Direct observation of optically injected spin-polarized currents in semiconductors

Quantum interference of one- and two-photon excitation of unbiased semiconductors yields ballistic currents of carriers. The magnitudes and directions of the currents and the spin orientations of the carriers are controlled by the polarization and relative phase of the exciting femtosecond laser fields. We provide direct experimental evidence for the spin polarization of the optically injected spin currents by detecting a phase-dependent spatial shift of the circularly polarized photoluminescence in cubic ZnSe.     

Spin injection into semiconductors using dilute magnetic semiconductors

So far, attempts at realizing spin-polarized current injection into a semiconductor using metallic ferromagnetic contacts have yielded unsatisfying results. In this paper, we present a simple model of diffusive transport, which shows that the principle reason for these negative results is a conductivity mismatch between the ferromagnetic contacts and the semiconductor. Moreover, we demonstrate that this problem can be addressed by using dilute magnetic semiconductor (DMS) contacts instead of metallic contacts. We present experimental results of optical measurements on a GaAs/AlGaAs diode fitted with a DMS spin injector contact. These measurements show a spin polarization of around 90% in the semiconductor. Furthermore, we discuss a novel magnetoresistance effect based on the suppression of one of the spin channels in the semiconductor which should allow the detection of a spin-polarized current by magnetoresistance measurements.     

Analytical study of magnetization dynamics driven by spin-polarized currents

An analytical approach is presented for the study of magnetization dynamics driven by spin-polarized currents. Two cases are considered: (i) magnetic layers with in-plane uniaxial anisotropy; (ii) magnetic layers with uniaxial anisotropy and applied field perpendicular to the layer plane. Theoretical predictions are obtained for the existence of stationary modes and self-oscillations of magnetization by solving the deterministic Landau-Lifshitz-Gilbert equation with Slonczewski spin-torque term. Thermal fluctuations are studied by deriving the corresponding Fokker-Planck equation for the magnetization probability distribution. Analytical procedures to estimate the effective potential barrier separating self-oscillatory regimes and/or stationary modes are proposed.     

极化子比率变化对自旋注入有机半导体性质的影响

从自旋扩散方程和欧姆定律出发研究了铁磁层到有机半导体的自旋注入,得到了系统的电流自旋极化率。有机半导体中的载流子为自旋极化子和不带自旋的双极化子,极化子比率在有机半导体内随输运距离变化。通过计算发现匹配的铁磁和有机半导体电导率有利于自旋注入;通过调节界面电阻自旋相关性,电流自旋极化率可获得很大程度提高;极化子比率衰减速率对有机半导体电流自旋极化率具有非常重要的影响。     

Effects of electric field and magnetic induction on spin injection into organic semiconductors

Spin-polarized injection and transport into ferromagnetic/organic semiconductor structure are studied theoretically in the presence of the external electric field and magnetic induction. Based on the spin-drift-diffusion theory and Ohm's law, we obtain the charge current polarization, which takes into account the special carriers of organic semiconductors. From the calculation, it is found that the current spin polarization is enhanced by several orders of magnitude by tuning the magnetic induction and electric fields. To get an apparent current spin polarization, the effects of spin-depended interfacial resistances and the special carriers in the organic semiconductor, which are polarons and bipolarons, are also discussed.     

Theoretical study of anisotropic mobility in ladder-type molecule organic semiconductors

The properties of two ladder-type semiconductors {M1: 2,2′-(2,7-dihexy1-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-4,9-diylidene) dimalononitrile and M2: 2,7-dihexy1-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-4,9-dione} as the n-type and ambipolar organic materials are systematically investigated using the first-principle density functional theory combined with the Marcus–Hush electron transfer theory. It is found that the substitution of M1 induces large changes in its electron-transfer mobility of 1.370 cm² V^?1 s^?1. M2 has both large electron- and hole-transfer mobility of 0.420 and 0.288 cm² V^?1 s^?1, respectively, which indicates that M2 is potentially a high efficient ambipolar organic semiconducting material. Both the M1 and M2 crystals show remarkable anisotropic behavior. A proper design of the n-type and ambipolar organic electronic materials, which may have high mobility performance, is suggested based on the investigated two molecules.     

