Electrical Injection Of Spin-polarized Electrons Research Articles

Spin injection through energy-band symmetry matching with high spin polarization in atomically controlled ferromagnet/ferromagnet/semiconductor structures

Electrical injection of spin-polarized electrons from ferromagnets into semiconductors has been generally demonstrated through a tunneling process with insulator barrier layers that can dominate the device performance, including the electric power at the electrodes. Here, we show an efficient spin injection technique for a semiconductor using an atomically controlled ferromagnet/ferromagnet/semiconductor heterostructure with low-resistive Schottky-tunnel barriers. On the basis of symmetry matching of the electronic bands between the top highly spin-polarized ferromagnet and the semiconductor, the magnitude of the spin signals in lateral spin-valve devices can be enhanced by up to one order of magnitude compared to those obtained with conventional ferromagnet/semiconductor structures. This approach provides a new solution for the simultaneous achievement of highly efficient spin injection and low electric power at the electrodes in semiconductor devices, leading to novel semiconductor spintronic architectures at room temperature.

Spin-injection-induced gain anisotropy in a polariton diode laser

Optical effects arising from spin-induced gain anisotropy such as threshold reduction and emission intensity enhancement, hitherto unobserved in electrically injected polariton lasers, are predicted theoretically for a bulk GaN-based exciton-polariton diode laser operated with electrical injection of spin-polarized electrons. These phenomena are deduced from a simplified spin-dependent rate equation model. We also demonstrate an electrical excitation scheme, which can amplify the degree of a deterministic circular polarization of the output emission by an order of magnitude, compared to the injected electron spin polarization, above threshold.

Room-Temperature Spin Polariton Diode Laser.

A spin-polarized laser offers inherent control of the output circular polarization. We have investigated the output polarization characteristics of a bulk GaN-based microcavity polariton diode laser at room temperature with electrical injection of spin-polarized electrons via a FeCo/MgO spin injector. Polariton laser operation with a spin-polarized current is characterized by a threshold of ∼69 A/cm^{2} in the light-current characteristics, a significant reduction of the electroluminescence linewidth and blueshift of the emission peak. A degree of output circular polarization of ∼25% is recorded under remanent magnetization. A second threshold, due to conventional photon lasing, is observed at an injection of ∼7.2 kA/cm^{2}. The variation of output circular and linear polarization with spin-polarized injection current has been analyzed with the carrier and exciton rate equations and the Gross-Pitaevskii equations for the condensate and there is good agreement between measured and calculated data.

Anomalous scaling of spin accumulation in ferromagnetic tunnel devices with silicon and germanium

The magnitude of spin accumulation created in semiconductors by electrical injection of spin-polarized electrons from a ferromagnetic tunnel contact is investigated, focusing on how the spin signal detected in a Hanle measurement varies with the thickness of the tunnel oxide. An extensive set of spin-transport data for Si and Ge magnetic tunnel devices reveals a scaling with the tunnel resistance that violates the core feature of available theories, namely, the linear proportionality of the spin voltage to the injected spin current density. Instead, an anomalous scaling of the spin signal with the tunnel resistance is observed, following a power law with an exponent between 0.75 and 1 over 6 decades. The scaling extends to tunnel resistance values larger than 10 9 �μ m 2 , far beyond the regime where the classical impedance mismatch or back flow into the ferromagnet play a role.Thisscalingisincompatiblewithexistingtheoryfordirecttunnelinjectionofspinsintothesemiconductor.It also demonstrates conclusively that the large spin signal does not originate from two-step tunneling via localized states near the oxide/semiconductor interface. Control experiments show that spin accumulation in localized states within the tunnel barrier or artifacts are also not responsible. Altogether, the scaling results suggest that, contrary to all existing descriptions, the spin signal is proportional to the applied bias voltage, rather than the (spin) current.

Electrical injection of spin-polarized electrons and electrical detection of dynamic nuclear polarization using a Heusler alloy spin source

We demonstrated electrical spin injection from a half-metallic Heusler alloy Co${}_{2}$MnSi electrode into a GaAs channel through observation of a spin-valve signal and a Hanle signal in the four-terminal nonlocal geometry. Furthermore, we electrically detected a nuclear field acting on electron spins, which was produced by the dynamic nuclear polarization, through observation of transient oblique Hanle signals. Samples with a Co${}_{2}$MnSi spin source exhibited higher spin-injection efficiency and a larger nuclear field compared to samples with a Co${}_{50}$Fe${}_{50}$ spin source, suggesting that the spin polarization of Co${}_{2}$MnSi is higher. This higher polarization is promising for realizing future spintronic devices and for understanding spin interactions as well as spin-dependent transport properties in a semiconductor channel.

