Session P33: Focus Session: Mostly Spin Injection in Si

Generation, Modulation and Electrical Detection of Spin Currents in Silicon in a Lateral Transport Geometry

The electron's spin angular momentum is one of several alternative state variables under consideration on the \textit{International Technology Roadmap for Semiconductors}. Electrical injection / transport of spin-polarized carriers is prerequisite for developing such an approach. While significant progress has been realized in GaAs, little has been made in Si. Electrical injection of spin-polarized electrons is demonstrated in Fe/Al2O3/Si (001) n-i-p structures by measuring the circular polarization of the electroluminescence (EL). The EL polarization tracks the Fe magnetization, confirming spin injection into the Si, and reflects Fe majority spin, consistent with the common delta{\_}1-symmetry of the Fe and Si bands. The Si spin polarization is $\sim $30{\%} at 5K, with significant polarization extending to at least 125K. These results are confirmed in Fe/Al2O3/Si/AlGaAs/GaAs quantum well structures -- the GaAs EL shows that spin transport occurs despite poor crystalline quality of Si epilayers on GaAs, the 0.3 eV Si/AlGaAs CB offset, and air exposure of the interfaces. Lateral transport structures and non-local detection techniques are used to create a spin current which flows separately from the spin-polarized charge current. This spin diffusion current is sensitive to the relative magnetizations of the injecting and detecting contacts, and can be modulated by a perpendicular magnetic field (Hanle effect) which causes precession in the transport channel. The generation of spin currents, coherent spin precession and electrical detection using magnetic tunnel barrier contacts and a simple lateral device geometry compatible with ``back-end'' silicon processing will facilitate development of silicon-based spintronic devices. \newline Refs: \textit{Nature Physics} \textbf{3}, 542 (2007); \textit{Appl. Phys. Lett} \textbf{91}, 212109 (2007).   

Electrical Spin injection into Silicon: a comparison between Fe/Schottky and Fe/Al$_{2}$O$_{3}$ tunnel contacts

We have recently demonstrated successful electrical injection of spin-polarized electrons from an Fe film through an Al$_{2}$O$_{3}$ tunnel barrier into Si [1]. The spin polarization in the Si is $\sim $30{\%} at 5K, with significant polarization sustained to at least 125K. In this study we compare electrical spin injection from Fe into MBE grown Si n-i-p heterostructures using different tunnel barriers- a reversed biased Fe/Si Schottky contact and an Fe/Al$_{2}$O$_{3}$ barrier. For both types of structures the electroluminescence (EL) spectra are dominated by transverse acoustic and optical phonon emissions in the Si. The surface emitted circular polarization of the EL due to radiative recombination in the Si tracks the Fe magnetization, confirming that the spin-polarized electrons originate from the Fe for both types of samples. However, the polarization is lower for the Fe/Si contact than that of the Fe/Al$_{2}$O$_{3}$/Si system. Systematic TEM analysis has been performed to correlate the interface structure with the observed optical polarization, and reveals some Fe/Si intermixing which is absent in the Fe/ Al$_{2}$O$_{3}$/Si structure. [1] B.T. Jonker et al., Nature Physics \textbf{3}, 542 (2007). This work was supported by ONR and core programs at NRL.   

Spin transport through n-type doped silicon using electrical methods

In this presentation, we report on all-electrical injection, transport, and detection of spin-polarized electrons through a 3um n-type Phosphorus-doped single-crystal silicon device. Using our hot-electron methods, we demonstrate both spin-valve behavior in an in-plane magnetic field and spin precession in a perpendicular magnetic field. Voltage spectroscopy reveals the effects of charge screening and band bending in the spin transport layer which are not evident in the operation of our previously-studied undoped silicon devices [1,2]. \newline References \newline [1] Ian Appelbaum et al. Nature 447, 295 (2007). \newline [2] Biqin Huang et al. Phys. Rev. Lett. 99, 177209 (2007).   

Electrical injection and detection of spin-polarized carriers in silicon in a lateral transport geometry

Significant progress has recently been made on spin injection into the technologically important semiconductor, Si, using vertical device structures. $^{1,2}$ We present the electrical injection, detection and magnetic field modulation of lateral diffusive spin transport through silicon using surface contacts. Fe/Al2O3 tunnel barrier contacts are used to create and analyze the flow of pure spin current in a silicon transport channel. Non-local detection techniques show that the spin current detected after transport through the silicon is sensitive to the relative orientation of the magnetization of the injecting and detecting contacts. Hanle effect measurements demonstrate that the spin current can be modulated by a perpendicular magnetic field, which causes the spin to precess and dephase in the transport channel. The realization of efficient electrical injection and detection using a tunnel barriers and a simple device geometry compatible with ``back-end'' Si processing should greatly facilitate development of Si-based spintronics. This work was supported by ONR and core NRL programs. 1. Jonker et. al., Nat. Phys. 3, 542 (2007) 2. Applebaum et. al., Nat. 447, 295 (2007)   

Spin Transport in Silicon

Silicon has been broadly viewed as the ideal material for spintronics due to its low atomic weight, lattice inversion symmetry, and near lack of nuclear spin, resulting in exceptionally long spin lifetime. Despite this appeal, however, the experimental difficulties of achieving coherent spin transport in silicon were overcome for the first time only recently, by using unique spin-polarized hot-electron injection and detection techniques. [1] Our subsequent observations of very long spin lifetimes and transit lengths [2] have impact on prospects for Silicon spintronics as the basis for a new paradigm of information processing. \newline \newline [1] Ian Appelbaum, Biqin Huang, and Douwe J. Monsma, ``Electronic measurement and control of spin transport in silicon,'' Nature 447, 295 (2007). \newline [2] Biqin Huang, Douwe J. Monsma, and Ian Appelbaum, ``Coherent spin transport through a 350-micron-thick silicon wafer,'' Phys. Rev. Lett. 99, 177209 (2007).   

