B. D. Schultz

Spin injection from perpendicular magnetized ferromagnetic δ-MnGa into (Al,Ga)As heterostructures

Electrical spin injection from ferromagnetic δ-MnGa into an (Al,Ga)As p-i-n light-emitting diode (LED) is demonstrated. The δ-MnGa layers show strong perpendicular magnetocrystalline anisotropy, enabling detection of spin injection at remanence, without an applied magnetic field. The bias and temperature dependence of the spin injection are found to be qualitatively similar to Fe-based spin LED devices. A Hanle effect is observed and demonstrates complete depolarization of spins in the semiconductor in a transverse magnetic field.

Optical pumping in ferromagnet-semiconductor heterostructures: Magneto-optics and spin transport

Epitaxial ferromagnetic metal-semiconductor heterostructures are investigated using polarization-dependent electroabsorption measurements on GaAs p-type and n-type Schottky diodes with embedded In12xGaxAs quantum wells. We have conducted studies as a function of photon energy, bias voltage, magnetic field, and excitation geometry. For optical pumping with circularly polarized light at energies above the band edge of GaAs, photocurrents with spin polarizations on the order of 1% flow from the semiconductor to the ferromagnet under reverse-bias. For optical pumping at normal incidence, this polarization may be enhanced significantly by resonant excitation at the quantum well ground state. Measurements in a side-pumping geometry, in which the ferromagnet can be saturated in very low magnetic fields, show hysteresis that is also consistent with spin-dependent transport. Magneto-optical effects that influence these measurements are discussed.

Interfacial reactions of Mn/GaAs thin films

Mn thin films were deposited in situ on molecular beam epitaxy grown GaAs(001) surfaces at 0 °C. Postgrowth anneals of the Mn/GaAs samples were done at 200, 300, and 400 °C for times ranging from 0.5 to 30 h. Rutherford backscattering spectrometry, x-ray diffraction, and transmission electron microscopy show that for samples annealed at 300 °C the interfacial reactions initially result in the formation of an epitaxial two phase region (Mn2As and MnGa) with an average composition of Mn0.6Ga0.2As0.2. The rate of reaction between the Mn and GaAs shows a square root of time dependence, indicating that the reactions are diffusion limited.

Electron spin dynamics and hyperfine interactions in Fe Al0.1Ga0.9As GaAs spin injection heterostructures

We have studied hyperfine interactions between spin-polarized electrons and lattice nuclei in Al_0.1Ga_0.9As/GaAs quantum well (QW) heterostructures. The spin-polarized electrons are electrically injected into the semiconductor heterostructure from a metallic ferromagnet across a Schottky tunnel barrier. The spin-polarized electron current dynamically polarizes the nuclei in the QW, and the polarized nuclei in turn alter the electron spin dynamics. The steady-state electron spin is detected via the circular polarization of the emitted electroluminescence. The nuclear polarization and electron spin dynamics are accurately modeled using the formalism of optical orientation in GaAs. The nuclear spin polarization in the QW is found to depend strongly on the electron spin polarization in the QW, but only weakly on the electron density in the QW. We are able to observe nuclear magnetic resonance (NMR) at low applied magnetic fields on the order of a few hundred Oe by electrically modulating the spin injected into the QW. The electrically driven NMR demonstrates explicitly the existence of a Knight field felt by the nuclei due to the electron spin.

Optically pumped transport in ferromagnet-semiconductor Schottky diodes (invited)

Optical pumping is used to generate spin-polarized carriers in epitaxial ferromagnet-GaAs Schottky diodes with InyGa1−yAs quantum wells placed in the depletion region. A strong dependence of the photocurrent on the polarization state of the incident light is observed, and a series of measurements as a function of excitation energy, bias voltage, magnetic field, and excitation geometry are used to distinguish spin-dependent transport from a variety of background effects. The spin polarization of the photocurrent for pumping energies at and above the band gap of GaAs is of order 0.5% or less. Much larger polarization dependence is observed for excitation energies near the quantum well ground state. Although background effects are very large in this regime, the field dependence of the polarization signal for several samples is suggestive of spin-dependent transport.

