Electron Spin Relaxations in the Electric-Field Applied GaAs/InGaAs Heterostructure

We have investigated the electron spin relaxation rates in GaAs/InGaAs heterostructure in the presence of electric field by time-resolved photoluminescence (PL) measurements at 10 K. Spin-polarized electrons were optically generated in the bulk GaAs region, drifted driven by the electric field, and captured in two InGaAs quantum wells which work as spin detectors. The comparison of the degrees of PL polarizations from two wells, by adjusting excitation energy and electric field, enables us to investigate the electron spin relaxation rates in the different parts of the sample separately. We have found that the spin relaxation during the drift transport in the bulk region is accelerated in the high electric fields and that a significant spin relaxation takes place when electrons are captured into the quantum well.     

Electron Spin Relaxations in the Electric-Field Applied GaAs/InGaAs Heterostructure

We have investigated the electron spin relaxation rates in GaAs/InGaAs heterostructure in the presence of electric field by time-resolved photoluminescence (PL) measurements at 10 K. Spin-polarized electrons were optically generated in the bulk GaAs region, drifted driven by the electric field, and captured in two InGaAs quantum wells which work as spin detectors. The comparison of the degrees of PL polarizations from two wells, by adjusting excitation energy and electric field, enables us to investigate the electron spin relaxation rates in the different parts of the sample separately. We have found that the spin relaxation during the drift transport in the bulk region is accelerated in the high electric fields and that a significant spin relaxation takes place when electrons are captured into the quantum well.     

Spin Relaxation in a Two-Dimensional Electron Gas in Si Quantum Wells

Electron spin resonance of two-dimensional (2D) electron gas in Si/SiGe quantum wells allows to evaluate both the longitudinal and the dephasing spin relaxation time. Diakonov–Perel (DP) relaxation, caused by Bychkov–Rashba (BR) spin orbit coupling, occurs to be the dominant mechanism in high mobility samples. For low mobility the Elliott–Yaffet mechanism dominates the longitudinal spin relaxation. When the BR effect is small, inhomogeneous broadening caused by potential fluctuations is seen. We compare spin relaxation of the 2D electron gas in Si and in GaAs quantum wells with respect to applications of these materials in spintronics.     

Spin relaxation in the channel of a spin field-effect transistor

We examine the major spin relaxation mechanism (D'yakonov-Perel') in the channel of a spin field-effect transistor (SPINFET) and show analytically that it can be completely eliminated if the channel is a quantum wire and transport is strictly single moded (only the lowest subband is occupied). Single-moded transport also produces the largest "on" to "off" conductance ratio and the largest transconductance of the transistor.     

Spin Dynamics and Spin Transport

Spin-orbit (SO) interaction critically influences electron spin dynamics and spin transport in bulk semiconductors and semiconductor microstructures. This interaction couples electron spin to dc and ac electric fields. Spin coupling to ac electric fields allows efficient spin manipulating by the electric component of electromagnetic field through the electric dipole spin resonance (EDSR) mechanism. Usually, it is much more efficient than the magnetic manipulation due to a larger coupling constant and the easier access to spins at a nanometer scale. The dependence of the EDSR intensity on the magnetic field direction allows measuring the relative strengths of the competing SO coupling mechanisms in quantum wells. Spin coupling to an in-plane electric field is much stronger than to a perpendicular field. Because electron bands in microstructures are spin split by SO interaction, electron spin is not conserved and spin transport in them is controlled by a number of competing parameters, hence, it is rather nontrivial. The relation between spin transport, spin currents, and spin populations is critically discussed. Importance of transients and sharp gradients for generating spin magnetization by electric fields and for ballistic spin transport is clarified.     

Spin Dynamics and Spin Transport

Spin-orbit (SO) interaction critically influences electron spin dynamics and spin transport in bulk semiconductors and semiconductor microstructures. This interaction couples electron spin to dc and ac electric fields. Spin coupling to ac electric fields allows efficient spin manipulating by the electric component of electromagnetic field through the electric dipole spin resonance (EDSR) mechanism. Usually, it is much more efficient than the magnetic manipulation due to a larger coupling constant and the easier access to spins at a nanometer scale. The dependence of the EDSR intensity on the magnetic field direction allows measuring the relative strengths of the competing SO coupling mechanisms in quantum wells. Spin coupling to an in-plane electric field is much stronger than to a perpendicular field. Because electron bands in microstructures are spin split by SO interaction, electron spin is not conserved and spin transport in them is controlled by a number of competing parameters, hence, it is rather nontrivial. The relation between spin transport, spin currents, and spin populations is critically discussed. Importance of transients and sharp gradients for generating spin magnetization by electric fields and for ballistic spin transport is clarified.     

Anisotropic Spin Splitting and Spin Relaxation in AlGaAs/GaAs Quantum Structures

Spin relaxation due to the D'yakonov–Perel' mechanism is intimately related with the spin splitting of the electronic states. We calculate the spin relaxation rates from anisotropic spin splittings of electron subbands in n-(001)-AlGaAs/GaAs quantum structures obtained in a self-consistent multiband approach. The giant anisotropy of spin relaxation rates found for different spin components in the (001) plane can be ascribed to a mutual compensation of terms because of the asymmetry of the bulk crystal and the quantum well structure.     

