A. Koeder

Universität Ulm, Ulm, Baden-Württemberg, Germany

Are you A. Koeder?

Claim your profile

Publications (20)40.32 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the electronic and magnetic properties of ferromagnetic GaMnAs single layers as well as multilayer structures fabricated by low-temperature molecular beam epitaxy. The electrochemical capacitance–voltage profiling as well as Raman spectroscopy measurements in GaMnAs single layers reveal a vertical gradient in the carrier concentration and therefore a gradient in Curie temperature measured by superconducting quantum interference device. Detailed studies of carrier density and ferromagnetic transition before and after annealing of the layers are presented. We also report on an enhancement of the Curie temperature in GaMnAs/InGaMnAs superlattices, which is due to thin InGaMnAs or InGaAs films embedded into the GaMnAs layers. The pronounced increase of the Curie temperature is strongly correlated to the In concentration in the embedded layers. Curie temperatures up to 110 K are observed in such structures compared to 60 K in GaMnAs single layers grown under the same conditions. A further increase in TC up to 130 K can be achieved using post-growth annealing at temperatures near the growth temperature.
    International Journal of Nanoscience 11/2011; 03(01n02). DOI:10.1142/S0219581X04001882
  • ChemInform 01/2006; 37(6). DOI:10.1002/chin.200606232
  • T. Weiers · G. Denninger · A. Koeder · W. Schoch · A. Waag
    [Show abstract] [Hide abstract]
    ABSTRACT: Various types of Mn in epitaxially grown (Ga,Mn)As have been investigated. Ionized interstitial manganese donors MnI2+ have been found to occur in (Ga,Mn)As at dopant concentrations as low as 0.5%. A comparison with spectra from interstitial MnI2+ inside bulk-doped GaAs:Mn yields a slight decrease of 1.5% in the hyperfine splitting with increasing dopant concentration. This is attributed to an increase in the lattice constant of (Ga,Mn)As with increasing manganese concentration. Contrary to Mn interstitials, Mn dopants on Ga lattice sites acts as acceptors. It is shown that Mn on Ga lattice sites MnGa2+ is non-uniformly distributed. A low percentage of isolated Mn acceptors can be distinguished from the exchange broadened MnGa2+ signal. Thus, electron paramagnetic resonance is a promising tool for the investigation of (Ga,Mn)As and the classification of various types of Mn dopants responsible for the magnetic properties of (Ga,Mn)As.
    Solid State Communications 08/2005; 135(7):416-419. DOI:10.1016/j.ssc.2005.05.035 · 1.70 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The effect of annealing at 250 C on the carrier depth profile, Mn distribution, electrical conductivity, and Curie temperature of (Ga,Mn)As layers with thicknesses > 200 nm, grown by molecular-beam epitaxy at low temperatures, is studied by a variety of analytical methods. The vertical gradient in hole concentration, revealed by electrochemical capacitance-voltage profiling, is shown to play a key role in the understanding of conductivity and magnetization data. The gradient, basically already present in as-grown samples, is strongly influenced by post-growth annealing. From secondary ion mass spectroscopy it can be concluded that, at least in thick layers, the change in carrier depth profile and thus in conductivity is not primarily due to out-diffusion of Mn interstitials during annealing. Two alternative possible models are discussed. Comment: 8 pages, 8 figures, to appear in Phys. Rev. B
    Physical Review B 03/2005; 71(20). DOI:10.1103/PhysRevB.71.205213 · 3.74 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We show that upon exposure to a remote dc hydrogen plasma, the magnetic and electronic properties of the dilute magnetic semiconductor Ga1-xMnxAs change qualitatively. While the as-grown Ga1-xMnxAs thin films are ferromagnetic at temperatures T less, similar 70 K, the samples are found to be paramagnetic after the hydrogenation, with a Brillouin-type magnetization curve even at T=2 K. Comparing magnetization and electronic transport measurements, we conclude that the density of free holes p is significantly reduced by the plasma process, while the density of Mn magnetic moments does not change.
