W. Szuszkiewicz

Structural properties of cubic MnTe layers grown by MBE

Abstract We report on the growth of zinc-blende MnTe on (001) GaAs substrates by MBE. Layers with thickness up to 8 μm were grown. The growth was performed for various ratios of Mn and Te fluxes. For high values of the Te Mn flux ratio, Raman scattering spectra showed a presence of tellurium lines corresponding to crystalline tellurium. For low flux ratios the presence of Te precipitates was less evident. These results were confirmed by X-ray diffraction studies. The lattice parameter of cubic MnTe and its temperature dependence were determined by X-ray diffraction. From the analysis of the rocking curves, for various Bragg reflections, the fluctuations of the lattice parameter and mosaicity were separated out.

Secondary phase segregation in heavily transition metal implanted ZnO

With micro-Raman spectroscopy and x-ray diffraction (XRD), we studied ZnO crystals implanted with Mn, Fe, Co, and Ni, respectively, with implantation concentrations from 4 up to 16 at. %. Using thermal treatments in air up to 700 °C, we analyzed the annealing effect on the ZnO crystal lattice as well as the onset of secondary phases and their microstructure on the sample surface. While the 500 °C treatment induces a considerable annealing, secondary phases are observed for transition metal (TM) concentrations ≥8 at. % after the treatment at 700 °C. Their microstructure strongly depends on the TM species. Various stoichiometric and nonstoichiometric TM oxide precipitates as well as elemental TM clusters are identified by their Raman and XRD signatures and their possible magnetic impact is discussed.

Ferromagnetic GaMnAs/GaAs superlattices—MBE growth and magnetic properties

Abstract We have studied the magnetic properties of (GaMnAs) m (GaAs) n superlattices with magnetic GaMnAs layers of thickness between eight and 16 molecular layers (ML) (23–45 A), and with non-magnetic GaAs spacers from 4 to 10 ML (11–28 A). While previous reports state that GaMnAs layers thinner than 50 A are paramagnetic in the whole Mn composition range achievable using MBE growth (up to 8% Mn), we have found that short period superlattices exhibit a paramagnetic-to-ferromagnetic phase transition with a transition temperature which depends on both the thickness of the magnetic GaMnAs layer and the non-magnetic GaAs spacer. The neutron scattering experiments have shown that the magnetic layers in superlattices are ferromagnetically coupled for both thin (below 50 A) and thick (above 50 A) GaMnAs layers.

Zn1−xMnxTe Diluted Magnetic Semiconductor Nanowires Grown by Molecular Beam Epitaxy

It is shown that the growth of II-VI diluted magnetic semiconductor nanowires is possible by the catalytically enhanced molecular beam epitaxy (MBE). Zn(1-x)MnxTe NWs with manganese content up to x=0.60 were produced by this method. X-ray diffraction, Raman spectroscopy, and temperature dependent photoluminescence measurements confirm the incorporation of Mn(2+) ions in the cation substitutional sites of the ZnTe matrix of the NWs.

Band structure of β-HgS from Shubnikov–de Haas effect

Abstract The Shubnikov–de Haas oscillations were measured in zinc–blende Hg 0.98 TM 0.02 S (TM= Co, Mn, Fe) at temperatures ranging from 1.6 to 30 K, in magnetic fields up to 13 T. From standard harmonic analysis the values of the effective masses at the Fermi level versus electron concentration were determined. The analysis of this dependence within the frame of Kane model for narrow-band semiconductors yielded a low temperature value for energy gap E 0 =−0.110 eV ±0.040 eV and momentum matrix element P=7.1×10 −8 eV cm ±0.2×10 −8 eV cm .

All wurtzite (In,Ga)As-(Ga,Mn)As core-shell nanowires

Structural and magnetic properties of (In,Ga)As-(Ga,Mn)As core-shell nanowires grown by molecular beam epitaxy on GaAs(111)B substrate with gold catalyst have been investigated. (In,Ga)As core nanowires were grown at high temperature (500 °C) whereas (Ga,Mn)As shells were deposited on the {11̅00} side facets of the cores at much lower temperature (220 °C). High-resolution transmission electron microscopy images and high spectral resolution Raman scattering data show that both the cores and the shells of the nanowires have wurtzite crystalline structure. Scanning and transmission electron microscopy observations show smooth (Ga,Mn)As shells containing 5% of Mn epitaxially deposited on (In,Ga)As cores containing about 10% of In without any misfit dislocations at the core-shell interface. With the In content in the (In,Ga)As cores larger than 5% the (In,Ga)As lattice parameter is higher than that of (Ga,Mn)As and the shell is in the tensile strain state. Elaborated magnetic studies indicate the presence of ferromagnetic coupling in (Ga,Mn)As shells at the temperatures in excess of 33 K. This coupling is maintained only in separated mesoscopic volumes resulting in an overall superparamagnetic behavior which gets blocked below ∼ 17 K.

Structural and optical properties of self-catalytic GaAs:Mn nanowires grown by molecular beam epitaxy on silicon substrates

Mn-doped GaAs nanowires were grown in the self-catalytic growth mode on the oxidized Si(100) surface by molecular beam epitaxy and characterized by scanning and transmission electron microscopy, Raman scattering, photoluminescence, cathodoluminescence, and electron transport measurements. The transmission electron microscopy studies evidenced the substantial accumulation of Mn inside the catalyzing Ga droplets on the top of the nanowires. Optical and transport measurements revealed that the limit of the Mn content for self-catalysed growth of GaAs nanowires corresponds to the doping level, i.e., it is much lower than the Mn/Ga flux ratio (about 3%) used during the MBE growth. The resistivity measurements of individual nanowires confirmed that they are conductive, in accordance with the photoluminescence measurements which showed the presence of Mn(2+) acceptors located at Ga sites of the GaAs host lattice of the nanowires. An anomalous temperature dependence of the photoluminescence related to excitons was demonstrated for Mn-doped GaAs nanowires.

Peculiarities of the Lattice Dynamics of Cubic Mercury Chalcogenides

The lattice dynamics of cubic mercury chalcogenides (HgTe, HgSe, β-HgS) was investigated by IR reflectivity, Raman scattering and inelastic neutron scattering measurements. The important role of two-phonon Raman scattering related to TA excitations was confirmed for HgSe and β-HgS and explained by the particular TA dispersion for mercury compounds. Analysis of the optical spectra taken for HgSe demonstrates that the resonant two-phonon difference process (widely accepted for HgTe, HgSe and the mixed crystals grown on the basis of these compounds) is not able to explain the present experimental data.

Growth and Structural Characterization of Zinc Blende HgS

The zinc blende (metastable) modification of mercury sulphide was stabilized by the partial substitution of Hg atoms by Mn, Fe, and Co. The structure of such crystals grown by the modified Bridgman method was investigated by X-ray diffraction and electron transmission spectroscopy techniques. The single-phase character of the crystals was demonstrated, the presence of defects resulting from the disorder in the stacking sequence of (111) layers was evidenced.

Magnons in layered MnTe/CdTe structures

An experimental and theoretical analysis of spin-wave excitations (magnons) in ultra-thin films of a magnetic semiconductor has been performed. Cubic MnTe, which possesses a frustrated long-range antiferromagnetic order when deposited in thick layers. was the material selected for such studies. In the low-frequency part of the Raman spectra. a complex structure has been observed in crossed polarization - z(x, y)z at low temperature on (MnTe) k /(CdTe) m , multilayers (where m ≥ 8). This structure can be attributed to magnons in thin MnTe slabs.