C.A. Sébenne

Effect of Cu deposition on structural and electronic properties of cleaved Si(111) surfaces

Photoemission yield spectroscopy (PYS), LEED and AES measurements are performed on a set of UHV cleaved Si(111)−2 × 1 reconstructed surfaces from n- and p-type crystals as a function of Cu coverage θ. The Cu layers are obtained by evaporation at calibrated rates on the substrate kept at room temperature in UHV. θ varies from about 10−2 monolayer (ML) to several tens of ML. In LEED, one observes successively the Si(111)−2 × l at θ = 0, a Si(111)−4 × l from θ = 0.5 to 5 ML, a Cu(111)−√3 × √3 from 5 to 10 ML and a Cu(111)−1 × 1 beyond 10 ML. In AES, the Si peak at 91 eV is split at low Cu coverages and remains observable up to 50 ML. Both the work function and the ionization energy decrease by about 0.1 eV in the 1 ML coverage range, while the effective density of surface states is markedly modified by Cu adsorption. These results are understood within a two-step Cu deposition process: a two-dimensional alloy between Cu and Si forms first; then an epitaxial growth of Cu is observed.

Influence of defects on surface scattering of UV photoemitted electrons in near perfect Si(111)1×1-H

Chemically hydrogenated Si(111) surfaces have been investigated in ultrahigh vacuum by photoemission yield spectroscopy ( PYS ) and characterized by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). From Kanes theory [1] the valence state contribution to photoemission yield at threshold in Si follows a power law Y(hv) (hv-E T ) in which m greater than I indicates electron scattering: E T being the threshold energy. By combining AES and LEED indications with the values of Er and m as determined by a fit with measured yield spectra, it is shown that traces of oxygen chemisorbed in the Si top layer can be distinguished for similar amounts adsorbed along the Si surface as OH radicals. Other effects of trace defects are also shown.

Study of the heterointerfaces InSe on GaSe and GaSe on InSe

Abstract InSe and GaSe thin films are grown on freshly cleaved (00.1) substrates of GaSe and InSe, respectively, by molecular beam epitaxy. They are studied in situ by X-ray photoelectron spectroscopy (XPS) and reflection high energy electron diffraction (RHEED). From the attenuation curves of the XPS substrate core level peaks, the quasi layer-by-layer growth is shown during the first stages of deposition in agreement with RHEED results. But both interfaces are not totally symmetrical. For InSe on GaSe(00.1), the sharpness of the interface is shown and the conditions of growth are well established. For GaSe on InSe(00.1), the sharpness of the interface can also be suggested although it is less clear; this is related to the growth conditions.

Effect of room-temperature adsorption of Sn on Si(100) surface properties

The initial steps of ultrahigh-vacuum deposition of Sn on clean Si(100) surfaces have been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS). While the initial 2×1 reconstruction reduces to 1×1 between 0.5 and 1 monolayer (1 ML=6.8×10 14 atoms per cm 2 along the (100) face of Si), Sn forms a uniform overlayer up to 2 ML and then grows as metallic islands. The ionization energy and work function decrease in parallel, and the largest part of the decrease occurs upon 0.25 ML of Sn. New features start to grow in the band of filled surface states beyond 0.25 ML of Sn. The results are compared to Sn adsorption on cleaved 2×1 reconstructed Si(111).

Water vapour adsorption on the Si(111)-(7 × 7) surface

Abstract Clean Si(111)-(7 × 7) reconstructed surfaces have been exposed to increasing doses of H 2 O up to 15 000 L at room temperature. At each stage the surface was studied by electron energy-loss spectroscopy (EELS), Auger electron spectrometry (AES), photoemission yield spectroscopy (PYS) and low-energy electron diffraction (LEED). The dissociative adsorption of H and OH onto the Si(111)-(7 × 7) surface dangling bonds, and the formation of SiOSi bonds are revealed through the changes in structural compositional and electrical properties. At water doses lower than 150 L, the (7 × 7) pattern is not affected by the reaction. The dangling-bond saturation by H and OH is dominant, but the formation of SiOSi bonding is significant. At higher doses, the former dangling bonds become mostly H-saturated, and the oxygen uptake consists mainly of the formation of SiOSi bonds through a process in which the surface becomes fully disordered.