Driving Spin and Charge in Quantum Wells by Surface Acoustic Waves

Recent experiments have shown the potential of surface acoustic waves as a mean for transporting charge and spin in quantum wells. In particular, they have proven highly effective for the coherent transport of spin‐polarized wave packets, suggesting their potential in spintronics applications. Motivated by these experimental observations, the spin and charge dynamics in a quantum well under surface acoustic waves is theoretically studied. It is shown that the dynamics acquires a simple and transparent form in a reference frame co‐moving with the surface acoustic wave. The results, e.g., the calculated spin relaxation and precession lengths, are in excellent agreement with recent experimental observations.     

Coherent Spin Dynamics and Spin Polarized Transport in Doped Semiconductors

We provide an overview of measurements that elucidate the effects of interactions, quantum confinement, reduced dimensionality, and interfacial geometries on coherent electronic spin dynamics and spin transport in doped semiconductors. The experiments focus on a variety of doped semiconductor systems, ranging from bulk n-GaAs crystals to modulation doped II-VI magnetic semiconductor quantum wells. In particular, the latter provide model systems in which electron gases are strongly exchange-coupled to an engineered distribution of magnetic moments, hence allowing one to systematically tailor spin interactions between confined electronic states, magnetic ions, and nuclei. Two complementary techniques including state-of-the-art spin dynamical probes having high temporal (~100 fs) and spatial (~100 nm) resolution, and low-temperature magneto-transport, are used to survey a variety of physical phenomena in these systems.     

[111] Heterostructures for Spin Devices

The symmetry properties of [111] quantum wells (QWs) and superlattices, and in particular the fact that the symmetry is not reduced when bulk inversion asymmetry (BIA) is supplemented with structural inversion asymmetry [SIA (Rashba)], have important consequences for spin dynamics and transport. We compute the effective spin Hamiltonians for [111] and [110] structures, and show that for the [111] case the splitting can be made to vanish to lowest order when BIA and SIA effects are of equal strength. As a consequence, the Dyakonov–Perel spin relaxation mechanism is suppressed for all spin components. This effect forms the base for an improved version of the Datta–Das and a recently proposed family of spin transistors. For [110]-grown QWs, the effect of SIA is to augment the spin relaxation rate of the component perpendicular to the well. We derive analytical expressions for the spin lifetime tensor and its proper axes, and see that they are dependent on the relative magnitude of the BIA- and SIA-induced splittings.     

[111] Heterostructures for Spin Devices

The symmetry properties of [111] quantum wells (QWs) and superlattices, and in particular the fact that the symmetry is not reduced when bulk inversion asymmetry (BIA) is supplemented with structural inversion asymmetry [SIA (Rashba)], have important consequences for spin dynamics and transport. We compute the effective spin Hamiltonians for [111] and [110] structures, and show that for the [111] case the splitting can be made to vanish to lowest order when BIA and SIA effects are of equal strength. As a consequence, the Dyakonov–Perel spin relaxation mechanism is suppressed for all spin components. This effect forms the base for an improved version of the Datta–Das and a recently proposed family of spin transistors. For [110]-grown QWs, the effect of SIA is to augment the spin relaxation rate of the component perpendicular to the well. We derive analytical expressions for the spin lifetime tensor and its proper axes, and see that they are dependent on the relative magnitude of the BIA- and SIA-induced splittings.     

Electrical Detection of Spin Precession in Freely Suspended Graphene Spin Valves on Cross‐Linked Poly(methyl methacrylate)

Spin injection and detection is achieved in freely suspended graphene using cobalt electrodes and a nonlocal spin‐valve geometry. The devices are fabricated with a single electron‐beam‐resist poly(methyl methacrylate) process that minimizes both the fabrication steps and the number of (aggressive) chemicals used, greatly reducing contamination and increasing the yield of high‐quality, mechanically stable devices. As‐grown devices can present mobilities exceeding 10⁴ cm² V^?1 s^?1 at room temperature and, because the contacts deposited on graphene are only exposed to acetone and isopropanol, the method is compatible with almost any contacting material. Spin accumulation and spin precession are studied in these nonlocal spin valves. Fitting of Hanle spin precession data in bilayer and multilayer graphene yields a spin relaxation time of ～125‐250 ps and a spin diffusion length of 1.7‐1.9 μm at room temperature.     

Giant enhancement of the carrier mobility in silicon nanowires with diamond coating

We show theoretically that the low-field carrier mobility in silicon nanowires can be greatly enhanced by embedding the nanowires within a hard material such as diamond. The electron mobility in the cylindrical silicon nanowires with 4-nm diameter, which are coated with diamond, is 2 orders of magnitude higher at 10 K and a factor of 2 higher at room temperature than the mobility in a free-standing silicon nanowire. The importance of this result for the downscaled architectures and possible silicon-carbon nanoelectronic devices is augmented by an extra benefit of diamond, a superior heat conductor, for thermal management.     

