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We present a study of the modifications of the electronic properties of β-gallium oxide crystals by 2.5-MeV electron irradiation. This type of irradiation produces exclusively local point defects in Ga2O3, predominantly gallium vacancies, which act as acceptor centers. Starting with a highly n-doped sample, we establish a quantitative linear relation between the irradiation dose and the concentration of generated acceptor centers. This gives the possibility to tune the Fermi level position within the bandgap by choosing an appropriate irradiation dose. At high doses, with a very deep position of the Fermi level, the n-type sample becomes compensated, reaching a semi-insulating state. The downward shift of the Fermi level with irradiation allows us to reveal the presence of latent impurities of transition metals (like Cr and Fe), which are inactive in electron paramagnetic resonance and luminescence spectra of pristine samples. This study confirms the potential of electron irradiation as a tool for tailoring the electronic properties of gallium oxide.

Using high-frequency electron paramagnetic resonance (EPR), we have observed non-Kramers ions with giant fine structure splitting of the order of 100 GHz in n-type β-Ga2O3 crystals. These EPR spectra were assigned to Fe2+ ions 5 D (3 d 6) with S = 2. This interpretation was supported by experiments on Fermi level displacement induced by high-energy electron irradiation and photoexcitation of irradiated samples with 405-nm laser light. The values and signs of the basic parameters of the spin Hamiltonian for ions, namely Cr3+ ( S = 3/2) and Fe3+ ( S = 5/2), were identified, and the order of their spin levels was established.

International audience; We present a semi-classical model for spin-injected vertical-cavity surface-emitting lasers (spin-VCSELs) with local optical anisotropies. Particular focus is put on highly-anisotropic spin-lasers with broad application potential. A generalized matrix formalism for extraction of the laser modes is introduced, which enables to calculate spatial distribution of vectorial modes in arbitrary spin-VCSELs. Time-dependence of such laser modes is further studied using the generalized coupled mode theory (CMT). It is the natural anisotropic generalization of the conventional modedecomposition approach. We use the circularly-polarized optical modes as the basis for CMT, which leads to extension of the well-known spin-flip model (SFM). In contrary to conventional SFM, the only input parameters are the geometric and local optical properties of the multilayer structure and properties of the gain media. The advantages of the theory are demonstrated on design and optimization of spin-VCSEL structure with high-contrast grating. We show that the proposed structures can be used for i) polarization modulation in THz range with tremendous applications for future ultrafast optical communication and ii) as perspective compact THz sources.

The chiral tunneling asymmetry for the transmission through a semiconductor interface with spin-orbit coupling (SOC) and antiparallel magnetization is presented from the perspective of the transfer Hamiltonian approach. We explicitly show that the expression for the tunneling rate contains terms which are odd in the momentum along the interface thus contributing to the skew tunneling and the transverse current along the interface. These terms contain the scalar chirality that is the mixed vector product of the effective SOC and exchange fields acting on the tunneling electron or hole. The crucial role of the evanescent states in the tunneling asymmetry is revealed. It is shown that the chiral Hall effect also results in a spin injection across the interface. We finally come up with a more general view on the chiral tunneling as a universal phenomenon expected in various systems.

Self-forming, vertically-aligned, ZnO moth-eye-like nanoarrays were grown by catalyst-free pulsed laser deposition on a-SiO2/Si (1 1 1) substrates. X-Ray Diffraction (XRD) and Cathodoluminescence (CL) studies indicated that nanostructures were highly c-axis oriented wurtzite ZnO with strong near band edge emission. The nanostructures were used as templates for the growth of non-polar GaN by metal organic vapor phase epitaxy. XRD, scanning electron microscopy, energy dispersive X-ray microanalysis and CL revealed ZnO encapsulated with GaN, without evidence of ZnO back-etching. XRD showed compressive epitaxial strain in the GaN, which is conducive to stabilization of the higher indium contents required for more efficient green light emitting diode (LED) and photovoltaic (PV) operation. Angular-dependent specular reflection measurements showed a relative reflectance of less than 1% over the wavelength range of 400–720 nm at all angles up to 60°. The superior black-body performance of this moth-eye-like structure would boost LED light extraction and PV anti-reflection performance compared with existing planar or nanowire LED and PV morphologies. The enhancement in core conductivity, provided by the ZnO, would also improve current distribution and increase the effective junction area compared with nanowire devices based solely on GaN.

