Session F5: Spin Transport in Semiconductors

Spin-orbit fields at semiconductor interfaces

Solids without space inversion symmetry exhibit spin-orbit fields, which are emerging manifestations of spin-orbit coupling of the underlying atomic structure. Primary examples of spatially asymmetric systems are interfaces, which are omnipresent in electronic devices. As the device dimensions scale down, interfaces imprint their symmetries into the transport channel by proximity effects. Proximity spin-orbit fields already play important roles in anisotropic magnetoresistance of ultrathin structures such as Fe/GaAs [1], in the physics of Majorana fermions [2,3] and Andreev reflection [4] of semiconductor/superconductor junctions, in Skyrmion textures [5] in ferromagnets, or in spin-orbit torques [6]. It is thus of vital interest to gain qualitative insight and realistic quantitative description of the interfacial spin-orbit fields for various materials hybrid settings. We have proposed a methodology to extract spin-orbit fields, both their magnitudes and directions, and applied it to investigate Fe/GaAs junctions [7]. Only at low momenta the traditional description of the fields in terms of linear Rashba and Dresselhaus works. At generic momenta the fields exhibit what we call “butterfly” patterns, conforming to the interfacial symmetry. Remarkably, the spin-orbit fields depend rather strongly on the magnetization orientation. We will also discuss our recent results on the spin-orbit coupling in zinc-blende and wurtzite semiconductor nanostructures. \newline \newline [1] T. Hupfauer {\em et al.}, Nat. Nanocommun. {\bf 6}, 7374 (2014). \newline [2] S. Nadj-Perge, {\em et al.}, Science {\bf 346}, 602 (2014); R. Pawlak et al., arXiv:1505.06078. \newline [3] V. Mourik, {\em et al.}, Science {\bf 336}, 1003 (2012). \newline [4] P. H\"ogl, A. Matos-Abiague, I. Zutic, and J. Fabian, Phys. Rev.Lett. {\bf 115}, 116601 (2015). \newline [5] S. M\"uhlbauer {\em et al.}, Science {\bf 323}, 915 (2013). \newline [6] K.-S. Lee {\em et al.}, Phys. Rev. B {\bf 91}, 144401 (2015). \newline [7] M. Gmitra, A. Matos-Abiague, C. Draxl, and J. Fabian, Phys. Rev. Lett. {\bf 111}, 036603 (2013).   

Realization of an all-electric spin transistor using quantum point contacts

The spin field effect transistor envisioned by Datta and Das opens a gateway to spin information processing. Although the coherent manipulation of electron spins in semiconductors is now possible, the realization of a functional spin field effect transistor for information processing has yet to be achieved, owing to several fundamental challenges such as the low spin-injection efficiency due to resistance mismatch, spin relaxation, and the spread of spin precession angles. Alternative spin transistor designs have therefore been proposed, but these differ from the field effect transistor concept and require the use of optical or magnetic elements, which pose difficulties for the incorporation into integrated circuits. Here, we present an all-electric all-semiconductor spin field effect transistor, in which these obstacles are overcome by employing two quantum point contacts as spin injectors and detectors. Distinct engineering architectures of spin-orbit coupling are exploited for the quantum point contacts and the central semiconductor channel to achieve complete control of the electron spins---spin injection, manipulation, and detection---in a purely electrical manner. Such a device is compatible with large-scale integration and hold promise for future spintronic devices for information processing. Ref: P. Chuang et al., Nat. Nanotechnol. 10, 35 (2015).   

Current-induced spin polarization in InGaAs epilayers with varying doping densities

Current-induced spin polarization (CISP) is a phenomena in which an applied electric field produces a bulk spin polarization in the plane of the sample. As this is thought to arise from the spin-orbit coupling, it was originally predicted that the magnitude of CISP should be proportional to the spin-orbit (SO) splitting [1]. However, crystal axis-dependent measurements of the CISP and SO fields showed a negative differential relationship between these two quantities [2]. To develop a phenomenological understanding of the factors affecting the magnitude of CISP, we performed measurements on three In$_{0.025}$Ga$_{0.975}$As epilayers, Si-doped at 0.67, 9.6, and 14.1 x 10$^{17}$ cm$^{-3}$. We will discuss the effects of the doping density and electron mobility on the magnitudes of the SO fields and CISP. [1] V. Edelstein, Solid State Commun. \textbf{73}, 233 (1990). [2] Norman, B. M., et al., Phys. Rev. Lett. \textbf{112}, 056601 (2014).   

