D. M. Chen

Three‐stage lattice relaxation of Ge islands on Si(111) measured by tunneling microscopy

We use the tunneling microscope to measure the surface lattice spacing of Ge islands grown on Si(111) as a function of their height. It changes in three stages: (I) (0–50 layers tall) Rapid relaxation from near the bulk Si value, at the end of which the lattice spacing atop some of the islands exceeds that of bulk Ge. (II) (50–80 layers) Rapid decrease in surface lattice spacing, to nearly 2% below the bulk Ge value. (III) (≳80 layers) Gradual relaxation to the bulk value. Additional observations of dislocations and analysis of island widths are used to explain this behavior.

Pinhole formation in solid phase epitaxial film of CoSi2 on Si(111)

The long-standing pinhole problem in solid phase epitaxial growth of a CoSi2 film on Si(111) has been revisited with in situ scanning tunneling microscopy. While the as-deposited film with 5 A of Co at room temperature shows a smooth granular texture with original substrate terraces remaining intact, annealing at 580 °C produces an epitaxial CoSi2 film with large pinholes enclosed by a thin ring CoSi2, exhibiting a volcano feature. Quantitative analysis shows that the formation of pinholes is a result of rapid Si outward diffusion from bulk to surface, and of the subsequent Si reaction with Co on the outer surface. Evidence suggests that inhibiting the Si diffusion channels during the thermal annealing process is the key to solving the pinhole problem.

An ultrahigh vacuum dual-tip scanning tunneling microscope operating at 4.2 K

The design and performance of an ultrahigh vacuum compatible cryogenic dual-tip scanning tunneling microscope is described. The microscope is attached at the bottom of a low-loss liquid helium Dewar and can be operated down to 4.2 K. The coarse positioning system consists of five linear steppers driven by piezo-tubes. The displacement of each stepper can be monitored by its own embedded capacitive position sensor with a submicron resolution, thus allowing accurate control of the tip navigation process. An alignment procedure, using a specimen made of three mutually nonparallel planes, is introduced to bring the two tips into overlapped scan ranges without the help of an additional guiding device such as an electron microscope. The overall system exhibits good mechanical rigidity and atomic resolution has been achieved with either tip. This instrument is well suited for investigating low temperature quantum properties of atomically clean nanostructures in a three-terminal configuration.

Arsenic and gallium atom location on silicon (111)

The x‐ray standing‐wave method is a powerful technique for accurately locating the position of impurity atoms at surfaces, interfaces, and in bulk crystals. Using UHV to prepare crystal surfaces and i n s i t u x‐ray experiments, we have addressed a number of problems in surface physics. Specifically the location of As atoms on Si(111) has been accurately established and has allowed for the first time a direct test of theoretical,density functional, total energy minimization, calculation of atom positions. For As, with only nearest neighbor relaxations, there is excellent agreement between theory and experiment. For Ga on silicon (111) the situation is much more complex. At low coverages (∼1/3 monolayer) the adatom T 4 site for Ga has been accurately measured. X‐ray standing waves were used to establish the vertical position and the in‐plane registration site of Ga to the substrate silicon atoms was established both by standing waves and tunneling microscopy. At higher coverages, Ga occupies substitutional sites in the top half of the Si(111) double plane, but contracted inwards by about one‐half of the distance between the Si(111) double planes. The consequences of the large surface strain generated by this contraction is discussed in the context of the standing wave and tunneling microscopy results. In addition, the atom positions that prevail when both Ga and As atoms are deposited on Si(111) are described.

Characterization of spinel iron-oxide nanocrystals grown on Fe whiskers

Passive iron-oxide nanocrystals are grown on Fe(100) and Fe(110) facets of single-crystal Fe whiskers. Transmission electron microscopy and electron diffraction characterize the oxide spinel structure and their epitaxial growth on Fe whiskers. Iron-oxide nanocrystals grown on Fe(100) facets have sizes close to that of the single magnetic domain Fe3O4 particles, which is supported by our preliminary magnetic force microscopy measurement at room temperature.

Cluster shapes in STM images of isolate clusters and cluster materials

The formation of cluster images in a scanning tunneling microscope (STM) has been modeled using isolate C60 molecules deposited on top of crystalline monolayers of C60 on Ge(100) and Si(100) containing vacancies. C60 molecules occupy positions above vacancies and depending on the dimensions of the latter protrude to a different height above the surface. A simple model of the image formation is suggested and the variation of the shape of C60 molecules in STM images as a function of their apparent height is explained by the convolution of their images with that of the tip and by scattering of tunneling electrons on vacancies below C60 molecules.

Dual-probe scanning tunneling microscope for study of nanoscale metal-semiconductor interfaces

Using a dual-probe scanning tunneling microscope, we have performed three-terminal ballistic electron emission spectroscopy on Au∕GaAs(100) by contacting the patterned metallic thin film with one tip and injecting ballistic electrons with another tip. The collector current spectra agree with a Monte-Carlo simulation based on modified planar tunneling theory. Our results suggest that it is possible to study nanoscale metal-semiconductor interfaces without the requirement of an externally-contacted continuous metal thin film.

Anisotropic Metal-Insulator Transition in Epitaxial Thin Films

By comparing the properties of In and Pb quantum wells in a scanning tunneling microscopy subsurface imaging experiment, we found the existence of lateral bound states, a 2D Mott-Hubbard correlation gap, induced by transverse confinement. Its formation is attributed to spin or charge overscreening of quasi-2D excitations. The signature of the 2D confinement-deconfinement transition is also experimentally observed, with the correlation gap being pinned in the middle of the conduction band. A self-organized 2D Anderson lattice is suggested as a new ground state.

Formation of crystalline islands of C60 on Si(111)

The formation of epitaxial crystalline islands of C60 on both the Si(111)7×7 and Si(111)√3×√3‐B surfaces is shown to follow an initial growth of a disordered layer and islands. This process is associated with a disordered‐to‐ordered interface transition at a critical coverage of three monolayers. Such a transition is driven and stabilized by the interlayer molecular couplings owing to the short range van der Waals interactions between the C60 molecules.

Probing atomically inhomogeneous surfaces with tunneling microscopy

Abstract The connection between observed, physical properties of adsorbed surfaces and their microscopic origins requires structural determination at the atomic level. Yet, investigation of such systems at that scale reveals local inhomogeneities in both reconstruction and individual atomic species at various stages of adsorbate incorporation. We have studied the submonolayer growth of Ga and B on Si(111) with tunneling microscopy and spectroscopy, and we show for each known phase assumed by these systems that images of mixed-phase surfaces provide structural information for new reconstructions which would not be accessible from images of a homogeneous, single-phase surface. Where mixing of adsorbate and substrate atoms occurs within a surface phase, individual atoms of two species may be identified. We demonstrate that controlled surface doping of Si(111) by boron induces identifiable changes in the local character of specific adatom sites leading to device characteristics associated with the presence of the tunneling tip over particular atoms.