台阶量子阱的结构和电场对Rashba自旋分裂的影响

Spin splitting of conduction subbands in Al0.3Ga0.7As/GaAs/AlxGa1-xAs/Al0.3Ga0.7As step quantum wells induced by interface and electric field related Rashba effects is investigated theoretically by the method of finite difference.The dependence of the spin splitting on the electric field and the well structure,which is controlled by the well width and the step width,is investigated in detail.Without an external electric field,the spin splitting is induced by an interface related Rashba term due to the built-in structure inversion asymmetry.Applying the external electric field to the step QW,the Rashba effect can be enhanced or weakened,depending on the well structure as well as the direction and the magnitude of the electric field.The spin splitting is mainly controlled by the interface related Rashba term under a negative and a stronger positive electric field,and the contribution of the electric field related Rashba term dominates in a small range of a weaker positive electric field.A method to determine the interface parameter is proposed.The results show that the step QWs might be used as spin switches.     

Simulations of polaron spin inversion in an organic semiconductor: Comparison of two mechanisms

We present a model study of the effects of two mechanisms, the Rashba spin–orbit coupling and the spin-flip term, on the polaron spin inversion in an organic semiconductor. We find that, while both mechanisms can impact the polaron spin by changing the polaron level from a spin eigenstate to a spin superposition state, substantial difference can be observed in the static and dynamical properties of the polaron. Given the values of model parameters relevant to conjugated polymers, the magnitude of the polaron spin inversion caused by the spin–orbit coupling is much smaller than that by the spin-flip term. When the dynamical properties of the polaron are considered, spin oscillations induced by both mechanisms are observed when the polaron moves along the polymer chain driven by external electric field. Interestingly, the length of the polaron motion during one spin oscillation period remains constant in the case of spin–orbit coupling, while it is enhanced with increasing the driven electric field in the case of spin-flip term, in which larger spin diffusion length and longer spin relaxation time can be expected.     

Rashba spin splitting and exchange enhancement of the g factor in InAs/AlSb heterostructures with a two-dimensional electron gas

The exchange enhancement of the g factor of quasiparticles in InAs/AlSb heterostructures with a two-dimensional electron gas exhibiting Rashba spin splitting is investigated using the 8-band k · p Hamiltonian. It is shown that, in low magnetic fields, Rashba spin splitting yields a profound increase in the amplitude of oscillations of the quasiparticle g factor renormalized by exchange interaction. From analysis of Shubnikov-de Haas oscillations at the temperature 250 mK, the energy of Rashba splitting and the g factor of quasiparticles are determined. The values determined experimentally are in good agreement with the results of theoretical calculations carried out with consideration for the asymmetric built-in electric field in InAs/AlSb heterostructures.     

Weak antilocalization and spin-orbit interaction in a In0.53Ga0.47As/InP quantum well in the persistent photoconductivity state

Low-field quantum magnetoresistance of two-dimensional electron gas at the In_0.53Ga_0.47As/InP interface was studied in the persistent photoconductivity state. The sign-alternating property of the dependences of the magnetoresistance on the magnetic field indicates that the spin-orbit interaction affects the quantum well conductivity. The mechanism caused by the electric field built in at the interface was shown to contribute dominantly to the spin-orbit scattering frequency 1/τ_so. This is the Rashba mechanism, which is linear in the electron wave vector. These data allowed us to estimate the parameters of spin-orbit splitting of the energy spectrum as α=(84±10) Ų (by the Rashba mechanism) and γ=(73±5) eV ų (by the Dyakonov-Perel and Dresselhaus mechanisms).     

Spin response of 2D electrons to a lateral electric field

The spin orientation of two-dimensional (2D) electrons by a lateral electric field is considered. The electron dispersion law is assumed to contain linear terms due to the spin-orbit band splitting in an asymmetric quantum well. The coefficient of spin orientation in a DC electric field is found. The mean electron spin is oriented in the sample plane perpendicularly to the electric field. The interaction of an AC electric field with spins of 2D electrons is studied. It is shown that transitions between different spin states give rise to a narrow absorption band. These states are mixed with 2D plasmons in the frequency range related to these transitions, with the result being that the plasmon spectrum is modified and a new type of oscillations arises (spin-plasmon polariton). The problem of the generation of spin-plasmon polaritons by an external field is solved.     

