A. T. Hanbicki

Efficient electrical spin injection from a magnetic metal/tunnel barrier contact into a semiconductor

We report electrical spin injection from a ferromagnetic metal contact into a semiconductor light emitting diode structure with an injection efficiency of 30% which persists to room temperature. The Schottky barrier formed at the Fe/AlGaAs interface provides a natural tunnel barrier for injection of spin polarized electrons under reverse bias. These carriers radiatively recombine, emitting circularly polarized light, and the quantum selection rules relating the optical and carrier spin polarizations provide a quantitative, model-independent measure of injection efficiency. This demonstrates that spin injecting contacts can be formed using a widely employed contact methodology, providing a ready pathway for the integration of spin transport into semiconductor processing technology.

Analysis of the transport process providing spin injection through an Fe/AlGaAs Schottky barrier

Electron-spin polarizations of 32% are obtained in a GaAs quantum well via electrical injection through a reverse-biased Fe/AlGaAs Schottky contact. An analysis of the transport data using the Rowell criteria demonstrates that single-step tunneling is the dominant transport mechanism. The current–voltage data show a clear zero-bias anomaly and phonon signatures corresponding to the GaAs-like and AlAs-like LO phonon modes of the AlGaAs barrier, providing further evidence for tunneling. These results provide experimental confirmation of several theoretical analyses, indicating that tunneling enables significant spin injection from a metal into a semiconductor.

Magnetoresistance of Mn:Ge ferromagnetic nanoclusters in a diluted magnetic semiconductor matrix

We have fabricated a thin film magnetic system consisting of nanoscale Mn11Ge8 ferromagnetic clusters embedded in a MnxGe1−x dilute ferromagnetic semiconductor matrix. The clusters form for growth temperatures of ∼300 °C with an average diameter and spacing of 100 and 150 nm, respectively. While the clusters dominate the magnetic properties, the matrix plays a subtle but interesting role in determining the transport properties. Variable range hopping at low temperatures involves both nanoclusters and MnGe sites, and is accompanied by a negative magnetoresistance attributed in part to spin-dependent scattering analogous to metallic granular systems.

Comparison of Fe/Schottky and Fe/Al2O3 tunnel barrier contacts for electrical spin injection into GaAs

We compare electrical spin injection from Fe films into identical GaAs-based light-emitting diodes (LEDs) using different tunnel barriers—a reverse-biased Fe/AlGaAs Schottky contact and an Fe/Al2O3 barrier. Both types of structures are formed in situ using a multichamber molecular-beam epitaxy system. A detailed analysis of the transport data confirms that tunneling occurs in each case. We find that the spin polarization achieved in the GaAs using the Al2O3 barrier is 40% (best case; 30% typical), but the electrical efficiency is significantly lower than that of the Fe Schottky contact.

Valley polarization and intervalley scattering in monolayer MoS2

We probe the degree of circular polarization of the emitted photoluminescence from a single layer of MoS2 as a function of the circularly polarized photo-excitation energy. A Single layer of MoS2 has strong emission at around 1.9 eV associated with a direct transition at the K-point of the Brillouin zone. The circular polarization of the photoluminescence is very high for excitation near the bandgap and has a power-law decrease as the excitation energy increases. We identify phonon-assisted intervalley scattering as the primary spin relaxation mechanism and present a model that explains the wide variation in values for the polarization reported in the literature.

Reduction of spin injection efficiency by interface defect spin scattering in ZnMnSe/AlGaAs-GaAs spin-polarized light-emitting diodes.

We report the first experimental demonstration that interface microstructure limits diffusive electrical spin injection efficiency across heteroepitaxial interfaces. A theoretical treatment shows that the suppression of spin injection due to interface defects follows directly from the contribution of the defect potential to the spin-orbit interaction, resulting in enhanced spin-flip scattering. An inverse correlation between spin-polarized electron injection efficiency and interface defect density is demonstrated for ZnMnSe/AlGaAs-GaAs spin-LEDs with spin injection efficiencies of 0 to 85%.

Quantifying electrical spin injection: Component-resolved electroluminescence from spin-polarized light-emitting diodes

The spin-polarized light-emitting diode (spin-LED) is a very effective tool for accurately quantifying electrical spin injection in a model independent manner. We resolve and identify various components which occur in the electroluminescence (EL) spectra of GaAs quantum-well-based spin-LEDs, and examine the circular polarization of each. While a number of components exhibit significant circular polarization, the values do not necessarily reflect the electrical spin injection efficiency. We show that a reliable measure of spin injection efficiency can be obtained only if one takes care to spectroscopically resolve and accurately identify the free exciton or free carrier components of the EL spectrum, and exclude other components.

Determination of interface atomic structure and its impact on spin transport using Z-contrast microscopy and density-functional theory.

We combine Z-contrast scanning transmission electron microscopy with density-functional-theory calculations to determine the atomic structure of the interface in spin-polarized light-emitting diodes. A 44% increase in spin-injection efficiency occurs after a low-temperature anneal, which produces an ordered, coherent interface consisting of a single atomic plane of alternating Fe and As atoms. First-principles transport calculations indicate that the increase in spin-injection efficiency is due to the abruptness and coherency of the annealed interface.

Synthesis of Large-Area WS2 monolayers with Exceptional Photoluminescence

Monolayer WS2 offers great promise for use in optical devices due to its direct bandgap and high photoluminescence intensity. While fundamental investigations can be performed on exfoliated material, large-area and high quality materials are essential for implementation of technological applications. In this work, we synthesize monolayer WS2 under various controlled conditions and characterize the films using photoluminescence, Raman and x-ray photoelectron spectroscopies. We demonstrate that the introduction of hydrogen to the argon carrier gas dramatically improves the optical quality and increases the growth area of WS2, resulting in films exhibiting mm2 coverage. The addition of hydrogen more effectively reduces the WO3 precursor and protects against oxidative etching of the synthesized monolayers. The stoichiometric WS2 monolayers synthesized using Ar + H2 carrier gas exhibit superior optical characteristics, with photoluminescence emission full width half maximum (FWHM) values below 40 meV and emission intensities nearly an order of magnitude higher than films synthesized in a pure Ar environment.

Optical control of charged exciton states in tungsten disulfide

A method is presented for optically preparing WS2 monolayers to luminescence from only the charged exciton (trion) state–completely suppressing the neutral exciton. When isolating the trion state, we observed changes in the Raman A1g intensity and an enhanced feature on the low energy side of the E12g peak. Photoluminescence and optical reflectivity measurements confirm the existence of the prepared trion state. This technique also prepares intermediate regimes with controlled luminescence amplitudes of the neutral and charged exciton. This effect is reversible by exposing the sample to air, indicating the change is mitigated by surface interactions with the ambient environment. This method provides a tool to modify optical emission energy and to isolate physical processes in this and other two-dimensional materials.