Currents In Semiconductors Research Articles

The impact of semiconductor surface states on vacuum field emission

This work presents a theoretical analysis of the impact of surface states on vacuum field emission currents in semiconductors. In wide and ultra-wide bandgap semiconductors such as GaN and AlGaN, low electron affinity has been proposed as a benefit for field emission into vacuum. However, in these materials, the surface Fermi level at the surface is pinned well below the conduction band, and the surface depletion barriers due to the surface Fermi level pinning can be comparable to or higher than the electron affinity. Therefore, analysis of field emission requires consideration of not only the vacuum potential barrier set by electron affinity, but also the depletion region near the semiconductor surface. In this paper, we develop analytical models to predict field emission currents with careful consideration of the impact of surface states on the energy band alignment. The results are used to provide guidelines for design of field emitters that could benefit from the low electron affinity of semiconductors such as Al(Ga)N.

Perspective on phase-controlled currents in semiconductors driven by structured light

Controlling electrons with ever-greater precision is central to both classical and quantum electronics. Since the invention of the laser, virtually every property of coherent light has been tamed, making it one of the most precise tools available to science, technology, and medicine. Coherent control involves the transduction of an exquisitely defined property of light to an electronic system, imparting coherence to an attribute of its constituent electrons. Early developments in coherent control utilized Gaussian laser beams and spatially averaged measurements. The spatial structure and orbital angular momentum of laser light provide additional degrees of freedom for steering electronic and quasiparticle excitations in condensed matter systems. In this Perspective, we first introduce the concept of coherent control in semiconductors. We then proceed to discuss the application of structured light beams to coherent control and the requirement for spatially resolved current detection. Subsequently, we present an overview of recent experiments that were performed using cylindrical vector beams and laser beams with structured phase fronts. Finally, we provide an outlook on the horizons that have emerged with these developments and future directions of interest.

Vectorized optoelectronic control and metrology in a semiconductor

The increasingly prominent role of light in information processing makes optoelectronic devices a technology of fundamental importance. Coherent control of currents in semiconductors using synthesized optical waveforms provides a sensitive and robust means to transfer information from light to an electronic circuit. Currents driven by Gaussian laser beams are spatially uniform in direction, offering limited technological utility. Full control over the transverse spatial distribution of currents excited in a material would vastly increase the versatility and impact of optoelectronic devices. Here we simultaneously control the waveform and vectorial arrangement of optical fields, enabling precise manipulation of the spatial distribution of currents in a semiconductor. As a direct application, we drive loop currents, embodying a new ultrafast magnetic field source. Subsequently, we demonstrate a scheme for generating an arbitrary superposition of two orthogonal current arrangements via subcycle adjustment of the optical waveform. Engineering of the spatial distribution of currents in a semiconductor is demonstrated using vectorial arrangement of optical fields, enabling an ultrafast magnetic field source.

Rashba semiconductor as spin Hall material: Experimental demonstration of spin pumping in wurtziten-GaN:Si

Pure spin currents in semiconductors are essential for implementation in the next generation of spintronic elements. Heterostructures of III-nitride semiconductors are currently employed as central building blocks for lighting and high-power devices. Moreover, the long relaxation times and the spin-orbit coupling (SOC) in these materials indicate them as privileged hosts for spin currents and related phenomena. Spin pumping is an efficient mechanism for the inception of spin current and its conversion into charge current in nonmagnetic metals and semiconductors with Rashba SOC. We report on the generation at room temperature (RT) of pure spin current in nonmagnetic degenerate $n$-GaN:Si from a magnetic permalloy layer driven to ferromagnetic resonance (FMR) conditions. The FMR signal and the generated Hall voltages due to spin Hall effect, inverse spin Hall effect (ISHE), and spurious mechanisms are detected simultaneously. No spin-pumping-induced voltage could be measured below RT, despite the persistence of FMR signal. After eliminating the spurious (not due to ISHE) components contributing to the generated voltage, we find for $n$-GaN:Si a spin Hall angle ${\ensuremath{\theta}}_{\mathrm{SH}}=3.03\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, exceeding by one order of magnitude those reported for other semiconductors, pointing at III-nitrides as particularly efficient spin current generators.