    Physical Review Letters 07/2004; 92(22):227202. DOI:10.1103/PhysRevLett.92.227202 · 7.51 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ga1-xMnxAs layers with Mn fractions 0 = x = 2.8 % grown on GaAs(001) substrates by low-temperature molecular beam epitaxy were investigated using micro-Raman spectroscopy and far-infrared (FIR) reflectance spectroscopy. The Raman and FIR spectra are strongly affected by the formation of coupled modes of longitudinal optical phonons and hole plasmons. A full line-shape analysis of the spectra was performed within a Lindhard–Mermin model for the dielectric function, including intraband and interband hole transitions. Annealing at temperatures between 250 and 500°C results in a decrease of the hole density with increasing annealing temperature and total annealing time. Simultaneously, a reduction of the number of Mn atoms on Ga lattice sites is deduced from high-resolution X-ray diffraction. After annealing at 450°C the Raman lines of elemental As are observed, which are due to the precipitation of As on the sample surface.
    Journal of Superconductivity 06/2004; 17(3):417-420. DOI:10.1023/B:JOSC.0000034268.96764.7e
  • [Show abstract] [Hide abstract]
    ABSTRACT: The effects of hydrogen and deuterium on ferromagnetic GaAs doped with high concentrations of Mn (approximate to10(21) cm(-3)) are studied. Secondary ion mass spectroscopy depth profiles show that D is incorporated in the same concentration as Mn. The epilayers change from metallic to semiconducting behavior upon hydrogenation. Fourier transform-infrared absorption measurements show the As-H and As-D local vibrational modes characteristic for the complexes of hydrogen with group-II acceptors in GaAs. (C) 2004 American Institute of Physics.
    Applied Physics Letters 03/2004; 84(13). DOI:10.1063/1.1690470 · 3.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We studied the effect of the V/III flux ratio and substrate temperature on magnetotransport properties and lattice parameters of Ga0.96Mn0.04As grown by molecular beam epitaxy. For all the substrate temperatures, the conductivities and Curie temperatures of the layers were found to increase as the V/III flux ratio approaches 1. The Curie temperature as high as 95 K was achieved for the Ga0.96Mn0.04As samples grown at 240C and a V/III ratio of about 1.5. The lattice parameter of Ga0.96Mn0.04As increased with decreasing V/III ratio and/or increasing growth temperature. Possible reasons for the effect of V/III ratio on the magnetotransport properties and lattice parameter of GaMnAs are discussed.
    Journal of Applied Physics 01/2004; 98(2). DOI:10.1063/1.1991971 · 2.19 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on an enhancement of the Curie temperature in GaMnAs/InGaMnAs superlattices grown by low-temperature molecular beam epitaxy, which is due to thin InGaMnAs or InGaAs films embedded into the GaMnAs layers. The pronounced increase of the Curie temperature is strongly correlated to the In concentration in the embedded layers. Curie temperatures up to 110 K are observed in such structures compared to 60 K in GaMnAs single layers grown under the same conditions. A further increase in T$_C$ up to 130 K can be achieved using post-growth annealing at temperatures near the growth temperature. Pronounced thickness fringes in the high resolution X-ray diffraction spectra indicate good crystalline quality and sharp interfaces in the structures. Comment: 4 pages, 4 figures, submitted to Appl. Phys. Lett
    Applied Physics Letters 08/2003; 85(5). DOI:10.1063/1.1771802 · 3.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The effect of short-time and long-time annealing at 250C on the conductivity, hole density, and Curie temperature of GaMnAs single layers and GaMnAs/InGaMnAs heterostructures is studied by in-situ conductivity measurements as well as Raman and SQUID measurements before and after annealing. Whereas the conductivity monotonously increases with increasing annealing time, the hole density and the Curie temperature show a saturation after annealing for 30 minutes. The incorporation of thin InGaMnAs layers drastically enhances the Curie temperature of the GaMnAs layers. Comment: 4 pages, 6 figures, submitted to Physica E
    Physica E Low-dimensional Systems and Nanostructures 07/2003; DOI:10.1016/j.physe.2003.11.173 · 1.86 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on detailed investigations of the electronic and magnetic properties of ferromagnetic Ga1−xMnxAs layers, which have been fabricated by low-temperature molecular-beam epitaxy. Superconducting quantum interference device measurements reveal a decrease of the Curie temperature from the surface to the Ga1−xMnxAs/GaAs interface. While high-resolution x-ray diffraction clearly shows a homogeneous Mn distribution, a pronounced decrease of the carrier concentration from the surface towards the Ga1−xMnxAs/GaAs interface has been found by Raman spectroscopy as well as electrochemical capacitance–voltage profiling. The gradient in Curie temperature seems to be a general feature of Ga1−xMnxAs layers grown at low temperature. Possible explanations are discussed. © 2003 American Institute of Physics.
