Photoinduced Inverse spin-Hall Effect Research Articles

Hole and electron spin lifetime in lightly n-doped silicon at low temperatures

We report on photoinduced inverse spin-Hall effect (ISHE) measurements as a function of the incident photon energy in the 4–50 K temperature range for a Pt/n-doped Si junction. Optical spin injection allows generating a spin-oriented population of electrons and holes around the Δ valleys and Γ point of the Si Brillouin zone, respectively. Spin-polarized carriers cross the Pt/Si contact and then enter the Pt overlayer, where spin-to-charge conversion occurs by means of spin-dependent scattering with Pt nuclei. For temperatures T up to 20 K, most of the dopants are not ionized, so that the electric field, stemming from the contact potential between Pt and Si, extends to the whole Si substrate, which becomes insulating, and only spin-oriented holes reach the Pt layer and contribute to the ISHE spectra. For T>20 K, donors are partially ionized, and the resulting space charge close to the Pt/Si interface leads to the formation of a Schottky contact where the electric field rapidly vanishes within a few micrometers. As a consequence, also spin-polarized electrons enter Pt by means of thermionic emission, contributing to the ISHE signal. We numerically solve the one-dimensional spin drift-diffusion equations for holes and electrons and estimate the temperature dependence of the spin lifetime in Si for both populations, demonstrating that Si may serve as a versatile platform for spintronic applications, able to leverage both electrons and holes.

Emerging Nonlinear Photocurrents in Lead Halide Perovskites for Spintronics.

Lead halide perovskites (LHPs) containing organic parts are emerging optoelectronic materials with a wide range of applications thanks to their high optical absorption, carrier mobility, and easy preparation methods. They possess spin-dependent properties, such as strong spin-orbit coupling (SOC), and are promising for spintronics. The Rashba effect in LHPs can be manipulated by a magnetic field and a polarized light field. Considering the surfaces and interfaces of LHPs, light polarization-dependent optoelectronics of LHPs has attracted attention, especially in terms of spin-dependent photocurrents (SDPs). Currently, there are intense efforts being made in the identification and separation of SDPs and spin-to-charge interconversion in LHP. Here, we provide a comprehensive review of second-order nonlinear photocurrents in LHP in regard to spintronics. First, a detailed background on Rashba SOC and its related effects (including the inverse Rashba-Edelstein effect) is given. Subsequently, nonlinear photo-induced effects leading to SDPs are presented. Then, SDPs due to the photo-induced inverse spin Hall effect and the circular photogalvanic effect, together with photocurrent due to the photon drag effect, are compared. This is followed by the main focus of nonlinear photocurrents in LHPs containing organic parts, starting from fundamentals related to spin-dependent optoelectronics. Finally, we conclude with a brief summary and future prospects.

Photo-induced inverse spin Hall effect of the top and bottom Dirac surface states of three-dimensional topological insulators Sb2Te3

Inverse spin Hall effect (ISHE) provides an important way to manipulate and detect spin current via spin–orbit coupling. However, it is difficult to distinguish the ISHE of the top and bottom surface states of topological insulators (TIs). Photo-induced inverse spin Hall effect (PISHE) is a powerful tool to investigate the ISHE of the Dirac surface states of TIs. In this work, PISHE of three-dimensional topological insulators Sb 2 Te 3 with a thickness of 7 and 30 quintuple layer (QL) has been investigated. It is revealed that both of the PISHE currents of the top and bottom surface states are observed in the 7-QL and the 30-QL samples. Besides, a model has been developed to separate the PISHE current of the top and bottom surface states. It is found that the PISHE currents of the top and bottom surface states of the Sb 2 Te 3 films almost do not change with temperature in the range of 77 to 300 K. It is demonstrated that the PISHE current changes linearly with light power, which agrees well with the theoretical model. This study provides a method utilizing circularly polarized light to manipulate and detect the ISHE of the top and bottom surface states of three-dimensional topological insulators. • Both of the PISHE currents of the top and bottom surface states are observed in the 7-QL and the 30-QL Sb 2 Te 3 films. • A model has been developed to separate the PISHE current of the top and bottom surface states. • The PISHE currents of the top and bottom surface states of the Sb 2 Te 3 films almost do not change with temperature in the range of 77 to 300 K.

Giant photoinduced inverse spin Hall effect of the surface states in three dimensional topological insulators Bi2Te3 with different thickness.