Driven injection of a polymer into a spherical cavity: A Langevin dynamics simulation study

The injection of a self-avoiding flexible polymer into a spherical cavity under a driving force is studied by using Langevin dynamics simulation. For given polymer length (N) and driving force (f), the polymer can be completely injected into the cavity only when the radius of the cavity is larger than a transition radius (R_eC). The dependence of R_eC on N and f can be described by a scaling relation R_eC∝N^1/3f^-δ. The value of δ changes from 4/15 in the small f region to 1/6 in the moderate f region due to the screening of the excluded-volume interaction between monomers. We find the complete injection time (τ) decreases monotonously with increasing the cavity radius or decreasing the polymer length. The simulation results are in good agreement with the theoretical predictions from the free energy analysis and a simple kinetic model.     

Theoretical study of the lattice thermal conductivity in Ge framework semiconductors

The lattice thermal conductivity of Ge clathrates is investigated by evaluating the linear response theory heat current correlation functions using molecular dynamics. Clathrate crystals with and without guest atoms in their fullerane cages are studied. In comparison with that of diamond-phase Ge, the clathrate conductivity is reduced by approximately 1 order of magnitude due to the open framework itself. The addition of an encapsulated (rattling) Sr guest atom produces a further order of magnitude reduction in the conductivity, making it comparable to that of amorphous Ge. Our results are consistent with experiments, and have impact on the search for improved thermoelectric materials.     

GaAs极化电子源实验研究

对GaAs极化电子源中GaAs晶片的激活过程进行了实验研究，着重采用2种不同的激活过程摸索了实验过程中的相关物理参量，以达到最佳的激活状态，并获得了一些有意义的实验结果.     

The electrical properties of dislocations in semiconductors

According to the theory of Read, closely spaced acceptor sites occur at dangling bonds along dislocations in semiconductors and lead to the formation of space-charge tubes which scatter electrons. In order to test Read's theory, parallel arrays of dislocations have been introduced by plastic bending and extensive electrical and galvanomagnetic measurements have been made on n-type germanium crystals. The experimental results are not in agreement with Read's theory; however, it is shown that Read's basic model is applicable but it must be modified by the assumption of non-specular electron scattering at the edges of the space-charge tubes—the electrical and galvanomagnetic consequences of the theory plus modification are worked out and good agreement with experiments obtained. The following quantities are derived: conductivity (σ), Hall constant (R), and magnetoresistance (MR) for current both parallel (‖) and perpendicular (⊥) to the long axis of the tubes with magnetic field (H) at right angles to the current. The quantities R, σ_‖ and σ_⊥ were measured versus temperature from 78°K to 300°K and MR was measured at 78°K as a function of the angle between H and the normal to the neutral plane (N). It was found that the variation of the electrical properties in the sample, particularly along the N direction, is so great that to obtain valid data one must use small samples cut from bent crystals; data are presented for whole samples, as well as small samples, in order to illustrate this effect. The dislocation acceptor level in n-type germanium is found to lie at ～0.50 ev below the conduction band edge. By using the above theory one can determine values of the dislocation density (N) from the result of electrical measurements on semiconductors; this was done in order to determine, on deformed n-type germanium, variations in N from region to region of bent samples as a function of temperature and time of deformation and annealing. In addition, other experiments were performed on plastically bent indium antimonide; it was found that the electrical effects of dislocations, although considerably different from those in germanium, can also be explained by the theory presented here.     

