We report on injection of optically created spin-polarized carriers into CdTe-based materials. The injected spins are initially aligned in a diluted magnetic semiconductor CdMnTe layer located on the top of CdMgTe layer in CdMnTe/CdMgTe spintronic generic model structures. A critical discussion of possible artifacts that may complicate the spin detection and its quantitative analysis is given. Although the spin injection efficiency, 80%, has been found by us to be basically independent of the thickness of the spin detecting layer, there is an essential difference between thin and wide detectors related to the strain-induced lifting of the valence band degeneracy in the former, when assessing the efficiency of the spin injection. Most importantly, we observe an effect of switching the spin injection process on and off by an external magnetic field variation within a relatively narrow field range. This effect can be achieved by a careful design of the interface between the diluted magnetic semiconductor and the non-magnetic semiconductor.
Coupled low dimensional structures have potential applications in quantum computing and spintronics. Using molecular beam epitaxy we fabricated three kinds of systems of coupled quantum wells and quantum dots with different energy order: wells at higher energy than dots, resonant structures, and dots at higher energy than wells. By analysis of photoluminescence and reflectivity spectra, we conclude that there is a possibility of effective carrier tunneling between structures, which opens possibility of subsequent testing of spin transfer efficiency.
The valence band structure E(k) for CdTe (111) Cd side surface 2 x 2 reconstructed was investigated along the Γ-L direction of the bulk Brillouin zone by high-resolution angle-resolved photoemission spectroscopy method in the energy range between 9.5 eV and 30.0 eV. The E(k) dependence was determined as well for bulk as for some of the surface states in the whole width of the valence band. Obtained results are compared with available calculated bulk band structure along Γ-L direction.
Halogen transport method was applied to grow the crystals of solid solutions of ZnSe and transition metals at the temperature far below the melting point and phase transition temperature. The large crystals of ZnMnSe, ZnFeSe, ZnNiSe and ZnFeSSe were obtained. The technological parameters and shape of the quartz reactor were chosen for growth of a large crystal by self-nucleation; the transparent quartz furnace enabled the control of nucleation by visual observation. The parameters of crystal growth were determined. The crystal quality was estimated by X-ray diffraction method. The composition of crystals was determined by electron microprobe analysis and energy dispersive X-ray fluorescence analysis.
We report on epitaxial growth of diluted magnetic semiconductor (Zn,Co)Te. Reflectivity spectra reveal excitonic transition which split under magnetic field due to giant Zeeman effect. Magnetooptical effects can be described using literature data.
Collective excitations have been analyzed for Cd_{1-x}Mn_{x}Te epilayers (0.66 ≤ x ≤ 1) by the Raman scattering studies performed at temperatures from 7 K to 295 K. Apart from the lattice optical modes magnetic excitations (magnons) were observed at sufficiently low temperatures.
In this work we present the band gap engineering, epitaxial growth and optical characterization of CdSe/Cd_{0.9}Mg_{0.1}Se and Cd_{0.9}Mg_{0.1}Se/Cd_{0.85}Mg_{0.15}Se quantum wells with a thickness ranging from 1 to 15 nm. These structures exhibit strong near-band-gap photoluminescence from helium up to room temperature. The emission energy is tuned in the range from 1.74 to 2.1 eV at 7 K, depending on the thickness and well composition. The most intense photoluminescence (both at 7 and 300 K) was observed for 10 nm thick CdSe/Cd_{0.9}Mg_{0.1}Se wells. Such a structure gives also a sharp emission line (FWHM = 20 meV) at low temperature. The presented quantum wells are well suited for being embedded in lattice matched ZnTe based microcavities.
Resonant photoemission spectroscopy was applied to determine the Mn 3d derived contribution to the valence band density of states of Mn_{0.44} Mg_{0.56}Te grown by molecular beam epitaxy on a GaAs(001) substrate. The modifications of the valence band density-of-states distribution are discussed as a consequence of the substitution of Mg ions for Mn ions.
We report on growth by molecular beam epitaxy of cubic MnTe(111) layers on BaF_{2} (111) substrates. Layers as thick as 0.2-1.0 μm were grown. Basic characterization by X-ray diffraction shows that the cubic crystal structure is deformed to orthorhombic symmetry.
In this paper we discuss a possibility of an optical characterization of thin semiconductor epilayers by Raman scattering measurements. As an example zinc blende Cd_{1-x}Mn_{x}Te epilayers (0.66 ≤ x ≤ 1.0) have been grown by molecular beam epitaxy method and investigated by Raman scattering and X-ray diffraction. Information resulting from both methods is compared and discussed.
Temperature dependence of the magnon frequency was studied for cubic MnTe epilayers by the Raman scattering measurements. Experimental data are compared to the results of theoretical calculations performed within the framework of the Heisenberg model using Green's function formalism.