The photoinduced inverse spin Hall effect (PISHE) has been studied in three dimensional (3D) topological insulator (TI) Bi2Te3 thin films with different thicknesses (3, 5, 12 and 20 quintuple layer (QL)). The sign of the PISHE current flips only once in the 3- and 20-QL Bi2Te3 films, but it flips three times in the 5-, 7- and 12-QL samples. The three-times sign flip is due to the superposition of the PISHE current of the top and bottom surface states in Bi2Te3 films. By analyzing the x-ray photoelectron spectroscopy (XPS) of the Bi2Te3 films, we find that the top surface of the 3- and 20-QL Bi2Te3 films are severely oxidized, leading to only one sign flip in the PISHE. The PISHE contributed by the top and bottom surface states in Bi2Te3 films have been successfully separated by fitting a theoretical model to the PISHE current. The impact of the bulk states on PISHE current has been determined. The PISHE current is also measured at different light powers, and all the measurement results are in good agreement with the theoretical model. In addition, it is found that the PISHE current in Bi2Te3 films grown on Si substrate is more than two orders larger than that grown on SrTiO3 substrates, which can be attributed to the larger absorption coefficient for Bi2Te3/Si samples. It is revealed that the PISHE current in 3D TI Bi2Te3 is as large as 140 nA/W in the 3-QL Bi2Te3 film grown on Si substrate, which is more than one order larger than that reported in GaAs/AlGaAs heterojunction (about 2 nA/W) and GaN/AlGaN heterojunction (about 1.7 nA/W). The giant PISHE current demonstrates that the TIs with strong SOC may have good application prospects in spintronic devices with high spin-to-charge conversion efficiency.

Modeling of the photocurrent induced by inverse spin Hall effect under local circularly polarized photoexcitation

The inverse spin Hall effect (ISHE) has been recently demonstrated through the photoinduced inverse spin Hall effect (PISHE), where a focused laser normal to the device plane generates photocurrent that depends on the helicity of the photoexcitation. Here, we have employed a finite element method to rigorously simulate the helicity-dependent photocurrent (HDPC) under local helical photoexcitation, taking into account the complications of minority carriers, metal contact junctions, and spin relaxation. We found that the ISHE-induced electromotive force is inversely proportional to the doping level of the material, caused by the diffusion and drift current balance. Furthermore, the HDPC near the metal contact can either increase or decrease, because of the competing mechanisms of fast carrier recombination and spin relaxation at the contact. These simulation results provide insightful visualization of charge, spin, electric field, and current density distributions and deeper understanding of photoinduced ISHE.

Inverse spin-Hall effect in GeSn

Due to the long spin lifetime and its optical and electrical properties, GeSn is a promising candidate for the integration of spintronics, photonics, and electronics. Here, we investigate the photoinduced inverse spin-Hall effect in a GeSn alloy with 5% Sn concentration. We generate a spin-polarized electron population at the Γ point of the GeSn conduction band by means of optical orientation, and we detect the inverse spin-Hall effect signal coming from the spin-to-charge conversion in GeSn. We study the dependence of the inverse spin-Hall signal on the kinetic energy of the spin-polarized carriers by varying the energy of the impinging photons in the 0.5–1.5 eV range. We rationalize the experimental data within a diffusion model which explicitly accounts for momentum, energy, and spin relaxation of the spin-polarized hot electrons. At high photon energies, when the spin relaxation is mainly driven by phonon scattering, we extract a spin-Hall angle in GeSn which is more than ten times larger than the one of pure Ge. Moreover, the spin–charge interconversion for electrons lying at the Δ valleys of GeSn results to be ≈4.3 times larger than the one for electrons at L valleys.

Temperature dependent spin Hall conductivity in n-GaAs epitaxial layers measured by inverse spin Hall effect

The photo-induced inverse spin Hall effect (ISHE) experiments are conducted in heavily doped n-GaAs epitaxial layers by measuring the transverse electric current generated through the diffusion of optically injected spin orientations over a temperature range of 10–300 K. ISHE origin of the measured signal is confirmed through meticulous checks including the characteristic dependence of magnitude of signal on the angle of incidence. The measured value of ISHE current (IISHE) is observed to fall with the increase in temperature. Furthermore, the value of spin current density is theoretically estimated by solving the spin diffusion equation with appropriate boundary conditions for an epitaxial layer. It is shown that by near resonant excitation and subsequent solution of diffusion equation, the spin Hall angle (γ) and spin Hall conductivity (σSH) can be estimated, provided the effective life time of spin polarized electrons(τS) is known independently. By using the numerically calculated value of τS, the proposed method is implemented to estimate the values of γ and σSH. It is found that the fall in the values of IISHE at high temperatures is not governed by σSH, rather by a rapid decrease in the values of τS. In fact, σSH is seen to increase with the temperature, which is compared with existing literature. The present work provides the necessary insight into material parameters which are essential for the development of advanced spin-photonic semiconductor devices.

