[en] This paper is an account of some early experiments on optical pumping in semiconductors realized in the late sixties and early seventies. It describes the optical pumping of free carriers as it emerged when we benefited from the hospitality and interest of Claude Benoit a la Guillaume. It briefly presents the orientation of excitons by circularly and linearly polarized light, a field in which he played a decisive role. (orig.)

[en] At low energy, a longitudinally spin-polarized electron beam impinges on an ultrathin, self-supported ferromagnetic target, consisting of a 1 nm-thick cobalt film sandwiched between 21 and 2 nm-thick gold layers, and which is magnetized perpendicularly to the surface. The current transmitted by the target depends on the spin of the electrons. Cesium deposition on both sides of the target increases the transmission ratio from about 1 x 10^-5 up to 3 x 10^-4 and also increases the transmission spin-asymmetry from 15 to about 40%. Such a structure is well suited to the construction of convenient and compact spin-detectors. (authors). 4 figs., 9 refs

No abstract available

[en] There has been a large amount of experimental and theoretical work on optical excitation between quantized levels of superlattices or quantum wells. Most results deal with level spectroscopy, but fewer are concerned with the transport of electrons perpendicular to the layers in the absence of an electric field. The authors report photoemission measurements in a GaAs/GaAlAs superlattice covered by a GaAs surface activated to negative electron affinity by cesium and oxygen coadsorption. The original experimental apparatus allows heating up to 900 K during sample activation and cooling down to 30 K by helium circulation. The photocurrent vs light excitation energy, i.e. the yield curve, shows definite steplike structures, which appear at the same energy positions as those present in the luminescence excitation spectrum. They are, therefore, attributed to electrons photoexcited from the valence quantized states into the conduction quantized states of the superlattice, which are transported toward the surface and then into vacuum. The amplitude of the contribution to the photocurrent of the electrons from the wells and its temperature variation are interpreted in terms of their tunneling through the superlattice barriers. This photoemission study opens a new field in the direct investigation of the dynamics of electrons in layered structures

[en] We analyze the operation of a spin-polarized electron source, consisting of a 100 A GaAs cap on top of Al/sub 0.3/Ga/sub 0.7/As, 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

No abstract available

[en] We present an original technique to investigate spin-dependent electron interactions in ferromagnetic metals, and place it in the context of previous studies. Our technique is based on spin-polarized electron transmission through ultrathin, free-standing, metal foils. A longitudinally spin-polarized, quasimonoenergetic, free-electron beam impinges onto a ferromagnetic target consisting of a few atomic layers of cobalt sandwiched between gold layers, for an overall thickness of the order of 25 nm. It is remanently magnetized perpendicular to the film plane. The current transmitted through the foil is energy analyzed and its dependence on the relative orientation between the spin polarization of the primary beam and the magnetization direction of the cobalt layer is measured. The experiments are performed over a wide primary energy range, starting from the vacuum level of the target; the work function of the target can be lowered down to 2 eV by cesium deposition. We demonstrate a spin-filter effect, favoring the transmission of majority electrons. It is very large at low primary energy, when the electrons travel close to the 3d bands. Perspectives for compact and highly discriminative spin detectors are discussed. copyright 1996 American Institute of Physics

[en] 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

[en] We have monitored the cathodoluminescence (CL) emitted upon injection of free electrons into a hybrid structure consisting of a thin magnetic Fe layer deposited on a p-GaAs substrate, in which InGaAs quantum wells are embedded. Electrons transmitted through the unbiased metal/semiconductor junction recombine radiatively in the quantum wells. Because of the electron spin-filtering across the Fe/GaAs structure, the CL intensity, collected from the backside, is found to depend on the relative orientation between the injected electronic spin polarization and the Fe layer magnetization. The spin asymmetry of the CL intensity in such junction provides a compact optical method for measuring spin polarization of free electrons beams or of hot electrons in solid-state devices

[en] Why a chapter on photoelectronic processes in semiconductors studied by negative electron affinity photoemission, in a volume on hot electrons in semiconductors? It is well-known that standard photoemission uses excitations with energies larger than the work function (≥5 eV) for a clean semi-conductor surface to overcome the surface energy barrier. This corresponds to UV light, so that such an excitation is absorbed in the first few angstroems next to the semiconductor-vacuum interface. Standard photoemission then essentially probes surface properties and is not suited for the analysis of energy relaxation. (author). 115 refs., 38 figs