Session X42: Spin Transport in III-V and Group IV Semiconductors

Giant spin signals in two-terminal ferromagnet/2DEG/ferromagnet spin-valve devices.

A ferromagnet/nonmagnet/ferromagnet (FM/NM/FM) structure constitute a basic spintronic device, with its two-terminal resistance depending on the spin state of the charge carriers. The primary example here is the Datta-Das spin field effect transistor (sFET)\footnote{S. Datta and B. Das, Appl. Phys. Lett. \textbf{56}, 665 (1990)}, with the nonmagnetic channel defined within a two-dimensional electron gas (2DEG) confined in a semiconductor (SC) structure. Efficient operation of such a device requires a large magnetoresistance signal, defined as $MR=\Delta R/R_P$, where $\Delta R$ is resistance difference between parallel (AP) and antiparallel (AP) configurations of magnetization in ferromagnetic source and drain contacts. In devices with semiconductor channels, however, measured MR signals are usually very low, well below 1\%, because highly resistive tunnel FM/SC interfaces are required to succesfully inject spins into semiconductors.\footnote{A. Fert \textit{et al.}, IEEE Transactions on Electron Devices, \textbf{54}(5), 921 (2007)} In this talk I will present the results of our recent experiments on lateral FM/2DEG/FM devices with a 2DEG channel embedded at GaAs/AlGaAs interface and with ferromagnetic (Ga,Mn)As/GaAs Esaki diodes as source and drain contacts.\footnote{M. Oltscher \textit{et al.}, Phys. Rev. Lett. \textbf{113}, 236602 (2014)} We observed very large two-terminal spin valve signals in such devices, with $\Delta R$ in order of 1k$\Omega$ and MR reaching even up to 80\% in the region of the Esaki dip. I will discuss a strong bias dependence of the signal in terms of the effect of the electric field on the detection sensitivity of the ferromagnetic contacts. I will demonstrate also that the MR signal can be additionally tuned by means of an electric gate, used to modify a distribution of spin accumulation in the 2DEG channel.   

Electrical Spin Injection and Helicity Modulation in a Room Temperature Polariton Laser

Room temperature elliptically polarized inversionless coherent emission, or polariton lasing, is observed from a bulk GaN-based edge-emitting microcavity diode operated with spin-polarized electrical injection. The low nonlinear threshold for polariton lasing occurs at 69 A/cm$^{\mathrm{2}}$ in the light-current characteristics, accompanied by a collapse of the emission linewidth to 1 meV and a blueshift of 1.87 meV of the emission peak. Sub-threshold angle-resolved measurements confirm strong-coupling regime of operation of the microcavity diodes (cavity Q $=$ 3200) with a cavity-to-exciton detuning of -- 11.6 meV and a vacuum-field Rabi splitting of 36.4 meV. Laser operation with a spin-polarized current, after in-plane magnetization, results in a maximum degree of output circular and linear polarization of 47 {\%} and 33 {\%} respectively, above the condensation threshold, at a field of $+$ 1.6 kOe. The magnitude and helicity of the output circular polarization is deterministically governed by the in-plane H field. The results have been analyzed using the Gross-Pitaevskii equations for the spinor exciton-polariton condensate and the calculated results agree with the measured data.   

Spin Lasers: From Microscopic Description to Rate Equations

Commercial spintronic devices usually rely on magnetoresistive effects. Injecting spin-polarized carriers into semiconductor lasers enables devices that use different operating principles and can overcome limitations of conventional (spin-unpolarized) lasers [1]. The conservation of angular momentum and spin-orbit coupling leads to the transfer of angular momentum from spin-polarized carriers to emitted circularly-polarized light. Using microscopic optical gain calculations [2] based on accurate electronic structure and phenomenological rate equations [3], we show, considering wurzite GaN quantum wells, how these complementary methods can elucidate the operation of such spin lasers and their advantages [4]. [1] M. Holub et al., PRL 98, 146603 (2007); M. Lindemann et al., APL 108, 042404 (2016); J. Rudolph et al., APL 82, 4516 (2003); J. Frougier et al., APL 103, 252402 (2013); J.-Y. Cheng et al., Nat. Nanotech. 9, 845 (2014). [2] P. Faria Junior, G. Xu, J. Lee, N. Gerhardt, G. Sipahi, I. Zutic, PRB 92, 075311 (2015). [3] J. Lee, S. Bearden, E. Wasner, I. Zutic, APL 105, 042411 (2014). [4] P. Faria Junior, G. Xu, Y.-F. Chen, G. Sipahi, I. Zutic, preprint.   

