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

Advances in the accuracy and efficiency of first-principles electronic structure calculations have allowed detailed studies of the energetics of materials under high pressures. At the same time, improvements in the resolution of powder x-ray diffraction experiments and more sophisticated methods of data analysis have revealed the existence of many new and unexpected high-pressure phases. The most complete set of theoretical and experimental data obtained to date is for the group-IVA elements and the group-IIIA--VA and IIB--VIA compounds. Here the authors review the currently known structures and high-pressure behavior of these materials and the theoretical work that has been done on them. The capabilities of modern first-principles methods are illustrated by a full comparison with the experimental data.

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High-pressure phases of group-IV, III–V, and II–VI compounds

In this paper, we report a systematic investigation of band-edge photoluminescence for Cd1-xMnxTe crystals grown by the vertical Bridgman method. The near-band-edge emissions of neutral acceptor-bound excitons (labeled as L1) were systematically investigated as a function of temperature and of alloy composition. The parameters that describe the temperature variation of the energy were evaluated by the semiempirical Varshni relation. From the temperature dependence of the full width at half maximum of the L1 emission line, the broadening factors Γ(T) were determined from the fit to the data. The activation energies of thermal quenching were obtained for the L1 peak from the temperature dependence of the bound exciton peaks and were found to decrease with increasing Mn concentration.

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Photoluminescence characteristics of Cd1-xMnxTe single crystals grown by the vertical Bridgman method.

Structural, FTIR and photoluminescence studies of Cu doped ZnO nanopowders by co-precipitation method

If the medium surrounding a nano-grain does not support the vibrational wavenumbers of a material, the optical and acoustic phonons get confined within the grain of the nanostructured material. This leads to interesting changes in the vibrational spectrum of the nanostructured material as compared to that of the bulk. Absence of periodicity beyond the particle dimension relaxes the zone-centre optical phonon selection rule, causing the Raman spectrum to have contributions also from phonons away from the Brillouin-zone centre. Theoretical models and calculations suggest that the confinement results in asymmetric broadening and shift of the optical phonon Raman line, the magnitude of which depends on the widths of the corresponding phonon dispersion curves. This has been confirmed for zinc oxide nanoparticles. Microscopic lattice dynamical calculations of the phonon amplitude and Raman spectra using the bond-polarizability model suggest a power-law dependence of the peak-shift on the particle size. This article reviews recent results on the Raman spectroscopic investigations of optical phonon confinement in several nanocrystalline semiconductor and ceramic/dielectric materials, including those in selenium, cadmium sulphide, zinc oxide, thorium oxide, and nano-diamond. Resonance Raman scattering from confined optical phonons is also discussed. Copyright © 2007 John Wiley & Sons, Ltd.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jrs.1684

Raman spectroscopy of optical phonon confinement in nanostructured materials

Photoluminescence and reflectivity measurements have been carried out on ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se solid solutions in the complete range of crystal compositions, 0\ensuremath{\le}x\ensuremath{\le}0.55. The features of the photoluminescence and reflectivity data near the band edge enabled us to determine values of the fundamental band gap as a function of Mn molar fraction x and temperature (8 K\ensuremath{\le}T\ensuremath{\le}300 K). The energy gap in crystals with compositions in the vicinity of the zinc-blende--wurtzite structural transition (0.2\ensuremath{\lesssim}x\ensuremath{\lesssim}0.3) exhibited a scatter of values, probably associated with the presence of various polytypes (as evidenced by transmission-electron-microscopy measurements). The dependence of ${E}_{g}$ on x is anomalous for 0\ensuremath{\le}x\ensuremath{\le}0.2, showing a minimum in the ${E}_{g}$ vs x curve. A simple model (in terms of second-order perturbation theory in s-d and p-d interactions) relates this minimum to a maximum observed in the magnetic susceptibility as a function of x. For samples with x\ensuremath{\gtrsim}0.35, the ${E}_{g}$ dependence on T shows an onset of an additional blue shift as the temperature is lowered below \ensuremath{\sim}150 K. This onset, though not reproduced by our simple calculation, appears to be also related to the presence of s-d and p-d interactions.

