DSpace at MIT
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Mostrando recursos 1 - 20 de 122
1.
Ballistic Transport in Nanostructures from First-Principles Simulations - Marzari, Nicola
We developed and implemented a first-principles based theory of the Landauer ballistic conductance, to determine the transport properties of nanostructures and molecular-electronics devices. Our approach starts from a quantum-mechanical description of the electronic structure of the system under consideration, performed at the density-functional theory level and using finite-temperature molecular dynamics simulations to obtain an ensemble of the most likely microscopic configurations. The extended Bloch states are then converted into maximally-localized Wannier functions to allow us to construct the Greenâ??s function of the conductor, from which we obtain the density of states (confirming the reliability of our microscopic calculations) and the...
2.
Characterization and Modeling of Stress Evolution During Nickel Silicides Formation - Liew, K.P.; Li, Yi; Yeadon, Mark; Bernstein, R.; Thompson, Carl V.
An curvature measurement technique was used to characterize the stress evolution during reaction of a Ni film and a silicon substrate to form nickel silicide. Stress changes were measured at each stage of the silicide growth. When the nickel films were subjected to long-time isothermal annealing, stresses that developed during silicide formation gradually relaxed. Fitting the experimental results with a kinetic model provides insight into the volumetric strain and relaxation behavior of the reacting film and the reaction product.
3.
Charge Storage Mechanism and Size Control of Germanium Nanocrystals in a Tri-layer Insulator Structure of a MIS Memory Device - Teo, L.W.; Ho, Van Tai; Tay, M.S.; Lei, Y.; Choi, Wee Kiong; Chim, Wai Kin; Antoniadis, Dimitri A.; Fitzgerald, Eugene A.
A method of synthesizing and controlling the size of germanium nanocrystals is developed. A tri-layer metal-insulator-semiconductor (MIS) memory device structure comprising of a thin (~5nm) silicon dioxide (SiO₂) layer grown using rapid thermal oxidation (RTO), followed by a layer of Ge+SiO₂ of varying thickness (3 - 6 nm) deposited using a radio frequency (rf) co-sputtering technique, and a capping SiO₂ layer (50nm) deposited using rf sputtering is investigated. It was verified that the size of germanium (Ge) nanocrystals in the vertical z-direction in the trilayer memory device was controlled by varying the thickness of the middle (cosputtered Ge+SiO₂) layer. From...
4.
Computation and Simulation of the Effect of Microstructures on Material Properties - Carter, W. Craig
Many material properties depend on specific details of microstructure and both optimal material performance and material reliability often correlate directly to microstructure. In nano- and micro-systems, the material's microstructure has a characteristic length scale that approaches that of the device in which it is used. Fundamental understanding and prediction of material behavior in nano- and micro-systems depend critically on methods for computing the effect of microstructure. Methods for including the physics and spatial attributes of microstructures are presented for a number of materials applications in devices. The research in our group includes applications of computation of macroscopic response of material...
5.
A continuum theory of amorphous solids undergoing large deformations, with application to polymeric glasses - Anand, Lallit
This paper summarizes a recently developed continuum theory for the elastic-viscoplastic deformation of amorphous solids such as polymeric and metallic glasses. Introducing an internal-state variable that represents the local free-volume associated with certain metastable states, we are able to capture the highly non-linear stress-strain behavior that precedes the yield-peak and gives rise to post-yield strain-softening. Our theory explicitly accounts for the dependence of the Helmholtz free energy on the plastic deformation in a thermodynamically consistent manner. This dependence leads directly to a backstress in the underlying flow rule, and allows us to model the rapid strain-hardening response after the initial...
6.
Effect of Wafer Bow and Etch Patterns in Direct Wafer Bonding - Spearing, S. Mark; Turner, K.T.
Direct wafer bonding has been identified as an en-abling technology for microelectromechanical systems (MEMS). As the complexity of devices increase and the bonding of multiple patterned wafers is required, there is a need to understand the factors that lead to bonding failure. Bonding relies on short-ranged surface forces, thus flatness deviations of the wafers may prevent bonding. Bonding success is determined by whether or not the surface forces are sufficient to overcome the flatness deviations and deform the wafers to a common shape. A general bonding criterion based on this fact is developed by comparing the strain energy required to...
7.
Evolution of AlN buffer layers on Silicon and the effect on the property of the expitaxial GaN film - Zang, Keyan; Wang, Lianshan; Chua, Soo-Jin; Thompson, Carl V.