A study of the dark currents of InSb charge injection devices

The dark currents of InSb metal-insulator-semiconductor (MIS) charge injection devices (CIDs) in the temperature range 30 to 120K are studied. It is found that a thermal-generating process dominates the dark current in the high temperature region. When the device is under small bias voltage, a bump in the trap emission current appears below 70K. This current is due to carrier capture and emission processes of a hole trap state located in the bulk material, and the measured activation energy is 50meV. For larger bias conditions, the band-to-band tunnelling current gradually overcomes the trap emission current. It smears out the bump in the trap emission current, and shows a slightly temperature-dependent behaviour in the plot. The effects of field electrodes are also studied. It is found from experimental results that the edge of the electric field around the device periphery plays an important role in the band-to-band tunnelling process, and a field electrode with suitable bias can improve the dark current response drastically.     

Optical imaging of electrical carrier injection into individual InAs quantum dots

We image the micro-electroluminescence (EL) spectra of self-assembled InAs quantum dots (QDs) embedded in the intrinsic region of a GaAs p-i-n diode and demonstrate optical detection of carrier injection into a single QD. Tunneling of electrons and holes into the QDs at bias voltages below the flat-band condition leads to a spectrum of sharp EL lines from a small number of bright spots on the diode surface, characteristic of emission from individual QDs. We explain this behavior in terms of Coulomb interaction effects and the selective excitation of a small number of QDs within the ensemble due to preferential tunneling paths for carriers.     

Theoretical interpretation of spin-polarized X-ray absorption spectra of Mn in MnP

On the basis of the multiple scattering approach in combination with the spin-polarized self-consistent potential calculation, an interpretation is proposed for the spin-polarized x-ray absorption spectra near the Mn K edge in MnP. The effect of the core vacancy potential on the spectra is analyzed and found to be insignificant. The method used for the calculations allowed the separation of the effect of the dipole transition matrix element on the spectra and the effect of the density of unoccupied electron states. It is shown that the transition matrix element causes an intensity redistribution only near the absorption jump, while the difference in the densities of states is most pronounced in the energy region 35–55 eV away from the main edge and leads to a shift in energy for the spectra corresponding to the two spin directions. The effect of the spin-dependent broadening caused by the dependence of the mean free path (as a function of energy) on the photoelectron spin is studied. It is shown that this factor considerably affects only the intensities of the peaks in the energy region lying within less than 12 eV from the main edge.     

Theoretical aspects of hydrogen in crystalline semiconductors

Hydrogen in semiconductors displays behavior quite different from any other impurity, causing effects which can be either beneficial or detrimental to the electronic properties. Theoretical calculations have been able to elucidate many puzzling aspects of this behavior, by providing a microscopic picture of the interactions of hydrogen with the host lattice and with other impurities. I will critically review the available theoretical techniques, and address the following topics: microscopic structure of isolated interstitial hydrogen, as well as various hydrogen-impurity complexes; electronic structure, including the relative stability of different charge states; diffusion; and H motion around impurities.     

Quantum interference control of ballistic pure spin currents in semiconductors

We demonstrate all-optical quantum interference injection and control of a ballistic pure spin current (without an accompanying charge current) in GaAs/AlGaAs quantum wells, consisting of spin-up electrons traveling in one direction and spin-down electrons traveling in the opposite direction. This current is generated through quantum interference of one- and two-photon absorption of approximately 100 fs phase-locked pulses that have orthogonal linear polarizations. We use a spatially resolved pump-probe technique to measure carrier movement of approximately 10 nm. Results agree with recent theoretical predictions.     

Theoretical study of field emission currents from W(001) surface

We calculated the field emission of the W(001) surface based on a local density functional method. In this method, the electronic structure and the potential under an external electric field are calculated self-consistently using the pseudopotential method. The method is easy to implement and requires minimal computation time beyond that of a standard density functional calculation, while at the same time fully compatible with the density functional formalism. Results such as the enhancement factors are in good agreement with experiments and the effect of the surface reconstruction and the choice of exchange-correlation functionals on the field emission current will be discussed.