Interaction of ionized hydrogen with cleaved GaAs. Comparison with atomic hydrogen interaction

The interaction of hydrogen ions H+2 with the (110) surface of GaAs prepared by cleavage under ultrahigh vacuum has been studied by photoemission yield spectroscopy and controlled by Auger electron spectroscopy and low energy electron diffraction. The ions, having a kinetic energy lower than 100 eV, were produced by an excitation cell connected to the working chamber by a hole 0.5 mm in diameter and could be deflected onto the sample surface. The interaction of H+2 with GaAs(110) at progressively increasing doses begins with the two successive stages which have been characterized from the interaction of atomic hydrogen with the same substrate : first an adsorption stage during which hydrogen binds covalently to both Ga and As, then a decomposition stage with formation of Ga metal and, most likely, arsine AsH3 which remains adsorbed on the surface. As the dose of hydrogen ions keeps on increasing, a layer by layer etching of the surface is observed with removal of the hydrogenated compound.L'interaction des ions hydrogène H+2 avec la face (110) de GaAs clivé sous ultravide a été étudiée par spectroscopie de rendement de photoémission et contrôlée par spectroscopie des électrons Auger et diffraction des électrons lents. Les ions, dont l'énergie cinétique est inférieure à 100 eV, sont produits dans une cellule d'excitation reliée à la chambre de travail par un trou de 0,5 mm de diamètre et peuvent être défléchis sur la surface de 1'echantillon. L'interaction de H2+ avec GaAs(110) à doses progressivement croissantes commence par les deux étapes successives caractérisées lors de l'interaction de l'hydrogène atomique avec le même substrat: tout d'abord une étape d'adsorption au cours de laquelle l'hydrogène forme des liaisons covalentes avec Ga et As, ensuite une étape de décomposition avec formation de métal Ga et, très probablement, d'arsine AsH3 qui reste adsorbée sur la surface. Lorsque la dose d'ions hydrogène continue à augmenter, on observe une érosion couche par couche avec départ du composé hydrogéné

Room temperature adsorption of au on cleaved GaAs (110)

The early stages of ultrahigh vacuum deposition of Au, from 10-3 to about 102 monolayers (ML) on an initially clean cleaved (110) surface of GaAs kept at room temperature are studied. Crystallographic, compositional and electronic surface data are obtained using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron loss spectroscopy (ELS) and photoemission yield spectroscopy. After surface reconstruction effects at coverages below 0.1 ML, the formation of a two-dimensional compound is observed which brings a specific density of states determining the Fermi level position; a model is developed leading to a compound about 8 A thick with four Au atoms per GaAs surface unit cell. The formation of more and more spread islands of Au follows.On étudie les premières étapes du dépôt sous ultravide de l'or, de 10 -3 à environ 102 monocouches (mc), sur une surface (110) initialement propre de GaAs clivé maintenu à température ambiante. Des renseignements d'ordre cristallographique, compositionnel et électronique sur la surface sont obtenus à partir de la diffraction des électrons lents, de la spectroscopie des électrons Auger, de la spectroscopie de perte d'énergie des électrons, et de la spectroscopie de rendement de photoémission. Après des effets dus à la reconstruction de surface à des recouvrements inférieurs à 0,1 mc, on observe la formation d'un composé bidimensionnel qui introduit une densité spécifique d'états déterminant la position du niveau de Fermi; un modèle est proposé conduisant à une épaisseur de 8 A environ et une composition de quatre atomes d'or par maille superficielle de GaAs. Ensuite, des ilots d'or de plus en plus larges se forment sur la surface

HREELS investigation of hydrogenated GaAs(110) surface

The interaction of atomic hydrogen with the cleaved GaAs(110) surface has been investigated by high-resolution electron energy loss spectroscopy (HREELS), at primary energies of 5 and 15 eV, analysing the losses associated with three different mechanisms: (i) low-energy surface collective excitations (surface TO phonon and dopant-derived free-carrier plasmon); (ii) the stretching of the Ga-H and As-H bonds: and (iii) the electronic losses above the fundamental gap, involving both surface and bulk electronic states. The first kind of loss is very sensitive to H exposure and shows that hydrogen induces a band bending at the lowest exposures. The vibrational part of the spectrum indicates that the exposure of 104 L corresponds to a coverage of one monolayer and that H bonds to both Ga and As over the whole coverage range. The region of the electronic transitions indicates the disappearance of transitions of the clean surface and the appearance of new transitions characteristic of the H covered surface. At high exposures the growth of a very strong background is consistent with the presence of small metallic Ga clusters. This result is also consistent with the modifications, at the same exposures, of the low-energy region of the spectrum

Trace element distributions in soils developed in loess deposits from northern France

A pedo-geochemical survey was carried out in the Nord-Pas de Calais region (France) on soils developed in loess deposits. Total concentrations of Al, Fe and 18 trace elements, as well as common soil characteristics, were determined in samples from 52 surface and 97 deep horizons developed in these loess deposits. The Pb isotopic composition was determined in two sola. The composition of deep horizons, compared with that of the upper continental crust, with that of horizons developed from 21 other sedimentary rocks from the region and with that of loess from various parts of the world, confirms that loess from the Nord-Pas de Calais region derives from multi-recycled and well-mixed ancient sedimentary rocks. Correlation analysis shows that least mobile (i.e. ionic potential (Z/r) is between 3 and 7) geogenic elements (Bi, Co, Cr, Cu, In, Ni, Pb, Sn, Tl, V, Zn) are associated with the fraction <2 µm (which we define as 'lutum'). More mobile elements (As, Cd, Hg, Mn, Mo, Sb, Se) are less associated with this fraction. Cadmium is particularly linked to Mn. The distribution of [trace element]/([Al] or [Fe]) in the French loess gives the background content for soils developed from most sedimentary materials in northwestern Europe. Topsoils are enriched with all the trace elements examined, except Co, Cr and Ni. Enrichments with Cd, Cu, Mn and Zn are greater in cultivated soils than in forest soils. Enrichments with Pb and with Cu, Hg, Mo, Sb, Se and Sn are mainly due to human contamination through atmospheric fallout. Organic matter seems to act as a sink for all the exogenous trace elements

Modifications of space charge layer on InP(110) surface induced by atomic hydrogen at very low exposures

The HREEL spectrum of InP(110) in the quasi-elastic peak region is strongly modified by the exposure to atomic hydrogen. From a detailed analysis of these modifications we have shown that (i) cleavage induced states in InP can have acceptor-like nature, as an “unavoidable” small upwards band bending was always present even on mirror-like cleaved surfaces; (ii) atomic hydrogen at very low exposures can neutralize these acceptor states, reducing to almost zero the pre-existing band bending; (iii) at high exposures, H induces new acceptor states inside the gap and, consequently, a new and larger band bending occurs at the surface, which reaches about 0.6 eV above 1000 L