R. Plass

Morphology of MgO(111) surfaces: artifacts associated with the faceting of polar oxide surfaces into neutral surfaces

Abstract It is shown by optical, atomic force, scanning and transmission electron microscopies that the MgO(111) surface does not facet into neutral (100) type planes upon annealing. The triangular pyramidal pits that Henrich [V.E. Henrich Surf. Sci., 57 (1976) 385] associates with the polar (111) surface faceting into (100) planes turn out to be artifacts of the acid etch used in the sample preparation process. It is shown that the pyramidal pits have facets sloped at 10.8±2.8° with respect to the (111) surface, corresponding to sets of vicinal surfaces. The pit edges are confirmed by transmission electron microscopy to be along the three equivalent 〈110〉-type directions.

Submonolayer Au on Si(111) phase diagram

Based on a review of the current literature, a surface phase diagram is proposed for the submonolayer Au on Si(111) system. Kinetic considerations are reviewed and key surface phase diagram features such as the Θ < 0.4 ML metastable Si(111)-(3 × 3)R30°-Au structure and the high temperature Si(111)-(3 × 3)R30°-Au to Si(111)-(1 × 1)Au second order phase transition are discussed. Experiments to verify certain portions of the phase diagram are proposed.

Atomic force microscopy and scanning electron microscopy study of MgO(110) surface faceting

Abstract Phosphoric- and nitric-acid etching of the MgO(110) surface generates vicinal faceting in both the 〈001〉 and 〈110〉 directions. Vacuum annealing (to 1000°C) does not introduce thermal faceting, and does not alter the chemical-etch morphology. Three types of acid-induced faceting (early-stage pits, later-stage grooves, and inverted trapezoidal pyramids) are seen as a function of etching time. Facet-angle analysis by atomic force microscopy (AFM) and scanning electron microscopy (SEM) shows the etch morphology to be vicinal, with angles in the range of 9° to 23°, not the low-energy {100} planes expected from minimization of surface energy.

Imaging surface structures by direct phasing

We report success in applying direct phasing methods to produce images of surface structures at the atomic scale from intensity data collected using transmission electron diffraction.

UHV transmission electron microscopy structure determination of the Si(111)-(√3 × √3)R30°Au surface

Abstract Details of the average atomic structure and the possible nature of the surface domain walls of the Si(111)-√3 × √3)R30°Au surfaces are presented. Transmission electron diffraction data supports the missing top layer twisted trimer [Surf. Sci. 256 (1991) 135] model in which gold and silicon atoms in the top two layers form sets of like-atom trimers rotated in the same direction about p3 symmetry centers. However, the average degree of trimer rotation and the interatomic spacings within the structure vary significantly with the sharpness of the structures diffraction spots. These variations in structure parameters among data sets as well as large values of the fitted Debye-Waller terms indicate substantial static disorder in the structure and hence any parameters determined can only be averages of locally varying parameters. Silicon displacements from the bulk positions continue through the second silicon double layer. Charge transfer in the structure is evident from a better fit obtained using Au + scattering factors than with those for neutral gold, confirming a total energy cluster calculation [Surf. Rev. Lett. 1 (1994) 273]. The presence of gold trimers is confirmed by the local symmetry seen in high resolution micrographs which also show surface domain morphology differences between diffuse and sharp diffraction spot regions. These images additionally indicate that the surface domain walls must be either vacancy type, or the gold-to-gold spacing in and near the walls must be close to 3.84 A. From these observations, models for the Si(111)-(√3 × √3)R30°Au domain walls and the Si(111)-(6 × 6) Au structure are proposed.

UHV-HREM and diffraction of surfaces

Characterization of the structure of surfaces is very important in order to develop a fundamental understanding of the electronic, mechanical and chemical properties of a material. While transmission electron microscopy imaging (TEM) and diffraction (TED) techniques are capable of providing surface structural information at the atomic level, such data would be suspect if obtained under conventional vacuum conditions (10-6–10-8 Torr). Ultrahigh vacuum (UHV) conditions are imperative during both preparation and observation of clean surfaces/interfaces. Conventional TEM techniques are very powerful for UHV-TEM investigations; however, the marriage of surface science and conventional TEM to yield an UHV-TEM is a complex task. These complexities and some of the results obtained using UHV-TEM and UHV-TED techniques for surfaces i.e. solid-vacuum interfaces will be illustrated.

Partially coherent and holographic contrast transfer theory

Abstract A more complete form of contrast transfer theory is derived using a partial coherency approach for both electron holography and conventional high resolution electron microscopy.

Room temperature deposition of gold onto the diffuse and sharp diffraction spot Si(111)-(3 × 3)R30° Au surfaces

Abstract Room temperature gold depositions onto Si (111)-( 3 × 3 ) R 30° Au surfaces with diffuse and sharp diffraction spots [Surf. Sci. 242 (1991) 73] (diffuse and sharp 3 × 3 Au hereafter) under UHV conditions has been monitored using transmission electron diffraction (TED). Both systems display an increase in surface structure diffraction spot intensities up to the completion of 1.0 monolayer (ML) after which the surface beams display an exponential decrease in intensity with coverage. The exponential decay rate decreases after roughly 1.33 ML. These results can be attributed to gold initially diffusing to and filling 3 × 3 Au gold trimer sites in vacancy type surface domain walls [Surf. Sci. 342 (1995) 233], then filling one of three possible sites on the 3 × 3 Au structure with essentially no surface diffusion, disrupting nearby gold trimers. Gold deposition onto the diffuse type structure caused the formation and expansion of satellite arcs around the strongest 3 × 3 beams similar to those seen by others [Surf. Sci. 242 (1991) 73; Jpn. J. Appl. Phys. 16 (1977) 891; J. Vac. Sci. Technol. A 10 (1992) 3486] at elevated temperatures while the sharp structure displayed only a modest shoulder formation near the strongest 3 × 3 beams.