Spin injection and detection in silicon

Using Light to Polarize and Detect Electron Spins in Silicon

Despite decades of research, demonstration of all-optical detection and control of free electron spins in silicon remains elusive. Here, we directly probe the electron spin properties in bulk silicon by measuring the polarization of luminescence following circularly polarized light excitation. The all-optical experiments performed for both direct and indirect gap excitation allow not only an experimental determination of the optical selection rules in silicon for the different phonon-assisted transitions but they also lead to the measurement of the spin relaxation of electrons in conditions that are not accessible using transport techniques. We also measure the spin properties of free excitons in bulk silicon, a very little explored field.

Spin-gapless van der Waals heterostructure for spin gating through magnetic injection devices

Spin-gapless semiconductors (SGSs) are new magnetic zero-bandgap materials whose band structure is extremely sensitive to external influences (pressure or magnetic fields) and have great potential for high-speed and low-energy spintronics applications. The first-principles method was used to systematically study the heterostructures constructed of an asymmetric surface-functionalized Janus MXene material, Cr₂NOF, and a two-dimensional hexagonal lattice (2DH) semiconductor material and to study the effects of the electronic structure, Curie temperature, magnetism, and the design of unusual band structures and magnetic injection in the bilayer to obtain an SGS structure. Through the design and construction of Cr₂NOF/2DH van der Waals heterojunction spintronic devices, the spin-filtering effect of the devices can reach 100%, especially, realizing spin gating through magnetic injection. We report the transport mechanism of the heterojunction spintronic devices to achieve the goal of a controllable optimization of the device functions, which provides a theoretical basis for the design of MXene van der Waals heterojunctions for high-efficiency and low-power-consumption spintronic devices.

Spin Laser Local Oscillators for Homodyne Detection in Coherent Optical Communications

We numerically investigate spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) for local oscillators, which are based on an injection locking technique used in coherent optical communications. Under the spin polarization modulation of an injection-locked spin-VCSEL, frequency-shifted and phase-correlated optical sidebands are generated with an orthogonal polarization against the injection light, and one of the sidebands is resonantly enhanced due to the linear birefringence in the spin-VCSEL. We determine that the peak strength and peak frequency in the spin polarization modulation sensitivity of the injection-locked spin-VCSEL depend on detuning frequency and injection ratio conditions. As a proof of concept, 25-Gbaud and 16-ary quadrature amplitude modulation optical data signals and a pilot tone are generated, and the pilot tone is used for the injection locking of a spin-VCSEL. An orthogonally-polarized modulation sideband generated from the injection-locked spin-VCSEL is used as a frequency-shifted local oscillator (LO). We verify that the frequency-shifted LO can be used for the homodyne detection of optical data signals with no degradation. Our findings suggest a novel application of spin-VCSELs for coherent optical communications.

Observation of the Magnetization Reorientation in Self-Assembled Metallic Fe-Silicide Nanowires at Room Temperature by Spin-Polarized Scanning Tunneling Spectromicroscopy

The quasi-periodic magnetic domains in metallic Fe-silicide nanowires self-assembled on the Si(110)-16 × 2 surface have been observed at room temperature by direct imaging of both the topographic and magnetic structures using spin-polarized scanning tunneling microscopy/spectroscopy. The spin-polarized differential conductance (dI/dV) map of the rectangular-sectional Fe-silicide nanowire with a width and height larger than 36 and 4 nm, respectively, clearly shows an array of almost parallel streak domains that alternate an enhanced (reduced) density of states over in-plane (out-of-plane) magnetized domains with a magnetic period of 5.0 ± 1.0 nm. This heterostructure of magnetic Fe-silicide nanowires epitaxially integrated with the Si(110)-16 × 2 surface will have a significant impact on the development of Si-based spintronic nanodevices.

Observation of layered antiferromagnetism in self-assembled parallel NiSi nanowire arrays on Si(110) by spin-polarized scanning tunneling spectromicroscopy

The layered antiferromagnetism of parallel nanowire (NW) arrays self-assembled on Si(110) have been observed at room temperature by direct imaging of both the topographies and magnetic domains using spin-polarized scanning tunneling microscopy/spectroscopy (SP-STM/STS). The topographic STM images reveal that the self-assembled unidirectional and parallel NiSi NWs grow into the Si(110) substrate along the [Formula: see text] direction (i.e. the endotaxial growth) and exhibit multiple-layer growth. The spatially-resolved SP-STS maps show that these parallel NiSi NWs of different heights produce two opposite magnetic domains, depending on the heights of either even or odd layers in the layer stack of the NiSi NWs. This layer-wise antiferromagnetic structure can be attributed to an antiferromagnetic interlayer exchange coupling between the adjacent layers in the multiple-layer NiSi NW with a B2 (CsCl-type) crystal structure. Such an endotaxial heterostructure of parallel magnetic NiSi NW arrays with a layered antiferromagnetic ordering in Si(110) provides a new and important perspective for the development of novel Si-based spintronic nanodevices.