Effect of electric field on the ratio between the rashba and dresselhaus parameters in III–V heterostructures

The size-quantized energy subbands and envelope wave functions for [001] quantum wells based on zinc-blende III–V semiconductors are numerically calculated using the eight-band Kane model and finite-difference discretization scheme in coordinate space. The effect of the quantum-well band parameters and external electric field oriented along the structure growth direction on the ratio between the Rashba and Dresselhaus spin–orbit coupling parameters is studied. It is demonstrated that at certain electric-field values the spin–orbit coupling parameters in GaAs/InGaAs structures can be equal, which ensures the condition for forming stable spin helices. In addition, it is established that the spin–orbit coupling linear in wave vector in symmetric GaAs/InGaAs wells can disappear under certain well widths and barrier chemical compositions.     

Ballistic Spin Hall Transistor Using a Heterostructure Channel and Its Application to Logic Devices

In a ballistic spin transport channel, spin Hall and Rashba effects are utilized to provide a gate-controlled spin Hall transistor. A ferromagnetic electrode and a spin Hall probe are employed for spin injection and detection, respectively, in a two-dimensional Rashba system. We utilize the spin current of which polarization direction is controlled by the gate electric field which determines the strength of the Rashba effective field. By observing the spin Hall voltage, spin injection and coherent spin precession are electrically monitored. From the original Datta–Das technique, we measure the channel conductance oscillation as the gate voltage is varied. When the magnetization orientation of the injector is reversed by 180°, the phase of the Datta–Das oscillation shifts by 180° as expected. Depending on the magnetization direction, the spin Hall transistor behaves as an n- or p-type transistor. Thus, we can implement the complementary transistors which are analogous to the conventional complementary metal oxide semiconductor transistors. Using the experimental data extracted from the spin Hall transistor, the logic operation is also presented.     

Structure Inversion Asymmetry and Rashba Effect in Quantum Confined Topological Crystalline Insulator Heterostructures

Structure inversion asymmetry is an inherent feature of quantum confined heterostructures with non-equivalent interfaces. It leads to a spin splitting of the electron states and strongly affects the electronic band structure. The effect is particularly large in topological insulators because the topological surface states are extremely sensitive to the interfaces. Here, the first experimental observation and theoretical explication of this effect are reported for topological crystalline insulator quantum wells made of Pb_1−xSn_xSe confined by Pb_1−yEu_ySe barriers on one side and by vacuum on the other. This provides a well defined structure asymmetry controlled by the surface condition. The electronic structure is mapped out by angle-resolved photoemission spectroscopy and tight binding calculations, evidencing that the spin splitting decisively depends on hybridization and, thus, quantum well width. Most importantly, the topological boundary states are not only split in energy but also separated in space—unlike conventional Rashba bands that are splitted only in momentum. The splitting can be strongly enhanced to very large values by control of the surface termination due to the charge imbalance at the polar quantum well surface. The findings thus, open up a wide parameter space for tuning of such systems for device applications.     

含Rashba自旋轨道耦合二维电子气结构中电子反常位移及自旋分离

研究了含Rashba自旋轨道耦合的磁电调制半导体二维电子气中弹道电子的反常位移 （Goos-H?nchen位移,即GH位移）。计算中发现,通过调节结构的各个参数包括入射角、磁场强度和Rashba自旋轨道耦合系数,可以有效地调控GH位移,并且在一定条件下可以为负。电子的GH位移和自旋极化态有密切关系,这个自旋相关的位移可以用来分离不同自旋极化的电子束。基于这种现象,提出了一种利用GH位移在半导体2DEG中分离不同自旋极化电子的方法。     

Anisotropy of the electron g factor in quantum wells based on cubic semiconductors

A new mechanism for the spin splitting of electron levels in asymmetric quantum wells based on GaAs-type semiconductors relative to rotations of the magnetic field in the well plane is suggested. It is demonstrated that the anisotropy of the Zeeman splitting (linear in a magnetic field) arises in asymmetric quantum wells due to the interface spin-orbit terms in the electron Hamiltonian. In the case of symmetric quantum wells, it is shown that the anisotropy of the Zeeman splitting is a cubic function of the magnitude of the magnetic field, depends on the direction of the magnetic field in the interface plane as the fourth-order harmonic, and is governed by the spin-orbit term of the fourth order by the kinematic momentum in the electron Hamiltonian of a bulk semiconductor.     

