Vasyl Motsnyi

Electrical spin injection in a ferromagnet/tunnel barrier/semiconductor heterostructure

We demonstrate experimentally the electrical electron spin injection from a ferromagnetic metal/tunnel barrier contact into a III–V semiconductor light emitting diode (LED). The injected electrons have in-plane spin orientation. By applying a relatively small oblique external magnetic field this spin orientation within the semiconductor can be manipulated to have a nonzero out-of-plane component. By measuring the resulting circular polarization of the emitted light, we observe injected spin polarization in excess of 9% at 80 K in a CoFe/AlOx/(Al,Ga)As/GaAs surface-emitting spin-LED.

Very high spin polarization in GaAs by injection from a (Ga,Mn)As Zener diode

We demonstrate an electrically injected electron spin polarization in GaAs of 80% at 4.6 K by interband tunneling from the valence band of (Ga,Mn)As into an (Al,Ga)As light-emitting diode. The polarization is analyzed by the oblique Hanle effect and vanishes at 120 K, the Curie temperature of the (Ga,Mn)As injector. The temperature and the bias dependence of the polarization are explained in terms of the properties of the (Ga,Mn)As/GaAs diode.

Technology and materials issues in semiconductor-based magnetoelectronics

There has been an increased interest in the introduction of magnetic thin films into semiconductors. This interest is motivated by the benefit found in using the magnetic thin-film properties (giant or tunnelling magnetoresistance and hysteresis) in magnetic memory (MRAM) products. Furthermore, the use of the electron spin in electronic, spintronic devices requires intimate ferromagnetic/semiconductor combinations. We review the technology and materials aspects of both the MRAM and spintronics fields that highlight the challenges that must be overcome in order to make magnetic (multilayer) films a standard ingredient in future electronics.

Low-temperature photoluminescence study of thin epitaxial GaAs films on Ge substrates

Thin epitaxial GaAs films, with thickness varying from 140to1000nm and different Si doping levels, were grown at 650°C by organometallic vapor phase epitaxy on Ge substrates and analyzed by low-temperature photoluminescence (PL) spectroscopy. All spectra of thin GaAs on Ge show two different structures, one narrow band-to-band (B2B) structure at an energy of ∼1.5eV and a broad inner-band-gap (IB) structure at an energy of ∼1.1eV. Small strain in the thin GaAs films causes the B2B structure to be separated into a light-hole and a heavy-hole peak. At 2.5K the good structural quality of the thin GaAs films on Ge can be observed from the narrow excitonic peaks. Peak widths of less than 1meV are measured. GaAs films with thickness smaller than 200nm show B2B PL spectra with characteristics of an n-type doping level of approximately 1018at.∕cm3. This is caused by heavy Ge diffusion from the substrate into the GaAs at the heterointerface between the two materials. The IB structure observed in all films consists ...

Highly efficient room temperature spin injection in a metal-insulator-semiconductor light-emitting diode

We demonstrate highly efficient spin injection at low and room temperature in an AlGaAs/GaAs semiconductor heterostructure from a CoFe/AlOX tunnel spin injector. We use a double-step oxide deposition for the fabrication of a pinhole-free AlOX tunnel barrier. The measurements of the circular polarization of the electroluminescence in the Oblique Hanle Effect geometry reveal injected spin polarizations of at least 24% at 80 K and 12% at room temperature.

Hybrid epitaxial structures for spintronics

The use of electron spin in future spintronic applications requires hybrid ferromagnetic/semiconductor structures with well controlled materials properties. Besides the control of the magnetic properties there are strict requirements for the material aspects of these devices: single phase, single crystal, perfect interfaces, defect control. Molecular beam epitaxy (MBE) has been used very successfully over the past 10 years to produce the most interesting spintronic heterostructures. In this paper we review our effort in using MBE to fabricate spintronic materials and heterostructures. We also discuss some aspects of future spintronic devices with the focus on the material aspects of transferring the electron spin information from the magnetic side of the heterostructure into the semiconductor side.

Mg doping of GaN by molecular beam epitaxy

We present a systematic study on the influence of growth conditions on the incorporation and activation of Mg in GaN layers grown by plasma-assisted molecular beam epitaxy. We show that high quality p-type GaN layers can be obtained on GaN-on-silicon templates. The Mg incorporation and the electrical properties have been investigated as a function of growth temperature, Ga?:?N flux ratio and Mg?:?Ga flux ratio. It was found that the incorporation of Mg and the electrical properties are highly sensitive to the Ga?:?N flux ratio. The highest hole mobility and lowest resistivity were achieved for slightly Ga-rich conditions. In addition to an optimal Ga?:?N ratio, an optimum Mg?:?Ga flux ratio was also observed at around 1%. We observed a clear Mg flux window for p-type doping of GaN?: 0.31% < Mg?:?Ga < 5.0%. A lowest resistivity of 0.98???cm was obtained for optimized growth conditions. The p-type GaN layer then showed a hole concentration of 4.3 ? 1017?cm?3 and a mobility of 15?cm2?V?1?s?1. Temperature-dependent Hall effect measurements indicate an acceptor depth in these samples of 100?meV for a hole concentration of 5.5 ? 1017?cm?3. The corresponding Mg concentration is 5 ? 1019?cm?3, indicating approximately 1% activation at room temperature. In addition to continuous growth of Mg-doped GaN layers we also investigated different modulated growth procedures. We show that a modulated growth procedure has only limited influence on Mg doping at a growth temperature of 800??C or higher. This result is thus in contrast to previously reported GaN?:?Mg doping at much lower growth temperatures of 500??C.

Electrical spin injection in a ferromagnetic metal/insulator/semiconductor tunnel heterostructure

We demonstrate experimentally the electrical spin injection from a ferromagnetic metal/tunnel barrier contact into a semiconductor III–V heterostructure. The injected electrons have an in-plane spin orientation. We show that by applying an oblique external magnetic field this spin orientation can be manipulated within the semiconductor, and a nonzero perpendicular spin component arises. This perpendicular component can be easily monitored by optical means (circular polarization of the emitted light). In a CoFe/AlOx/(Al,Ga)As/GaAs heterostructure we observe injected spin polarization in access of 9% at 80 K and optimized structures have recently shown spin injection up to room temperature.

Oblique Hanle effect for reliable assessment of electrical spin injection

Summary form only given. One important aspect of spintronic devices is the effective electrical spin injection into a semiconductor. Optical methods have proven their efficiency for creation and detection of a spin polarized electrons in III-V semiconductors. Due to the high refractive index of such materials the detected light characterizes only the component of electron spin perpendicular to the surface. However, a vast amount of common ferromagnetic thin films, interesting as sources for electrical spin injection, show in-plane magnetic anisotropy, thus making direct optical measurements practically impossible. In order to overcome this limitation we introduce the oblique Hanle effect (OHE) technique to assess spin injection. The application of the OHE to assess successful electrical spin injection from ferromagnetic contact into semiconductors will be presented.

Efficient spin injection in metal-insulator-semiconductor (MIS) light emitting diodes (invited)

In this paper, we review our results on the MIS (Metal- InsulatorSemiconductor) spin-source approach including some results on a NiMnSb/Schottky barrier spin-source.