Embedded growth mode of thermodynamically stable metallic nanoparticles on III-V semiconductors

The epitaxial growth of ErSb on GaSb(001) surfaces occurs by an embedded growth mode where stable nanometer sized islands are nucleated within the semiconductor via a displacement reaction on the surface. The ErSb islands extend up to four atomic layers beneath the surface before growing laterally and coalescing into a continuous film. The growth mode is not governed by epitaxial strain or surface, bulk, and interfacial energy differences, instead the surface morphologies resulting from this growth mode are shown to depend on structural similarities, thermodynamics, and diffusion.

Phase formation in the thin film Fe∕GaAs system

Interfacial reactions between epitaxial films of Fe and GaAs were examined using Rutherford backscattering spectrometry, x-ray diffraction, and transmission electron microscopy as a function of postgrowth annealing conditions. At 450°C, two stable binary phases were observed at the Fe∕GaAs(001) interface with DO3Fe3Ga forming near the Fe interface and Fe2As forming near the GaAs interface. The diffusion rate of Fe in the reacted interface was found to be of similar magnitude to that of Ga and As resulting in a rough and intermixed interfacial region.

Nuclear magnetic resonance in a ferromagnet–semiconductor heterostructure

We report the observation of nuclear magnetic resonance in a ferromagnet–semiconductor heterostructure in the presence of a spin-polarized current. Spin-polarized electrons injected from a metallic ferromagnet dynamically polarize a large nuclear spin population in a GaAs quantum well. The characteristic time for the polarization process is approximately 20 s, and the nuclear polarization can persist for several minutes after the current is turned off. Resonant depolarization is observed in the presence of an ac magnetic field or when the injection current is modulated at the nuclear magnetic resonance frequency.

Control of magnetic anisotropy in Fe1−xCox films on vicinal GaAs and Sc1−yEryAs surfaces

We demonstrate that two distinct surface contributions to the magnetocrystalline anisotropy can be used to control the magnetic properties of thin films of bcc Fe1−xCox grown on GaAs (100) and Sc1−yEryAs (100). The bare GaAs (100) surface has twofold symmetry, and Fe1−xCox films grown directly on it show a strong uniaxial magnetic anisotropy. Fourfold symmetry is restored in films grown on interlayers of Sc1−yEryAs, in which the rock-salt structure provides a fourfold symmetric surface. A uniaxial magnetic anisotropy can be induced in this case by miscutting the substrate towards a {111} plane, so that vicinal steps run along a 〈011〉 direction. A simple Neel pair-bonding model describes the evolution of the anisotropy with the degree of miscut. For miscut GaAs (100) surfaces without interlayers, both the intrinsic anisotropy originating from the surface bonding and a step-induced term contribute to the total magnetic anisotropy. Depending on the orientation of the miscut, the step contribution can either ...

Growth temperature dependence of Mn∕GaAs surfaces and interfaces

In situ x-ray photoelectron spectroscopy and scanning tunneling microscopy were combined to examine the formation of the reacted region at the Mn∕GaAs(001) interface during deposition. Thin films of Mn were grown on GaAs c(4×4) surfaces by molecular beam epitaxy at substrate temperatures of 40, 95, and 250°C. The attenuations of the Ga and As photoemission peak intensities as a function of Mn deposition indicate the formation of a reacted layer at the interface with an average composition of Mn0.6Ga0.2As0.2, and submonolayer coverages of As were found to segregate to the sample surface independent of the growth temperature. The extent of the Mn–Ga–As interfacial reactions that occur during the growth of Mn on GaAs strongly depends upon the growth temperature. At growth temperatures of 95°C and below, the interfacial reacted layer reaches a thickness that is sufficient to limit any additional diffusion of Mn into the GaAs. During growth at 250°C the rate of diffusion is higher and the interfacial reacted l...