Modeling the photo-induced inverse spin-Hall effect in Pt/semiconductor junctions

We show that the photon energy dependence of the photo-induced inverse spin-Hall effect (ISHE) signal at Pt/semiconductor junctions can be reproduced by a model that explicitly accounts for the electron spin diffusion length Ls in the semiconductor. In particular, we consider the Pt/GaAs, Pt/Ge, and Pt/Si systems: although optical spin injection and transport of spin-polarized electrons in the conduction band of these semiconductors are ruled by different mechanisms, a simple one dimensional analytical diffusion model, where Ls is the free parameter, can reproduce the ISHE data in all cases. This highlights the potentialities of the photo-induced ISHE spectra as a tool to directly address fundamental spin transport properties in semiconductors.

Photoinduced Inverse Spin Hall Effect of Surface States in the Topological Insulator Bi2Se3.

The three-dimensional (3D) topological insulator (TI) Bi2Se3 exhibits topologically protected, linearly dispersing Dirac surface states (SSs). To access the intriguing properties of these SSs, it is important to distinguish them from the coexisting two-dimensional electron gas (2DEG) on the surface. Here, we use circularly polarized light to induce the inverse spin Hall effect in a Bi2Se3 thin film at different temperatures (i.e., from 77 to 300 K). It is demonstrated that the photoinduced inverse spin Hall effect (PISHE) of the top SSs and the 2DEG can be separated based on their opposite signs. The temperature and power dependence of the PISHE also confirms our method. Furthermore, it is found that the PISHE in the 2DEG is dominated by the extrinsic mechanism, as revealed by the temperature dependence of the PISHE.

Pure spin currents in Ge probed by inverse spin-Hall effect

We perform photoinduced inverse spin-Hall effect (ISHE) measurements on a Pt/Ge(001) junction at room temperature. The spin-oriented electrons are photogenerated at the Γ point of the Ge Brillouin zone using circularly-polarized light. After the ultrafast Γ−L scattering in the Ge conduction band, which partially preserves the spin polarization, electrons diffuse into the Pt layer where spin-dependent scattering with Pt nuclei yields a transverse electromotive field EISHE. The ISHE signal dependence as a function of the incident photon energy is investigated and interpreted in the frame of a one-dimensional spin drift-diffusion model. This allows estimating the electron spin lifetime at the L-valleys to be τs=1 ns.

Erratum: “Photon energy dependence of photo-induced inverse spin-Hall effect in Pt/GaAs and Pt/Ge” [Appl. Phys. Lett. 106, 232402 (2015)

Erratum: “Photon energy dependence of photo-induced inverse spin-Hall effect in Pt/GaAs and Pt/Ge” [Appl. Phys. Lett. <b>106</b>, 232402 (2015)

Spin diffusion in Pt as probed by optically generated spin currents

We investigate the dependence of the photoinduced inverse spin-Hall effect (ISHE) signal in a set of Pt/Ge(001) junctions as a function of the Pt thickness ${t}_{\mathrm{Pt}}$ in the 1--20-nm range at room temperature. The spin-polarized electrons, photoexcited through optical orientation at the direct gap of bulk Ge, diffuse towards the Pt layer, where spin-dependent scattering yields a transverse electromotive field ${\mathbf{E}}_{\mathrm{ISHE}}$. We show that the experimental data can be properly interpreted in the frame of the spin diffusion model, provided that we consider a variation of the spin diffusion length ${L}_{\mathrm{s}}$ in platinum when the Pt film thickness is increased. Indeed, for ${t}_{\mathrm{Pt}}g10$ nm we estimate ${L}_{\mathrm{s}}=8.2\ifmmode\pm\else\textpm\fi{}3.5$ nm, whereas for ${t}_{\mathrm{Pt}}l10$ nm the spin diffusion length decreases to ${L}_{\mathrm{s}}=3.5\ifmmode\pm\else\textpm\fi{}2.1$ nm. Moreover, the experimental results clearly indicate that for ${t}_{\mathrm{Pt}}l10$ nm the spin Hall angle $\ensuremath{\gamma}$ is three times larger than the value corresponding to thick Pt layers, suggesting a drastic change of the spin-dependent scattering mechanism for very thin Pt films.

