Thin layers of GaMnAs ferromagnetic semiconductor grown by molecular beam epitaxy on GaAs(001) substrates were studied. To improve their magnetic properties the post-growth annealing procedures were applied, using the surface passivation layers of amorphous arsenic. This post-growth treatment effectively increases the ferromagnetic-to-paramagnetic phase transition in GaMnAs, and provides surface-rich MnAs layer which can be used for formation of low-dimensional structures such as superlattices. If the surface rich MnAs layer consists of MnAs dots, then it is possible to grow Mn-doped GaAs nanowires.
We compare the results of electrically detected magnetic resonance in a 2D electron gas in Si/SiGe quantum wells with transport and magnetic resonance measurements on ferromagnetic Ga_{1-x}Mn_xAs. The results lead us to the conclusion that observation of electrically detected magnetic resonance is possible only in the case of a slow spin relaxation, where the microwave resonant absorption leads to a noticeable change of spin magnetization.
The structural properties of MBE grown YbTe layers were investigated by X-ray diffraction methods and photoluminescence measurements. YbTe films were grown on the ZnTe and CdTe buffer layers crystallised on the GaAs(100) 2° off oriented substrates and on the BaF_{2}(100) substrates. In the case of GaAs substrates the two-dimensional growth mode of YbTe was observed on reflection high energy electron diffraction picture. Results of the X-ray rocking curve and photoluminescence excitation measurements indicate that the structural properties of YbTe films are comparable to the properties of the MBE grown ZnTe and CdTe layers on the GaAs(100) substrates. The measured values of the YbTe lattice constant parallel and perpendicular to the growth plane show that the 1 μm thick layers are partially strained. The full width at half maximum values of the X-ray rocking curves are the smallest (900 arc seconds) for the YbTe films crystallised on the 2 μm thick CdTe bucher layer grown on the GaAs(100) substrate. In the case of BaF_{2}(111) substrate the two-dimensional MBE growth mode of YbTe was not observed.
Positron annihilation spectroscopy can be used to determine the role of vacancy defects in semiconductors, by identification and quantification of the vacancies and their chemical surroundings. We have studied 0.5-0.8μm thick low temperature MBE GaMnAs layers with Mn content 0.5--5% and different As_2 partial pressures at growth. The Doppler broadening results show that the Ga vacancy concentration in the layers decreases with increasing Mn content and decreasing As_2 partial pressure.
A major progress has been made in the last decade in the understanding of molecular mechanisms involved in plant responses to stresses. This has been achieved in great part by application of a high-throughput genomics tools. Transcriptome analysis based on the DNA microarray technique has proved its effectiveness in studies of genome level responses to environmental changes and genetic manipulations. From the perspective of a decade, DNA microarrays serve as an invaluable tool for global gene expression analyses, unravelling new information about genes, pathways and their transcriptional regulation networks. In this review, we present applications of the DNA microarrays and a basic analysis of microarray data in relation to the transcriptional response of plants to ozone and drought stresses.
Static and dynamic reflection high energy electron diffraction (RHEED) has been applied for studying the initial growth processes of ZnTe crystallized by molecular beam epitaxy (MBE) on vicinal surfaces of GaAs(100) substrates. Atomically smooth ZnTe epilayers have been grown by MBE when in situ thermal desorption of the substrate protecting oxide layer was performed in the ultra high vacuum environment of the vacuum growth chamber just before the growth of ZnTe started. By gradual increasing of the substrate temperature of the crystallized ZnTe epilayers from 300°C to 420°C, when recording the RHEED intensity oscillations at these and eleven intermittent temperatures, it has been shown that the transition from the 2D-nucleation growth mechanism to the step-flow growth mechanism of ZnTe occurs at 410°C. Measuring periods of RHEED intensity oscillations recorded during the MBE growth processes it has been demonstrated that the growth rate of ZnTe at constant fluxes of the constituent elements decreases with increasing temperature from 0.37 ML/s at 300°C to 0.22 ML/s at 400°C.