    Applied Physics Letters 05/2003; 82(19):3278-3280. DOI:10.1063/1.1573369 · 3.52 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report ferromagnetic resonance experiments on Ga <sub>1-x</sub> Mn <sub>x</sub> As thin films. For the dc magnetic field perpendicular to the sample plane, we observe up to eight distinct resonances, which we attribute to spin wave modes. To account for the spacing of the resonances, we infer a linear gradient in the magnetic properties, which is ascribed to a linear variation of the uniaxial magnetic anisotropy with film thickness. Values of D=(1±0.4)×10<sup>-9</sup>  Oe cm <sup>2</sup> for the spin stiffness and J<sub> MnMn </sub>≈1  meV for the exchange integral between Mn spins are obtained. © 2003 American Institute of Physics.
    Applied Physics Letters 03/2003; 82(5-82):730 - 732. DOI:10.1063/1.1539550 · 3.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report on ferromagnetic resonance measurements of Ga1-xMnxAs thin films with Mn contents 0.022 less than or equal to x less than or equal to 0.051. For x greater than or equal to 0.036 and the external magnetic field normal to the thin film, we observe several resonances, which we identify as spin wave resonances. The nonquadratic mode spacing can be consistently explained by a linear gradient in the magnetic properties of the films. From the measurements, the exchange constant A can be deduced for different Mn contents x.
    Journal of Superconductivity 02/2003; 16(1). DOI:10.1023/A:1023284617994
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report on ferromagnetic resonance measurements of Ga1-xMnxAs thin films with Mn contents 0.022 ≤ x ≤ 0.051. For x ≥ 0.036 and the external magnetic field normal to the thin film, we observe several resonances, which we identify as spin wave resonances. The non-quadratic mode spacing can be consistently explained by a linear gradient in the magnetic properties of the films. From the measurements, the exchange constant A can be deduced for different Mn contents x.
    Journal of Superconductivity 01/2003; 16(1).
  • [Show abstract] [Hide abstract]
    ABSTRACT: The vibrational and electronic properties of Ga1-xMnxAs layers with Mn fractions 0less than or equal toxless than or equal to2.8%, grown on GaAs(001) substrates by low-temperature molecular-beam epitaxy, are investigated by micro-Raman spectroscopy and far-infrared (FIR) reflectance spectroscopy. The Raman and FIR spectra are strongly affected by the formation of a coupled mode of the longitudinal optical phonon and the hole plasmon. The spectral line shapes are modeled using a dielectric function where intraband and interband transitions of free holes are included. In addition to the coupled mode, the contributions of a surface depletion layer as well as a symmetry forbidden TO phonon have to be taken into account for the Raman spectra. Values for the hole densities are estimated from a full line-shape analysis of the measured spectra. Annealing at temperatures between 250 and 500 degreesC results in a decrease of the hole density with increasing annealing temperature and total annealing time. Simultaneously, a reduction of the fraction of Mn atoms on Ga lattice sites is deduced from high-resolution x-ray diffraction.