Spin-Hall Voltage over a Large Length Scale in Bulk Germanium

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 a low temperature. We show that a large spin density up to 400  μm^{-3} can be achieved at the edges of the 100-μm-wide Ge channel for an applied electric field lower than 5  mV/μm. We find that the spin density linearly decreases toward the center of the Ge bar, due to the large spin diffusion length, and such a decay is much slower than the exponential one observed in III-V semiconductors, allowing very large spin accumulations over a length scale of tens of micrometers. This lays the foundation for multiterminal spintronic devices, where different spin voltages can be exploited as inputs for magnetologic gates on the same Ge platform.

Strong confinement-induced engineering of the g factor and lifetime of conduction electron spins in Ge quantum wells

Control of electron spin coherence via external fields is fundamental in spintronics. Its implementation demands a host material that accommodates the desirable but contrasting requirements of spin robustness against relaxation mechanisms and sizeable coupling between spin and orbital motion of the carriers. Here, we focus on Ge, which is a prominent candidate for shuttling spin quantum bits into the mainstream Si electronics. So far, however, the intrinsic spin-dependent phenomena of free electrons in conventional Ge/Si heterojunctions have proved to be elusive because of epitaxy constraints and an unfavourable band alignment. We overcome these fundamental limitations by investigating a two-dimensional electron gas in quantum wells of pure Ge grown on Si. These epitaxial systems demonstrate exceptionally long spin lifetimes. In particular, by fine-tuning quantum confinement we demonstrate that the electron Landé g factor can be engineered in our CMOS-compatible architecture over a range previously inaccessible for Si spintronics.

Electrical detection of spin transport in Si two-dimensional electron gas systems

Spin transport in a semiconductor-based two-dimensional electron gas (2DEG) system has been attractive in spintronics for more than ten years. The inherent advantages of high-mobility channel and enhanced spin-orbital interaction promise a long spin diffusion length and efficient spin manipulation, which are essential for the application of spintronics devices. However, the difficulty of making high-quality ferromagnetic (FM) contacts to the buried 2DEG channel in the heterostructure systems limits the potential developments in functional devices. In this paper, we experimentally demonstrate electrical detection of spin transport in a high-mobility 2DEG system using FM Mn-germanosilicide (Mn(Si0.7Ge0.3)x) end contacts, which is the first report of spin injection and detection in a 2DEG confined in a Si/SiGe modulation doped quantum well structure (MODQW). The extracted spin diffusion length and lifetime are l sf = 4.5 μm and [Formula: see text] at 1.9 K respectively. Our results provide a promising approach for spin injection into 2DEG system in the Si-based MODQW, which may lead to innovative spintronic applications such as spin-based transistor, logic, and memory devices.

Electric control of spin injection into a ferroelectric semiconductor

Electric-field control of spin-dependent properties has become one of the most attractive phenomena in modern materials research due to the promise of new device functionalities. One of the paradigms in this approach is to electrically toggle the spin polarization of carriers injected into a semiconductor using ferroelectric polarization as a control parameter. Using first-principles density-functional calculations, we explore the effect of ferroelectric polarization of electron-doped BaTiO3 (n-BaTiO3) on the spin-polarized transmission across the SrRuO3/n-BaTiO3(001) interface. Our study reveals that, in this system, the interface transmission is negatively spin polarized and that ferroelectric polarization reversal leads to a change in the transport spin polarization from -65% to -98%. Analytical model calculations demonstrate that this is a general effect for ferromagnetic-metal-ferroelectric-semiconductor systems and, furthermore, that ferroelectric modulation can even reverse the sign of spin polarization. The predicted effect provides a nonvolatile mechanism to electrically control spin injection in semiconductor-based spintronics devices.

