31 research outputs found
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 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 BiSe due to SML by employing spin
sensitive FM tunnel contacts. The current-induced spin polarization on the
BiSe 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