In our previous paper [T. Fordos, et al., J. Opt. 16 (2014) 065008] we have proposed a new approach for modeling of polarized light emission from anisotropic multilayers with active dipole layers. The method is suitable to model spin-polarized light emitting diodes (spin-LED) and spin-lasers. This paper deals with generalization of the approach to scattering matrix (S-matrix) formalism and to laterally periodic structures in the frame of rigorous coupled wave algorithm (RCWA). We use expansion of the permittivity tensor in a grating layer into Fourier series and the periodic electromagnetic field in the structure is expressed using a matrix method including appropriate boundary conditions. The new approach based on S-matrix formalism is also suitable for modeling of monomode emission from MQW laser structures with multiple source layers.

The anisotropic properties of thermal transport in insulating or conducting ferromagnets are derived on the basis of the Onsager reciprocity relations applied to a magnetic system. It is shown that the angular dependence of the temperature gradient takes the same form as that of the anisotropic magnetoresistance, including anomalous and planar Hall contributions [1]. The experimental study [2] shows that the voltage measured between the extremities of the non-magnetic electrode in thermal contact to the Py or YIG ferromagnetic layers follows the predicted angular dependence. Furthermore, the sign and the amplitude of the magneto-voltaic signal measured is in agreement with the thermocouples calculated from the corresponding Seebeck coefficients, for the three different electrodes used in the study (Pt, Cu, Bi).

Équipe 101 : Nanomagnétisme et électronique de spin; International audience; The anisotropic properties of thermal transport in insulating or conducting ferromagnets are derived on the basis of the Onsager reciprocity relations applied to a magnetic system. It is shown that the angular dependence of the temperature gradient takes the same form as that of the anisotropic magnetoresistance, including anomalous and planar Hall contributions. The measured thermocouple generated between the extremities of the nonmagnetic electrode in thermal contact to the ferromagnet follows this same angular dependence. The sign and amplitude of the magnetovoltaic signal is controlled by the difference of the Seebeck coefficients of the thermocouple.

International audience; Self-forming, vertically-aligned, arrays of black-body-like ZnO moth-eye nanostructures were grown on Si(111), c-Al2O3, ZnO and high manganese austenitic steel substrates using Pulsed Laser Deposition. X-ray diffraction (XRD) revealed the nanostructures to be well-crystallised wurtzite ZnO with strong preferential c-axis crystallographic orientation along the growth direction for all the substrates. Cathodoluminescence (CL) studies revealed emission characteristic of the ZnO near band edge for all substrates. Such moth-eye nanostructures have a graded effective refractive index and exhibit black-body characteristics. Coatings with these features may offer improvements in photovoltaic and LED performance. Moreover, since ZnO nanostructures can be grown readily on a wide range of substrates it is suggested that such an approach could facilitate growth of GaN-based devices on mismatched and/or technologically important substrates, which may have been inaccessible till present.

This work reports on investigations into the possibility of harvesting energy from the piezoelectric response of millimetric ZnO rods to movement. SEM & PL studies of hydrothermally grown ZnO rods revealed sizes ranging from 1 - 3 mm x 100 - 400 microns and suggested that each was a wurtzite monocrystal. Studies of current & voltage responses as a function of time during bending with a probe arm gave responses coherent with those reported elsewhere in the literature for ZnO nanowires or micro-rod single wire generators. The larger scale of these rods provided some advantages over such nano- and microstructures in terms of contacting ease, signal level & robustness.

GaN was grown on ZnO-buffered c-sapphire (c-Al2O3) substrates by Metal Organic Vapor Phase Epitaxy. The ZnO then served as a sacrificial release layer, allowing chemical lift-off of the GaN from the c-Al2O3 substrate via selective wet etching of the ZnO. The GaN was subsequently direct-wafer-bonded onto a glass substrate. X-Ray Diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray microanalysis, Room Temperature Photoluminescence & optical microscopy confirmed bonding of several mm2 of crack-free wurtzite GaN films onto a soda lime glass microscope slide with no obvious deterioration of the GaN morphology. Using such an approach, InGaN based devices can be lifted-off expensive single crystal substrates and bonded onto supports with a better cost-performance profile. Moreover, the approach offers the possibility of reclaiming and reusing the substrate.

