Yang Xu

Physical models for coupled electromechanical analysis of silicon nanoelectromechanical systems

Nanoelectromechanical systems (NEMS) can be designed and characterized by understanding the interaction and coupling between the mechanical, electrical, and the van der Waals energy domains. In this paper, we present physical models and their numerical simulation for coupled electrical and mechanical analysis of silicon NEMS. A nonlinear continuum elastic model is employed for mechanical analysis. The material properties required in the continuum model are extracted from molecular-dynamics simulations. We present three electrostatic models—namely, the classical conductor model, the semiclassical model, and the quantum-mechanical model, for electrostatic analysis of NEMS at various length scales. The electrostatic models also account for the corrections to the energy gap and the effective mass due to the strain in the silicon nanostructure. A continuum layer approach is introduced to compute the van der Waals forces. The coupling between the mechanical, electrical, and the van der Waals energy domains as w...

Nucleation and growth in the initial stage of metastable titanium disilicide formation

Initial stage of the C49–TiSi2 formation was investigated at 530u2009°C and at a rate of 10u2009°C/m using transmission electron microscopy. Morphological studies reveal that the C49 phase first separately nucleates at the interface between amorphous silicide and crystalline silicon, then followed by simultaneous lateral and vertical growth. The growth proceeds very fast until the formation of a continuous layer of C49–TiSi2. Local chemical analysis shows that the composition range of the amorphous silicide is narrowed due to the C49 formation. For isothermal annealing, a linear density of the C49 nuclei is about 6.7×10−3/A, and remains the same upon prolonged annealing. In the case of annealing at 10u2009°C/m, the linear density depends on temperature, reaching a maximum of 7.2×10−3/A at around 575u2009°C.

Inducing electronic changes in graphene through silicon (100) substrate modification.

We have performed scanning tunneling microscopy and spectroscopy (STM/STS) measurements as well as ab initio calculations for graphene monolayers on clean and hydrogen(H)-passivated silicon (100) (Si(100)/H) surfaces. In order to experimentally study the same graphene piece on both substrates, we develop a method to depassivate hydrogen from under graphene monolayers on the Si(100)/H surface. Our work represents the first demonstration of successful and reproducible depassivation of hydrogen from beneath monolayer graphene flakes on Si(100)/H by electron-stimulated desorption. Ab initio simulations combined with STS taken before and after hydrogen desorption demonstrate that graphene interacts differently with the clean and H-passivated Si(100) surfaces. The Si(100)/H surface does not perturb the electronic properties of graphene, whereas the interaction between the clean Si(100) surface and graphene changes the electronic states of graphene significantly. This effect results from the covalent bonding between C and surface Si atoms, modifying the π-orbital network of the graphene layer. The local density of states shows that the bonded C and Si surface states are highly disturbed near the Fermi energy.

Pull-in/out analysis of nano/microelectromechanical switches with defective oxide layers

We investigate the effect of surface and interior defects such as vacancies and broken bonds on the performance of nano/microelectromechanical (N/MEMS) switches. By combining multiscale electrostatic analysis with mechanical analysis, we compute the capacitance-voltage and pull-in/out voltages of N/MEMS switches in the presence of defects in the dielectric oxide layer. Our results indicate that both surface and interior defects can change the pull-in/out voltages leading to significant voltage offsets. These voltage offsets can lead to an eventual failure of the N/MEMS switch.

Carbon nanotube screening effects on the water-ion channels.

A self-consistent tight-binding method is used to investigate the screening effects of semiconducting and metallic single-wall carbon nanotubes (SWCNTs) when the water molecules and various charged ions pass through the nanotubes. The trajectories of ions and water molecules are obtained from molecular dynamics simulations. It is shown that metallic SWCNTs have much stronger screening abilities than semiconducting SWCNTs. Our results indicate that it is possible to distinctly identify different ions and also to differentiate between armchair and zig-zag nanotubes.

Low‐temperature solid‐phase heteroepitaxial growth of Ge‐rich SixGe1−x alloys on Si (100) by thermal annealing a‐Ge/Au bilayers

Heteroepitaxial layers of SixGe1−x alloys were grown on Si(100) by thermal annealing bilayers of a‐Ge/Au deposited on single‐crystal Si (sc‐Si) substrates at 300u2009°C. During annealing, Ge dissolves into the Au layer and then grows epitaxially to the substrate, with the final structure changing from a‐Ge/Au/sc‐Si to Au/SixGe1−x/sc‐Si. The SixGe1−x layer was found to be Ge rich (x≊0.15) from AES and STEM analysis and to be epitaxial to the Si(100) substrate from the x‐ray rocking curve and RBS channeling measurements. Stacking faults and microtwins are the major defects in the epitaxial film, as observed by cross‐sectional TEM. High‐resolution TEM analysis of the SixGe1−x/Si interface shows that the growth initiates at specific areas of the original Au/Si interface. This work demonstrates for the first time both heteroepitaxial growth and the growth of SixGe1−x alloys on Si(100) using solid phase epitaxy with an eutectic‐forming metal as the transport medium.

Automated interferometric synthetic aperture microscopy and computational adaptive optics for improved optical coherence tomography.

In this paper, we introduce an algorithm framework for the automation of interferometric synthetic aperture microscopy (ISAM). Under this framework, common processing steps such as dispersion correction, Fourier domain resampling, and computational adaptive optics aberration correction are carried out as metrics-assisted parameter search problems. We further present the results of this algorithm applied to phantom and biological tissue samples and compare with manually adjusted results. With the automated algorithm, near-optimal ISAM reconstruction can be achieved without manual adjustment. At the same time, the technical barrier for the nonexpert using ISAM imaging is also significantly lowered.

Detection of defective DNA in carbon nanotubes by combined molecular dynamics/tight-binding technique

A tight-binding method combined with molecular dynamics (MD) is used to investigate the electrostatic signals generated by DNA segments inside short semiconducting single-wall carbon nanotubes (CNTs). The trajectories of DNA, ions, and waters, obtained from MD, are used in the tight-binding method to compute the electrostatic potential. The electrostatic signals indicate that when the DNA translocates through the CNT, it is possible to identify the total number of base pairs and the relative positions of the defective base pairs in DNA chains. Our calculations suggest that it is possible to differentiate Dickerson and hairpin DNA structures by comparing the signals.

Computational optical coherence tomography [Invited]

Optical coherence tomography (OCT) has become an important imaging modality with numerous biomedical applications. Challenges in high-speed, high-resolution, volumetric OCT imaging include managing dispersion, the trade-off between transverse resolution and depth-of-field, and correcting optical aberrations that are present in both the system and sample. Physics-based computational imaging techniques have proven to provide solutions to these limitations. This review aims to outline these computational imaging techniques within a general mathematical framework, summarize the historical progress, highlight the state-of-the-art achievements, and discuss the present challenges.

Filtering for unwrapping noisy Doppler optical coherence tomography images for extended microscopic fluid velocity measurement range

In this Letter, we report the first application of two phase denoising algorithms to Doppler optical coherence tomography (DOCT) velocity maps. When combined with unwrapping algorithms, significantly extended fluid velocity dynamic range is achieved. Instead of the physical upper bound, the fluid velocity dynamic range is now limited by noise level. We show comparisons between physical simulated ideal velocity maps and the experimental results of both algorithms. We demonstrate unwrapped DOCT velocity maps having a peak velocity nearly 10 times the theoretical measurement range.