Design Challenges for the Detection of Dynamic Nuclear Polarization with Semiconductor Spin Diode Arrays

The success of the processes of electrical and optical injection of electron spins across the interface between semiconductor and other materials in the past 15 years, has also opened the door to exploitation of the measurement from NMR family of techniques on a semiconductor chip. Specifically, dynamic nuclear polarization and related phenomena are detectable in spin diodes under certain experimental conditions. Initial results that demonstrated the presence of DNP effects have relied on the NMR-like detection of Ga and As isotopes naturally present in Ga and As wafers. This Report is driven by the need to develop applications that would extend such detection to atoms and molecules that are external to the natural GaAs surface, but in touch with it, in its immediate vicinity. We therefore report on the design, nanofabrication and initial tests of a semiconductor spin ``DNP diode'' arrays, where we aim to detect the DNP-like effects between the operating GaAs spin diodes and externally applied molecules. In addition to the recommendations on the selection of molecules which are detectable through such process, we elaborate on the design of the spin diodes that may enhance the DNP signal.   

Detection of the spin injection into silicon by broadband ferromagnetic resonance spectroscopy

Silicon (Si) based spintronics was eagerly studied to realize spin metal-oxide-semiconductor field-effect-transistors (MOSFETs) since it has long spin lifetime and gate tunability. The operation of n-type Si spin MOSFET was successfully demonstrated [1], however, their resistivity is still too low for practical applications and a systematic study of spin injection properties (such as spin lifetime, spin injection efficiency and so on) from the ferromagnet into the Si with different resistivity is awaited for further progress in Si spintronics. In this study, we show the spin injection by spin pumping technique in the NiFe(Py)/Si system. Broadband FMR measurement was carried out to see the enhancement of the Gilbert damping parameter with different resistivity of the Si channel. Additional damping indicated the successful spin injection by spin pumping and observed even for the Si channel with high resistivity, which is necessary for the gate operation of the device. [1] T. Sasaki, M. Shiraishi et al., Phys. Rev. Appl. 2, 034005 (2014).   

Spin Hall effect in p-Si

The spin-Hall effect is considered to be an efficient method for generation of pure spin current for spintronics applications instead of spin injection from a ferromagnetic source. Si is the apex semiconductor for electronics devices and considered to be the ideal material for spintronics as well due to relatively long spin diffusion length. The p-Si is proposed to exhibit the spin Hall effect and, currently, there is no experimental evidence of spin-Hall effect in p-Si. In this study, we report interactions and coupling of spin, charge and thermal transport behavior in p-Si. The spin Hall angle in p-Si is 10$^{\mathrm{-4}}$, which leads to insignificant spin-Hall magnetoresistance. Instead, we rely on magneto thermal transport behavior to discover spin Hall effect in p-Si. The spin-phonon interactions are proposed to be the underlying mechanism for the observed behavior, which is supported by Raman spectroscopy measurements. The observed magneto-thermal transport behavior is called as spin Hall magneto thermal resistance or SMTR. The spin-phonon interactions, presented in this work, may allow thermal manipulation of spin current, which essential for energy efficient spintronics, spin caloritronics and energy conversion applications.   

Surface Magnetism on pristine silicon thin film for spin and valley transport

The spin and valley degree of freedom for an electron have received tremendous attention in condensed matters physics because of the potential application for spintronics and valleytronics. It has been widely accepted that $d^{\mathrm{0}}$ light elemental materials of single component are not taken as ferromagnetic candidates because of the absence of odd paired electrons. The ferromagnetism has to be introduced by ferromagnetic impurity, edge functionalization, or proximity with ferromagnetic neighbors \textit{etc}. These special surface or interface structures require atomically precise control which significantly increases experimental uncertainty and theoretical understanding. By means of density functional theory (DFT) computations, we found that the spin- and valley- polarized state can be introduced in pristine silicon thin films without any alien components. The key point to this aim is the formation of graphene-like hexagonal structures making a spin-polarized Dirac fermion with half-filling. The resulting fundamental physics such as quantum valley Hall effect (QVHE), quantum anomalous Hall effect (QAHE) and magnetoelectric effect will be discussed.   