GeTe中铁电极化对自旋霍尔电导的调控研究

利用电场控制电荷的自旋流与电流相互转换是自旋电子器件的关键所在,而这种控制机制在铁电半导体GeTe中可以得到实现,因为其铁电极化可以改变自身的自旋织构。基于密度泛函理论计算,我们发现可以通过铁电极化可以进一步调节自旋霍尔电导(spinHallconductivity,简记为SHC),通过计算得到自旋霍尔电导的一个分量σxyz在带边缘附近可以达到100?/e(?cm)-1的量级,其主要原因在于电极化改变了能带结构。该研究工作为可控的自旋输运的实验和理论研究具有重要的价值,必将推动自旋电子学的进一步发展。     

Effect of spin-orbit coupling on the spectrum of two-dimensional electrons in a magnetic field

The electron states in semiconductors with zinc-blende structure in an external magnetic field are studied taking into account the intrinsic spin splitting. The Hamiltonian of the spin-orbit coupling is written out using the method of invariants. Effects of modification of the energy spectrum of two-dimensional electrons in single and double quantum wells in a magnetic field oriented parallel to the interface planes are considered.     

具有Stubs结构量子波导中电子的自旋传输特性

利用递归格林函数法研究了含Rashba自旋轨道耦合效应的具有Stubs结构的量子波导中电子的自旋极化传输特性.结果表明在含一个stub的量子波导系统中, 由于stub和Rashba自旋轨道耦合引起的势阱导致系统电导出现Fano共振形式的“山谷”和“针尖”结构, 通过改变自旋轨道耦合的强度可以调节它们的大小. 同时,在同样的位置自旋极化率也出现Fano共振或反共振结构. 当系统中出现多个周期性的stubs时, 在Fano共振点附近电导中出现一些小的带隙结构.但是,当系统加上磁场后, stubs和自旋轨道耦合带来的效应都被抑制, 系统的电导重新出现量子化台阶结构. 同时由于子带间干涉效应变小, 自旋电导也出现台阶结构.     

Multisubband electron mobility in a parabolic quantum well structure under the influence of an applied electric field

We study the multisubband electron mobility in a barrier delta doped AlχGal-χAs parabolic quantum well structure under the influence of an applied electric field perpendicular to the interface plane. We consider the alloy fraction χ = 0.3 for barriers and vary x from 0.0 to 0.1 for the parabolic well. Electrons diffuse into the well and confine within the triangular like potentials near the interfaces due to Coulomb interaction with ionized donors. The parabolic structure potential, being opposite in nature, partly compensates the Coulomb potential. The external electric field further amends the potential structure leading to an asymmetric potential profile. Accordingly the energy levels, wave functions and occupation of subbands change. We calculate low temperature electron mobility as a function of the electric field and show that when two subbands are occupied, the mobility is mostly dominated by ionised impurity scattering mediated by intersubband effects. As the field increases transition from double subband to single subband occupancy occurs. A sudden enhancement in mobility is obtained due to curtailment of intersubband effects. Thereafter the mobility is governed by both impurity and alloy disorder scatterings. Our analysis of mobility as a function of the electric field for different structural parameters shows interesting results.     

低阈值高效率InAlGaAs量子阱808 nm激光器

以Al0.3Ga0.7As/InAlGaAs/Al0.3Ga0.7As压应变量子阱代替传统的无应变量子阱作为有源区,实现降低808 nm半导体激光器的阈值电流,并提高器件的效率。首先优化设计了器件结构,并利用金属有机物化学气相淀积(MOCVD)进行了器件的外延生长。通过优化外延生长条件,保证了5.08 cm片内的量子阱(QW)光致发光(PL)光谱峰值波长均匀性达0.1%。对于条宽为50μm,腔长为750μm的器件,经镀膜后的阈值电流为81mA,斜率效率为1.22 W/A,功率转换效率达53.7%。变腔长实验得到器件的腔损耗仅为2 cm-1,内量子效率达90%。结果表明,压应变量子阱半导体激光器具有更优异的特性。     