    Physical review. B, Condensed matter 11/2002; 66(20). DOI:10.1103/PhysRevB.66.205209 · 3.66 Impact Factor
  • R Kling · A Köder · W Schoch · S Frank · M Oettinger · W Limmer · R Sauer · A Waag
    [Show abstract] [Hide abstract]
    ABSTRACT: For spin injection, detection and manipulation, diluted magnetic semiconductors are interesting materials. GaMnAs is one of these candidates, though the ferromagnetism occurs only at temperatures below 110 K. In order to possibly increase the Curie temperature, nitride based III–V semiconductors have been proposed. Here, we report results on the structural, magnetic and electronic properties of Ga1−xMnxNyAs1−y. In GaMnNAs, the valence band maximum is supposed to increase in energy with N concentration, and since Mn is a relative deep acceptor with an activation energy of 110 meV, this would lead to a reduction of the activation energy and hence to a higher doping level. GaMnNAs thin films have been grown by low-temperature molecular-beam epitaxy. We present experimental data on the transport properties of this compound, indicating that it is ferromagnetic at low nitrogen concentrations. Samples with higher N concentration show a decrease of the Curie temperature or even no ferromagnetism at all and a high ohmic resistance compared to GaMnAs reference layers. This is most probably due to a compensation of the holes, which are needed for the ferromagnetic coupling.
    Solid State Communications 10/2002; 124(5-6):207-210. DOI:10.1016/S0038-1098(02)00483-0 · 1.70 Impact Factor
  • Source
    W. Limmer · M. Glunk · W. Schoch · A. Köder · R. Kling · R. Sauer · A. Waag
    [Show abstract] [Hide abstract]
    ABSTRACT: Ga1-xMnxAs layers with Mn concentrations 0% less than or equal to x less than or equal to 2.7% grown on GaAs(001) substrates by low temperature molecular beam epitaxy were studied using micro-Raman spectroscopy, The layers were identified as p-type and an estimate of the hole densities was made by a full line shape analysis of the coupled plasmon-LO-phonon modes. The line width of the TO-phonon mode as a function of the Mn concentration was determined by spatially resolved measurements on cleaved (1 1 0) side faces.
    Physica E Low-dimensional Systems and Nanostructures 03/2002; 13(2):589-592. DOI:10.1016/S1386-9477(02)00192-3 · 1.86 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In this contribution we report on the incorporation of nitrogen into GaMnAs, leading to novel semimagnetic quaternary GaMnNAs compounds. The incorporation of nitrogen into GaAs is well known to change the band structure substantially, causing a pronounced band gap bowing. The motivation for this study is to identify the influence of the nitrogen incorporation on the exchange coupling mechanisms in the semimagnetic GaMnNAs. Possible relevant aspects in this respect are a change of hole masses, a change of the position of the valence band edge relative to the Mn acceptor levels, as well as a change of the distance between the magnetic Mn ions, which could eventually lead to an increase of the Curie temperatures relative to GaMnAs.GaMnNAs layers have been fabricated by MBE, with Mn and N concentrations of around 0.5–1.5%. The structures have been analyzed by X-ray diffraction as well as magnetotransport at magnetic fields of up to 6T. The anomalous Hall effect has been analyzed, indicating that this material is ferromagnetic. Compared to GaMnAs, the Curie temperature is slightly increased. Possible reasons for this behavior are discussed.
    Physica E Low-dimensional Systems and Nanostructures 03/2002; 13(2):593-596. DOI:10.1016/S1386-9477(02)00193-5 · 1.86 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Ferromagnetic Ga1-xMnxAs films containing up to 5.1 at%Mn were grown by low-temperature MBE. The structural, electrical, and magnetic properties of the layers are reported. At x > 0.01, the materials show a ferromagnetic behavior. The Curie temperature reaches 80 K at 5.1at% Mn. We propose the use of a n+-GaAs/p+-GaMnAs Esaki-diode (ferromagnetic Esaki-diode, FED) to provide injection of spin-polarized electrons via interband tunneling. Under reverse bias, spin-polarized electrons at the Fermi level in the valence band of GaMnAs tunnel to the conduction band of GaAs in contrast to the injection of spin-polarized holes used before.
    Proceedings of SPIE - The International Society for Optical Engineering 01/2002; DOI:10.1117/12.514534 · 0.20 Impact Factor