Recently, there has been a surge of activity in the development of next-generation transparent thin film transistors for use in applications such as electronic paper and flexible organic light emitting diode panels. Amongst the transparent conducting oxides attracting the most interest at present are Amorphous Oxide Semiconductors (AOS) based on ZnO because they exhibit enhanced electron mobility (μ), superior capacity for processability in air and improved thermodynamic stability compared with conventional covalent amorphous semiconductors and existing AOS. Moreover, they give excellent performance when fabricated at relatively low temperature and can readily be made in large area format. Thus, they are projected to resolve the trade-off between processing temperature and device performance and thereby allow fabrication on inexpensive heatsensitive substrates. For the moment, however, an undesireable post-deposition annealing step at a temperature of about 200°C is necessary in order to obtain suitable electrical and optical properties. This paper demonstrates the possibility of directly engineering amorphous ZnO with relatively high conductiviy at room temperature on paper and mylar substrates using pulsed laser deposition.

Hot-electron transport in indium phosphide is studied by means of energy-resolved field-assisted photoemission of an Ag/InP Schottky diode. The work function of the thin silver top layer is lowered by cesium and oxygen coadsorption. A quasimonoenergetic electron distribution is created close to the bottom of the conduction band in the bulk of the weakly doped p-type InP crystal by near-band-gap light excitation. The energy-distribution curves of the photoemitted electrons are measured for different values of the bias applied to the Ag/InP contact. It is demonstrated that this set of photoemission spectra constitutes a direct measurement of the evolution of the initial distribution during transport in the high electric field of the band-bending region. From this spectroscopic image of the complex multivalley transport, the relevant energy and momentum relaxation processes are clearly identified. Simple models based on the energy balance between the gain in the electric field and the losses by phonon emission account remarkably well for the experimental results. From the comparison between theory and experiment, the characteristic phonon-scattering times are deduced throughout the first conduction band.

We present the last developments of the approach of spin-transfer in terms of mesoscopic non-equilibrium thermodynamics. The modification of the ferromagnetic states due to the presence of a spin-accumulation process is investigated. The spin accumulation (also responsible for giant magnetoresistance) is generated by an electric current at the interfaces with a ferromagnetic nanostructure. The dynamical coupling that links the ferromagnetic degree of freedom to the spin of the conduction electrons is investigated with taking into account longitudinal and transverse spin accumulation. It seems that the coupling with the transverse spin-accumulation may be equivalent to the usual spin-transfer-torque mechanisms deduced from microscopic transmission- reflection coefficients at the interface. In contrast, the coupling with the longitudinal spin- accumulation generates fluctuations and seems to play important role in the activation process at larger time scales only.

Energy analysis of the electrons photoemitted from p-type doped (100) InP crystals is performed with 20-meV resolution, at 120 K. The samples are activated to low electron affinity. Laser excitation in the photon energy range 1.96\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}3.53 eV is used. The locations of the subsidiary minima ${\mathit{L}}_{6}$ and ${\mathit{X}}_{6}$ of the first conduction band and ${\mathit{X}}_{7\mathit{c}}$ of the second one are unambiguously measured (respectively, 0.67, 0.90, and 1.18 eV above the bottom of the conduction band). The energy dispersion of the three upper valence bands and of the first conduction band is probed over a large portion of the Brillouin zone. These experimental results are accurately described in the framework of the k\ensuremath{\cdot}p Kane model and the value of the spin-orbit-split-band mass ${\mathit{m}}_{7}$=(0.19\ifmmode\pm\else\textpm\fi{}0.01)${\mathit{m}}_{0}$ (where ${\mathit{m}}_{0}$ is the free-electron mass) is directly obtained.