Early stages of H2O adsorption on clean Si(100)

Abstract Clean and smooth Si(100) surfaces have been exposed to low doses of water vapor, up to 100 L at most, under ultra-high vacuum. Structural and electronic property changes of the surface have been studied by LEED, AES and photoemission yield spectroscopy. Special attention has been payed in order to get a quantitative indication of the oxygen coverage, by a careful calibration of the Auger lines. It is found that the first stage of adsorption saturates when all the dangling bonds are removed, at an oxygen coverage of 0.25 monolayer. This result is in contradiction with the model where H 2 O dissociates into H and OH, each one removing one dangling bond, which would give 0.5 monolayer at saturation. In agreement with similar observations made in the dissociative interaction of NH 3 with Si(100), we propose that either H or OH is able to remove the two dangling bonds of a surface Si dimer.

Electronic properties of cleaved CdTe(110) surfaces

Abstract Photoemission yield spectroscopy measurements were performed on a set of n- and p-doped CdTe single crystals. The surfaces were obtained by cleavage in ultrahigh vacuum and characterized by low energy electron diffraction and Auger electron spectroscopy. On clean and properly cleaved surfaces, no band bending was found, neither on n- nor on p-type samples, showing the absence of intrinsic surface states in the gap. The ionization energy is found at 5.80±0.05 eV. Oxygen adsorption removes defect-induced surface states on the valence band side of the gap and develops a band bending on n-type samples which indicates the presence of acceptor surface states in the gap down to 0.70 eV below the conduction band edge. The ionization energy remains constant.

Investigation of the Ag/InP(110) interface formation

Photoemission yield spectroscopy (PYS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) measurements are performed on a set of UHV cleaved InP(110) surfaces from n- and p-type crystals as a function of Ag coverage θ which is varied from about 10−3 monolayer (ML) to 100 ML. Ag is shown to form an abrupt interface and no pseudo epitaxy of Ag is observed below 100 ML. The variations upon interface formation of the atomic structure, the work function φ, the ionization energy Φ and the semiconductor band bending are analysed together with the changes of the density of filled surface states in the gap and in the valence band within about 0.6 eV from the top. The pinning of the Fermi level observed on n-type samples at ≅4.8 eV, giving then a band bending of 0.35 eV, and the decrease of the work function on p-type samples with θ are related to the observed interface states the possible origin of which is discussed and compared to the AgGaAs(110) system.

Effect of NH3 on Si(100) vicinal surfaces

Abstract The structural and electronic properties of clean vicinal surfaces of Si(100) upon interaction with NH3 have been studied using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and photoemission yield spectroscopy (PYS) in situ, under ultrahigh vacuum (UHV). Two eases of vicinal surfaces were compared to the nominal (100) face, namely one with a 6° angle towards the [011] direction, the other with a 5° angle towards the [010] direction. It is shown that the rougher the surface, the faster the common saturation coverage θs is reached, where θs corresponds to one NH3 molecule per two surface Si dimers. The dipole formed upon adsorption leads to a similar decrease of the work function, which can reach 0.9 eV at saturation. The best ordered surfaces (here 6 ° vicinal) show upon NH3 saturation a well pronounced electronic structure near the valence band edge, the origin of which is discussed.

Valence band of Cd1−xFexSeFe in resonant photoemission spectra

Valence band of Cd 1-x Fe x Se/Fe in resonant photoemission spectra Resonant photoemission spectra of Fe atoms (3p-3d) were measured for freshly cleaved Cd 0-86 Fe 0.14 Se crystal (110) surface and for the same surface with sequentially deposited Fe atoms in the range 0.3-80 ML. The synchrotron radiation (LURE, Orsay, France) was used to measure valence band density of state distributions and Cd 4d core level for the radiation energy range 50-60 eV, which includes the Fe 3p-3d resonant energy equal to 56 eV. At the lowest stages of Fe atom deposition (up to about 3 ML), Fe atoms replace Cd atoms in the crystal, making a Cd 1-x Fe x Se alloy with a higher x and leaving extra Cd as small islands or clusters. For the next stages of Fe deposition (about 3-10 ML) the dissociated Cd atoms create a Cd-Fe alloy. At higher stages of Fe deposition (above about 10 ML) a Fe thin film is formed.