Shot noise at spin-dependent hopping in tunnel junctions with ferromagnetic electrodes

Although resistance modulation with magnetic field due to spin-dependent resonant tunneling [1] is likely responsible for a larger signal than expected from spin accumulation measured within a three-terminal spin injection setup [2], the reason for the discrepancy is still under scrutiny [3]. An additional characteristic capable to distinguish between spin-dependent trap-assisted tunneling and spin accumulation due to direct tunneling is needed. We calculate the shot noise at spin-dependent hopping and demonstrate that, due to the Pauli spin blockade in a magnetic field parallel to the magnetization of the ferromagnetic contacts, the Fano factor is significantly enhanced above its value at direct tunneling. Only in this case the spin-dependent tunneling rates are determined by the two spin-up and spin-down eigenvalues of a 4$\times$4 non-symmetric transition matrix, while in the general case all four eigenvalues contribute into the transition rate. This explains the discrepancy between [1] and [4], where only the two eigenstates are considered. 1.Y.Song and H.Dery, PRL {\bf 113}, 047205 (2014). 2.R.Jansen, Nature Materials {\bf 11}, 400 (2012). 3.A.Spiesser {\it et al.}, Appl.Phys.Express {\bf 9}, 103001 (2016). 4.Z.Yue {\it et al.}, PRB {\bf 91}, 195316 (2015).   

Non-local opto-electrical spin injection and detection in germanium at room temperature

Non-local charge carriers injection/detection schemes lie at the foundation of information manipulation in integrated systems. The next generation electronics may operate on the spin instead of the charge and germanium appears as the best hosting material to develop such spintronics for its compatibility with mainstream silicon technology and long spin lifetime at room temperature. Moreover, the energy proximity between the direct and indirect bandgaps allows for optical spin orientation. In this presentation, we demonstrate injection of pure spin currents in Ge, combined with non-local spin detection blocks at room temperature [F. Rortais et al., submitted to Nature Nanotechnology (2016)]. Spin injection is performed either electrically through a magnetic tunnel junction (MTJ) or optically, by using lithographed nanostructures to diffuse the light and create an in-plane polarized electron spin population. Pure spin current detection is achieved using either a MTJ or the inverse spin-Hall effect across a Pt stripe.   

Crossover of Equilibrium and Nonequilibrium Carrier Density in Germanium Two-Dimensional Hole Gases

We performed a detailed study on the enhancement-mode two-dimensional hole gases (2DHGs) in the Ge/Ge$_{1-x}$Si$_{x}$ (0.15 \textless x \textless 0.4) heterostructures. High mobility of 200,000 cm$^{2}$/V-s was achieved with clear quantum Hall plateaus observed. By varying the depth of 2DHG layer, a cross-over of equilibrium and non-equilibrium 2DHG density is reported in any two-dimensional system for the first time. Furthermore, an unexpectedly low effective capacitance in the shallow-channel devices was also observed. Shubnikov-de Haas oscillations showed beatings in a very shallow 2DHG (\textasciitilde 9 nm) due to second subband population.   

Spin-Hall effect in bulk germanium

Germanium is one of the most appealing candidate for spintronic applications, thanks to its compatibility with the Si platform, the long electron spin lifetime and the optical properties matching the conventional telecommunication window. Electrical spin injection schemes have always been exploited to generate spin accumulations and pure spin currents in bulk Ge. However, it is well known that ferromagnetic injection or detection blocks can introduce parasitic effects at the metal/semiconductor interface, which are still under debate. Here, we exploit the spin-Hall effect to generate a uniform pure spin current in an epitaxial n-doped Ge channel and we detect the electrically-induced spin accumulation, transverse to the injected charge current density, with polar magneto-optical Kerr microscopy at low temperature. We show that a giant spin density up to 200 \textmu m$^{\mathrm{-3}}$ can be achieved at the edges of the Ge channel for a low-voltage applied bias. Such a giant spin density is almost two orders of magnitude larger than the one achievable in III-V semiconductors. We have also characterized the electrically-induced spin voltage as a function of the applied bias and temperature, revealing that the spin-to-charge conversion in bulk Ge is preserved up to 120 K.   

Effective lattice model for collective modes in a Fermi liquid with spin-orbit coupling

A Fermi liquid (FL) with spin-orbit coupling (SOC) supports special type of collective modes--spin-chiral waves--which are oscillations of magnetization even in the absence of the external magnetic field. In this work, we study the collective spin modes of a two-dimensional (2D) FL in the presence of both SOC (of Rashba and Dresselhaus type) and in-plane magnetic field. We map the system of coupled kinetic equations for the angular harmonics of the distribution function onto an effective one-dimensional tight-binding model, in which the lattice sites correspond to values of the angular momentum. Linear-in-momentum coupling ensures that the effective tight-binding model has only nearest-neighbor hopping. In this language, the continuum of spin-flip excitations becomes a lattice band. The FL interaction, characterized by the harmonics of the Landau function, produces defects of both on-site and off-site type. The collective modes correspond to bound states produced by these defects. We find the dispersions of the collective modes both analytically, for the case when the Landau function can be approximated by the zeroth harmonic, and numerically, for the most general case.