Ferromagnetism and interlayer exchange coupling in short period (Ga,Mn)As/GaAs superlattices

Magnetic properties of (Ga,Mn)As/GaAs superlattices are investigated. The structures contain magnetic (Ga,Mn)As layers, separated by thin layers of non-magnetic GaAs spacer. The short period Ga$_{0.93}$Mn$_{0.07}$As/GaAs superlattices exhibit a paramagnetic-to-ferromagnetic phase transition close to 60K, for thicknesses of (Ga,Mn)As down to 23 \AA. For Ga$_{0.96}$Mn$_{0.04}$As/GaAs superlattices of similar dimensions, the Curie temperature associated with the ferromagnetic transition is found to oscillate with the thickness of non magnetic spacer. The observed oscillations are related to an interlayer exchange interaction mediated by the polarized holes of the (Ga,Mn)As layers.Comment: REVTeX 4 style; 4 pages, 2 figure

Laser-induced photoconductivity of ferromagnetic semiconductors

The influence of a strong electromagnetic wave on the kinetic phenomena in ferromagnetic semiconductors (FMSC) is considered. To sequentialy consider this influence, we obtained quantum kinetic equations for electrons and magnons involved into the process of electron-magnon interaction at presence of intense high-frequency electromagnetic field. The new mechanism of a photoconduction not bound with a change of the number of carriers in the conduction band of FMSC and hot conduction electrons is offered. The density of electric current in the FMSC provided by strong variable electromagnetic field and feeble constant electric field has been calculated. The valuation of FMSC electric conductivity in the infrared spectral range is made. It is shown that magnification of the amplitude of variable electromagnetic filed results in increasing the conductivity of FMSC at some fixed temperature

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

Ferromagnetic GaMnAs/GaAs superlattices - MBE growth and magnetic properties

We have studied the magnetic properties of (GaMnAs)m/(GaAs)n superlattices with magnetic GaMnAs layers of thickness between 8 and 16 molecular layers (ML) (23-45 \AA), and with nonmagnetic GaAs spacers from 4 ML to 10 ML (11-28 \AA). While previous reports state that GaMnAs layers thinner than 50 \AA 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 nonmagnetic GaAs spacer. The neutron scattering experiments have shown that the magnetic layers in superlattices are ferromagnetically coupled for both thin (below 50 \AA) and thick (above 50 \AA) GaMnAs layers.Comment: Proceedings of 4th International Workshop on Molecular Beam Epitaxy and Vapour Phase Epitaxy Growth Physics and Technology, September 23 - 28 (2001), Warszawa, Poland, to appear in Thin Solid Films. 24 pages, 8 figure

Features of conduction electrons motion in the field of coherent light beams

Conduction electrons moving in a non-homogeneous field of coherent light beams (CLB) are investigated. It is shown that a conduction electron simultaneously takes part in two oscillations. More exactly, an electron oscillate with the frequency w and in addition, it carry out a forced oscillation with the frequency wz << w (w is the CLB frequency) inside a one-dimensional potential well that is induced in the semiconductor by the CLB

treating the main topic of modelling the

This licentiate thesis is composed of three different parts: a brief introduction, a repor

Beibwanger On the design of a 55 GHz Si/SiGe HBT frequency doubler operating close to f

r his supervision. I would also like to thank Prof. Sören Berg for accepting me as Ph. D. student. A list of all my colleagues at Signals and Systems would unfortunately be too long to fit the space here, but they are all greatly acknowledged for inspiring discussions regarding both professional aspects and topics of more general nature. A special thanks goes to Prof. Anders Ahlen for providing a very nice atmosphere. I would also like to thank Prof. Hermann Schumacher and all the co-workers at Department of Electron Devices and Circuits at University of Ulm in Germany for a very friendly atmosphere and for helping me in all ways during my stays there. Special thanks to Prof. Hermann Schumacher, Dr. Uwe Erben and Dr. Andreas Trasser for many fruitful discussions and to M. Sc. Martin Birk for many interesting discussions and for helping me as well in the university as outside. I am grateful to Dr. Moonil Kim at JPL, for his work on frequency doublers which I have benefited from. I&apos;m in

Effect of layer relaxation on the internal photoemission in Pt/Si1-xGex Schottky barrier type infrared detectors

A Schottky type infrared detector fabricated on a p-type Si1-xGex substrate has a higher cut-off wavelength than one on a pure Si substrate because the barrier height of the Schottky junction on p-type Si1-xGex decreases with the Ge content and the induced strain in the Si1-xGex layer. We have studied the effect of the strain relaxation on the internal photoemission and I-V characteristics of a Pt/Si1-xGex Schottky junction with x=0.14. It is shown that the cut-off wavelength of the diode made on a strained Si0.86Ge0.14 layer is higher than that on a Si substrate as expected. This shows the possibility of tuning the range of these detectors in the midinfrared region. However, the thermal relaxation in the Si0.86Ge0.14 layer is found to reduce the cut-off wavelength to lower values, showing that the difference between the Fermi level of the metal and the valence band edge increases with the layer relaxation. This effect should be taken into account when a Schottky type infrared detector is manufactured on a strained Si1-xGex film. I-V characteristics of the junctions also indicate an increase of the barrier height with the relaxation of Si1-xGex. These results demonstrate the band edge movements in a Si1-xGex layer experimentally agree with the expected changes in the band structure of the Si1-xGex layer with strain relaxation

Photoemission study of GaAs (100) grown at low temperature

GaAs(100) layers grown by low-temperature (LT) molecular beam epitaxy were studied by means of valence-band and core-level photoelectron spectroscopy. Small differences were found between valence-band spectra from the LT layers and corresponding layers grown at high temperature. In the Ga 3d spectra a new component was found in the LT-GaAs. The relative intensity of this component was found to be practically constant with varying probing depth. It is proposed that this component represents sites coordinated to the five-atom As clusters formed at As-Ga antisites. This interpretation implies a higher density of antisite defects in the near-surface region than typically found in the bulk