The morphology evolution of high-temperature grown AlN nucleation layers on (111) silicon has been studied using atomic force microscopy (AFM). The structure and morphology of subsequently grown GaN film were characterized by optical microscopy, scanning electron microscopy, x-ray diffraction, and photoluminescence measurement. It was found that a thicker AlN buffer layer resulted in a higher crystalline quality of subsequently grown GaN films. The GaN with a thicker buffer layer has a narrower PL peak. Cracks were found in the GaN film which might be due to the formation of amorphous SiNx at the AlN and Si interface.
8.
Fabrication of Highly Ordered Nanoparticle Arrays Using Thin Porous Alumina Masks - Lei, Y.; Teo, L.W.; Yeong, K.S.; See, Y.H.; Chim, Wai Kin; Choi, Wee Kiong; Thong, J.T.L.
Highly ordered nanoparticle arrays have been successfully fabricated by our group recently using ultra-thin porous alumina membranes as masks in the evaporation process. The sizes of the nanoparticles can be adjusted from 5-10 nm to 200 nm while the spacing between adjacent particles can also be adjusted from several nanometers to about twice the size of a nanoparticle. The configuration of the nanoparticles can be adjusted by changing the height of the alumina masks and the evaporation direction. Due to the high pore regularity and good controllability of the particle size and spacing, this method is useful for the ordered...
9.
First Principles Modeling for Research and Design of New Materials - Ceder, Gerbrand
First principles computation can be used to investigate an design materials in ways that can not be achieved with experimental means. We show how computations can be used to rapidly capture the essential physics that determines the useful properties in different applications. Some applications for predicting crystal structure, thermodynamic and kinetic properties, and phase stability are discussed. This first principles tool set will be demonstrated with applications from rechargeable batteries and hydrogen storage materials.
10.
GaN Based Nanomaterials Fabrication with Anodic Aluminium Oxide by MOCVD - Wang, Yadong; Sander, Melissa; Peng, Chen; Chua, Soo-Jin; Fonstad, Clifton G. Jr.
A highly self-ordered hexagonal array of cylindrical pores has been fabricated by anodizing a thin film of Al on substrate and subsequent growth of GaN and InGaN in these nanoholes has been performed. This AAO template-based synthesis method provides a low cost process to fabricate GaN-based nanomaterials fabrication.
11.
Laser Fabrication by Using Photonic Crystal - Vajpeyi, Agam P.; Chua, Soo-Jin; Fitzgerald, Eugene A.
This paper involves the calculation for composition of different layer used in laser structure and the simulation of cavity, formed by creating air columns in the InGaAsP medium, for square lattice. The aim of this project is to fabricate approximately zero threshold current lasers. This project involves FDTD simulation for optimizing dimension of the device, fabrication of laser structure and finally characterization of the device structure.
12.
Mechanical properties of La-based bulk amorphous alloy and composites - Lee, Irene Mei Ling; Li, Yi; Carter, W. Craig
Influence of different microstructure of La-based fully amorphous samples and its composites on the impact fracture energy were investigated and discussed. Results showed improvement in fracture energy of glassy metals with the presence intermetallic phases, but deteriorated in the presence of dendrite phases and high volume % of crystalline phases.
13.
MEMS Materials and Processes: a research overview - Spearing, S. Mark
An overview is provided of materials and processes research currently being conducted in support of MEMS device design at MIT. Underpinning research is being conducted in five areas: room temperature strength characterization, elevated temperature strength characterization, processing of Si/SiC hybrid structures, modeling of wafer bonding processes and development of high temperature fluid interconnections. Emphasis is placed on the key areas of materials science and engineering.
14.
Metallic Cluster Coalescence: Molecular Dynamics Simulations of Boundary Formation - Takahashi, A. R.; Thompson, Carl V.; Carter, W. Craig
During the evaporative deposition of polycrystalline thin films, the development of a tensile stress at small film thicknesses is associated with island coalescence. Several continuum models exist to describe the magnitude of this tensile stress but the coalescence stress becomes significant at small enough thicknesses to draw the continuum models into question. For nanometer-sized islands, we perform atomistic simulations of island coalescence to determine if the atomistic methods and continuum models are mutually consistent. The additional detail provided by the atomistic simulations allows for study of the kinetics of island coalescence and the treatment of different crystallographic orientations. We find...
15.
Novel CMOS-Compatible Optical Platform - Pitera, Arthur J.; Groenert, M. E.; Yang, V. K.; Lee, Minjoo L.; Leitz, Christopher W.; Taraschi, G.; Cheng, Zhiyuan; Fitzgerald, Eugene A.