Spin relaxation and weak localization of two-dimensional electrons in asymmetric quantum Wells

The anomalous alternating-sign magnetoresistance in a two-dimensional electron gas on an In_0.53Ga_0.47As/InP heterostructure was investigated experimentally at liquid-helium temperatures in a wide range of electron densities, including the case of two filled quantum-well subbands. The data obtained are analyzed in terms of a theory that takes into account terms in the spin splitting of the electron spectrum which are cubic and linear in the wave vector. The linear term is related to the asymmetry of the quantum well, i.e., the presence of an electric field at the heterojunction. It is shown that the new theoretical model describes the experiment better. Fiz. Tekh. Poluprovodn. 31, 459–467 (April 1997)     

纤锌矿GaN/AlxGa1-xN量子阱中自由极化子能级

采用Lee-Low-Pines (LLP)变分法研究了纤锌矿GaN/Al0.3Ga0.7N量子阱中自由极化子能量和电子-声子相互作用对自由极化子能量的影响.理论计算中考虑了量子阱中定域声子模(Confined phonon modes)和半空间声子模(Half-space phonon modes)的影响以及电子有效质...     

Influence of Electric Field on Microstructures of Pentacene Thin‐Films in Field‐Effect Transistors

We report on electric‐field‐induced irreversible structural modifications in pentacene thin films after long‐term operation of organic field‐effect transistor (OFET) devices. Micro‐Raman spectroscopy allows for the analysis of the microstructural modifications of pentacene in the small active channel of OFET during device operation. The results suggest that the herringbone packing of pentacene molecules in a solid film is affected by an external electric field, particularly the source‐to‐drain field that parallels the a–b lattice plane. The analysis of vibrational frequency and Davydov splitting in the Raman spectra reveals a singular behavior suggesting a reduced separation distance between pentacene molecules after long‐term operations and, thus, large intermolecular interactions. These results provide evidence for improved OFET performance after long‐term operation, related to the microstructures of organic semiconductors. It is known that the application of large electric fields alters the semiconductor properties of the material owing to the generation of defects and the trapping of charges. However, we first suggest that large electric fields may alter the molecular geometry and further induce structural phase transitions in the pentacene films. These results provide a basis for understanding the improved electronic properties in test devices after long‐term operations, including enhanced field‐effect mobility, improved on/off current ratio, sharp sub‐threshold swing, and a slower decay rate in the output drain current. In addition, the effects of source‐to‐drain electric field, gate electric field, current and charge carriers, and thermal annealing on the pentacene films during OFET operations are discussed.     

GaN/GaAlN宽量子阱的二类激子特征

考虑了内建电场的影响,用变分法计算了GaN/GaAlN量子阱(QW)的电子子带和激子结合能.结果表明,对于阱宽较大情形,电子和空穴高度局域在QW边沿附近.内建电场造成的电子空穴空间的较大分离使QW激子表现出二类阱特征.重空穴基态结合能对Al浓度变化不敏感.     

Electroabsorption modulation at 1.3 /spl mu/m on GaAs substrates using a step-graded low temperature grown InAlAs buffer

We have achieved quantum confined Stark effects (QCSE) on In/sub 0.38/Ga/sub 0.62/As-In/sub 0.38/Al/sub 0.62/As multiple-quantum-well (MQW) structures, operating at 1.3 /spl mu/m grown on GaAs substrates. A quantum confined Stark shift of the exciton absorption peak of 47 meV was obtained with an applied electric field of 190 KV/cm, measured on surface normal PIN diodes. The structure is grown by MBE on a novel three-stage, compositionally step graded, In/sub x/Al/sub 1-x/As buffer, doped with Si to 5/spl middot/10/sup 17//cm/sup 3/, on an n-type GaAs substrate. The total thickness of the buffer is 0.3-0.6 mm, which is considerably smaller than that of linearly graded buffer layers. This structure can be used in both waveguide modulators and surface normal F-P type modulators on GaAs substrates.