Template-directed atomically precise self-organization of perfectly ordered parallel cerium silicide nanowire arrays on Si(110)-16 × 2 surfaces

The perfectly ordered parallel arrays of periodic Ce silicide nanowires can self-organize with atomic precision on single-domain Si(110)-16 × 2 surfaces. The growth evolution of self-ordered parallel Ce silicide nanowire arrays is investigated over a broad range of Ce coverages on single-domain Si(110)-16 × 2 surfaces by scanning tunneling microscopy (STM). Three different types of well-ordered parallel arrays, consisting of uniformly spaced and atomically identical Ce silicide nanowires, are self-organized through the heteroepitaxial growth of Ce silicides on a long-range grating-like 16 × 2 reconstruction at the deposition of various Ce coverages. Each atomically precise Ce silicide nanowire consists of a bundle of chains and rows with different atomic structures. The atomic-resolution dual-polarity STM images reveal that the interchain coupling leads to the formation of the registry-aligned chain bundles within individual Ce silicide nanowire. The nanowire width and the interchain coupling can be adjusted systematically by varying the Ce coverage on a Si(110) surface. This natural template-directed self-organization of perfectly regular parallel nanowire arrays allows for the precise control of the feature size and positions within ±0.2 nm over a large area. Thus, it is a promising route to produce parallel nanowire arrays in a straightforward, low-cost, high-throughput process.

Measurement of electron spin lifetime and optical orientation efficiency in germanium using electrical detection of radio frequency modulated spin polarization

We propose and demonstrate a technique for electrical detection of polarized spins in semiconductors in zero applied magnetic fields. Spin polarization is generated by optical injection using circularly polarized light which is modulated rapidly using an electro-optic cell. The modulated spin polarization generates a weak time-varying magnetic field which is detected by a sensitive radio-frequency coil. Using a calibrated pickup coil and amplification electronics, clear signals were obtained for bulk GaAs and Ge samples from which an optical spin orientation efficiency of 4.8% could be determined for Ge at 1342 nm excitation wavelength. In the presence of a small external magnetic field, the signal decayed according to the Hanle effect, from which a spin lifetime of 4.6±1.0  ns for electrons in bulk Ge at 127 K was extracted.

Spin polarization measurement of homogeneously doped Fe(1-x)Co(x)Si nanowires by Andreev reflection spectroscopy

We report a general method for determining the spin polarization from nanowire materials using Andreev reflection spectroscopy implemented with a Nb superconducting contact and common electron-beam lithography device fabrication techniques. This method was applied to magnetic semiconducting Fe(1-x)Co(x)Si alloy nanowires with x̅ = 0.23, and the average spin polarization extracted from 6 nanowire devices is 28 ± 7% with a highest observed value of 35%. Local-electrode atom probe tomography (APT) confirms the homogeneous distribution of Co atoms in the FeSi host lattice, and X-ray magnetic circular dichroism (XMCD) establishes that the elemental origin of magnetism in this strongly correlated electron system is due to Co atoms.

Introduction to spin-polarized ballistic hot electron injection and detection in silicon

Ballistic hot electron transport overcomes the well-known problems of conductivity and spin lifetime mismatch that plague spin injection attempts in semiconductors using ferromagnetic ohmic contacts. Through the spin dependence of the mean free path in ferromagnetic thin films, it also provides a means for spin detection after transport. Experimental results using these techniques (consisting of spin precession and spin-valve measurements) with silicon-based devices reveals the exceptionally long spin lifetime and high spin coherence induced by drift-dominated transport in the semiconductor. An appropriate quantitative model that accurately simulates the device characteristics for both undoped and doped spin transport channels is described; it can be used to recover the transit-time distribution from precession measurements and determine the spin current velocity, diffusion constant and spin lifetime, constituting a spin 'Haynes-Shockley' experiment without time-of-flight techniques. A perspective on the future of these methods is offered as a summary.

Spin-orbit symmetries of conduction electrons in silicon

We derive a spin-dependent Hamiltonian that captures the symmetry of the zone edge states in silicon. We present analytical expressions of the spin-dependent states and of spin relaxation due to electron-phonon interactions in the multivalley conduction band. We find excellent agreement with experimental results. Similar to the usage of the Kane Hamiltonian in direct band-gap semiconductors, the new Hamiltonian can be used to study spin properties of electrons in silicon.

