A semimagnetic semiconductor with rare earth like magnetic ion is studied: Pb1-xGdxTe. Very high Hall mobilities were found up to 107 cm2/V.s. The magnetization is well fitted at 4.2K by a Curie-Weisslaw, with a Gaj's parameter of 1K. The value of JNN' the exchange in a Gd pair deduced from the susceptibility measurements is small, as expected, due to the specific character of Gadolinium (S state and 4f shell): JNN/k = - O.3°K for x = 0.025.

Gd2Y1-xN is an instantiation of the broad series of diluted magnetic materials composed of lanthanide nitrides in a Sc, Y or La nitride host. These lanthanoid nitrides are interstitial compounds which all form in the Bi structure and with similar lattice constants, so solid solutions are readily obtained. We have prepared single-crystal thin films by epitaxial growth on sapphire substrates, and obtained material with moderate carrier densities (n Hall∼10 21 cm-3) and mobilities (μHall∼10 cm2V-1sec-1). Below the ordering temperature Tc,∼15 K the GdxY1-x,N films (0.25<x<0.40) exhibit a ferromagnetic-like M-H loop. The saturation magnetization measured at 15 kOe and 4.2 K is anomalously low, representing an effective moment of only 4.2 μB per Gd compared to the expected value of at least 7.2 μB. The piecewise-linear M-H curve is inconsistent with conventional models for a simple anisotropic ferromagnet. These features illustrate some of the unusual properties we are beginning to identify in the behavior of these new materials.

Data are presented yielding evidence on the long-range character of magnetic interactions in Diluted Magnetic Semiconductors. The radial dependence of these interactions is deduced from the concentration dependence of the freezing temperature. It is shown that calculations of the thermodynamic properties, which include this long-range interaction, are in fair agreement with the experimental data.

Several experimental methods: absorption, photoemission and transport measurements were used to determine the energy position of substitutional Fe2+ (3d6) donor state in the band structure of the semimagnetic semiconductor Hg1-v-xCdvFexSe for 0≤v≤0.7 and v+x=l, and x≤0.15. For v≤0.40, Fe2+(3d6) level is a resonant donor located in the conduction band. For v=O (HgSe) we obtain 230 meV for the position of Fe2+(3d6) level with respect to the bottom of the conduction band which coincides with the position of the Fermi level for electron concentration N ≅5x1018 cm-3. Surprisingly, the mobility of free electrons (T∼4.2K) is abnormally high and the Dingle temperature measured in quantum magnetoresistivity oscillations (SdH effect) and magnetooptical measurements is abnormally low. Because of the Coulomb interaction between the ionized donors, at low T, there will appear some correlation of their positions. This may lead to a kind of “liquefying” of the system of ions and to its “crystallisation” (i.e. formation of a superlattice or hyperlattice of ionized donors) at even lower T. The space-ordering of ionized donors influences dramatically the free-carrier scattering and correspondingly explains the high mobility and low Dingle temperature. Finally, we shall also present some magnetic properties of these new semimagnetic materials.

The electrical resistivity, electron concentration, and mobility of Hg1-xFexSe are reported for 4.2K < T < 300K and for 0.0001 < x < 0.12. 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 value of the donor energy as a function of the temperature and the crystal composition. The low temperature electron mobility for ∼ 0.0003 ≤ x ≤ 0.01 is considerably higher than expected and indicates a reduction of the charged impurity scattering effects at low temperatures.

A number of spectroscopic techniques that exploit synchrotron radiation are providing new insight into the properties of ternary semimagnetic semiconducting alloys. Photon energy-dependent photoemission methods are being used to characterize the electronic structure and the elemental and orbital contributions to the valence states. Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) is providing a detailed understanding of the atomic structure and of the local phenomena that make the virtual crystal approximation inadequate. The new experimental information is stimulating novel theoretical work on the electronic structure, lattice stability and thermodynamics of these materials. We will review the results of a number of recent experimental studies and emphasize systematic trends and promising new areas of research.