Photon energy dependence of photo-induced inverse spin-Hall effect in Pt/GaAs and Pt/Ge

We report the photon energy dependence of photo-induced inverse spin Hall effect (ISHE) in Pt/GaAs and Pt/Ge Schottky junctions. The experimental results are compared with a spin drift-diffusion model, which highlights the role played by the different spin lifetime in the two semiconductors, in determining the energy dependence of the ISHE signal detected in the Pt layer. The good qualitative agreement between experiments and modelling indicates that photo-induced ISHE can be used as a tool to characterize spin lifetime in semiconductors.

Temperature dependence of the photo-induced inverse spin Hall effect in Au/InP hybrid structures

Photo-induced Inverse Spin Hall Effect (ISHE) measurements on Au/InP hybrid structures are performed over a temperature range of 45 to 300 K. Dependence of the spin current density on the degree of circular polarization and also on the angle of incidence of laser beam confirms the ISHE origin of measured signal. The magnitude of ISHE increases with sample cooling. A numerical model based on the spin relaxation of non-equilibrium spin-polarized electrons is proposed for predicting the temperature dependence of ISHE. Our results indicate that the proposed device can be used as a spin photodetector over a wide temperature range.

Experimental evaluation of the spin-Hall conductivity in Si-doped GaAs

We performed photoinduced inverse spin-Hall effect (ISHE) measurements on Si-doped bulk GaAs at room temperature. The spin current is optically injected in the sample by means of circularly polarized light and is detected through spin-dependent scattering by Si impurities, which provide an electromotive field ${\mathbf{E}}_{\mathrm{ISHE}}$ close to the sample surface. We measured the ISHE signal as a function of the incident photon energy, and we determined the corresponding averaged spin current, flowing through the sample, to experimentally determine the spin-Hall conductivity.

Photoinduced inverse spin Hall effect in Pt/Ge(001) at room temperature

We performed photoinduced inverse spin Hall effect (ISHE) measurements on a Pt/Ge(001) junction at room temperature. The spin-oriented electrons, photogenerated at the direct gap of Ge using circularly polarized light, provide a net spin current, which yields an electromotive field EISHE in the Pt layer. Such a signal is clearly detected at room temperature despite the strong Γ to L scattering, which electrons undergo in the Ge conduction band. The ISHE signal dependence on the exciting photon energy is in good agreement with the electron spin polarization expected for optical orientation at the direct gap of Ge.

Direct conversion of light-polarization information into electric voltage using photoinduced inverse spin-Hall effect in Pt/GaAs hybrid structure: Spin photodetector

The direct conversion of light-polarization information into electric voltage has been demonstrated using the photoinduced inverse spin-Hall effect in a Pt/GaAs hybrid structure. In the GaAs layer, spin-polarized carriers are generated by the illumination of circularly polarized light, which induces a pure-spin current in the Pt layer through the interface. The pure-spin current is converted into an electromotive force using the inverse spin-Hall effect (ISHE) in the Pt layer. The electromotive force due to the photoinduced ISHE was found to be proportional to the degree of circular polarization of the illuminated light outside the sample in spite of the presence of the Pt top layer, which is consistent with a calculation based on the analysis for light propagation in multilayer structures. This conversion of light-polarization information into electric voltage works at room temperature without bias voltage and magnetic fields, and thus can be used as a spin photodetector.

Photoinduced inverse spin-Hall effect: Conversion of light-polarization information into electric voltage

The photoinduced inverse spin-Hall effect was observed in a Pt/GaAs hybrid structure. In the GaAs layer, circularly polarized light generates spin-polarized carriers, inducing a pure spin current into the Pt layer through the interface. This pure spin current is, by the inverse spin-Hall effect in the Pt layer, converted into electric voltage. By changing the direction and ellipticity of the circularly polarized light, the electromotive force varies systematically, consistent with the prediction of the photoinduced inverse spin-Hall effect. The observed phenomenon allows the direct conversion of circular-polarization information into electric voltage; this phenomenon can be used as a spin photodetector.