Electrical Spin Injection into InN Semiconductor Nanowires

We report on the conditions necessary for the electrical injection of spin-polarized electrons into indium nitride nanowires synthesized from the bottom up by molecular beam epitaxy. The presented results mark the first unequivocal evidence of spin injection into III-V semiconductor nanowires. Utilizing a newly developed preparation scheme, we are able to surmount shadowing effects during the metal deposition. Thus, we avoid strong local anisotropies that arise if the ferromagnetic leads are wrapping around the nanowire. Using a combination of various complementary techniques, inter alia the local Hall effect, we carried out a comprehensive investigation of the coercive fields and switching behaviors of the cobalt micromagnetic spin probes. This enables the identification of a range of aspect ratios in which the mechanism of magnetization reversal is single domain switching. Lateral nanowire spin valves were prepared. The spin relaxation length is demonstrated to be about 200 nm, which provides an incentive to pursue the route toward nanowire spin logic devices.

Nuclear spin‐polarization in single InGaAs quantum dots through electrical and optical spin‐injection in spin‐LEDs

AbstractWe investigate nuclear spin polarization in a single InGaAs quantum dot via electrical injection of spinpolarized electrons in a spin‐injection light‐emitting diode. The Overhauser shift in the single‐dot electroluminescence is compared with results obtained for optical spin injection in the same structure.The results clearly demonstrate the possibility of efficient nuclear spin polarization by purely electrical means which might be useful for future spintronic applications. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Lateral Spin Injection in Germanium Nanowires

Electrical injection of spin-polarized electrons into a semiconductor, large spin diffusion length, and an integration friendly platform are desirable ingredients for spin-based devices. Here we demonstrate lateral spin injection and detection in germanium nanowires, by using ferromagnetic metal contacts and tunnel barriers for contact resistance engineering. Using data measured from over 80 samples, we map out the contact resistance window for which lateral spin transport is observed, manifestly showing the conductivity matching required for spin injection. Our analysis, based on the spin diffusion theory, indicates that the spin diffusion length is larger than 100 mum in germanium nanowires at 4.2 K.

Efficient spin injection into semiconductor from an Fe/GaOx tunnel injector

We examined the electrical injection of spin-polarized electrons into a GaAs-based light-emitting diode structure from a Fe/GaOx tunnel injector whose electron-charge injection efficiency was comparable to that of a conventional Fe/n+-AlGaAs ohmic injector. A high circular polarization of electroluminescence up to 20% was observed at 2 K. The combination of effective spin-and charge-injection efficiencies makes GaOx a promising tunnel barrier for GaAs-based spintronic devices.

Spin injection light-emitting diode with vertically magnetized ferromagnetic metal contacts

We analyze the electrical injection of spin-polarized electrons into a (GaIn)As∕GaAs light-emitting diode. Using an Fe∕Tb multilayer structure with perpendicular magnetic anisotropy and a reverse-biased Schottky contact, we demonstrate spin injection even in remanence between 90 and 260K. The maximum degree of circular polarization of the emitted light is 0.75% at 90K.

Electrical spin pumping of quantum dots at room temperature

We report on electrical control of the spin polarization of InAs∕GaAs self-assembled quantum dots (QDs) at room temperature. This is achieved by electrical injection of spin-polarized electrons from an Fe Schottky contact. The circular polarization of the QD electroluminescence shows that a 5% electron spin polarization is obtained in the InAs QDs at 300K, which is remarkably insensitive to temperature. This is attributed to suppression of the spin-relaxation mechanisms in the QDs due to reduced dimensionality. These results demonstrate that practical regimes of spin-based operation are clearly attainable in solid-state semiconductor devices.