Recent calculations of the electronic structure and magnetic interactions in Mn - alloyed II-VI diluted magnetic semiconductors (DMS) are summarized. Detailed band structure results are obtained using an empirical tight-binding, coherent potential approximation approach with input from experiment and local spin density band calculations. The dominant magnetic interactions in these systems result from hybridization between spin-split Mn d states and sp valence bands. Superexchange between Mn moments is well described by a simple three-level model which yields accurate Mn - Mn exchange constants for a variety of II-VI DMS as well as the rocksalt insulators MnO and α-MnS.

We have performed spin-polarized, self-consistent local spin density total energy and band structure calculations for the prototype semimagnetic semiconductor alloy Cd1-xMnxTe. Based on the calculated band structures and taking into account the many body effects of localized states, we propose a schematic energy level diagram to interpret the d→d*, p→d, and photoemission transitions in Cd1-xMnxTe.

Gaussian-based Self Consistent Field (SCF) MOs are obtained at various nuclear geometries for the MnS4 cluster in the external potential of cubic ZnS. Electronic relaxation and d-shell electron correlation effects are then included. For Td symmetry, quartet d-d excitation energies calculated as functions of R(Mn-S) qualitatively resemble the simple CF diagram. While separations between successive quartet levels agree closely with experiment, the threshold is about 0.5 eV high; much of this discrepancy is removed by including additional correlation effects from charge-transfer states. Large Jahn-Teller (JT) splittings of 4T1 and 4T2 levels are found with D2d distortion. Difficulty in accurately evaluating force constants interferes with predicting the corresponding deformations and stabilization energies. Estimates, encorporating the observed Stokes'; shift and JT stabilization energy, are that R(Mn-S) decreases by ∼0.1 Å and the S-Mn-S dihedral angle increases by 7sim;8°, in the lowest quartet level.

A large magnetoresistance was observed in n-type Cdl-xMnxSe (x ≤ 0.10) at low temperatures. The concomitant changes in the Half coefficient as a function of H were also studied. These magnetotransport effects are caused by the s-d interaction. Some specific magnetoresistance mechanisms are discussed. The most likely dominant mechanisms are related to the giant spin splitting of the conduction band.

DC- and FIR-magneto-transport measurements on Zn(1−x)Mn(x)Se for x<0.05 are interpreted in terms of nonresonant spin-flip hopping and resonant spin-flip conductivity. In combination with magnetization data the exchange-matrix element of the conduction band is determined to α = 0.32 eV.

We have calculated the effect of strains on the valence band offset and bandgap in the CdTe/(Cd,Mn)Te superlattice system for various growth directions and alloy compositions. We find that strain can modify the effect of alloy fluctuations on band offset and bandgap. The consequences of this for the formation of interface states will be discussed.

EXAFS has been used to determine bond lengths in the diluted magnetic semiconductor Zn1−xMnxSe. These alloys change from a cubic (zincblende) crystal structure to a hexagonal (wurtzite) structure as a function of composition, with the transition at x ≈ 0.3. Although x-ray diffraction measurements show the lattice parameter to change by approximately 0.2Å through the series and the crystal structure changes from cubic to hexagonal, the nearestneighbor bond distances show no change within the experimental uncertainty of 0.0Å.

The dilute magnetic semiconductor CdMnTe has been synthesized by ion implantation of Mn into CdTe. Samples of CdTe have been implanted with Mn ions of 60 keV energy to fluences in the range 1 × 1013 cm−2 to 2×1016 cm−2 and subsequently annealed, using rapid thermal annealing, for 10–15 sec at temperatures 300 ≤ TA ≤ 730°C. The successful formation of a near surface layer of CdMnTe is demonstrated by studies of the structural, electronic and magnetic properties of the ion implanted and annealed sampies; Rutherford backscattering-channeling analysis of the radiation-induced damage indicates complete recovery of lattice order after annealing at 700°C. Photoluminescence measurements, performed at 2K, reveal an increase in the energy band gap of the ion implanted alloy with respect to CdTe. Application of magnetic fields, up to 8T, produce both the characteristic energy shift of the excitonic recombination peak and polarization of the emitted radiation (in the Faraday configuration) previously observed in bulk–grown CdMnTe material.