The aim of this paper is to demonstrate the relevance of the a non-equilibrium stochastic approach in the context of spin-transfer mechanism. Spin-transfer is the generic name for the effect of magnetization reversal (or magnetic excitation) produced by the injection of a spin-polarized current in a ferromagnetic layer. Deterministic vs. stochastic approaches are first defined in the context of the Landau-Lifshitz Gilbert equation of the magnetization. We then present a model based on non-equilibrium thermodynamics in which the spin-accumulation at the interface appears as a diffusion term in the Landau-Lifshitz equation. The expression of the critical current I c is derived from this diffusive process and compared to the experimental results. Due to the definition of the critical current in terms of activation process, the phenomenological expression of I c is identical to that derived in the deterministic case, after introducing an "efficiency parameter". Only the specific form of this efficiency parameter allows one to discriminate between the different models. The direct link to the integral of the magnetoresistance of the junction derived here allows some highly specific behaviour to be predicted.

We present an experimental study of the effect of an electric field upon the photoemission properties of Ag/InP metal-semiconductor structures. The electron affinity of the structure is lowered by cesium and oxygen coadsorption so that photoemission can be observed under near bandgap light excitation in a p-type sample. Two distinct effects occur upon reverse biasing. The raising of the energy of the conduction band in the bulk with respect to the position of the vacuum level produces a strong increase of the photoemission yield of the system. Furthermore, in the high electric field region near the surface, photoelectrons are transferred to indirect valleys by emission of optical phonons. This latter effect is demonstrated in a spectacular way by high resolution energy analysis of the emitted electrons: above a reverse bias of 0.8 V, one observes a strong modification of the shape of the energy distribution curve which is a direct proof for the relevance of intervalley transfers.

Spin injection in metallic normal/ferromagnetic junctions is investigated taking into account the anisotropic magnetoresistance occurring in the ferromagnetic layer. On the basis of a generalized two-channel model, it is shown that there is an interface resistance contribution due to anisotropic scattering, besides spin accumulation and giant magnetoresistance. The corresponding expression of the thermoelectric power is derived and compared with the expression accounting for the thermoelectric power produced by the giant magnetoresistance. The results of this study show that, while the giant magnetoresistance and the corresponding thermoelectric power indicate the role of spin-flip scattering, the observed anisotropic magneto-thermoelectric power might be the fingerprint of interband s-d relaxation mechanisms.

A spin‐dependent transport experiment in which hot electrons pass through a ferromagnetic metal / semiconductor Schottky diode has been performed. A spin‐polarized free‐electron beam, emitted in vacuum from a GaAs photocathode, is injected into the thin metal layer with an energy between 5 and 1000 eV above to the Fermi level. The transmitted current collected in the semiconductor substrate increases with injection energy because of secondary ‐ electron multiplication. The spin‐dependent part of the transmitted current is first constant up to about 100 eV and then increases by 4 orders of magnitude. As an immediate application, the solid‐state hybrid structure studied here leads to a very efficient and compact device for spin polarization detection.

The effects of the thermoelastic and piezoelectric strain exerted by an active polymer matrix on a Ni nanowire (NW) are studied at the nanoscale by measuring the inverse magnetostriction of single-contacted Ni NWs. The reorientation of the magnetization is measured by anisotropic magnetoresitance. In the absence of strain, the Ni NW exhibits a typical uniform rotation of the magnetization as a function of the external field. When piezoelectric or ther-moelelastic strain is present in the polymer matrix, the hysteresis loop becomes strongly modified by the inverse magnetostriction of Ni. It is shown that the ferromagnetic NW plays then the role of a mechanical probe that allows the effects of the mechanical strain to be characterized and described qualitatively and quantitatively. Moreover the stress exerted by the polycarbonate matrix on the NW is found to be isotropic while the one produced by the PVDF matrix is anisotropic.