Theory of spin-dependent phonon-assisted optical transitions in silicon

Silicon is an ideal material choice for spintronics devices due to its relatively long spin relaxation time and mature technology. To date, however, there are no parameter-free methods to accurately determine the degree of spin polarization of electrons in silicon. This missing link is established with a theory that provides concise relations between the degrees of spin polarization and measured circular polarization for each of the dominant phonon-assisted optical transitions. The phonon symmetries play a key role in elucidating recent spin injection experiments in silicon.

Oscillatory spin-polarized tunnelling from silicon quantum wells controlled by electric field

Spin-dependent electronic transport is widely used to probe and manipulate magnetic materials and develop spin-based devices. Spin-polarized tunnelling, successful in ferromagnetic metal junctions, was recently used to inject and detect electron spins in organics and bulk GaAs or Si. Electric field control of spin precession was studied in III-V semiconductors relying on spin-orbit interaction, which makes this approach inefficient for Si, the mainstream semiconductor. Methods to control spin other than through precession are thus desired. Here we demonstrate electrostatic modification of the magnitude of spin polarization in a silicon quantum well, and detection thereof by means of tunnelling to a ferromagnet, producing prominent oscillations of tunnel magnetoresistance of up to 8%. The electric modification of the spin polarization relies on discrete states in the Si with a Zeeman spin splitting, an approach that is also applicable to organic, carbon-based and other materials with weak spin-orbit interaction.

Theory of the spin relaxation of conduction electrons in silicon

A realistic pseudopotential model is introduced to investigate the phonon-induced spin relaxation of conduction electrons in bulk silicon. We find a surprisingly subtle interference of the Elliott and Yafet processes affecting the spin relaxation over a wide temperature range, suppressing the significance of the intravalley spin-flip scattering, previously considered dominant, above roughly 120 K. The calculated spin relaxation times T1 agree with the spin resonance and spin injection data, following a T(-3) temperature dependence. The valley anisotropy of T1 and the spin relaxation rates for hot electrons are predicted.

Spin polarized electron transport near the Si/SiO2 interface

Using long-distance lateral devices, spin transport near the interface of Si and its native oxide (SiO(2)) is studied by spin-valve measurements in an in-plane magnetic field and spin precession measurements in a perpendicular magnetic field at 60 K. As electrons are attracted to the interface by an electrostatic gate, we observe shorter average spin transit times and an increase in spin coherence, despite a reduction in total spin polarization. This behavior, which is in contrast with the expected exponential depolarization seen in bulk transport devices, is explained using a transform method to recover the empirical spin current transit-time distribution and a simple two-stage drift-diffusion model. We identify strong interface-induced spin depolarization (reducing the spin lifetime by over 2 orders of magnitude from its bulk transport value) as the consistent cause of these phenomena.

Chemical synthesis and magnetotransport of magnetic semiconducting Fe1-xCoxSi alloy nanowires

We report single-crystal nanowires of magnetic semiconducting Fe1-xCoxSi alloys synthesized using a two-component single source precursor approach. Extending our previous syntheses of FeSi and CoSi nanowires from Fe(SiCl3)2(CO)4 and Co(SiCl3)(CO)4 precursors, we found that a homogeneous solution formed upon mixing these two precursors due to melting point suppression. This liquid constitutes the single-source precursor suitable for delivery through chemical vapor deposition, which enables the chemical synthesis of Fe1-xCoxSi alloy nanowires on silicon substrates covered with a thin (1-2 nm) SiO2 layer. Using scanning and transmission electron microscopy and energy dispersive X-ray spectroscopy and mapping, we demonstrate two homogenously mixed alloy nanowire samples with very different Co substitution concentrations (x): 6+/-5%, the ferromagnetic semiconductor regime, and 44+/-5%, the helical magnetic regime. The magnetotransport properties of these alloy nanowires are pronouncedly different from that of the host structures FeSi and CoSi, as well as from one another, and consistent with the physical properties as expected for their corresponding compositions. These novel magnetic semiconducting silicide nanowires will be important building blocks for silicon-based spintronic nanodevices.

Thermodynamics of carrier-mediated magnetism in semiconductors

We propose a model of carrier-mediated ferromagnetism in semiconductors that accounts for the temperature dependence of the carriers. The model permits analysis of the thermodynamic stability of competing magnetic states, opening the door to the construction of magnetic phase diagrams. As an example, we analyze the stability of a possible reentrant ferromagnetic semiconductor, in which increasing temperature leads to an increased carrier density such that the enhanced exchange coupling between magnetic impurities results in the onset of ferromagnetism as temperature is raised.

