Y. L. Chen

Electrostatic particle-particle interactions in electrorheological fluids

A model for the particle‐particle interaction in a chain of spherical particles in an electrorheological (ER) fluid is developed based on an expansion of the potential which satisfies the Laplace equation. The electric‐field distribution and force‐distance relationship between particles are calculated as a function of Kp/Kf (the dielectric constant of the sphere relative to that of the medium) and particle‐particle spacing using this model. The calculations predict electric‐field concentrations of about an order of magnitude near particle contacts. This gives a force between particles that is approximately an order of magnitude higher than predicted by the classical point‐dipole approximation, and is in reasonable agreement with that measured experimentally. Subtle effects of the carrier fluid on the interaction force and an explanation for the non‐Ohmic behavior of ER fluids are deduced in terms of its dielectric constant and dielectric strength.

Stability of C54 titanium germanosilicide on a silicon‐germanium alloy substrate

The stability of C54 Ti(Si1−yGey)2 films in contact with Si1−xGex substrates was investigated. The C54 Ti(Si1−yGey)2 films were formed from the Ti‐Si1−xGex solid phase metallization reaction. It was determined that initially C54 Ti(Si1−yGey)2 forms with a Ge index y approximately the same as the Ge index x of the Si1−xGex substrate (i.e., y≊x). After the formation of the C54 titanium germanosilicide, Si and Ge from the Si1−xGex substrate continue to diffuse into the C54 layer, presumably via lattice and grain boundary diffusion. Some of the Si diffusing into the C54 lattice replaces Ge on the C54 lattice and the Ge index of the C54 Ti(Si1−yGey)2 decreases (i.e., y<x). We propose that this process is driven by a reduction in C54 crystal energy which accompanies the replacement of Ge with Si on the C54 lattice. The excess Ge diffuses to the C54 grain boundaries where it combines with Si1−xGex from the substrate and precipitates as Si1−zGez which is Ge‐rich relative to the substrate (z≳x). This segregation a...

Silicide formation and stability of Ti SiGe and Co SiGe

Abstract The formation and stability of the products of Ti and Co reacting with Si1 − x Gex substrates were investigated. For the Ti SiGe system, when a C54 Ti(Si1 − yGey)2 layer forms, the Ge index y is initially the same as the Ge index of the Si1−xGex substrate (i.e. y = x). Thereafter Si1 − xGex from the substrate continues to diffuse into the C54 layer via lattice and grain-boundary diffusion. Some of the Si which diffuses into the C54 lattice replaces Ge in the lattice, and the C54 Ti(Si1 − yGey)2 becomes silicon enriched (i.e. y Co SiGe system, it was determined that a silicon-enriched Co(Si1 − yGey) layer was formed at ~ 400 °C. As the annealing temperature was increased, the reacted layer became even more Si enriched. For both materials systems, Ge-enriched Si1 − zGe(z > x) islands were observed. It was found that for Co Si 1 − xe x the reacted layer consisted of CoSi2 and Si1 − zGez, after high-temperature annealing (≈700 °C). We propose that these processes are driven by a reduction in the crystal energy of the C54 Ti(Si1 − yGey)2 phase in the Ti SiGe system and the Co(Si1 − yGey) phase in the Co SiGe system which accompanies the replacement of Ge with Si.

EFFECT OF COMPOSITION ON PHASE FORMATION AND MORPHOLOGY IN TI-SI1-XGEX SOLID PHASE REACTIONS

The effects of Si{sub 1{minus}{ital x}}Ge{sub {ital x}} alloy composition on the Ti--Si{sub 1{minus}{ital x}}Ge{sub {ital x}} solid phase reaction have been examined. Specifically, effects on the titanium germanosilicide phase formation sequence, C54 Ti(Si{sub 1{minus}{ital y}}Ge{sub {ital y}}), nucleation temperature, and C54 (TiSi{sub 1{minus}{ital y}}Ge{sub {ital y}}), morphology were examined. It was determined that the Ti--Si{sub 1{minus}{ital x}}Ge{sub {ital x}} reaction follows a ``Ti--Si like`` reaction path for Si rich Si{sub 1{minus}{ital x}}Ge{sub {ital x}} alloys and follows a ``Ti--Ge like`` reaction path for Ge rich Si{sub 1{minus}{ital x}}Ge{sub {ital x}} alloys. The coexistence of multiple titanium germanosilicide phases were observed during Ti--Si{sub 1{minus}{ital x}}Ge{sub {ital x}} reactions for Si{sub 1{minus}{ital x}}Ge{sub {ital x}} alloys in an intermediate composition range. The morphology and stability of the resulting C54 germanosilicides were directly correlated to the Ti--Si{sub 1{minus}{ital x}}Ge{sub {ital x}} reaction path. Smooth continuous C54 titanium germanosilicde was formed for samples with Si{sub 1{minus}{ital x}}Ge{sub {ital x}} compositions in the ``Ti--Si-like`` regime. Discontinuous islanded C54 germanosilicides were formed for samples with Si{sub 1{minus}{ital x}}Ge{sub {ital x}} compositions in the mixed phase and ``Ti--Ge-like`` regimes. Using rapid thermal annealing techniques, it was found that the C54 titanium germanosilicides were stable to highermorexa0» temperatures. This indicated that the morphological degradation occurs after C54 phase formation. The C54 Ti(Si{sub 1{minus}{ital x}}Ge{sub {ital x}}){sub 2} formation temperature was examined as a function of alloy composition and was found to decrease by {approx}70 {degree}C as the composition approached x{approx}0.5. (Abstract Truncated)«xa0less

