Surface Reconstruction on Al(100) and Al(110) Surfaces

Abstract

Al surfaces grown epitaxially on GaAs(lOO) substrates exhibit a nearly atomically smooth surface morphology which permits the formation of various clean and impurityinduced surface reconstructions. The Al(100)-c(2x 2) reconstruction can be ascribed to a two-dimensional Fermi-surface instability. Arguments are given that suggest a charge-density-wave mechanism.PACS numbers: 73.20.Cw, 68.20.+t We report here several new results on singlecrystal Al surfaces, made possible by a novel method of their preparation. Among these results are the observations of various surface reconstructions on both the (100) and (110) surfaces, the first reconstructions found for a simple metal. In addition, filled and empty surface resonances have been studied by Auger-electron (AES) and electron-energy-loss (ELS) spectroscopies, respectively.Considerable activity has been devoted recently to the observation of surface phase transitions on clean noble 1 " 6 and transition metals 7 " 9 and to attempts at their theoretical characterization. 10 " 15 The present observations on a simple metal should provide a simpler case for theoretical analysis.The observed surface reconstructions were a c(2x2) and an impurity-stabilized hexagonal structure on the Al(100) surface, and a (5x1) reconstruction on the clean Al(110) surface. In addition, we observed an oxygen-induced c(2x2) structure on the (100) surface for coverages approaching a monolayer. The reconstructions were observed with reflection high-energy electron diffraction (RHEED) and appeared generally below ~ 300 K, with the fractional diffraction streaks increasing in intensity upon further cooling. The RHEED patterns of the c(2x2) are shown in Fig. 1(a). Although Al surfaces have been investigated over the years at these and lower temperatures, 16 the failure to observe these reconstructions may be attributed to the rough surface produced by conventional preparation methods. This difficulty is avoided by the method used here, which prepares the surfaces by heteroepitaxial overgrowth of Al on GaAs(100) surfaces in a molecular beam epitaxy (MBE) system equipped with in situ analytical measuring equipment. The observations were independent of the Al film thicknesses which ranged from ~0.1 to 1 /im.Detailed growth procedures will be discussed elsewhere 17 ; however, at room temperature the orientation of the Al overgrowth is determined by the nature of the GaAs surface; either Al(110) or Al(100) is produced depending on whether the starting surface exhibits the As-stabilized c(2 x8) or Ga-rich (4x6) structure. Extensive precautions for maintaining the vacuum during the Al deposition in the range (1-2) xlO" 10 Torr and the substrate temperature at or below room temperature resulted in a contamination-free, 18 single-crystalline overgrowth with a nearly atomically smooth surface morphology. The latter prop-FIG. 1. RHEED pattern for Al(100) along [100] and [110] azimuths for (a) c(2x2); (b) hexagonal; and (c) c(2x 2)-0 structures. Patterns for the clean unreconstructed surface are similar to ...

Relativistic Continuum Random Phase Approximation and Applications II. Applications

“…The distribution of IVGQR has a peak at 22.05 MeV, very close to the position of the experimental centroid energy at 22.0 MeV. [35] As in the dipole case, the isovector nature of this mode is clearly observed in the surface region.…”