Ultrafast and Gigantic Spin Injection in Semiconductors.

The injection of spin currents in semiconductors is one of the big challenges of spintronics. Motivated by the ultrafast demagnetization and spin injection into metals, we propose an alternative femtosecond route based on the laser excitation of superdiffusive spin currents in a ferromagnet such as Ni. Our calculations show that even though only a fraction of the current crosses the Ni-Si interface, the laser-induced creation of strong transient electrical fields at a ferromagnet-semiconductor interface allows for the injection of chargeless spin currents with record spin polarizations of 80%. Beyond that they are pulsed on the time scale of 100fs which opens the door for new experiments and ultrafast spintronics.

Spin and charge transport in materials with spin-dependent conductivity

The spin and charge transport in materials with spin-dependent conductivity has been studied. It has been shown that there is a charge accumulation along spin diffusion in a ferromagnetic metal, which causes a shortening of the spin diffusion length. It has been shown that there is a substantial interaction between the drift and diffusion currents in semiconductors. The effects of gain/damping of a spin current by a charge current and the existence of a threshold spin current in a semiconductor have been described. Because of the substantial magnitude, these new spintronics effects might be used for new designs of efficient spintronic devices. The influence of a spin drain on spin transport has been studied.

Optical effects of spin currents in semiconductors

A spin current has novel linear and second-order nonlinear optical effects due to its symmetry properties. With the symmetry analysis and the eight-band microscopic calculation we have systematically investigated the interaction between a spin current and a polarized light beam (or the "photon spin current") in direct-gap semiconductors. This interaction is rooted in the intrinsic spin-orbit coupling in valence bands and does not rely on the Rashba or Dresselhaus effect. The light-spin current interaction results in an optical birefringence effect of the spin current. The symmetry analysis indicates that in a semiconductor with inversion symmetry, the linear birefringence effect vanishes and only the circular birefringence effect exists. The circular birefringence effect is similar to the Faraday rotation in magneto-optics but involves no net magnetization nor breaking the time-reversal symmetry. Moreover, a spin current can induce the second-order nonlinear optical processes due to the inversion-symmetry breaking. These findings form a basis of measuring a pure spin current where and when it flows with the standard optical spectroscopy, which may provide a toolbox to explore a wealth of physics connecting the spintronics and photonics.

Optical studies of ballistic currents in semiconductors [Invited

We present a summary of recent studies of ballistic currents using nonlinear optical techniques. Quantum interference between one- and two-photon absorption pathways is used to inject and control ballistic currents in GaAs samples. With this, a pure charge current, pure spin current, or spin-polarized charge current can be injected by changing the polarization configuration of the two pump pulses. Such currents are temporally and spatially resolved using high-resolution pump–probe techniques, including a derivative-detection scheme, which allows detection of the motion of carriers as small as 0.1 nm. Observation of the intrinsic inverse spin Hall effect in the ballistic regime, a study of time-resolved ballistic spin-polarized charge currents, and a study of the efficiency of spin current injection by quantum interference were all achieved using these techniques. Additionally, we discuss demonstrations of second-order nonlinear optical effects induced by charge and spin currents, which allow for the nondestructive, noninvasive, and real-time imaging of currents.

Effect of interaction between deep impurity traps on thermally stimulated currents in semiconductors

We demonstrate that the problem of determining the concentrations of free and trapped charge carriers in wide-gap semiconductors from their thermally stimulated current (TSC) curves for m interacting levels in an implicit difference scheme for numerically solving differential rate equations of TSCs reduces to finding the roots of an algebraic equation of degree m + 1. For two interacting trap levels of the same nature (electron or hole traps), we present an algorithm for numerically solving differential rate equations of TSCs which allows the concentrations of free and trapped charge carriers to be determined. TSC curves can be divided into four types according to their shape (dependent on trap parameters and experimental conditions): “splitting” (two well-resolved peaks separated by a temperature range), “saddle” (a well-defined minimum between two peaks), shoulder (on the high- or low-energy side), and “coalescence-absorption” (one peak). The modeling results are used to interpret an experimental TSC curve for semiconducting InSe and to demonstrate that, to adequately interpret experimental TSC data, one should use a model for the interaction between levels.

