eV band gap—potentially meets the demand. Here we report the unusual angle dependences of Raman spectra on high-quality GeSe crystals. The polarization-resolved absorption spectra (400–950 nm) and polarization-sensitive photodetectors (532, 638, and 808 nm) both exhibited well-reproducible cycles, distinct anisotropic features, and typical absorption ratios αy/αx ≈ 1.09 at 532 nm, 1.26 at 638 nm, and 3.02 at 808 nm (the dichroic ratio Ipy/Ipx ≈ 1.09 at 532 nm, 1.44 at 638 nm, 2.16 at 808 nm). Obviously, the polarized measurement for GeSe showed superior anisotropic response at around 808 nm within the SW-NIR band. Besides, the two testing methods have demonstrated the superior reliability for each other. For the...

Short-Wave Near-Infrared Linear Dichroism of Two-Dimensional Germanium Selenide

Optical Properties of (Zn, Mn) and (Cd, Mn) Chalcogenide Mixed Crystals and Superlattices

Improving the thermoelectric efficiency is one of the greatest challenges in materials science. The recent discovery of excellent thermoelectric performance in simple orthorhombic SnSe crystal offers new promise in this prospect [Zhao et al. Nature 508, 373 (2014)]. By calculating the thermoelectric properties of orthorhombic IV-VI compounds GeS,GeSe,SnS, and SnSe based on the first-principles combined with the Boltzmann transport theory, we show that the Seebeck coefficient, electrical conductivity, and thermal conductivity of orthorhombic SnSe are in agreement with the recent experiment. Importantly, GeS, GeSe, and SnS exhibit comparative thermoelectric performance compared to SnSe. Especially, the Seebeck coefficients of GeS, GeSe, and SnS are even larger than that of SnSe under the studied carrier concentration and temperature region. We also use the Cahill's model to estimate the lattice thermal conductivities at the room temperature. The large Seebeck coefficients, high power factors, and low thermal conductivities make these four orthorhombic IV-VI compounds promising candidates for high-efficient thermoelectric materials.

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High-efficient thermoelectric materials: The case of orthorhombic IV-VI compounds

The electronic structure of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te (0\ensuremath{\le}x\ensuremath{\le}0.65) has been investigated by photoemission in the photon energy range from 20 to 140 eV. A sharp (\ensuremath{\approxeq}1 eV full width at half maximum) peak located 3.4 eV below the valence-band maximum (VBM) is assigned to emission from Mn 3${d}_{\ensuremath{\uparrow}}$ states with e symmetry. The ${t}_{2}$ components hybridize significantly with the Te 5p states and contribute therefore nearly uniformly to the top 6 eV of the valence bands. Weak structures below 6 eV of mixed Mn 3d--Te 5p character occur also due to the p-d hybridization. Cd and Te 4d core-level binding energies remain constant over the whole range of Mn concentrations when measured relative to the VBM. This implies that there are negligible chemical shifts and that the VBM is not affected by the replacement of Cd by Mn. The increase in the optical gap with x is thus due to an increase of the conduction-band energy, in agreement with a shift in the Te 4d absorption threshold as measured by partial-yield spectroscopy. A maximum of the Te 5p component in the density of empty conduction states is identified \ensuremath{\approxeq}2 eV above threshold. The Mn 3p\ensuremath{\rightarrow}3${d}_{\ensuremath{\downarrow}}$ excitations are atomiclike. The results are interpreted in terms of a schematic linear-combination-of-atomic-orbitals level scheme for the band structure of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te.

Synchrotron radiation study of Cd1-xMnxTe (0 <= x <= 0.65).

Photoelectron partial-yield and constant-initial-state (CIS) spectra of the layered-semiconductor GeS, GeSe, ${\mathrm{SnS}}_{0.9}$${\mathrm{Se}}_{0.1}$, and SnSe single crystals were investigated with use of synchrotron radiation. The cation d core (Ge 3d and Sn 4d) yield spectra show intense and sharp doublets due to cation-derived core excitons. The CIS spectra exhibit strong resonance due to an interference between the direct-recombination process of the core excitons and the direct-excitation process of the valence electrons. The strong resonance is characteristic of core excitons with a long lifetime and a fairly localized nature. The anion p and d core (S 2p and Se 3d) yield spectra are much broader than the cation d core spectra and reflect the density of states (DOS) of the conduction bands. Energies of the experimental DOS peaks compare favorably with conduction-band features derived from interband optical-absorption spectra.

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Core excitons and conduction-band structures in orthorhombic GeS, GeSe, SnS, and SnSe single crystals.

We have studied the contribution of Mn 3d states to the valence bands of ${\mathrm{Cd}}_{0.35}$${\mathrm{Mn}}_{0.65}$Te by means of the resonant enhancement of the Mn 3d photoionization cross section near the Mn 3${p}^{5}$3${d}^{6}$${(\mathrm{}}^{6}$P) core excitation (\ensuremath{\Elzxh}\ensuremath{\omega}=50 eV). The cross section of selected valence-band areas has a characteristic resonance shape as a function of photon energy that is well described by a Fano-type profile. From the strength of the resonance we conclude that there is appreciable hybridization with Mn 3d states throughout the valence bands. In addition, a new interpretation of the photoemission intensity with strong Mn 3d character that lies in the ionic gap of the CdTe valence bands is given. Based on an analysis of the final-state structure in terms of a configuration-interaction calculation performed on a ${\mathrm{MnTe}}_{4}$${\mathrm{}}^{6\mathrm{\ensuremath{-}}}$ cluster, we ascribe these features between 5 and 9 eV binding energy to ${d}^{4}$ final states (satellites), whereas the photoemission between 0 and 5 eV is due to Mn 3d multiplets where the ${d}^{4}$ final state is screened by charge transfer from the Te-derived valence-band states.

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M. Taniguchi

Papers

Synchrotron radiation study of Cd1-xMnxTe (0 <= x <= 0.65).

Core excitons and conduction-band structures in orthorhombic GeS, GeSe, SnS, and SnSe single crystals.

Manganese-derived partial density of states in Cd 1 − x Mn x Te