D.N. Dunn

Atomic imaging of surfaces in plan view

We report experimental results imaging the surface diffraction spots in the plan view geometry from the Si(111) surface and the Ir(001) surface. High quality images have been obtained using conventional large aperture high resolution electron microscopy (HREM), a smaller aperture to exclude the bulk diffraction spots and with highly tilted crystals. The experimental data indicates that there should be no major problems in obtaining atomic scale surface information in plan view.

UHV transmission electron microscopy of Ir(001): II. Atomic positions of the (5 × 1) reconstructed surface from HREM and R-factor refinements

Abstract A partial solution of the atomic scale structure of the Ir(001)−(5 × 1) reconstructed surface is reported combining direct high resolution electron microscopy and R -factor analyses of transmission electron diffraction data. From the high resolution imaging, the structure can be uniquely identified as a hexagonal monolayer on the surface with at most small distortions. R -factor minimization of the near surface structure based upon 8-bit digitization of the diffraction data shows systematic differences for models using single and multiple layers, demonstrating the existence of sub-surface strain fields. Unfortunately, the intrinsic error of 8-bit digital data is too large for a full multilayer minimization. Fitting was also performed using sub-surface relaxations constrained to match analytical strain solutions. These models indicate a two-fold bridge registry of the surface hexagonal layer with a three-fold hollow registry of the sub-surface atoms with respect to the surface layer and small sub-surface relaxations. The data reduction is quantitative with a 20–30% coverage, in agreement with experimental imaging data, and the accuracy of the atomic positions is about ±0.005 nm.

UHV transmission electron microscopy of Ir(001): I. Microstructure of the (1 × 1) and the reconstructed (5 × 1) surfaces

Experimental results on the unreconstructed Ir(001)-(1 × 1) and the reconstructed (5 × 1) surfaces using plan view ultra-high vacuum transmission electron microscopy (UHV-TEM) are presented. The contamination-stabilized Ir(001)-(1 × 1) surface is moderately well ordered with steps preferentially aligned along 〈110〉 and 〈100〉 directions, but is fairly rough on the atomic scale as evidenced by both dark field and high resolution imaging. The step evolution and the microstructure of the surface with repeated annealing are described. The Ir(001)-(5 × 1) reconstruction is determined by electron diffraction to be a simple quasi-hexagonal surface layer with very little distortion in a (5 × 1) registry with respect to the bulk. This surface is a much flatter one than the (1 × 1). The basic morphology of the reconstructed surface layer is shown by dark field microscopy, and the transition from (1 × 1) to (5 × 1) is a nucleation and growth process. Evidence for subsurface IrO2 precipitate particles is also presented.

UHV microscopy of the reconstructed Au(001) surface

Abstract We investigate the microstructure of the reconstructed Au(001) surface using ultra-high vacuum transmission electron microscopy (UHV-TEM). Bulk single crystal Au(001) surfaces were prepared via standard metallographic techniques followed by repetitive cleaning of the surface with ion milling and annealing. After a clean surface was obtained, the (001) surface was found to reconstruct into two nearly orthogonal domains of dimensions (5 × n ) where n ranges between 15 and 21. The unit cell vectors of the surface cell are parallel to the 〈110〉 directions of the unreconstructed fcc (001) surface. Analysis of the diffuse scattering and dark field micrographs indicates that the surface is sheared with a complicated domain and periodicity structure which depends upon the local geometry of the substrate.

Preparation and detection of reconstructed plan-view surfaces

It is shown that ion-beam thinning and cleaning of bulk single-crystal samples coupled with annealing can produce surfaces suitable for conventional plan-view imaging including HREM. The key elements are to ensure the absolute cleanliness of the sample preparation system, care in choosing proper ion beam energies, and choosing the appropriate annealing conditions so as to minimize coarsening by bulk diffusion. The presence of surface reconstructions can be readily detected in off-zone diffraction patterns and in on-zone two-beam bright- and dark-field images.

Problems with the use of surface reconstructions as indicators of a well-ordered surface

The presence of sharp surface reconstruction or diffraction spots is often taken as an indicator of a well-ordered surface. We present results obtained using transmission electron microscopy in ultra-high vacuum (UHV) which demonstrate that even if the surface is well-ordered and reconstructed, there may be very high defect concentrations just below the surface of the order of 1011-1013 cm-2. The key point is that surface ordering takes place via surface diffusion, but the temperature where this is active are too low to anneal out the near-surface defects through bulk diffusion.

Growth of Au on A Ge (111) Surface

We investigate the room temperature growth of evaporated Au thin films on both clean and dirty single crystal Ge (111) substrates. The annealing behavior of these films was then examined under low and high temperatures.

Uhv-Tem Studies of Laser-Induced Damage in Silicon

The effects of pulsed laser irradiation on silicon (111) single crystal thin samples were studied in a ultra-high vacuum transmission electron microscope. Samples were found to cleave along (110) planes under the laser beam. The formation of dislocation networks was also observed. The cleaving did not seem to originate from previously observed defect areas, but from random places, and is believed to be caused by thermal shock from laser beam heating. Bulk defects in the specimens, such as stacking fault tetrahedra and dislocations, were not observed to be affected by the laser treatment.

Ion Beam Damage of Clean Gold Surfaces

We investigate the ion beam damage of clean Au (001) single crystals prepared under standard surface science techniques using sputter - anneal cycles. Under initial ion milling conditions using 4 keV Xe ions a significant amount of Xe was implanted into the bulk of the crystal. After a short period of time a Xe super structure developed which yielded 12 A moire fringes under bright field imaging conditions. With a short anneal the implanted Xe concentration was reduced to the detection limit of Parallel Electron Energy Loss Spectroscopy (PEELS). With extended annealing, the bulk point defect concentration slowly decayed, independent of the surface reconstruction.