S. Nishigaki

Magic Numbers in a Mass Spectrum of Lithium Clusters Emitted from a Liquid Metal Ion Source

Lithium ion clusters Lin+ with n ranging up to 19 have been observed from liquid lithium ion sources by using a magnetic mass analyzer. The ion intensity as a function of cluster size showed local maxima at n=7, 9, 13, and 19. Only n=9 and 19 can be explained by the electronic shell model, which successfully predicts magic numbers of other alkali metal (Na, K) clusters. On the other hand, the geometrical atom packing model explains the observed magic numbers except for n=9.

Adlayer formation and its local charge states of Li on Si(111)7 × 7 surface studied by Δφ, MDS and AES

Abstract Initial states of Li adlayer formation on a Si(111)7×7 surface have been investigated by work function change (Δo), metastable-atom de-excitation spectroscopy (MDS) and Auger electron spectroscopy (AES), for the first time. MDS spectra detected remaining valence electrons on the adsorbed Li atom even in the earliest stage of adsorption, which provides clear evidence of the partial occupancy of a Li resonance. The charge state of adsorbates did not change in a low coverage region. The electronic structure of the substrate surface is little disturbed by the Li adsorption according to AES spectra. We observed two types of Auger transition occurring between deposited Li and substrate Si atoms, but no pure Li Auger peaks. It is suggested that the Li-Si interface grows by the process of initial adsorption followed by diffusion of Li beneath the Si surface.

Oxygen effects on ion-induced secondary ion and photon emissions from GaAs and Ga surfaces

Abstract Secondary ion and photon emissions from GaAs or Ga metal surfaces during oxygen flooding were simultaneously measured by the SIMS-SCANIIR technique. The oxygen coverage was sequentially monitored by ISS, utilizing a double focusing mass spectrometer of a SIMS instrument. With increasing oxygen coverage, the Ga+ intensity from the GaAs surface decreased slightly, whereas that from the Ga metal surface increased. The photon intensity due to excited Ga atoms sputtered from the surface, whether GaAs or Ga metal, was in proportion to the oxygen coverage. This indicates that Ga+ intensity depends on the Ga-O bonding configuration, as well as the oxygen coverage, whereas the photon intensity depends only on the oxygen coverage, implying that the mechanism of electronic excitation of Ga atoms is different from its ionization.

O2/K/Ge(100) 2 × 1 and O2/Cs/Ge(100) 2 × 1: puzzling behavior of K and Cs in the oxidation of germanium

Abstract The effect of alkali-metal overlayers (K, Cs) on the room-temperature oxidation of the Ge (100)2 × 1 surface in the presence of molecular oxygen was investigated by core-level and valence-band photoemission spectroscopy using synchrotron radiation. In strong contrast to the behavior observed for many other elemental or compound semiconductor surfaces, the presence of a K or Cs layer does not enhance the oxidation rate as expected but rather poisoned it since the K (Cs) covered Ge (100)2 × 1 surface exhibits significantly smaller amount of oxides for the same oxygen exposures. This behavior indicates that K does not act as a catalyst as observed in the cases of many other semiconductor surfaces, but as an oxygen acceptor, in agreement with a classical model of electron affinity.

Sequential SIMS-ISS measurements using a double focusing mass spectrometer

Abstract A new technique for sequential SIMS-ISS measurements was developed using the double focusing mass spectrometer of a SIMS instrument operated with a B2 / E linked scanning method and a modulation technique. Formulations that give the scattered ion energy and the mass resolution were also derived in the biased sample condition. The energy resolution of this system was assessed at nearly 40 with an ISS spectrum obtained from a GaAs monocrystal surface.

Accommodation of argon and oxygen gases on a GaAs(110) surface under Ar+ ion bombardment

Abstract Intensities of Ar+ ions scattered by Ga and As atoms on the GaAs(110) surface under a sputtering equilibrium condition attained by 7 keV Ar + ion beam irradiation were measured as a function of Ar and O2 pressures at room temperature. All scattered ion signals decayed linearly as the gas pressure increased, indicating that the surface concentrations of these gases are proportional to the gas pressure. This means that the ratios of the surface accommodation probability to the ion-induced removal cross section of these gases are constant. Results also showed that the As signal decayed faster than the Ga signal as the O2 pressure increased, while these signals decayed with the same slope as the Ar pressure increased. This suggests that an oxygen atom is preferentially trapped at an As vacancy site rather than at a Ga vacancy site on the sputtering front.

Cesium effect on ion-induced secondary photon emission from a GaAs surface

Abstract The ion-induced photon emission from excited Ga atoms sputtered from a GaAs single crystal was measured as a function of cesium coverage on the specimen surface. The results showed a remarkable non-linearity in photon yield against cesium coverage. This implies that the photon enhancement effect due to cesium adsorption is nonlocal and relates to the nonradiative deexcitation process of sputtered atoms.

Auger electron emission upon impact of excited helium atoms: Alkali-metal adsorbed Si(111) surfaces

Abstract Nonradiative deexcitation processes in excited He atom-surface collisions have been investigated by monitoring ejected Auger electrons. At a clean Si(111) surface, He ∗ atoms (mainly in the 2 3 S state) were deexcited by the resonance ionization followed by Auger neutralization (RIAN) path. Upon adsorption of lithium, they were forced to be directly Auger-deexcited (AD) at adsorbed-Li sites. The energy spectrum of electrons, thus ejected, reveals that the 2s resonance of adsorbed Li is partially occupied even in the limit of zero coverage. The conversion from RIAN to AD in the deexcitation of He ∗ was observed at exposed Si sites when the induced average work function change reached about −1.3 eV. The local work function at sites where He ∗ atoms approach and the local charge state of adsorbates are discussed.