Molecular beam epitaxy grown Ga_{1-x}Mn_xAs layers were investigated by means of magnetic resonances. With an increase in Mn concentration, x, the spectrum changes from the (i) paramagnetic one, with resolved fine and hyperfine structures, typical of S=5/2 spin of substitutional Mn^{2+} ions, for very diluted alloy, via (ii) paramagnetic spectrum, where the fine and hyperfine structures are averaged by a long range Mn^{2+}-Mn^{2+} exchange coupling, (iii) single, isotropic line of ferromagnetic resonance. Insulator to metal transition is accompanied with occurrence of (iv) a very complex spectrum of the ferrimagnetic resonance, accompanied with the well-resolved spin wave resonance. Reentrance to insulator phase for the most condensed alloys is accompanied with the reentrance to (v) ferromagnetic phase. The data confirm that the effective mass holes transfer the exchange interaction between localized Mn^{2+} spins.
Comparison of genome sequences has become an important approach to identify and understand biological significance of the variations and fluxes that occur through a genome. The main subject of the work concentrates on identification of indels and SNPs in large genomes and their potential application in biotechnology. Importantly, fine elaboration of genome structure and sequence polymorphism that results from resequencing promises to benefit breeding, biotechnology and medical research. The article also describes how the data extracted from comparative studies of genomes depends on phylogenetic distances of the species involved.
An optical pump - terahertz probe technique was used for measuring electron lifetime in various Ga_{1-x}Mn_xAs epitaxial layers with the subpicosecond temporal resolution. The measurements were performed on the samples with x up to 2%, which had large resistivities and were transparent in a THz frequency range. It has been found that an induced THz absorption relaxation is the fastest and electron lifetimes are the shortest for the samples with the smallest Mn content. For the samples with x=0.3% and x=2% this relaxation becomes much slower; its rate is comparable to the carrier recombination rate in Ga_{1-x}Mn_xAs substrate.
Different types of magnetic resonance observed in Ga_{1-x}Mn_xAs reflect three different magnetic phases: para-, ferro-, and ferrimagnetic. Ferromagnet is characterized by single isotropic resonance line. A complex spectrum in ferrimagnet can be described by g factor equal to 1.44 and a sum of an axial and cubic anisotropy field. The axial field is by an order of magnitude greater than the cubic one. The complex structure of ferrimagnetic resonance is attributed to spin-wave resonance. Quantitative analysis of the dispersion of spin wave shows that the range of exchange coupling is very long, of the order of 25 nm, while spin-wave stiffness and the total exchange field are very small. The exchange field as evaluated from spin wave is by two orders of magnitude smaller than the Zener field corresponding to the critical temperature.
The Mn/ZnO(0001) system was investigated by synchrotron radiation photoemission. The Mn/ZnO interface with 4 ML of manganese deposited onto the ZnO surface was annealed up to 500ºC. No Mn capping layer was found at the surface after annealing as was confirmed by scanning Auger spectroscopy experiment. We used a resonant photoemission to extract the Mn3d partial density of states in photoemission spectra. The Mn3d states contribute to the electronic structure of the system within 10 eV of the Fermi level. They show three features: a main peak at 3.8-4.5 eV, a valence structure at the top of the valence band (1-3 eV), and a broad satellite situated between 5.5 and 9 eV below E_F. The satellite/main branching ratio was determined to be 0.43, which is a fingerprint of strong hybridization between the Mn3d electrons and the valence band of the crystal. The hybridization effect in Zn_{1-x} Mn_xO surface alloy is comparable to Zn_{1-x}Mn_xS and much higher than in Zn_{1-x}Mn_xSe, Zn_{1-x}Mn_xTe, and Ga_{1-x}Mn_xAs semimagnetic compounds.
We analyse properties of thin SnTe layers and PbTe/SnTe heterostructures grown by MBE on BaF_{2}(111) substrates. Reflection high energy electron diffraction patterns registered during MBE growth of the samples as well as post-growth X-ray diffraction measurements evidence a high structural perfection of 0.6 μm thick SnTe layers and (50 Å PbTe)/(50 Å SnTe) superlattices. The full width at half maximum values of (222) X-ray rocking curves measured for these thin SnTe layers crystallized in the optimal MBE growth conditions are about 300 arcsec; the carrier concentrations can be tuned from 5×10^{19} cm^{-3} to 10^{2 1} cm^{-3} depending on the MBE process parameters.