Effective spin Hall properties of a mixture of materials with and without spin-orbit coupling: Tailoring the effective spin-diffusion length

We study theoretically the effective spin Hall properties of a composite consisting of two materials with and without spin-orbit (SO) coupling. In particular, we assume that SO material represents a system of grains of radius, $a$, and density, $n$, in a matrix with no SO. We calculate the effective spin Hall angle, $\theta^{\scriptstyle{SH}}_{\scriptstyle{\text{eff}}}$, and the effective spin diffusion length, $\lambda_{\scriptstyle{\text{eff}}}$, of the mixture. Our main qualitative finding is that, if the bare spin diffusion length, $\lambda$, is much smaller than $a$, then $\lambda_{\scriptstyle{\text{eff}}}$ is strongly {\em enhanced}, well beyond $\lambda/(na^3)^{1/2}$, which can be expected from purely ``geometrical" consideration. The physical origin of this additional enhancement is that, with small diffusion length, $\lambda \ll a$, the spin current mostly flows {\em around the grain} without suffering much loss. We also demonstrate that the voltage, created by a spin current, is sensitive to a very weak magnetic field directed along the spin current, and even reverses sign in a certain domain of fields. The origin of this sensitivity is that the spin precession, caused by magnetic field, takes place outside the grains where SO is absent.   

Spin relaxation via exchange with donor impurity-bound electrons

In the Bir-Aronov-Pikus depolarization process affecting conduction electrons in p-type cubic semiconductors, spin relaxation is driven by exchange with short-lived valence band hole states. We have identified an analogous spin relaxation mechanism in nominally undoped silicon at low temperatures, when many electrons are bound to dilute dopant ion potentials. Inelastic scattering with externally injected conduction electrons accelerated by electric fields can excite transitions into highly spin-orbit-mixed bound excited states, driving strong spin relaxation of the conduction electrons via exchange interaction. We reveal the consequences of this spin depolarization mechanism both below and above the impact ionization threshold, where conventional charge and spin transport are restored.   

Electrical spin injection and detection in Si nanowires with axial doping gradient

Due to the technological importance and potential long spin coherence time in silicon, there have been significant recent efforts to realize spin injection, coherent transport, and electrical spin detection in Si nanowires (NWs). The nature of the electronic transport at the interface and its resistance are crucial factors in realizing efficient spin injection/detection between a ferromagnet (FM) and a semiconductor (SC). In this work, we examine the effects on electrical spin injection and detection by FM/SC interfaces with well-defined Schottky barriers in Si NW devices. The Si NWs are synthesized via a vapor-liquid-solid method using silane and phosphine precursor gases for the growth and doping respectively, which results in a graded phosphorus doping profile along the length of the NW. The Si NWs are dispersed on a p$^{\mathrm{+}}$-Si/SiO$_{\mathrm{2}}$/SiN$_{\mathrm{x}}$ substrate, and a series of CoFe electrodes are defined along a Si NW with electron beam lithography and magnetron sputtering after the removal of the native oxide by HF treatment. As a consequence of the doping gradient, the FM electrodes form Ohmic and Schottky barrier contacts of varying heights along the length of a single NW. Two-terminal local and four-terminal non-local spin-valve measurements are performed to probe spin accumulation and transport at different FM contacts, enabling a study of the dependence of the spin signals on the Schottky barrier height and interface resistance on a single device. *Work supported by NSF grant DMR-1308613.   