We observe spin-dependent transmission of hot electrons through the ferromagnetic metallic layer of a Schottky diode. A 25% spin-polarized electron beam is produced in vacuum by a GaAs photocathode under illumination with circularly polarized light. The photoemitted electrons are then injected from the vacuum into the sample: a 3.5 nm-thick Fe layer deposited on a n-doped GaAs substrate. We measure a transmission coefficient of the order of a few 10-4. Using an incident energy above the Fermi level equal to 5 eV, a 5% asymmetry in the transmitted current is observed by changing the spin-polarization of the incident electrons (i.e. the light polarization) and/or reversing the Fe-layer magnetization. This corresponds to a transmission spin-dependence of 20% for a 100% spin-polarized incident beam.

We present an experiment in which the attenuation of a spin-polarized free electron beam is measured by direct transmission through an ultrathin ferromagnetic layer. The self-supported metal target consists of a 1-nm-thick cobalt film sandwiched between 21- and 2-nm-thick gold layers. Measurements are performed throughout a wide energy range (incident electron energies from 4 eV to 50 eV above the Fermi level). At low energy, close to the clean surface vacuum level, we find that the transmission coefficient of minority spin electrons is about 0.7 times that of majority spin electrons.

Analyse de l'energie en haute resolution de la photoemission pres de la bande interdite de la barriere de Schottky InP/Ag souspotentiel electrique. Cette technique permet d'etudier le transport d'electrons en champ intense. Estimation des temps d'emission de phonons dans un domaine d'energie recouvrant la premiere bande de conduction. Mise en evidence d'un transfert intervallee Γ 6 -X 6

Photoemission of spin-polarized conduction electrons allows to analyze the spin relaxation processes inside the semiconductor and to directly evidence the internal precession related to the lack of inversion symmetry in III-V compounds. It is the principle of GaAs polarized electron sources.

The use of photocathodes as intense monoenergetic electron sources seems very promising. Yet, many experimenters have observed large energy spreads, increasing with the emitted current, which remained unexplained. Here, we analyze a mechanism which accounts well for these effects.

We present a new technique for spectroscopic analysis of high-field electronic transport, as a function of both energy and distance. We measure the energy of electrons photoemitted into ultra-high vacuum from a biased InP/Ag metal-semiconductor structure after crossing the high electric field band bending region (BBR). The work function of this structure is lowered by cesium and oxygen coadsorption so that photoemission is obtained under light excitation at an energy equal to the bandgap. The study of the energy distribution curves of the emitted electrons versus reverse bias allows to obtain the energy distribution inside the solid as a function of distance travelled in the BBR. We evidence ballistic transport over a distance of 4500 A, over a kinetic energy range which approximately spans the entire first conduction band. A high-energy structure is resolved for a bias larger than 0.3 V. The study of the energy position of this structure as a function of bias allows direct visualization of intervalley transfer and to directly measure an estimate value of the mean free path associate with this phonon scattering process.

A comprehensive study of a compact electron spectrometer coupled with a Mott polarimeter is presented. This device is operated in ultrahigh vacuum (1×10−10 Torr) and optimized to investigate the polarized photoemission from negative electron affinity semiconductors. The electron energy dispersing device is made up of two 90° cylindrical electrostatic deflectors. It combines optimum geometry to allow spin‐polarization measurements with high resolution (∼20 meV) and high luminosity (peak output current up to 5 nA). Emphasis is placed on practical hints, limiting factors, and simple criteria to optimize the design of the electron selector, the electron optics, the power supplies, and the current detection. The proposed solutions lead to a very convenient, easy to operate, and low‐cost apparatus. Also the way of obtaining optimal performances is discussed. A new and simple method to measure the energy resolution of an electron spectrometer is described in detail. The problem of energy calibration is considere...

The work function of a semiconductor may be lowered using a surface treatment. Very efficient commercial photomultiplier tubes are built by this technique, starting from a semiconductor crystal. We first recall the principles of their operation and give a physical insight into the phenomena which determine their infrared energy cut-off. We show that this operation threshold can be extended towards smaller photon energies by application of an electric field. The use of the technique of high-resolution energy analysis of photoemitted electrons is discussed and appears to be a very powerful approach to the elucidation of the fundamental features of field-assisted photoemission. These well characterized systems, important for practical applications, are also very exciting from a fundamental point of view. The physical phenomena involves, which are directly connected with problems encountered in the design of ultra-fast electron devices, are discussed in the light of our experimental results obtained on Ag/InP metal-semiconductor structures.