Dependence of energy gap on x and T in Zn 1 − x Mn x Se: The role of exchange interaction

The electrical resistivity, electron concentration, and mobility of ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Fe}}_{\mathrm{x}}$Se are reported for 4.2lTl300 K and for 0.0001lxl0.12. The data are interpreted within an electronic band-structure model that assumes the existence of resonant donors (due to the presence of Fe ions) whose ground-state energy coincides with the conduction-band continuum. The electron-concentration data enable determination of the donor energy as a function of the temperature and crystal composition. Unexpectedly high values of the low-temperature electron mobility for \ensuremath{\sim}0.0003\ensuremath{\le}x\ensuremath{\le}0.01 indicate a reduction of the charged-impurity scattering rate at these temperatures. This effect is interpreted in terms of a spatial ordering of charges within the Fe-related impurity system.

Reduction of charge-center scattering rate in Hg 1-x Fe x Se

We report the results of a spin-flip Raman scattering study of electrons bound to shallow donors in the diluted magnetic semiconductors ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te and ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se for x\ensuremath{\lesssim}0.30 and in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$S for x=0.022 and 0.125. Spin-flip Raman scattering is observed at temperatures ranging from 1.8 K to as high as 160 K and at magnetic fields \ensuremath{\le}60 kG. The measured spin-flip energies are large and, for some samples, correspond to g values in excess of 100 at low temperatures. The enhancement of the Raman shifts results from the exchange coupling of the donor electron with the ${\mathrm{Mn}}^{2+}$ ions. The parameters of the mean-field approximation, used to describe the magnetic field and temperature dependence of g are shown to exhibit a temperature dependence for temperatures above 20 K. Finite Raman shifts are observed in the absence of a magnetic field and are attributed to the bound magnetic polaron and thermal fluctuations of the local magnetization. The temperature and magnetic field dependences of the spin-flip energies, Raman intensities, and polarizations were compared with the results of the statistical-mechanical model of Dietl and Spal/ek [Phys. Rev. B 28, 1548 (1983)]. The theory is in good agreement with the experimental results for x\ensuremath{\lesssim}0.15.

Spin-flip Raman scattering in n -type diluted magnetic semiconductors

Optical phonons of the mixed crystal ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se (0\ensuremath{\le}x\ensuremath{\le}0.33) in the zinc-blende phase are investigated by Raman scattering. The optical phonons of ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se exhibit behavior intermediate between the two-mode and one-mode situations. The LO phonon of ZnSe continuously evolves into that of the ``hypothetical'' zinc-blende MnSe whereas the TO phonon and one component of the impurity mode merge into the gap mode of MnSe. The other component of the impurity mode transforms into the TO phonon of MnSe. Under hydrostatic pressure, in contrast to pure ZnSe, the mixed crystals undergo a high-pressure phase transition between 28 and 52 kbar depending on the composition. A new Raman peak appears in the high-pressure phase, which shows marked softening. When the pressure is reduced, the reverse transition occurs at a significantly lower pressure. At pressures below the reverse transition, the mixed crystal appears to be a mixture of the zinc-blende and another phase whose structure is closely related to that of the high-pressure phase. The energy of the ${\mathrm{Mn}}^{2+}$ internal transition shows a negative pressure coefficient.

Raman-scattering study of the high-pressure phase transition in Zn1-xMnxSe.

We have measured ultrasonic transit times to determine the elastic constants of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te crystals with 0\ensuremath{\le}x\ensuremath{\le}0.52 and of ${\mathrm{Cd}}_{0.52}$${\mathrm{Zn}}_{0.48}$Te at 296 K using hydrostatic pressures up to 4 kbar. It is found that Mn, but not Zn, weakens the zinc-blende crystal structure and makes it less stable under pressure. Applying a modified Born criterion to our data, we deduce the pressure expected to cause the transition to the rocksalt structure in each compound. The influence of Mn we attribute to Mn 3d orbitals hybridizing into the tetrahedral bonds.

U. Debska

Papers

Dependence of energy gap on x and T in Zn 1 − x Mn x Se: The role of exchange interaction

Reduction of charge-center scattering rate in Hg 1-x Fe x Se

Spin-flip Raman scattering in n -type diluted magnetic semiconductors

Raman-scattering study of the high-pressure phase transition in Zn1-xMnxSe.

Elastic constants and their pressure dependences in Cd1-xMnxTe with 0 <= x <= 0.52 and in Cd0.52Zn0.48Te.