Room-temperature operation of Si spin MOSFET with high on/off spin signal ratio

Si spintronics is now one of the pivotal fields in semiconductor spintronics. After the first report on successful propagation of pure spin current in Si, much effort has been paid for realization of Si spin metal-oxide-semiconductor field-effect transistor (MOSFET), since Si spin MOSFETs allow constructing reconfigurable logic circuits with ultra-low energy consumption. Our group achieved the first operation of Si spin MOSFET at room temperature by using non-degenerate n-type Si [1]. However, the remaining issue to be solved was low on/off ratio of spin signals. In this presentation, we report on our experimental demonstration of Si spin MOSFET with high on/off ratio of spin signals. The on/off ratio is greater than 10$^{\mathrm{3}}$, whereas on/off ratio in a conventional MOSFET operation is ca. 10$^{\mathrm{5}}$. More importantly, the gate voltage dependence of the spin signals and the MOSFET signals are in good agreement [2]. This achievement can pave the way to a practical application of Si spin MOSFETs. References: [1] T. Sasaki, M. Shiraishi et al., Phys. Rev. Applied 2, 034005 (2014). [2] T. Tahara, M. Shiraishi et al., Appl. Phys. Express, in press (selected as Spotlight Paper).   

Electronic measurement of strain effects on spin transport in silicon

Spin transport in silicon is limited by the Elliott-Yafet spin relaxation mechanism, which is driven by scattering between degenerate conduction band valleys. Mechanical strain along a valley axis partially breaks this degeneracy, and will ultimately quench intervalley spin relaxation for transitions between states on orthogonal axes. Using a custom-designed and constructed strain probe, we study the effects of uniaxial compressive strain along the $\langle100\rangle$ direction on ballistic tunnel junction devices used to inject spin-polarized electrons into silicon. The effects of strain-induced valley splitting will be presented and compared to our theoretical model.   

Three-terminal experiments on epitaxial Si/MgO tunnel junctions

In the field of spin injection into semiconductors, experiments in a three-terminal (3T) Hanle geometry are widely used to determine spin life times and spin diffusion lengths. However, as charge and spin current are not separated in the 3T geometry, it is yet unclear how reliable 3T experiments are to reveal spin-related quantities of the semiconductor channel. In particular, the impact of defect states in the tunnel barrier or at its interfaces on measured 3T Hanle-like signals has intensely been discussed recently.\\ In our contribution, we compare 3T experiments on entirely MBE-grown epitaxial Si/MgO/Fe/Au and Si/MgO/Au, i.\,e. ferromagnetic and nonmagnetic tunnel junctions. Both sample types show a similar Lorentzian signal comparable to those obtained by the Hanle effect of a precessing and dephasing spin ensemble. In contrast to the ferromagnetic sample, the resistance of the nonmagnetic sample increases for increasing external magnetic field. We discuss the dependence of the signal on bias, temperature and orientation of the external magnetic field, taking into account the high crystalline quality of our epitaxial tunnel junctions with atomically sharp interfaces.   

Spin Transport and Precession in Epitaxial BaTiO$_{\mathrm{3}}$/Ge Heterostructures

Spintronics has opened up new possibilities to leverage the spin degree of freedom in electronic devices. Spin injection from ferromagnets into semiconductors has been realized by inserting a thin tunnel barrier layer, which adjusts the conductivity mismatch. However, limited functionality in conventional tunnel barriers hinders the control and manipulation of spin. Here we report the spin injection and detection in p-type Germanium (p-Ge) through a functional BaTiO$_{\mathrm{3}}$ (BTO) tunnel barrier. Epitaxial BTO thin films are grown on p-Ge by molecular beam epitaxy (MBE), followed by electron beam pattern generation (EBPG) to fabricate multi-terminal spin devices. Spin accumulation is demonstrated using the Hanle technique, where the spin signal shows a non-monotonic temperature dependence. Using this temperature dependence, we investigate the dominant spin damping pathways in each temperature regime. Furthermore, we discuss the possibility of manipulating spin transport using the BTO layer, which would allow one to integrate the unique functionalities of complex oxides with semiconductor spintronics devices.