Electrical spin injection from an n-type ferromagnetic semiconductor into a III–V device heterostructure

The use of carrier spin in semiconductors is a promising route towards new device functionality and performance. Ferromagnetic semiconductors (FMSs) are promising materials in this effort. An n-type FMS that can be epitaxially grown on a common device substrate is especially attractive. Here, we report electrical injection of spin-polarized electrons from an n-type FMS, CdCr(2)Se(4), into an AlGaAs/GaAs-based light-emitting diode structure. An analysis of the electroluminescence polarization based on quantum selection rules provides a direct measure of the sign and magnitude of the injected electron spin polarization. The sign reflects minority rather than majority spin injection, consistent with our density-functional-theory calculations of the CdCr(2)Se(4) conduction-band edge. This approach confirms the exchange-split band structure and spin-polarized carrier population of an FMS, and demonstrates a litmus test for these FMS hallmarks that discriminates against spurious contributions from magnetic precipitates.

Optical investigation of electrical spin injection into semiconductors

We investigate the electrical injection of spin-polarized electrons into a semiconductor [Al(GaAs)] heterostructure from ferromagnetic FeCo metal through an ${\mathrm{AlO}}_{x}$ tunnel barrier. We have developed the optical oblique Hanle effect approach for the quantitative analysis of electrical spin injection into semiconductors. This technique is based on the manipulation of the electron spins within a semiconductor when spin polarized electrons have been injected. This allows us to clearly separate the effects caused by spin injection from others, that are magneto-optical, Zeeman, etc. Simultaneously, the oblique Hanle effect approach provides additional information on the spin dynamics in the semiconductor. In the ${\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{o}/\mathrm{A}\mathrm{l}\mathrm{O}}_{x}/\mathrm{Al}(\mathrm{GaAs})$ heterostructures we observe spin injection of 21% and 16% at 80 and 300 K, respectively. The importance of electron thermalization effects and the impact of the doping level of the semiconductor for practical investigation of spin injection by optical means are demonstrated.

Progress toward electrical injection of spin-polarized electrons into semiconductors

The use of carrier spin as a new degree-of-freedom in semiconductor devices offers new functionality and performance. However, efforts to implement semiconductor spintronics have been crippled by the lack of an efficient and practical means to electrically inject spin polarized carriers into a semiconductor device heterostructure. This paper summarizes progress toward that end using magnetic semiconductors and ferromagnetic metals as spin injecting contacts. We describe a very successful approach which employs a ferromagnetic metal/tunnel barrier contact, where the tunnel barrier is simply a tailored Schottky barrier which forms naturally between the ferromagnetic metal and the semiconductor itself. Initial efforts have demonstrated electron spin polarizations of at least 32% in GaAs quantum-well LED heterostructures. Significantly higher spin injection efficiencies are anticipated in optimized structures. These results demonstrate that spin injecting contacts can be formed using a very familiar and widely employed contact methodology, providing a ready pathway for the integration of spin transport into semiconductor processing.

Electrical spin injection across air-exposed epitaxially regrown semiconductor interfaces

We have fabricated spin-polarized light-emitting diode structures via epitaxial regrowth of Zn1−xMnxSe on air-exposed surfaces of AlyGa1−yAs/GaAs quantum wells. No passivation procedures were used to protect or prepare the III–V surface. The electroluminescence is strongly circularly polarized due to the electrical injection of spin-polarized electrons from the ZnMnSe contact into the GaAs quantum well. An analysis of the optical polarization yields a lower bound of 65% for the spin injection efficiency. These results demonstrate the robustness of the spin injection process in the diffusive transport regime, and attest to the practicality of manufacturing semiconductor-based spin injection devices.

Experimental search for the electrical spin injection in a semiconductor

The electrical injection of spin-polarized electrons in a semiconductor can be achieved in principle by driving a current from a ferromagnetic metal, where current is known to be significantly spin polarized, into the semiconductor via Ohmic conduction. For detection a second ferromagnet can be used as a drain. We studied submicron lateral spin valve junctions, based on high-mobility InAs/AlSb two-dimensional electron gas, with Ni, Co, and permalloy as ferromagnetic electrodes. In the standard geometry it is very difficult to separate true spin injection from other effects, including local Hall effect, anomalous magnetoresistance contribution from the ferromagnetic electrodes and weak localization/antilocalization corrections, which can closely mimic the signal expected from spin valve effect. The reduction in size, and the use of a multiterminal nonlocal geometry allowed us to reduce the unwanted effects to a minimum. Despite all our efforts, we have not been able to observe spin injection. However, we find that this ``negative'' result in these systems is actually consistent with theoretical predictions for spin transport in diffusive systems.