Thermal Creation of Electron Spin Polarization in n-Type Silicon

Conversion of heat into a spin-current in electron doped silicon can offer a promising path for spin-caloritronics. Here we create an electron spin polarization in the conduction band of n-type silicon by producing a temperature gradient across a ferromagnetic tunnel contact. The substrate heating experiments induce a large spin signal of 95 $\mu$V, corresponding to 0.54 meV spin-splitting in the conduction band of n-type silicon by Seebeck spin tunneling mechanism. The thermal origin of the spin injection has been confirmed by the quadratic scaling of the spin signal with the Joule heating current and linear dependence with the heating power

Efficient Spin Injection into Silicon and the Role of the Schottky Barrier

Implementing spin functionalities in Si, and understanding the fundamental processes of spin injection and detection, are the main challenges in spintronics. Here we demonstrate large spin polarizations at room temperature, 34% in n-type and 10% in p-type degenerate Si bands, using a narrow Schottky and a SiO2 tunnel barrier in a direct tunneling regime. Furthermore, by increasing the width of the Schottky barrier in non-degenerate p-type Si, we observed a systematic sign reversal of the Hanle signal in the low bias regime. This dramatic change in the spin injection and detection processes with increased Schottky barrier resistance may be due to a decoupling of the spins in the interface states from the bulk band of Si, yielding a transition from a direct to a localized state assisted tunneling. Our study provides a deeper insight into the spin transport phenomenon, which should be considered for electrical spin injection into any semiconductor

Gate-tunable Hall sensors on large area CVD graphene protected by h-BN with 1D edge contacts

Graphene is an excellent material for Hall sensors due to its atomically thin structure, high carrier mobility and low carrier density. However, graphene devices need to be protected from the environment for reliable and durable performance in different environmental conditions. Here we present magnetic Hall sensors fabricated on large area commercially available CVD graphene protected by exfoliated hexagonal boron nitride (h-BN). To connect the graphene active regions of Hall samples to the outputs the 1D edge contacts were utilized which show reliable and stable electrical properties. The operation of the Hall sensors shows the current-related sensitivity up to 345 V/(AT). By changing the carrier concentration and type in graphene by the application of gate voltage we are able to tune the Hall sensitivity

Room Temperature Electrical Detection of Spin Polarized Currents in Topological Insulators

Topological insulators (TIs) are a new class of quantum materials that exhibit spin momentum locking (SML) of massless Dirac fermions in the surface states. Usually optical methods, such as angle and spin-resolved photoemission spectroscopy, have been employed to observe the helical spin polarization in the surface states of three-dimensional (3D) TIs up to room temperatures. Recently, spin polarized surface currents in 3D TIs were detected by electrical methods using ferromagnetic (FM) contacts in a lateral spin-valve measurement geometry. However, probing the spin texture with such electrical approaches is so far limited to temperatures below 125K, which restricts its application potential. Here we demonstrate the room temperature electrical detection of the spin polarization on the surface of Bi$_2$Se$_3$ due to SML by employing spin sensitive FM tunnel contacts. The current-induced spin polarization on the Bi$_2$Se$_3$ surface is probed at room temperature by measuring a spin-valve signal while switching the magnetization direction of the FM detector. The spin signal increases linearly with current bias, reverses sign with current direction, exhibits a weak temperature dependence and decreases with higher TI thickness, as predicted theoretically. Our results demonstrate the electrical detection of the spin polarization on the surface of 3D TIs, which could lead to innovative spin-based quantum information technology at ambient temperatures.Comment: Incl. Supplementary informatio

Origin and evolution of surface spin current in topological insulators

The Dirac surface states of topological insulators offer a unique possibility for creating spin polarized charge currents due to the spin-momentum locking. Here we demonstrate that the control over the bulk and surface contribution is crucial to maximize the charge-to-spin conversion efficiency. We observe an enhancement of the spin signal due to surface-dominated spin polarization while freezing out the bulk conductivity in semiconducting Bi1.5Sb0.5Te1.7Se1.3 below 100K. Detailed measurements up to room temperature exhibit a strong reduction of the magnetoresistance signal between 2 and 100K, which we attribute to the thermal excitation of bulk carriers and to the electron-phonon coupling in the surface states. The presence and dominance of this effect up to room temperature is promising for spintronic science and technology

Effect of high-k dielectric and ionic liquid gate on nanolayer black-phosphorus field effect transistors

Nanolayer black phosphorus (BP) is a direct bandgap semiconducting two dimensional crystal, showing immense promise for future nanoelectronic devices. Here, we report the effect of high-k dielectric and ionic-liquid gate in BP field effect transistors (BP FET). An ambipolar behavior is observed in pristine BP FETs with current modulation of 104. With a high-k HfO2 encapsulation, we observed identical switching performance in the BP FETs, however, with noticeable enhancement in mobility at room temperature. In comparison to the pristine device, the HfO2 encapsulation showed a contrasting decrease in mobility at lower temperatures. BP FETs with electric double layer ionic liquid gate showed a drastic improvement in the subthreshold swing (SS) to 173mV/dec and operation voltages less than 0.5V in comparison to solid state SiO2 back gated devices. Our results elucidate the effect of different electrostatic conditions on BP transistor channels and open up ways for further exploration of their prospects for nanoelectronic devices and circuits

Enhanced Tunnel Spin Injection into Graphene using Chemical Vapor Deposited Hexagonal Boron Nitride

The van der Waals heterostructures of two-dimensional (2D) atomic crystals constitute a new paradigm in nanoscience. Hybrid devices of graphene with insulating 2D hexagonal boron nitride (h-BN) have emerged as promising nanoelectronic architectures through demonstrations of ultrahigh electron mobilities and charge-based tunnel transistors. Here, we expand the functional horizon of such 2D materials demonstrating the quantum tunneling of spin polarized electrons through atomic planes of CVD grown h-BN. We report excellent tunneling behavior of h-BN layers together with tunnel spin injection and transport in graphene using ferromagnet/h-BN contacts. Employing h-BN tunnel contacts, we observe enhancements in both spin signal amplitude and lifetime by an order of magnitude. We demonstrate spin transport and precession over micrometer-scale distances with spin lifetime up to 0.46 nanosecond. Our results and complementary magnetoresistance calculations illustrate that CVD h-BN tunnel barrier provides a reliable, reproducible and alternative approach to address the conductivity mismatch problem for spin injection into graphene

One-dimensional ferromagnetic edge contacts to two-dimensional graphene/h-BN heterostructures

We report the fabrication of one-dimensional (1D) ferromagnetic edge contacts to two-dimensional (2D) graphene/h-BN heterostructures. While aiming to study spin injection/detection with 1D edge contacts, a spurious magnetoresistance signal was observed, which is found to originate from the local Hall effect in graphene due to fringe fields from ferromagnetic edge contacts and in the presence of charge current spreading in the nonlocal measurement configuration. Such behavior has been confirmed by the absence of a Hanle signal and gate-dependent magnetoresistance measurements that reveal a change in sign of the signal for the electron- and hole-doped regimes, which is in contrast to the expected behavior of the spin signal. Calculations show that the contact-induced fringe fields are typically on the order of hundreds of mT, but can be reduced below 100 mT with careful optimization of the contact geometry. There may be additional contribution from magnetoresistance effects due to tunneling anisotropy in the contacts, which need to be further investigated. These studies are useful for optimization of spin injection and detection in 2D material heterostructures through 1D edge contacts