Film thickness effects in the Ti–Si1−xGex solid phase reaction

The effects of film thickness on the Ti–Si1−xGex solid phase reaction were investigated. Thin C49 TiM2 (M=Si1−yGey) films were formed from the solid phase reaction of 400 A Ti or 100 A Ti with Si1−xGex alloys. It was determined that for films formed from 400 A Ti, the nucleation barrier of the C49‐to‐C54 transformation decreases with increasing germanium content, for alloy compositions with up to ≊40 at.u2009% germanium (i.e., x≤0.40). It was also observed that germanium segregates out of the TiM2 lattice, for both the C49 and C54 phases, and is replaced on the TiM2 lattice with Si from the substrate. The germanium segregation changes the Ge index y of the Ti(Si1−yGey)2. For films formed from a 100 A Ti layer it was observed that the C54 TiSi2 nucleation temperature was increased by ≥125u2009°C. The addition of germanium to the silicon increased the agglomeration of the C49 phase and caused the C54 TiM2 nucleation barrier to increase further. The results also indicate that the increased temperature required for t...

MORPHOLOGY OF SI(100) SURFACES EXPOSED TO A REMOTE H PLASMA

This study addresses the formation of roughness and near surface defects on Si(100) surfaces that are exposed to a remotely excited H plasma. The remote H plasma processing can be employed for in situ wafer cleaning. Atomic force microscopy, transmission electron microscopy, and residual gas analysis are used to measure the surface roughness, the near surface defects, and the etching, respectively. For remote H plasma exposures at substrate temperatures ≤300u2009°C, etching is observed along with a significant increase in the surface roughness and the formation of platelet defects in the near surface region. As the substrate temperature is increased to above 450u2009°C, etching is significantly reduced and no subsurface defects or increases in surface roughness are observed.

Transmission electron microscopy and vibrational spectroscopy studies of undoped and doped Si,H and Si,C:H films

Films of undoped and doped Si:H and Si,C:H alloys were deposited by remote plasma‐enhanced chemical vapor deposition onto thermally grown silicon oxide layers at a substrate temperature of 250u2009°C. The microstructure of the films, including the degree of crystallinity, and the distribution of carbon within the Si,C alloys films were characterized by transmission electron microscopy, Raman scattering, and infrared absorption spectroscopy. The degree of crystallinity depends on both the doping level and on the presence of carbon. For two‐phase μc‐Si or μc‐Si,C alloy films, the results indicate that (i) the crystallites are Si, and (ii) the amorphous encapsulating materials are a‐Si:H for μc‐Si,H, and a‐Si,C:H for the μc‐Si,C:H alloys. A relationship between microstructure, doping levels, and the measured dark conductivity is discussed.

Structural and electrical properties of (Ti0.9Zr0.1)Si2 thin films on Si(111)

Alloy films of Ti and up to 20% Zr were prepared by codeposition onto Si(111) surfaces in ultrahigh vacuum. After in situ thermal annealing at temperatures of ∼600u2009°C, the films form the C49 phase and are stable in this phase up to at least 910u2009°C. In contrast, Ti films on Si(111) initially react to form the C49 phase and transform to the C54 phase at ∼700u2009°C. The surfaces of the (Ti0.9Zr0.1)Si2 alloy films are studied by atomic force microscopy and are shown to be smoother than the surfaces of TiSi2 films on Si substrates. In addition the tendency to island formation is also not observed for annealing temperatures less than 910u2009°C. The sheet resistivity of the (Ti0.9Zr0.1)Si2 alloy films is found to be ∼46 μΩu2009cm for annealing temperatures from 600 to 910u2009°C.

Barrier‐limited transport in μc‐Si and μc‐Si,C thin films prepared by remote plasma‐enhanced chemical‐vapor deposition

Systematic variations of room‐temperature dark conductivities and dark conductivity activation energies for n‐ and p‐type μc‐Si and μc‐Si,C thin films with optical band gaps between 1.9 and 2.2 eV and deposited by remote plasma‐enhanced chemical‐vapor deposition are interpreted in terms of a band alignment model. This leads to an observation that the maximum attainable dark conductivities of these microcrystalline thin films are limited by either thermally assisted transport through, or over interfacial potential barriers between Si crystallites, c‐Si, and the encapsulating amorphous materials: a‐Si:H and a‐Si,C:H, respectively. As the doping is increased in n‐ or p‐type μc‐Si, there is a transition from thermal emission limited to thermally assisted tunneling transport. For all levels of doping so‐far achieved in the μc‐Si,C alloys, the transport is determined by thermionic emission over interfacial barriers at the c‐Si/a‐Si, C:H interface.

Effects of predeposition HF/NH4F treatments on the electrical properties of SiO2/Si structures formed by low‐temperature plasma‐assisted oxidation and deposition processes

Si(100) and (111) wafers were prepared by a standard RCA cleaning process followed by a treatment in HF/NH4F solutions with different pH values. SiO2/Si structures were formed on these surfaces by a two‐step low‐temperature, 200–300u2009°C, plasma‐assisted oxidation/deposition process. The SiO2/Si(111) interface, subjected to a predeposition rinse in a 40 wtu2009% NH4F solution, displayed a midgap interface trap density Dit of approximately 5×1010 cm−2u2009eV−1, which is significantly lower than is generally observed on thermally oxidized Si(111). The Dit values increased systematically up to about 2×1011 cm−2u2009eV−1 as the pH of the HF/NH4F treatment was decreased. For Si(100) wafers, the absolute level, and the energy distribution of Dit within the Si band gap, were essentially independent of the pH of the HF/NH4F treatment. The micromorphology and perfection of the H passivation of the Si surfaces after the HF/NH4F treatment were characterized by Auger electron spectroscopy and low‐energy electron diffraction, and w...