We analyze the operation of a spin‐polarized electron source, consisting of a 100 A GaAs cap on top of Al0.3Ga0.7As, excited at 300 or 120 K by a He‐Ne laser. The cap allows easy activation to negative electron affinity while the alloy permits gap matching to the light source, and thus large electron spin polarization (30% at 300 K, 36% at 120 K). We compare yield curves, energy distribution curves, and polarized energy distribution curves obtained on samples with 100 and 1000 A caps and on bulk GaAs. The X conduction minimum position in the alloy is also determined.

The measurement of the spin vector S of electrons photoemitted from a III-V compound semiconductor activated to negative electron affinity is shown to be a powerful means to explore band hybridization properties, surface effects, and to estimate characteristic times of electron relaxation. Because of the lack of inversion symmetry (i.e., to the conduction-band spin splitting), an internal spin precession occurs in the bulk semiconductor and also in the band-bending region (BBR) near the surface. From the modulus of S we determine the hot-electron mean free path. From the S direction we deduce the angle of spin precession ${\ensuremath{\theta}}_{p}$ due to the internal precession vector \ensuremath{\omega}. The experimental originality of the present work stands in the combination of the S measurement with the high-resolution energy analysis of the photoemitted electrons. The dependence of ${\ensuremath{\theta}}_{p}$ on the electron kinetic energy reflects the memory of the initial anisotropic distribution of electron momenta, and its modifications during thermalization and transit through the BBR. Our analysis relates the D'ymnikov-D'yakonov-Perel' formalism for the calculation of spin-polarization properties under circularly polarized light excitation with the widely used Kane k\ensuremath{\cdot}p? band description.

The energy distribution curves (EDC's) of the photoelectrons emitted from the (100) face of a p-type doped (\ensuremath{\sim}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) GaAs crystal, activated to negative electron affinity in ultrahigh-vacuum conditions, is investigated. The study is performed at 300 and 120 K under well-focused ${\mathrm{Kr}}^{+}$-laser excitation and with a very-high-energy resolution (20 meV). The analysis of the EDC's as a function of the photon energy, mainly at low temperature, is shown to provide a very direct picture of the GaAs band structure away from the Brillouin-zone center. The experimental results are well fitted by a spherical, nonparabolic k\ensuremath{\rightarrow}\ensuremath{\cdot}p\ensuremath{\rightarrow} perturbation calculation of the coupled conduction and valence bands, for electron kinetic energies up to 1 eV in the central \ensuremath{\Gamma} valley. The essential role played by the subsidiary L and X minima in the energy relaxation and photoemission processes is evidenced. The main contribution to the total emitted current is due to electrons which were thermalized in the bulk \ensuremath{\Gamma} minimum and have lost an average energy \ensuremath{\simeq}130 meV in the band-bending region prior to emission into vacuum. The band-bending value is shown to be \ensuremath{\ge}0.5 eV. The yield and time evolution of GaAs photocathodes are discussed. This detailed study leads to a reexamination of the pioneer work of L. W. James and J. L. Moll [Phys. Rev. 183, 740 (1969)] and to a good understanding of the photoemission properties of activated GaAs.

