Cary Y. Yang

Crystallography of decahedral and icosahedral particles: I. Geometry of twinning

The crystal structure of the tetrahedral twins in multiply-twinned particles with decahedral and icosahedral point group symmetries has been examined and correlated with the face-centered cubic structure. Details on the crystal structure as well as the geometrical relationships among twins in each particle are presented. These crystallographic facts serve as a basis for the interpretation of small particle images obtained with advanced methods of transmission electron microscopy.

The structure of small, vapor-deposited particles: I. Experimental study of single crystals and particles with pentagonal profiles

Abstract The crystallographic structure of small gold particles vapor-deposited on NaCl substrates was investigated using high resolution,0 selected-zone dark field (SZDF), Bragg reflection imaging (BRI), moire fringe imaging, and strong beam and weak beam dark field (WBDF) transmission electron microscopy (TEM) imaging techniques. Those particles that exhibit uniform contrast in bright field (BF) and have a distinct triangular shape are perfect single crystals with a face-centered cubic (fcc) structure. The particles with pentagonal profile were determined to be regular decahedra composed of five identical tetrahedral building units of non-fcc structure. Each unit has a body-centered orthorhombic crystal symmetry, slightly distorted from the fcc structure. The experimental evidence for this new particle model and for the conclusion that the older model based on fcc tetrahedral building units does not apply includes (a) SZDF-BRI micrographs demonstrating the nonexistence of gaps between the tetrahedral units; (b) WBDF images showing thickness fringes that exclude nonuniform lattice strains that would result from an fcc model with closed gaps; and (c) micrographs exhibiting characteristics moire fringes in well-defined parts of the composite particles.

Electron transport through metal-multiwall carbon nanotube interfaces

In this paper, we examine mechanisms of electron transport across the metal-carbon nanotube (CNT) interface for two different types of multiwall carbon nanotube (MWNT) architectures, horizontal or side-contacted MWNTs and vertical or end-contacted MWNTs. Horizontally aligned nanotube growth and electrical characteristics are examined with respect to their potential applications in silicon-based technologies. Recent advances in the synthesis techniques of vertical MWNTs have also enhanced the possibility for a manufacturable solution incorporating this novel material as on-chip interconnects or vias as copper interconnect feature sizes are scaled into the sub-100-nm regime. A vertical MWNT architecture is presented that may be suitable for integration into silicon-based technologies. The growth method for this architecture and its effect on electrical characteristics are examined. Through simulations, dc measurements, and comparison of our results with previous studies, we explain why high contact resistance is observed in metal-CNT-metal systems.

The structure of small, vapor-deposited particles: II. Experimental study of particles with hexagonal profile

Abstract “Multiply-twinned” gold particles with hexagonal bright field TEM profile were determined to be icosahedra composed of 20 identical and twin-related tetrahedral building units that do not have an fcc structure. The crystal structure of these slightly deformed tetrahedra is rhombohedral. Experimental evidence supporting this particle model was obtained by selected-zone dark field and weak beam dark field electron microscopy. In conjunction with the results of part I, it has been concluded that multiply- twinned gold particles of pentagonal or hexagonal profile that are found during the early stages of the vapor deposition growth process on alkali halide surfaces do not have an fcc crystal structure, which is in obvious contrast to the structure of bulk gold.

Crystallography of decahedral and icosahedral particles: II. High symmetry orientations

Abstract Based on the exact crystal structure of decahedral and icosahedral particles, high energy electron diffraction patterns and image profiles have been derived for various high symmetry orientations of the particles with respect to the incident beam. These results from a basis for the identification of small metal particle structures with advanced methods of transmission electron microscopy.

Structural and Electrical Characterization of Carbon Nanofibers for Interconnect Via Applications

We present temperature-dependent electrical characteristics of vertically aligned carbon nanofiber (CNF) arrays for on-chip interconnect applications. The study consists of three parts. First, the electron transport mechanisms in these structures are investigated using I-V measurements over a broad temperature range (4.4 K to 350 K). The measured resistivity in CNF arrays is modeled based on known graphite two-dimensional hopping electron conduction mechanism. The model is used because of the disordered graphite structure observed during high-resolution scanning transmission electron microscopy (STEM) of the CNF and CNF-metal interface. Second, electrical reliability measurements are performed at different temperatures to demonstrate the robust nature of CNFs for interconnect applications. Finally, some guidance in catalyst material selection is presented to improve the nanostructure of CNFs, making the morphology similar to multiwall nanotubes.

Amorphous and Crystalline Silicon Carbide and Related Materials

Although silicon carbide has been used for more than half a century, its potential as a high-temperature, corrosion-resistant semiconductor has only recently begun to be exploited. Both crystalline and amorphous forms of SiC offer several advantages over Si, GaAs, and InP for high-frequency, high-power, and high-speed circuits. This volume contains reports on high-temperature SiC MOSFETs and MESFETs, secondary harmonic generation in SiC, a-SiC emitter heterojunction bipolar transistors, and bulk crystal growth of 6H-SiC. For newcomers to the field it provides an up-to-date review of technological developments in SiC and related materials, while specialists will find here recent references and new insights into materials for high-temperature, high-power, and high-speed circuit applications.

Characteristics of aligned carbon nanofibers for interconnect via applications

Electrical properties of plasma-enhanced chemical vapor deposited carbon nanofibers (CNFs) are characterized with measurements over a broad temperature range (4-300 K). Temperature-dependent measurements of CNF via resistivity reveal a behavior resembling the mixture of graphite a-axis and c-axis transport mechanisms. For the first time, temperature-dependent characteristics of CNFs are measured and modeled based on previously developed models for electron conduction in graphite. Reliability measurements are performed to demonstrate the robust electrical and thermal properties of CNF vias for next-generation on-chip-interconnect designs.

Characterization of SiGe/Si heterostructures formed by Ge+ and C+ implantation

Formation of SiGe/Si heterostructures by germanium ion implantation was investigated. A germanium‐implanted layer was grown epitaxially in the solid phase by thermal annealing. Two kinds of crystalline defects were observed. One is a misfit dislocation, and the other is a residual dislocation caused by ion bombardment. The p‐n junction formed in the SiGe layer has a leakage current three orders of magnitude larger than that of a pure Si p‐n junction fabricated with an identical process except for the Ge+ implantation. Carbon doping in the SiGe layer improves its crystalline quality and the junction characteristics.

Relativistic scattered wave calculations on UF6

Self‐consistent Dirac–Slater multiple scattering calculations are presented for UF6. These are the first such calculations to be reported, and the results are compared critically to other relativistic calculations. The results of all molecular orbital calculations are in good qualitative agreement, as measured by energy levels, population analyses, and spin–orbit splittings. The overall charge distribution is computed to be U+1.5(F−0.25)6. Polarization functions are found to be qualitatively unimportant. A detailed comparison is made to the relativistic Xα (RXα) method of Wood and Boring, which also uses multiple scattering theory, but incorporates relativistic effects in a more approximate fashion. For the most part, the RXα results are in excellent agreement with the present results. Some differences of possible significance are noted in the lower valence and core energies.