Measurement of pure spin currents in semiconductors

A new field has emerged called spintronics (spin-based electronics), where the spin of electrons rather than the charge carries information. This may offer opportunities for a new generation of electronic devices. Measuring spin currents in semiconductors plays a key role in spintronics. In this review article, starting from the basic properties of spin currents and the current experimental techniques for spin current measurements, we briefly introduce our theory on optical effects of spin currents in the direct-gap semiconductors, i.e., the Faraday rotation and nonlinear optics, which provide a direct measurement of pure spin currents in semiconductors.

Oscillatory persistent currents in quantum rings: Semiconductors versus superconductors

Persistent currents are a hallmark of superconductivity in metals. To observe those dissipationless currents in a non-superconducting ring, the circumference of the ring must be short enough so that the phase coherence of the electronic wave functions is preserved around the loop. Recent progress in the fabrication of self-assembled semiconductor quantum rings (SAQRs), which can be filled with only a few (1–2) electrons, has offered the unique possibility to study the magnetic-field-induced oscillations in the persistent current carried by a single electron. In this paper, we discuss similarities and distinctions between the behavior of persistent currents in semiconductor and superconductor samples and give an overview of the recent results for oscillatory persistent currents in SAQRs. Although the real SAQR shape differs strongly from an idealized circular-symmetric open ring structure, the Aharonov–Bohm oscillations of the magnetization survive, as observed in low temperature magnetization measurements on In x Ga 1− x As/GaAs SAQRs.

Establishment of specific features of electron localization at U − centers in semiconductors from thermally stimulated currents

The specific features of thermally stimulated currents in semiconductors containing U − centers are treated theoretically in comparison with the corresponding features for semiconductors containing defects with a positive correlation energy or single-level centers. It is shown that the analysis of the curves of thermally stimulated currents normalized to the maximum current in respect to the shape and to the presence or absence of a shift of the temperature maximum of the curve in relation to the initial occupancy provides an extra necessary criterion for identifying defects with different parameters of binding of electrons. The results can be used in analyzing experimental data on thermally stimulated currents that exhibit features inexplicable in the context of the often used model of single-level centers.

All-optical coherently controlled terahertz ac charge currents from excitons in semiconductors

We have investigated the influence of excitonic effects on two-color coherently controlled electrical currents in semiconductors. We produce currents in CdSe and CdTe at temperature of 10 or 80 K using quantum interference between single- and two-photon absorption of fundamental and second-harmonic 150 fs optical pulses tuned over a wide energy range. Current injection is monitored via the emitted terahertz generation. For the highest photon energies wherein the injected electron-hole kinetic energy is large compared to the exciton binding energy, ``dc'' electrical current injection is observed and expected within the independent-particle approximation in which phase control of the current magnitude is governed by the optical phases only. However, as the photon energy decreases to the band-gap energy, features appear in the terahertz emission pattern that increasingly signal the breakdown of this model, in agreement with recent theoretical calculations that incorporate electron-hole interactions. In particular, when the excitation pulse bandwidth spans the $1s\text{\ensuremath{-}}2p$ exciton states, the terahertz emission characteristics are consistent with a theoretically predicted ac electrical current injection in which the phase of the current---but not its amplitude---is controlled by the relative phase of the optical pulses.

