E. Dynowska

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.

Photoluminescence study and structural characterization of p-type ZnO doped by N and/or As acceptors

ZnO doped with N and/or As layers was fabricated by thermal oxidation of ZnTe films grown by MBE on different substrates. Hall effect measurements demonstrated p-type conductivity with a hole concentration of ~5 × 1019 cm−3 for ZnO:As and ZnO:As:N on GaAs substrates and ~6 × 1017 cm−3 for ZnO:N on ZnTe substrates. The concentration of N and As atoms in ZnO is estimated to be ~1020 cm−3. This suggested that simple substitutional N atoms form acceptor impurities with a smaller efficiency than an As-related complex, probably AsZn–2VZn. In particular, we were able to distinguish between nitrogen and arsenic acceptor-related luminescence. Optical studies showed meaningful differences of the PL features in samples with different acceptors, grown on different substrates.

Amorphous state and pulsed laser deposition of YBa2Cu3O7−δ thin films

Pulsed laser deposition of high Tc compounds onto unheated substrates, resulting in amorphous thin films, preserves to a great extent the composition of matter ejected from the target. This composition is of primary interest, both for understanding the dynamics of laser–target interaction and for practical (optimization) reasons. We have investigated the structure of amorphous and crystalline YBaCuO films obtained both in on-axis and off-axis deposition geometries, and correlated the results with optical and transport properties of these films. X-ray scattering reveals in amorphous films the existence of: (i) amorphous continuum of spatially disordered atoms, (ii) small (10–40 A) amorphous clusters which can be considered as mesoscopic order fluctuations in the amorphous continuum, and (iii) slightly larger (50–250 A) crystalline clusters exhibiting quasi-two dimensional (00l) or (11l) long range order. Crystalline films are predominantly (00l) oriented. Optical spectra of both crystalline and amorphous f...

Fabrication and properties of YBa/sub 2/Cu/sub 3/O/sub 7//RE/sub 1-x/MnO/sub 3-y/ multilayers

The high pressure d.c. sputtering method has been used to fabricate (YBa/sub 2/Cu/sub 3/O/sub 7//Nd/sub 0.67/Sr/sub 0.33/MnO/sub 3-y/)//sub n/ [(YBCO/NSMO)/sub n/] superlattices. X-ray analyses show c-axis epitaxial growth on LaAlO/sub 3/ (100) substrates. Nd/sub 0.67/Sr/sub 0.33/MnO/sub 3-y/ films are strained due to some difference in lattice parameters between substrate and YBCO sublayers. Magnetization measurements done on single NSMO films show ferromagnetic order below 200 K. Magnetic moment measured for the superlattices below superconducting transition show ferromagnetic order and superconductivity. Such a behavior indicates on the co-existence of superconductivity and ferromagnetism in the total volume of heterostructures, where superconductivity is related to the YBCO sublayer and ferromagnetism to the NSMO sublayer.

ZnTe–ZnO core–shell radial heterostructures grown by the combination of molecular beam epitaxy and atomic layer deposition

ZnTe-ZnO core-shell radial heterostructures were grown using a new method of combining molecular beam epitaxy (MBE) and atomic layer deposition (ALD). Zinc telluride nanowires (core) were grown on a GaAs substrate using gold catalyzed vapor-liquid-solid mechanism. An atomic layer deposition technique using diethyl zinc and deionized water as precursors was applied for zinc oxide shell formation. The core-shell ZnTe-ZnO heterostructures thus obtained were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction and photoluminescence measurements.

Magnetism and magnetotransport of strongly disordered Zn1−xMnxGeAs2 semiconductor: The role of nanoscale magnetic clusters

We present systematic studies of magnetic and transport properties of Zn1−xMnxGeAs2 semimagnetic semiconductor with the chemical composition varying between 0.053≤x≤0.182. The transport characterization showed that all investigated samples had p-type conductivity strongly depending on the chemical composition of the alloy. The Hall effect measurements revealed carrier concentrations p≥1019 cm−3 and relatively low mobilities, μ≤15 cm2/(V s), also chemical composition dependent. The magnetic investigations showed the presence of paramagnet-ferromagnet phase transitions with transition temperatures greater than 300 K for the samples with x≥0.078. We prove by means of x-ray diffraction, nuclear magnetic resonance, and scanning electron microscopy techniques that the observed room temperature ferromagnetism is due to the presence of MnAs inclusions. The high field magnetoresistance showed the presence of giant magnetoresistance effect with maximum amplitudes around 50% due to the presence of nanosize ferromagn...

Ohmic Contact to n-GaN with TiN Diffusion Barrier

The formation of n-GaN/Ti ohmic contacts with TiN diffusion barriers has been investigated by electrical measurements, x-ray diffraction and SIMS. It has been shown that the onset of the ohmic behaviour is associated with the thermally induced phase transformation of Ti into TiN at the GaN/Ti interface. It is suggested that the process is accompanied by an increase in the doping level in the semiconductor subcontact region. The presence of a TiN barrier is found to inhibit excessive decomposition of GaN and to confine the reaction between n-GaN and Ti.

On the origin of stress-anneal-induced anisotropy in Finemet-type nanocrystalline magnets

Abstract It has recently been shown that the anisotropy in nanocrystalline Finemet -type magnets, induced by their crystallization under tensile stress, can originate from the magnetoelastic coupling within the crystallites (G. Herzer, IEEE Trans. Magn. 30 (1994) 4800) or can have its source in directional diatomic ordering which occurs also within the volume of nanocrystalline phase (Hofmann and Kronmuller, J. Magn. Magn. 152 (1996) 91). In the present work the temperature dependence of this anisotropy has been investigated for a series of initially amorphous Fe73.5Cu1Nb3Si15.5B7-ribbon samples (Vacuumschmelze, GmbH), nanocrystallized at various temperature-time-stress conditions. It has been assumed that the obtained results will allow to judge which of the mechanisms mentioned above governs in reality the considered anisotropy. An analysis of the results obtained shows that diatomic directional ordering within the volume of the nanocrystalline phase seems to be very likely the origin of the stress-anneal-induced anisotropy in FINEMET -type magnets. This analysis has been performed assuming that the anisotropy originating from the atomic ordering should scale with the square of saturation magnetization in the high-temperature range (in the vicinity of critical temperature-Curie point of nanocrystalline phase) and with the cube of this quantity at low temperatures. Quite satisfactory agreement has been obtained for the best fit of the experimental data to the polynomial consisting of the two above terms.

Giant Spin Splitting in Optically Active ZnMnTe/ZnMgTe Core/Shell Nanowires

An enhancement of the Zeeman splitting as a result of the incorporation of paramagnetic Mn ions in ZnMnTe/ZnMgTe core/shell nanowires is reported. The studied structures are grown by gold-catalyst assisted molecular beam epitaxy. The near band edge emission of these structures, conspicuously absent in the case of uncoated ZnMnTe nanowires, is activated by the presence of ZnMgTe coating. Giant Zeeman splitting of this emission is studied in ensembles of nanowires with various average Mn concentrations of the order of a few percent, as well as in individual nanowires. Thus, we show convincingly that a strong spin sp-d coupling is indeed present in these structures.