All-epitaxial heterostructure for tunneling spins into silicon

An all-epitaxial spin-tunnel structure has been constructed using molecular-beam epitaxy (MBE). The structure consists of an epitaxial layer of iron ($\sim $100 {\AA} thick) on commensurately strained SrTiO$_{3}$ ($\sim $20 {\AA} thick) on (100) Si. The thin SrTiO$_{3}$ layer serves simultaneously as a tunnel barrier for spin polarized currents and as a protective layer preventing the reaction between iron and the underlying silicon which would lead to the formation of an undesired iron silicide. While the iron film was grown in ultra high vacuum, the growth of the SrTiO$_{3}$ film on silicon was accomplished using molecular oxygen via a sequence of steps by which the formation of an interfacial amorphous silicon dioxide layer is kinetically suppressed. Magnetic measurements indicate strong magnetic anisotropy with the easy axis lying in the plane of the film and a curie temperature above 400 K. Electrical measurements probing spin injection and detection in microfabricated Fe-SrTiO$_{3}$-Si-SrTiO$_{3}$-Fe devices, where the ferromagnetic electrodes have different coercive fields due to size anisotropy, will be discussed.   

Tunneling Characteristics across Nano-Scale Metal Ferric Junction Lines into Doped Si

Tunneling properties were studied on nanofabricated metal ferric tunnel structures on phosphorus doped silicon by measuring $I-V$ characteristics and differential conductance versus bias over a wide temperature range between 80 K to 325 K. These properties were found to have very weak temperature dependences up to 250 K. Such temperature independencies in transport properties demonstrated tunneling characteristics from metal ferric nano-lines into Si via AlO$_{x}$ insulating barrier. Nanoscaled spin-dependent tunneling (STD) lines were patterned on doped Si with the injection contacts having the form of long strips with width and separation of 100 nm and several micron long patterned by e-beam lithography. The measured tunneling coefficient was nearly independent of the bias below 1.0 V, and abruptly increases above the threshold, indentifying such threshold as tunneling barrier height. The thermal transport of the active Si region demonstrated a direct correlation between thermal activation of deep levels (of 0.4 eV) in bulk Si and metal-semiconductor tunneling, revealing feasible mechanisms influencing the interfacial transport.   

Tunnel magnetoresistance of spin tunnel contacts to silicon.

For the development of silicon-based spintronic devices, careful design of the contacts between ferromagnet and semiconductor is crucial, as the resistance and potential energy landscape critically affects spin flow across the interface. One approach to engineer spin tunnel contacts to Si uses low work function materials, inserted between the ferromagnet (FM) and the insulator of FM/Al2O3/Si tunnel contacts [1]. Here we present another route to tune the properties of FM/Al2O3/Si contacts by exposure of the Si surface to Alkali metal atoms, such as Cs, prior to preparation of the tunnel barrier. This is surprisingly effective in reducing the Schottky barrier height, and we will present a series of measurements that elucidate the mechanism. Moreover, we show that the band bending near the contact can be inverted, leading to the formation of a two-dimensional electron gas observable in tunneling spectroscopy, and giving rise to novel tunnel magnetoresistance. [1] B.C. Min et al. Nature Materials 5, 817 (2006).   

Spin blockade at semiconductor/ferromagnet junctions

We study theoretically extraction of spin-polarized electrons at nonmagnetic semiconductor/perfect ferromagnet junctions. The outflow of majority-spin electrons from the semiconductor into the ferromagnet leaves a cloud of minority-spin electrons in the semiconductor region near the junction, forming a local spin-dipole configuration at the semiconductor/ferromagnet interface. This minority-spin cloud can limit the majority-spin current through the junction, creating a pronounced spin blockade at a critical current. We calculate the critical spin-blockade current in both planar and cylindrical geometries and discuss possible experimental tests of our predictions. [1] Yu. V. Pershin and M. Di Ventra, Phys. Rev. B \textbf{75}, 193301 (2007). [2] Yu. V. Pershin and M. Di Ventra, arXiv:0707.4475.   

Spin injection and transport in graphene layers

Graphene is an intriguing material system for spintronics research. Due to its low atomic number and low spin-orbit coupling, graphene is an excellent candidate for spin transport. In our past study, we have demonstrated spin-polarized transport in mesoscopic graphite by two-probe spin-valve devices [1]. Recently, we further investigated this topic and fabricated non-local spin-valve devices consisting of single-layer and few-layer graphene. Ferromagnet (FM) and nonmagnetic electrodes are formed by using electron beam (e-beam) lithography and e-beam evaporation. Thin tunnel barriers consisting of magnesium oxide are inserted between graphene layers and FM electrodes to overcome conductivity mismatch and enhance spin injection efficiency. Atomic force microscopy and Auger spectroscopy are used to characterize their morphology and chemical composition. We performed magneto-transport using the Johnson-Silsbee geometry in a cryogenic environment and observed non-local spin signal up to room temperature. This unambiguously demonstrates the spin injection, transport and detection in graphene materials. [1] W.-H. Wang, \textit{et. al.}, Phys. Rev. B (Rapid Communications), in press.