Proposal for the direct optical detection of pure spin currents in semiconductors

We suggest a different practical scheme for the direct detection of pure spin current by using the two-color Faraday rotation of optical quantum interference process (QUIP) in a semiconductor system. We demonstrate theoretically that the Faraday rotation of QUIP depends sensitively on the spin orientation and wave vector of the carriers, and can be tuned by the relative phase and the polarization direction of the $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ laser beams. By adjusting these parameters, the magnitude and direction of the spin current can be detected.

Nanocontact spin-electric effect

It is predicted that a spin signal can be converted into a change of the electric potential. This effect arises when spin-polarized electrons from a nonmagnetic circuit N penetrate into a magnetized magnetic attachment M with Zeeman splitting of the electron spectrum. Since M possesses a high density of spin states with the same direction of the spin, the penetration of spin-polarized electrons into M results in the appearance of an electric double layer at the N–M boundary and therefore a jump in the electric potential between M and N. The predicted effect can be used for direct detection of a spin signal in nonmagnetic metals and semiconductors as well as for solving a number of problems of spintronics because of the ease with which an electric field can be controlled by currents in semiconductors.

Electric field control photo-induced Hall currents in semiconductors

We generate spin-polarized carrier populations in GaAs and low temperature-grown GaAs (LT-GaAs) by circularly polarized optical beams and pull them by external electric fields to create spin-polarized currents. In the presence of the optically generated spin currents, anomalous Hall currents with an enhancement with increasing doping are observed and found to be almost steady in moderate electric fields up to ∼120 mV μm −1, indicating that photo-induced spin orientation of electrons is preserved in these systems. However, a field ∼300 mV μm −1 completely destroys the electron spin polarization due to an increase of the D’yakonov–Perel’ spin precession frequency of the hot electrons. This suggests that high field carrier transport conditions might not be suitable for spin-based technology with GaAs and LT-GaAs. It is also demonstrated that the presence of the excess arsenic sites in LT-GaAs might not affect the spin relaxation by Bir–Aronov–Pikus mechanism owing to a large number of electrons in n-doped materials.

Spin Currents in Semiconductors, Metals, and Insulators

Recent theoretical developments on the physics of spin currents in semiconductors, metals and insulators are reviewed with the stress on the role of relativistic spin–orbit interaction. The key concept is the interplay between the electric field and spin current, which might enable the spintronics without magnetic field/magnets. We present this principle discussing spin Hall effect and multiferroic behavior as two realizations.

Amplifying a Tiny Optical Effect

How do you measure a transverse shift of a light beam to within a nanometer? Recently, physicists predicted (1) that light could experience such a shift similar to what happens to electrical currents in semiconductors through the spin Hall effect (2–4). On page 787 of this issue, Hosten and Kwiat (5) use “weak measurement,” a controversial procedure from the foundations of quantum mechanics, to amplify this spin Hall effect in light (SHEL) rather than detecting it directly. In their experiment, they boost the tiny shift by a factor of 10,000, detecting it for the first time and characterizing it at the angstrom scale. This realizes one of the long-standing promises of weak measurement and demonstrates its potential in precision measurements.

Realization of a Room-Temperature Spin Dynamo: The Spin Rectification Effect

We demonstrate a room-temperature spin dynamo where the precession of electron spins in ferromagnets converts energy from microwaves to a bipolar current of electricity. The current/power ratio is at least 3 orders of magnitude larger than that found previously for spin-driven currents in semiconductors. The observed bipolar nature and intriguing symmetry are fully explained by the spin rectification effect via which the nonlinear combination of spin and charge dynamics creates dc currents.

Generating Spin Currents in Semiconductors with the Spin Hall Effect

We investigate electrically induced spin currents generated by the spin Hall effect in GaAs structures that distinguish edge effects from spin transport. Using Kerr rotation microscopy to image the spin polarization, we demonstrate that the observed spin accumulation is due to a transverse bulk electron spin current, which can drive spin polarization nearly 40 microns into a region in which there is minimal electric field. Using a model that incorporates the effects of spin drift, we determine the transverse spin drift velocity from the magnetic field dependence of the spin polarization.