Y. C. Chou

NANOMACHINING OF (110)-ORIENTED SILICON BY SCANNING PROBE LITHOGRAPHY AND ANISOTROPIC WET ETCHING

We have demonstrated that silicon nanostructures with high aspect ratios, having ∼400 nm structural height and ∼55 nm lateral dimension, may be fabricated by scanning probe lithography and aqueous KOH orientation-dependent etching on the H-passivated (110) Si wafer. The high spatial resolution of fabricated features is achieved by using the atomic force microscope based nano-oxidation process in ambient. Due to the large (110)/(111) anisotropic ratio of etch rate and the large Si/SiO2 etch selectivity at a relatively low etching temperature and an optimal KOH concentration, high-aspect-ratio gratings with (111)-oriented structural sidewalls as well as hexagonal etch pit structures determined by the terminal etch geometry can be obtained.

Local electric-field-induced oxidation of titanium nitride films

Nanometer-scale patterning of TiN films grown on SiO2/Si(001) has been demonstrated using the local electric-field-induced oxidation process with a conductive-probe atomic force microscope. The chemical composition of the modified TiN region was determined by micro-Auger electron spectroscopy and was found to consist of Ti, some trace amount of N, and O, suggesting the formation of titanium oxynitride in the near surface region. The dependence of the oxide height on the sample bias voltage with a fixed scanning speed shows a nonlinear trend in the high electric field regime, indicating that the growth kinetics might be significantly different from previous studies using other film materials.

A NEW DIRECT SURFACE STRUCTURAL PROBE: INVERSION OF MEASURED KIKUCHI ELECTRON PATTERNS

The technique of electron-emission holography (EEH) is reviewed with respect to its potential as a direct local structural probe. Direct inversion of the measured Kikuchi electron patterns is emphasized. The description of how the multiple scattering effects are eliminated by the integral-energy phase-summing method is presented. High-fidelity, artifact-free three-dimensional atomic images obtained by inverting simulated diffuse LEED and photoelectron diffraction patterns are shown. Direct inversion of the measured Kikuchi patterns shows clear images of the neighboring atoms within the range of the electron mean free path, thus yielding structural information with high resolution (~1 A) in all directions. Special attention is paid to the correct role of background subtraction in removing artifacts, and to the selection of minimal experimental data in the practical application of EEH. Moreover, direct inversion of diffraction patterns from many different local emitters is found to yield a three-dimensional Patterson function of the near-surface structure. This leads to direct surface structural determination by inverting Kikuchi electron patterns. Finally, the future of this direct method based upon inverting Kikuchi electron patterns, including the comparison with diffuse LEED and photoelectron holography, is discussed.

Theoretical modeling and experimental testing of a multimode optical system and energy analyzer for electron spectroscopy

An electron‐optical system and hemispherical electrostatic energy dispersing element for quantitative electron spectroscopy over a wide range of kinetic energies is described. The electron optics were modeled using several calculational techniques, in order to determine the theoretical conditions under which a fixed linear magnification could be obtained. By designing an optical system with a plane of reflection symmetry, fixed magnification focus was possible over a calculated range of retard ratios from 1/40 to 40/1. The optics can be run in two different modes, one with and one without a retarding field grid to achieve the energy retardation. Comparisons between the predictions made using the various computational methods are reported, as well as experimental verification of the actual performance of the electron optics and energy analyzer. A method is described by which the angular acceptance of the electron optics can be varied by changing the excitation potentials on the lenses. The completed system allows for the simple installation of single‐channel, multichannel, and spin‐polarization detectors without modification of the analyzer.

Divergence of dielectric constant near the percolation threshold

Abstract The d.c. capacitance of carbon-wax mixtures was studied near the percolation threshold. The critical behavior of the capacitance (or the effective dielectric constant) was observed only when the mixture and the copper electrodes were in good electric contact. The effective dielectric constant varied continuously through the percolation threshold if the copper electrodes were insulated from the mixture. The divergence of the measured capacitance is explained by the divergence of the effective area of the capacitor, when the percolative clusters are in good contact with the copper electrodes.

Surface-enhanced Raman scattering from tetracyanoethylene deposited on thin Ag films and charge transfer effects

Abstract Raman spectra of tetracyanoethylene (TCNE) deposited on rough Ag films were studied with an Ar + laser. TCNE was found to form a charge transfer com

XPS studies of several high temperature superconductors: Y-Ba-Cu-O and La-Ba-Cu-O systems

Abstract Copper 2P core level X-ray photoelectron spectra on several high Tc superconductors La1.85Ba0.15CuO4, Y1.2Ba0.8CuO4−δ and Y0.4B0.6CuO4−δ oxides have been measured. The results indicate the coexistence of Cu2+ and Cu3+ in these compounds.

Field effects on electron conduction through self-assembled monolayers

The electronic conduction through the self-assembled monolayer (SAM) can be modulated by the electric potential applied to the silicon gate electrode surrounding the SAM. The dependence of the current through SAM on the gate voltage can be explained that the renormalized molecular energy levels are swept through the window between the Fermi levels of the source and drain electrodes. The effects of the lowest unoccupied molecular orbital and a hybrid energy level near the Fermi level in the transmission spectrum can be identified.

Coulomb blockade oscillations in ultrathin gate oxide silicon single-electron transistors

Ultrathin oxide-gated (thickness ∼6nm) point-contact junctions have been fabricated to explore single-electron charging effects in strongly gate-dot-coupled polycrystallinesilicon transistors. Current–voltage (I–V) measurements show periodic current oscillations near room temperature. Analysis of the energy-level spacing relates the electron charging energy to a quantum dot of size ∼8nm, and also suggests electron tunneling is via the first excited state. These low-power ∼30pW and low-cost devices can be useful for the next generation nanoelectronics.

Electron-emission holography. A new direct method for surface structural determination

Abstract Direct inversion of measured Kikuchi and simulated photoelectron diffraction patterns shows clear images of the neighboring atoms within the range of the electron mean free path. More than ten nearby atoms are obtained for the Ag(100), Si(100) and (2 × 1) Na/Si (100) systems by the integral-energy phase-summing method. The key point in removing artifacts is a correct role of background subtraction. When this is achieved, the three-dimensional images are essentially high fidelity and artifact free. This demonstrates that electron-emission holography can be used as a direct local structural probe.