The spin polarization of the electrons emitted by a GaAs photocathode under circularly polarized light excitation is investigated as a function of the electron kinetic energy. The photocathode is activated by cesium and oxygen coadsorption under ultrahigh-vacuum conditions to achieve a negative electron affinity. The spin polarization is measured by Mott scattering. The study is performed with a very-high-energy resolution (20 meV), at 300 and 120 K, under well-focused ${\mathrm{Kr}}^{+}$-laser light excitation (photon energy ranging from 1.55 to 2.60 eV). The polarization-versus-energy distribution curves show typical features related to those observed in the energy distribution curves, which are analyzed in detail in the preceding paper [H.-J. Drouhin, C. Hermann, and G. Lampel, Phys. Rev. B 31, 3859 (1985)]. A model is developed to account for the largest measured polarization, which arises from electrons excited from the heavy-hole band and emitted without suffering any collision: A (2/3) maximum value is expected, which is reduced by spin precession in the internal D'yakonov and Perel' (DP) field, due to the absence of space-inversion symmetry in GaAs. An estimation of the hot-electron mean free path (\ensuremath{\sim}0.1 \ensuremath{\mu}m for photon energy above 1.96 eV) is deduced. The photoemission polarizations of the electrons excited from each of the two other valence bands are also calculated using a nonparabolic Kane band model. The L and X subsidiary minima give rise to polarization plateaus originating from energy relaxation in the band-bending region. The main contribution to the photocurrent is due to electrons which were thermalized in the central minimum of the bulk crystal and have relaxed their energy in the band-bending region prior to emission into vacuum. Their polarization is studied in relation with the luminescence polarization, measured on the same samples, in the framework of a one-dimensional diffusion model. An additional depolarization, occurring during the escape process, is evidenced and attributed to the DP relaxation mechanism in the band-bending region. Finally, the performances of GaAs photocathodes as monochromatic and polarized electron sources are analyzed with use of the physical concepts developed in the present paper and in the preceding one.

We have performed high resolution energy selection of photoelectrons emitted from negative electron affinity (NEA) GaAs crystals [1]. Under circularly polarized light excitation the spin polarization of the energy selected electrons has also been analyzed [2,3]. GaAs crystals (p-doped,~10l9cm−3, oriented parallel to the ( ) face) are activated by Cs and O2coadsorption. The sample is illuminated by a well-focused Kr+-Iaser. The photoelectrons are analyzed through a cylindrical electrostatic electron selector operated in the constant energy mode with a 20 meV resolution. The spin polarization is measured using a Mott Polarimeter.

The development of efficient spin-polarized electron sources, the improvement of electron spin polarimeters, and the progress in the fabrication of magnetic nanostructures and hybrid ferromagnet/semiconductor structures open up a wide field of investigation involving spin-dependent electron interactions in magnetic systems. This paper reviews the state of the art concerning spin-polarized electron sources based on semiconductor photocathodes. Their use is illustrated by the example of a new spin-dependent transmission experiment where a low-energy free-electron beam crosses a free-standing ferromagnetic thin film. It is shown that the transmission spin asymmetry reaches high values at very low primary electron energy. Possible applications of these spin-filter structures as compact and performant spin detectors are discussed, as well as perspectives for spin sensitive probes.

We present a new set of experiments in which the attenuation of a `monoenergetic', possibly spin-polarized, free-electron beam is measured by direct transmission through an ultra-thin metal layer. The self-supported metal target is either a reference gold sample or a ferromagnetic structure. The overall thickness is of the order of 25 nm. The magnetic structure consists of a 1 nm-thick cobalt film sandwiched between 21 and 2 nm-thick gold layers, with perpendicular magnetization. Measurements are performed throughout a wide energy range, with incident electron energies between 2 and 1000 eV above the Fermi level. The transmission of the gold layer is found to be substantially higher than that of the magnetic structure. In the latter case, at low energy, close to the clean surface vacuum level, we find that the majority spin electrons are more easily transmitted than the minority spin electrons. Cesium deposition on the exit side or on both sides of the target increases the overall transmitted current by almost an order of magnitude. In the case of the magnetic structure, this also increases the transmission spin-asymmetry from 16 to about 40%. Such structures appear to be well-suited to the construction of convenient and compact spin-detectors.

ZnO is a remarkable, multifunctional, biocompatible material with a distinctive set of properties including a direct bandgap of ~3.4eV, high transparency over the visible spectrum, a very wide range of obtainable conductivities & a strong piezoelectric response. Thus ZnO has many established & emerging applications including varistors, transparent contacts, surface acoustic wave devices and transparent thin film transistors [1]. ZnO also exhibits one of the largest families of nanostructures of all materials systems and a capacity to crystallise readily on amorphous and/or mismatched substrates [2].

The design of a sample transfer device for ultrahigh vacuum experiments is presented. In this device, the mobile sample holder has a wedge shape and can be attached to its place using a magnetic rod without delicate alignment. This device operates from close to nitrogen temperature to a temperature on the order of 700–800 °C.