A spin-dependent and electric-field tunable magnetoresistance (MR) of a semiconducting (SC) channel placed between two ferromagnetic (FM) contacts is a key ingredient in many novel spin-based device concepts. Whereas successful realization of such devices requires a large magnetoresistance signal, the signals measured in semiconductor-based devices are usually very low, well below 1\%, because of highly resistive tunnel FM/SC interfaces. In this talk, we will discuss how the finite electric fields effects in lateral FM/SC/FM devices lead to enhancement of the measured magnetoresistance by increasing the efficiency of the spin transport in the channel and boosting spin-to-charge conversion at the FM/SC interface. We will illustrate this discussion with the results of our recent experiments on lateral all-semiconductor spin valve devices with a transport channel formed in the two-dimensional electron system embedded in GaAs/(Al,Ga)As interface and with ferromagnetic (Ga,Mn)As/GaAs Esaki diodes as source and drain contacts [1]. We have measured very large two-terminal spin valve signals, in order of 1 kOhm in such devices, with MR reaching even up to 80\% in the nonlinear regime of the current-voltage characteristic [2]. We will also demonstrate that the MR signal can be additionally tuned by means of an electric gate, with the gating scheme based on switching between uni- and bidirectional spin diffusion, without resorting to spin–orbit coupling.
The work has been supported by Deutsche Forschungsgemeinschaft (DFG) through SFB689.
[1] M. Oltscher et al., Phys. Rev. Lett. 113, 236602 (2014)
[2] M. Oltscher et al., Nature Commun. 8, 1807 (2017)
KEYWORDS: Ferromagnetics, Semiconductors, Signal processing, Spintronics, Field effect transistors, Heterojunctions, Sensors, Instrument modeling, Magnetic semiconductors, Current controlled current source
Effective electrical spin injection into two-dimensional electron gas (2DEG) is a prerequisite for many new functionalities in spintronic device concepts, with the Datta-Das spin field effect transistor [1] being a primary example. Here we will discuss some of the results of our studies on spin injection devices with high mobility 2DEG confined in an inverted AlGaAs/GaAs heterojunction and a diluted ferromagnetic semiconductor (Ga,Mn)As employed as a source and a detector of spin-polarized carriers. Firstly we will show that nonlocal spin valve signal in such devices can significantly exceed the prediction of the standard model of spin injection based on spin drift-diffusion equations [2], what leads to conclusion that ballistic transport in the 2D region directly below the injector should be taken into account to fully describe the spin injection process [3]. Furthermore, we demonstrate also a large magnetoresistance (MR) signal of ~20% measured in local configuration, i.e., with spin–polarized current flowing between two ferromagnetic contacts. To our knowledge, this is the highest value of MR observed so far in semiconductor channels.
The work has been supported by Deutsche Forschungsgemeinschaft (DFG) through SFB689.
[1] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990)
[2] M. Oltscher et al., Phys. Rev. Lett. 113, 236602 (2014)
[3] K. Cheng and S. Zhang, Phys. Rev. B 92, 214402 (2015)
We study the contact resistance of strongly doped ferromagnetic/non-magnetic semiconductors structure (p+ - F/n+ -N), working as spin injectors and spin extractors. Because of the strong effect that the barrier shape has on the tunneling probabilities, we evaluate, as accurately as possible, the quantum-mechanical spin-dependent transmission across the Esaki barrier built up at the p-n interface. To simplify the discussion and calculation of transmission coefficients through the Esaki barrier, we concentrate here on the structure p+ - F/n+ -N, without a stop layer I. We evaluate the spin injection and spin extraction transmission coefficients T↑↑ and T↓↓, and the spin transmission polarization as functions of bias potential, exchange interaction energy and Fermi energy level for specific realizations of the structure p+-Ga1-xMnxAs/n+-GaAs.
The dichalcogenide MoS2, which is an indirect-gap semiconductor in its bulk form, was recently shown to become
an efficient emitter of photoluminescence as it is thinned to a single layer, indicating a transition to a direct-gap
semiconductor due to confinement effects. With its layered structure of weakly coupled, covalently bonded twodimensional
sheets, it can be prepared, just as graphene, using mechanical exfoliation techniques. With these
techniques, few- and single-layer flakes can be prepared. Raman spectroscopy is a sensitive tool to determine the
number of layers of a flake, as two characteristic Raman modes in MoS2 shift to higher or lower frequency with
the number of layers. In addition to previously reported Raman modes in MoS2, we observe an interlayer shear
mode at very low frequencies, which also shifts with the number of layers. We use scanning Raman spectroscopy
to map and characterize MoS2 flakes.
The dichalcogenide MoS2, which is an indirect-gap semiconductor in its bulk form, was recently shown to become
an efficient emitter of photoluminescence as it is thinned to a single layer, indicating a transition to a direct-gap
semiconductor due to confinement effects. With its layered structure of weakly coupled, covalently bonded
two-dimensional sheets, it can be prepared, just as graphene, using mechanical exfoliation techniques. Here, we
present temperature-dependent and time-resolved photoluminescence (PL) studies of single-layer MoS2 flakes.
Some of the flakes are covered with oxide layers prepared by atomic layer deposition (ALD). At low temperatures,
we clearly see two PL peaks in the as-prepared flakes without oxide layers, which we may assign to bound and
free exciton transitions. The lower-energy, bound exciton PL peak is absent in the oxide-covered flakes. In
time-resolved PL measurements, we observe very fast photocarrier recombination on the few-ps timescale at low
temperatures, with increasing photocarrier lifetimes at higher temperatures due to exciton-phonon scattering.
We report about scanning Raman experiments on, both, as deposited and nano-structured graphene flakes. The
Raman scans allow us to extract spatially resolved information about frequencies, intensities and linewidths of
the observed phonon modes. In nano-structured single-layer flakes, where periodic arrays of holes (antidots) were
fabricated by electron-beam lithography and subsequent etching, we find a systematic dependence of the phonon
frequencies, intensities and linewidths on the periods and hole sizes of the nano-patterned regions. A systematic
shift of the G mode frequency evidences a doping effect in the nano-patterned regions. In order to calibrate the
doping dependence of the G mode phonon frequency, we have investigated the position and linewidth of this
mode in a gated single-layer flake. With this calibration, we can quantitatively determine the doping level, which
is introduced via preparation of the periodic hole arrays into the samples. A comparison of G and 2D mode
frequencies allows us to identify the doping to be of p-type.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.