The structural disorder and annealing characteristics of dilute magnetic semiconductors synthesized by implantation of Mn ions into CdTe are studied. After implantation but prior to annealing, the implanted Mn ions reside at random sites in a highly disordered CdTe lattice. To separate magnetic and nonmagnetic effects we have implanted CdTe with nonmagnetic Zn ions under the same conditions as the Mn ions (energy 60 keV, fluence 1016 cm−2). Various annealing conditions are applied to monitor the decrease in intensity of the disorder-induced photoluminescence peaks and the increase of the peak intensity of the CdTe-like exciton line is reduced.

The successful thin film growth of diluted magnetic semiconductors (DMS) by molecular beam epitaxy has “nucleated” a new field of research in which the DMS material is incorporated in a variety of novel superlattice and quantum well structures. The observation of reflection high energy electron diffraction intensity oscillations in ZnSe and MnSe has enabled the fabrication of ultrathin layered structures involving the “hypothetical” zincblende magnetic semiconductor MnSe. The expected antiferromagnetic ordering of MnSe is increasingly inhibited as the MnSe layer thickness is reduced from ten monolayers to the quasi-2D limit of one monolayer. Further developments include new observations of the epitaxial growth and nucleation of ZnSe, utilizing GaAs epilayers as the substrate material.

Superlattices containing alternating layers of Hg1-xMnx Te and HgTe have been successfully grown by molecular beam epitaxy. These structures are the first superlattices containing layers of a mercury-based dilute magnetic semiconductor to be grown by any thin film technique. The optical and electrical properties of these novel magnetic multilayers are presented and discussed.

Hg-based superlattices (SL)-e.g., the HgTe/CdTe SL–exhibit interesting and important physical properties. Among these are the tunability of the band gap, achieved by varying the thicknesses of the two constituent materials; and the existence of the vajeqnce band offset, on which the details of the valence bands of the SL largely depen. Recently a new type of Hg-based SL, the Hg1-xMnxTe/HgTe SL, has been successfully prepared. In addition to the ability of varying its band gap by controlling the Mn content, this SL has the advantage that some of its important parameters, including the band offset, can be tuned by an external magnetic field because of the spin-spin exchange interaction in the Hg1-xMnxTe layers. So far, the electronic and optical properties of this type of SL have not been explored.

HgMnCdTe/CdTe photodiodes with responsivity cutoffs of up to 1.54 pm have been fabricated by liquid phase epitaxy (LPE). The mesa device structure consists of a boron-implanted mosaic fabricated on a p-type Hg1−x−yMnxCdyTe layer grown on a CdTe substrate. A reverse breakdown voltage (VB) of 50 V and a leakage current density of 1.5 × 10−4 A/cm2 at V = −10 V was measured at room temperature (295K). A 0.75 pF capacitance was also measured under a 5 V reverse bias at room temperature. This device performance based on the quaternary HgMnCdTe shows both theoretical and practical promise of superior performance for wavelengths in the range 1.3 to 1.8 μm for fiber optic applications.

Using absorption and photoluminescence investigations the energies of shallow acceptors are determined as follows: 0.11 eV for P; 0.12 eV for As; 0.17 eV for Cu and 0.18 eV for Au. Deep level energies are found as 0.28 eV for P; 0.29 eV for As; 0.41 eV and 0.92 eV for Cu; and 0.33 eV, 0.69 eV and 1.25 eV for Au dopants. Acceptor concentrations in the 1017cm−3 range are achieved for As and P dopants, but for Au and Cu high compensation is found.