The MBE grown ZnYbTe layers were characterized by X-ray diffraction, photoluminescence and reflectivity measurements. The MBE growth conditions allowing to obtain monocrystalline ZnYbTe layers were found to be metal-rich (MBE growth with excess of Zn flux). In optical measurements (photoluminescence, reflectivity), both transitions connected with ternary ZnYbTe compound and with Yb^{3+} ions were detected. The quality of ZnYbTe layers with Yb content of 3% and 1% is inferior to the quality of pure ZnTe MBE layers, which is clearly seen in the results of photoluminescence and reflectivity measurements. In the ZnYbTe layers with 3% Yb, exhibiting monocrystalline character in reflection high-energy electron diffraction and X-ray diffraction measurements, optical transitions characteristic of pure YbTe were detected. In ZnYbTe layers with 1% Yb, no transitions connected with YbTe were observed.
We report on cross-sectional transmission electron microscopy and magnetic force microscopy studies performed on self-organized MnAs nanoclusters embedded in GaAs. It was found that 10÷20 nm large MnAs ferromagnetic nanocrystals were formed during rapid thermal annealing of Ga_{1-x}Mn_xAs layers at 600ºC, leading to magnetic contrasts in magnetic force microscopy images.
The core-multishell wurtzite structure (In,Ga)As-(Ga,Al)As-(Ga,Mn)As semiconductor nanowires have been successfully grown on GaAs(111)B substrates using MBE technique. The nanowires cores were grown with gold eutectic catalyser in vapour-liquid-solid growth mode. The double shell overgrowth, on the side facets of nanowires, was performed using lower substrate temperature (about 400°C, and 230°C, for (Ga,Al)As, and (Ga,Mn)As shell growth, respectively). The polytypic ordering, defects, chemistry and geometric perfection of the core and the shells have been analysed at atomic level by advanced transmission electron microscope techniques with the use of axial and longitudinal section of individual nanowires prepared by focused ion beam. High quality cross-sections suitable for quantitative transmission electron microscope analysis have been obtained and enabled analysis of interfaces between the core and the shells with near atomic resolution. All investigated shells are epitaxial without misfit dislocations at the interface. Some of the shells thicknesses are not symmetric, which is due to the shadowing effects of neighbouring nanowires and directional character of the elemental fluxes in the MBE growth process.
Two (Ga,Mn)As samples having different magnetic anisotropy (one with in-plane easy axis and another one with out-of-plane easy axis) were studied by means of magnetotransport experiments. Anisotropy field B_{A} was determined for both samples as a function of temperature. For the sample having in-plane easy axis, an inversion of the direction of planar Hall effect hysteresis was observed upon increase of temperature. This result was simulated using the Stoner-Wohlfarth model.
Granular GaAs:(Mn,Ga)As films were prepared by annealing at 500°C under ambient and enhanced hydrostatic pressure (1.1 GPa), of Ga_{1-x}Mn_xAs/GaAs layers (x = 0.025, 0.03, 0.04, 0.05 and 0.063) grown at 230°C by molecular beam epitaxy method. Distinct influence of enhanced hydrostatic pressure applied during sample annealing on strain state of inclusions was found. An increase of lattice distortion and of strain of inclusions for the samples treated under hydrostatic pressure is related to different bulk moduli of GaAs and of MnAs
High-quality layers of the (Ga,Mn)(Bi,As) quaternary compound semiconductor have been grown by the low-temperature molecular-beam epitaxy technique. An effect of Bi incorporation into the (Ga,Mn)As ferromagnetic semiconductor and the post-growth annealing treatment of the layers have been investigated through examination of their magnetic and magneto-transport properties. Significant enhancement of the planar Hall effect magnitude upon addition of Bi into the layers is interpreted as a result of increased spin-orbit coupling in the (Ga,Mn)(Bi,As) layers.
We have observed a paramagnetic-to-ferromagnetic phase transition in short period InGaMnAs/InGaAs superlattices. The thicknesses of magnetic InGaMnAs layers in the structures studied was chosen to be 4 or 8 molecular layers (12Å or 24Å). The non-magnetic InGaAs spacer layers are 12Å thick. The composition (In content) in InGaMnAs and InGaAs was chosen in such a way that magnetic layers were: deep potential wells, high potential barriers, or shallow potential wells. For superlattices with 8 monolayer thick InGaMnAs magnetic layers and 4 monolayer thick InGaAs non-magnetic spacers the temperatures of paramagnetic-to-ferromagnetic phase transition do not depend on the band offsets between InGaMnAs and InGaAs adjusted by the In content.
The Lorentz off-axis electron holography technique is applied to study the magnetic nature of Mn rich nanoprecipitates in (Mn,Ga)As system. The effectiveness of this technique is demonstrated in detection of the magnetic field even for small nanocrystals having an average size down to 20 nm.
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