A research synopsis is presented summarizing work with integration of Ge and III-V semiconductors and optical devices with Si. III-V GaAs/AlGaAs quantum well lasers and GaAs/AlGaAs optical circuit structures have been fabricated on Si using Ge/GeSi/Si virtual substrates. The lasers fabricated on bulk GaAs showed similar output characteristics as those on Si. The GaAs/AlGaAs lasers fabricated on Si emitted at 858nm and had room temperature cw lifetimes of ~4hours. Straight optical links integrating an LED emitter, waveguide and detector exhibited losses of approximately 144dB/cm. A process for fabrication of a novel CMOS-compatible platform that integrates III-V or Ge layers with...
16.
Research Summary: Object Oriented Finite Element Analysis for Materials Science*: A Tool for Viscoelastic Polymer Composite Deformation Analysis - Raghavan, Rajesh; Carter, W. Craig
A public domain code "Object Oriented Finite element analysis for materials science" (OOF) has been extended to include tools for analysis of viscoelastic materials. Utility of these tools has been discussed along with possible applications in this publication. Added features in OOF include means to quantitatively analyze the spatiotemporal response of a composite polymeric material in dynamic as well as in static deformation conditions. These coupled with the existing features of OOF, in particular, the complete analysis of mechanical characteristics of materials provide a comprehensive tool for the studies of time dependent behavior of variety of materials including polymeric solid...
17.
Research on Polycrystalline Films for Micro- and Nano-Systems - Thompson, Carl V.
Polycrystalline films are used in a wide array of micro- and nano-scale devices, for electronic, mechanical, magnetic, photonic and chemical functions. Increasingly, the properties, performance, and reliability of films in these systems depend on nano-scale structure. In collaborative research with a number of SMA Fellows, Associates, and students, our group is carrying out research focused on probing, modeling and controlling nano-scale structural evolution during both vapor-phase and solid-phase polycrystalline film formation. In particular, high-sensitivity in-situ and real-time stress measurements are being used to study atomic scale forces and to characterize structure formation and evolution at the nano-scale. In other collaborative...
18.
SiGe-On-Insulator (SGOI): Two Structures for CMOS Application - Cheng, Zhiyuan; Jung, Jongwan; Lee, Minjoo L.; Nayfeh, Hasan; Pitera, Arthur J.; Hoyt, Judy L.; Fitzgerald, Eugene A.; Antoniadis, Dimitri A.
Two SiGe-on-insulator (SGOI) structures for CMOS application are presented: surface-channel strained-Si on SGOI (SSOI) and dual-channel SGOI structures. Comparisons between two structures are made from both device performance and CMOS process point of view. We have demonstrated both structures on SGOI, and have fabricated n-MOSFETâ??s and p-MOSFETâ??s on those two structures respectively. Device characteristics are presented. The devices show enhancement on both electron and hole mobilities.
19.
Solid State Thin Film Lithium Microbatteries - Shi, Z.; Lü, L.; Ceder, Gerbrand
Solid state thin film lithium microbatteries fabricated by pulsed-laser deposition (PLD) are suggested. During deposition the following process parameters must be considered, which are laser energy and fluence, laser pulse duration, laser pulse frequency, target composition, background gasses, substrate temperature, target-substrate distance and orientation. The effects of the variations of the process parameters can be obtained by measuring stoichiometry, thickness, phases and structure (grain size and texture), and stress of the deposited films. Electrochemical measurements will be conducted to test the microbattery properties through open-circuit voltage, charge-discharge cycling, cyclic voltammetry, and impedance analysis.
20.
Bulk Glass Formation in Eutectic of La-Cu-Ni-Al Metallic Alloys - Zhang, Yong; Tan, Hao; Li, Yi
A eutectic in La-rich La-Cu₀.₅Ni₀.₅-Al alloys was determined by studying the melting behaviors and the microstructure observations. The microstructures of the La-Cu-Ni-Al alloys prepared by Bridgman Solidification and copper mould casting were studied by using scanning electron microscope (SEM). The results show that La₆₆[Cu₀.₅Ni₀.₅]₂₀Al₁₄ alloy is very near to a pseudo-ternary eutectic. When the cooling rate is higher than 450 K/s, fully amorphous can be formed; when the cooling rate is within 15 K/s to 450 K/s, the alloy has a microstructure of dendrite plus amorphous, when the cooling rate is within 12 K/s to 1.5 K/s, the microstructures of...
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