Significant enhancement of hole mobility in [110] silicon nanowires compared to electrons and bulk silicon

Utilizing sp3d5s* tight-binding band structure and wave functions for electrons and holes we show that acoustic phonon limited hole mobility in [110] grown silicon nanowires (SiNWs) is greater than electron mobility. The room temperature acoustically limited hole mobility for the SiNWs considered can be as high as 2500 cm2/V s, which is nearly three times larger than the bulk acoustically limited silicon hole mobility. It is also shown that the electron and hole mobility for [110] grown SiNWs exceed those of similar diameter [100] SiNWs, with nearly 2 orders of magnitude difference for hole mobility. Since small diameter SiNWs have been seen to grow primarily along the [110] direction, results strongly suggest that these SiNWs may be useful in future electronics. Our results are also relevant to recent experiments measuring SiNW mobility.     

Fabrication and Characterization of Modulation-Doped ZnSe/(Zn,Cd)Se (110) Quantum Wells: A New System for Spin Coherence Studies

We describe the growth of modulation-doped ZnSe/(Zn,Cd)Se quantum wells on (110) GaAs substrates. Unlike the well-known protocol for the epitaxy of ZnSe-based quantum structures on (001) GaAs, we find that the fabrication of quantum well structures on (110) GaAs requires significantly different growth conditions and sample architecture. We use magnetotransport measurements to confirm the formation of a two-dimensional electron gas in these samples, and then measure transverse electron spin relaxation times using time-resolved Faraday rotation. In contrast to expectations based upon known spin relaxation mechanisms, we find surprisingly little difference between the spin lifetimes in these (110)-oriented samples in comparison with (100)-oriented control samples.     

Fabrication and Characterization of Modulation-Doped ZnSe/(Zn,Cd)Se (110) Quantum Wells: A New System for Spin Coherence Studies

We describe the growth of modulation-doped ZnSe/(Zn,Cd)Se quantum wells on (110) GaAs substrates. Unlike the well-known protocol for the epitaxy of ZnSe-based quantum structures on (001) GaAs, we find that the fabrication of quantum well structures on (110) GaAs requires significantly different growth conditions and sample architecture. We use magnetotransport measurements to confirm the formation of a two-dimensional electron gas in these samples, and then measure transverse electron spin relaxation times using time-resolved Faraday rotation. In contrast to expectations based upon known spin relaxation mechanisms, we find surprisingly little difference between the spin lifetimes in these (110)-oriented samples in comparison with (100)-oriented control samples.     

Spin transport in self assembled all-metal nanowire spin valves: a study of the pure Elliott-Yafet mechanism

We report experimental study of spin transport in all metal nanowire spin valve structures. The nanowires have a diameter of 50 nm and consist of three layers--cobalt, copper, and nickel. Based on the experimental observations, we determine that the primary spin relaxation mechanism in the paramagnet layer--copper--is the Elliott-Yafet mode associated with elastic scattering caused by charged states on the surface of the nanowires. This mode is overwhelmingly dominant over all other modes, so that we are able to study the pure Elliott-Yafet mechanism in isolation. We deduce that the spin diffusion length associated with this mechanism is about 16 nm in our nanowires and is fairly temperature independent in the range 1-100 K, which is consistent with the spin relaxation being associated with elastic scattering by surface states. The corresponding spin relaxation time is about 100 femtoseconds. We also find that the spin relaxation rate is fairly independent of the electric field driving the current in the field range 0.3-3 kV/cm.     

Spin relaxation in InGaN quantum disks in GaN nanowires

The spin relaxation time of photoinduced conduction electrons has been measured in InGaN quantum disks in GaN nanowires as a function of temperature and In composition in the disks. The relaxation times are of the order of ～100 ps at 300 K and are weakly dependent on temperature. Theoretical considerations show that the Elliott-Yafet scattering mechanism is essentially absent in these materials and the results are interpreted in terms of the D'yakonov-Perel' relaxation mechanism in the presence of Rashba spin-orbit coupling of the wurtzite structure. The calculated spin relaxation times are in good agreement with the measured values.     

Electron trapping in InP nanowire FETs with stacking faults

Semiconductor III-V nanowires are promising components of future electronic and optoelectronic devices, but they typically show a mixed wurtzite-zinc blende crystal structure. Here we show, theoretically and experimentally, that the crystal structure dominates the conductivity in such InP nanowires. Undoped devices show very low conductivities and mobilities. The zincblende segments are quantum wells orthogonal to the current path and our calculations indicate that an electron concentration of up to 4.6 × 10(18) cm(-3) can be trapped in these. The calculations also show that the room temperature conductivity is controlled by the longest zincblende segment, and that stochastic variations in this length lead to an order of magnitude variation in conductivity. The mobility shows an unexpected decrease for low doping levels, as well as an unusual temperature dependence that bear resemblance with polycrystalline semiconductors.     

A laser operating on the interband transitions in quantum wells with coherent electron transport

A new approach to the creation of a quantum cascade laser is suggested, which employs interband transitions in quantum wells with a coherent electron transport. The coherent electron transport is studied based on the simplest two-band Kane model, in which the interaction between states of the conduction and valence bands is described taking into account only states in the light hole subband of the valence band.