Detection of a spin accumulation in nondegenerate semiconductors

Electrical detection of a spin accumulation in a nondegenerate semiconductor using a tunnel barrier and ferromagnetic contact is shown to be fundamentally affected by the energy barrier associated with the depletion region. This prevents the ferromagnet from probing the spin accumulation directly, strongly suppresses the magnetoresistance in current or potentiometric detection, and introduces nonmonotonic variation of spin signals with voltage and temperature. Having no analogue in metallic systems, we identify energy mismatch as an obstacle for spin detection, necessitating control of the energy landscape of spin-tunnel contacts to semiconductors.

Coherent spin transport through a 350 micron thick silicon wafer

We use all-electrical methods to inject, transport, and detect spin-polarized electrons vertically through a 350-micron-thick undoped single-crystal silicon wafer. Spin precession measurements in a perpendicular magnetic field at different accelerating electric fields reveal high spin coherence with at least 13pi precession angles. The magnetic-field spacing of precession extrema are used to determine the injector-to-detector electron transit time. These transit time values are associated with output magnetocurrent changes (from in-plane spin-valve measurements), which are proportional to final spin polarization. Fitting the results to a simple exponential spin-decay model yields a conduction electron spin lifetime (T1) lower bound in silicon of over 500 ns at 60 K.

Semiconductor spintronics

Spintronics refers commonly to phenomena in which the spin of electrons in a solid state environment plays the determining role. In a more narrow sense spintronics is an emerging research field of electronics: spintronics devices are based on a spin control of electronics, or on an electrical and optical control of spin or magnetism. While metal spintronics has already found its niche in the computer industry—giant magnetoresistance systems are used as hard disk read heads—semiconductor spintronics is yet to demonstrate its full potential. This review presents selected themes of semiconductor spintronics, introducing important concepts in spin transport, spin injection, Silsbee-Johnson spin-charge coupling, and spin-dependent tunneling, as well as spin relaxation and spin dynamics. The most fundamental spin-dependent interaction in nonmagnetic semiconductors is spin-orbit coupling. Depending on the crystal symmetries of the material, as well as on the structural properties of semiconductor based heterostructures, the spin-orbit coupling takes on different functional forms, giving a nice playground of effective spin-orbit Hamiltonians. The effective Hamiltonians for the most relevant classes of materials and heterostructures are derived here from realistic electronic band structure descriptions. Most semiconductor device systems are still theoretical concepts, waiting for experimental demonstrations. A review of selected proposed, and a few demonstrated devices is presented, with detailed description of two important classes: magnetic resonant tunnel structures and bipolar magnetic diodes and transistors. In view of the importance of ferromagnetic semiconductor materials, a brief discussion of diluted magnetic semiconductors is included. In most cases the presentation is of tutorial style, introducing the essential theoretical formalism at an accessible level, with case-study-like illustrations of actual experimental results, as well as with brief reviews of relevant recent achievements in the field.

Electronic measurement and control of spin transport in silicon

The spin lifetime and diffusion length of electrons are transport parameters that define the scale of coherence in spintronic devices and circuits. As these parameters are many orders of magnitude larger in semiconductors than in metals, semiconductors could be the most suitable for spintronics. So far, spin transport has only been measured in direct-bandgap semiconductors or in combination with magnetic semiconductors, excluding a wide range of non-magnetic semiconductors with indirect bandgaps. Most notable in this group is silicon, Si, which (in addition to its market entrenchment in electronics) has long been predicted a superior semiconductor for spintronics with enhanced lifetime and transport length due to low spin-orbit scattering and lattice inversion symmetry. Despite this promise, a demonstration of coherent spin transport in Si has remained elusive, because most experiments focused on magnetoresistive devices; these methods fail because of a fundamental impedance mismatch between ferromagnetic metal and semiconductor, and measurements are obscured by other magnetoelectronic effects. Here we demonstrate conduction-band spin transport across 10 mum undoped Si in a device that operates by spin-dependent ballistic hot-electron filtering through ferromagnetic thin films for both spin injection and spin detection. As it is not based on magnetoresistance, the hot-electron spin injection and spin detection avoids impedance mismatch issues and prevents interference from parasitic effects. The clean collector current shows independent magnetic and electrical control of spin precession, and thus confirms spin coherent drift in the conduction band of silicon.