Structural and magnetic properties of GaMnAs layers with high Mn content grown by Migration Enhanced Epitaxy on GaAs(100) substrates

We have grown the ferromagnetic semiconductor GaMnAs containing up to 10% Mn by migration enhanced epitaxy at a substrate temperature of 150^oC. The alternate supply of As2 molecules and Ga and Mn atoms made it possible to grow single crystalline GaMnAs layers at very low substrate temperature, at which conventional molecular beam epitaxial growth under excess As supply is not possible due to As condensation. Secondary ion mass spectroscopy and X-ray diffraction measurements confirmed a higher Mn content in the films grown by this method in comparison to the GaMnAs layers grown by low temperature molecular beam epitaxy. The lattice constant of hypothetical zinc-blende structure MnAs is determined to be 5.9 \AA, which deviates somewhat from previously reported values. This deviation is ascribed to growth-condition dependent density of point defects. It is stressed that this effect must be taken into account for any assessment of Mn content from X-ray diffraction data. Magnetization measurements showed an onset of ferromagnetic ordering around 75 K for the GaMnAs layer with 10% Mn. This means that the trend of falling Curie temperatures with increasing Mn concentrations above 5.5% is broken. We tentatively assign this to the variation of the carrier concentration, including contributions from donor and acceptor centers formed by antisite defects and Mn doping, and increased density of magnetically active Mn ions.Comment: No LaTeX source; gzipped postscript text + 3 gzipped postscript figure

Magnetization of ultrathin (Ga,Mn)As layers

Kerr rotation and superconducting quantum interference device magnetometry measurements were performed on ultrathin (Ga0.95Mn0.05)As layers. The thinner layers (below 250 A) exhibit magnetic properties different than those of thicker ones, associated with different microstructure, and some degree of inhomogeneity. The temperature dependence of the field-cooled magnetization of the layers is recorded after successive low temperature annealings. While the Curie temperature of the thicker layer (250 A) is nearly unchanged, the critical temperature of the thinner layers is enhanced by more than 23 K after two annealings. Secondary ion mass spectrometry experiments on similar layers show that Mn is displaced upon annealing. The results are discussed considering a possible segregation of substitutional and interstitial Mn atoms at the surface of the (Ga,Mn)As layers

On-chip tensile testing of the mechanical and electro-mechanical properties of nano-scale silicon free-standing beams

A simple and versatile on-chip tensile testing method is proposed for the statistical evaluation of size effects on the mecanichal strenght of silicon thin films along with the simultaneous study of (from low to ultra) strain effects on the carrier transport. Mechanical results are presented on the fracture strenght of micro-nano scale silicon beams, followed with a discussion on interface states and problems facing reliable nano-electronic and nano-electromechanical characterizations

Mass transport growth and properties of hydride vapour phase epitaxy GaN

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Characterization of mass-transport grown GaN by hydride vapour-phase epitaxy

A comprehensive study of the morphological, optical and microstructural properties of mass-transport (MT) and conventionally grown GaN by hydride vapour-phase epitaxy is presented. Spatially resolved techniques have been utilized to reveal in a comparative way, the characteristics of the material grown either in predominant vertical or lateral growth modes. A strong donor-acceptor pair (DAP) emission is observed from the MT regions with a distinctive intensity contrast between the exciton and DAP emission bands from MT and nontransport regions. Secondary ion mass spectroscopy and imaging were employed to investigate the impurity incorporation into different regions. An increase of residual oxygen and aluminium impurity concentrations was found in the MT areas. In addition, positron annihilation spectroscopy showed a strong signal of Ga vacancy clusters in the MT grown material. The increase of the point defect concentrations of both Ga vacancy and oxygen impurity, most likely forming defect complexes, is related to the enhancement of the DAP emission.11 page(s