We show that post growth annealing of GaMnAs under As capping at temperatures in the range of 180-210ºC leads to significant surface modifications. Depending on GaMnAs layer thickness and composition, we obtain either a smooth continuous reacted (MnAs) surface layer or 3D islands (quantum dots). The surface modifications are due to a solid phase epitaxial process, in which Mn interstitials diffusing to the GaMnAs surface are bound with the As.
We report on growth by molecular beam epitaxy of thick layers of MnTe with zinc blende structure. Films as thick as 5.6 µm were obtained. Characterization by X-ray diffraction proved their good structural quality. We determined the lattice constant and its temperature dependence. Broad luminescence due to internal Mn^{2+}- transitions was observed. It showed an unexpected temperature dependence.
Second harmonic generation, created by nanosecond Nd : YAG laser pulses at 1.064 µm with relatively low intensity in Cd_{1-x}Mn_{x}Te bulk crystals and thin layers was measured in transmission geometry. The effect practically occurs in a very thin surface layer of the material and it is used as a relatively straightforward method of layer quality characterization. It is shown that the angular dependence of the second harmonic generation intensity in thin layers of CdTe with good crystallographic (and optical) quality agrees very well with the theory in contrary to the samples with some distortions from the ideal structure which exhibits large distortions from the theory.
The photoluminescence studies in CdTe/CdMnTe quantum wells are reported in the temperature range 10-300 K. The MnTe concentration in the barriers is x = 0.3, 0.5, 0.63 and 0.68. Thus the potential wells in our samples are very deep, of the order of ≈ 800 meV in the conduction band and ≈ 200 meV in the valence band in the case of the x = 0.68 sample. In spite of the large lattice mismatch (related to high x value) between the wells and the barriers the observed line widths are as narrow as 2 meV in the case of 100 Å. Clear manifestations of internal strain are observed. In particular, the temperature coefficient of the luminescence energies shows strong dependence on the width of wells.
The temperature dependence of the energy gap of MBE grown Cd_{1-x}Mn_{x}Te (0.6 < x ≤ 1.0) was measured for 2 K ≤ T ≤ 200 K and B ≤ 5 T. The results are interpreted in the frames of the model predicting that the exchange contribution to the band edge shift is proportional to the product of the magnetic susceptibility and the temperature.
We report MBE growth and properties of samples with self assembled quantum dots with single manganese ions and low density of quantum dots. Manganese concentration was calibrated using magneto-reflectivity measurements and the giant Zeeman effect in (Cd,Mn)Te and (Zn,Mn)Te layers. Successful incorporation of Mn in the CdTe/ZnTe quantum dots was confirmed using micro-photoluminescence measurements: single manganese ion in quantum dot manifests in sixfold splitting of exciton emission lines due to s, p-d exchange interaction.
One-magnon excitations in MBE-grown A_{1-x}Mn_{x}Te layers (where A = Cd, Zn, Mg and x>0.7) were investigated by means of the Raman scattering measurements at low temperatures (≈20 K). The composition dependence of the anisotropy energy - as extracted from these measurements - is discussed. Further, the elastic neutron scattering measurements were performed in layers of cubic MnTe, which constitute the end point material of the ternary alloys series. Abundance of variously oriented antiferromagnetic domains in MnTe layers as a function of temperature was studied. We confirm occurrence of a pronounced magnetostriction effect.
In this work we studied the influence of an external electric voltage on spatial dimensions of CdZnTe mixed crystals. In order to get an absolute magnitude of the sample thickness and to gain insight to the changes of lateral dimension, in quasi-bulk 3 μm thick CdZnTe layers grown by molecular beam epitaxy square craters were formed by ion sputtering in a secondary ion mass spectrometer. The vertical and lateral dimensions of the craters were studied by the atomic force microscopy. The atomic force microscopy measurement revealed that the thickness of the CdZnTe layer increases in a result of applying a single voltage pulse to the sample surface and decreases reversibly after applying reversely biased voltage. The voltage triggering was high enough to switch the conductivity state of the sample i.e., the effect of thickness change is accompanied by the effect of conductivity switching. The thickness change is significant, reaching several percents of the entire layer thickness.
We report on n-type indium doping of CdTe films grown by molecular beam epitaxy on (001) GaAs substrates. By adjusting the flux of In atoms we can precisely control the carrier concentration over three orders of magnitude - from 8 × 10^{14} up to 1.3 × 10^{18} cm^{-3}. In agreement with earlier reports we confirmed that Cd overpressure plays an important role in the doping process. The doping appears to be most effective for Cd/Te pressure ratio of 1.5. For this value of Cd/Te pressure ratio essentially 100% efficiency of doping is achieved at low In concentrations (< 10^{18} cm^{-3}). At higher In concentrations acceptor impurities compensate shallow donors limiting the concentration of free carriers.
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