Nanotechnology Modeling Laboratory
| UW EE
Current Research Projects
Silicon Solar Cells
Photovoltaics have great potential to address the world's energy needs while reducing carbon emission. We study both silicon and polymer photovoltaics, using integrated process, optical and device modeling to both understand fundamental limits of performance and to design technologies to minimize cost, while maximizing efficiency and reliability. Silicon is currently the dominant material for photovoltaics, with more than 90% of the market. In order to continue to reduce cost and improve performance of silicon solar cells, it is essential to understand how extended defects and impurities affect performance. This understanding enables the development of high efficiency cells using low cost materials and processes. Of particular interest is understanding recombination via point and extended defects, particularly the synergistic effects by which impurities can either passivate or activate recombination at grain boundaries and dislocations. Closely coupled with this is understanding and modeling of gettering processes to isolate unwanted impurities such as metals to locations where they cannot severely degrade cell performance.
Organic (Polymer) Photovoltaics
Organic photovoltaics currently have substantially lower efficiencies than Si, but have tremendous potential for very low cost production and broad application due to possibility of roll-to-roll processing on flexible substrates. Our work on polymer photovoltaics is in collaboration with faculty in MSE and Chemistry. Specific issues we are investigating include novel optical engineering schemes and structure optimization to increase exciton generation, use of thin interfacial dipole layers modify band offsets, and the impact of nanostructure of "bulk heterojunction" layers to simultaneously enhance exciton dissociation and carrier extraction. We are also investigating the fundamentals of carrier transport in organic materials.
Dopant Diffusion and Activation in Ultrashallow Junctions
Simultaneously reducing junction depth while keeping parasitic source/drain resistance low is a difficult challenge that must be overcome for the effective performance of deep submicron (<100nm) CMOS devices. A great deal of attention has been focused on making very shallow junctions; however, the greater difficulty lies in simultaneously maintaining a sufficiently low parasitic resistance, which in turn requires concentrations of electrically-active dopants above the equilibrium solubility. This task is aimed at developing and quantifying models for coupled dopant/defect diffusion and extended defect formation, as well as their mutual interactions. The work will build on our ongoing efforts toward development of fundamental models for the evolution of extended defects during device fabrication. There are three main aspects to the proposed work: continued advancement of our understanding of coupled dopant/defect diffusion under non-equilibrium conditions, modeling the kinetics of dopant activation/deactivation during junction formation, and understanding dopant deactivation during conditions typical of back-end processes (300-700C). The work will require a combination of experimental work involving the WTC or EE Microfabrication Lab with development and implementation of models in the form of differential equations describing the physical processes.
Effect of Anisotropic Stress on Dopant Diffusion and Activation
Anisotropic stress has been identified as an effective mechanism to enhance mobility in semiconductor devices. To reliably fabricate high performance nanoscale devices incorporating high stress levels, it is necessary to understand the effect of stress on active doping concentrations. We use ab-initio density functional theory (DFT) calculations to analyze the effect of the anisotropic (tensor) stress on the energies of substitutional dopants and dopant/defect complexes, as well as transition states for dopant migration and reaction via use of nudged elastic bands method. These results are then used to develop predictive models for the changes in dopant diffusion and activation kinetics due to stress.
Atomistic Modeling of Deep Submicron VLSI Device Fabrication
As device sizes continue to shrink deeper into the nanometer regime, the fundamentally discrete nature of semiconductor devices becomes of critical importance. This work is aimed at developing and utilizing tools for modeling VLSI device fabrication at the atomic scale. The research involves a multiscale modeling approach, with ab-initio (DFT) calculations used as the basis for molecular dynamics potentials and kinetic lattice Monte Carlo (KLMC) transition rates. We are currently focusing our efforts on the effects of composition on point defect incorporation during solid phase epitaxial regrowth and extended defect evolution in the presence of ion implant damage and anisotropic stress.
Modeling and Simulation of Silicon Wafer Preparation
This project represents the Modeling and Simulation thrust for the NSF Industry/University Cooperative Research Center on Silicon Wafer Engineering and Defect Science (SiWEDS). The aim of this work is to develop a predictive simulator for initial growth and subsequent preparation of silicon wafers for microelectronics applications. The initial focus of the work is two-fold: implementing models for metal gettering via segregation and relaxation mechanisms, and developing moment-based models for oxygen precipitation in the presence of carbon and nitrogen.
Conduction of Metal Nanowires
As metal wire dimensions shrink below about 100nm, their resistivity increases due to surface and grain boundary scattering. The aim of this work is the development of fundamental understanding and quantitative models for this conductivity degradation, with the aim of assisting the development of processes for metal wire fabrication that maximize conductivity.
Direct Write of Semiconductor Nanowires
The localized high-field reaction of diphenylgermane via AFM has demonstrated the direct-write of Ge and Si nanofeatures. To better understand the high field chemistry occurring near the tip-sample interface, we attempt to elucidate the fundamental processes occurring in the high-field reaction of diphenylgermane (DPG) by comparing experimental results from AFM writing and electron ionization mass spectroscopy with simulations. We propose a model that involves electron field emission from the tip, followed by electron attachment with formation of temporary negative ions and high field fragmentation. We thus confirm that the reaction of the diphenylgermane precursor at the tip-sample interface is neither purely electrochemical in nature nor purely high-field activated and follows the same trends as the previously reported carbon deposition from organic precursors. This work contributes to the understanding of the possible reactions occurring at the tip-sample interface and can be used to expand the scope of this technique. Examples include using different organometallic precursors for the direct-write of a broad variety of materials.
Tunable Photonic Bandgap Structures
There is a huge and growing market for high performance optical switching systems, particularly those related to WDM, which involves the transmission of a large number of signals with different closely-spaced frequencies on the same optical channel. Photonic band gap structures have tremendous potential in enabling photonic devices with the very strong frequency selectivity required for these applications. In this work, we aim to extend that potential by developing photonic bandgap (PBG) structures whose properties can be modulated at high speeds. These structures will be integrated into waveguides in order to produce a new generation of switches, multiplexers, sensors and lasers to enable the rapid reconfiguration of optical switching networks. The work will integrate periodic photonic structures with microelectronic and micromechanical elements selected to modulate the optical properties of specific device regions. These modulations will shift the device resonances allowing rapid tuning of PBG properties. Published efforts at tunable PBG structures have been to large millimeter or centimeter-wave structures or slow and difficult to control processes such as infusion of a liquid within a photonic crystal (e.g., inverse opal). In this work, we will focus on structures where the resonances at infrared frequencies can be rapidly and reliable modulated via electrical signals. The effort will have two primary components: (1) device design via computational modeling of optical, electronic and mechanical properties as well as fabrication processes; and (2) fabrication of selected devices in the WTC and EE Microfabrication Laboratories. The objective of the research will be identify through modeling the most promising and practical device structures and to build prototypes of those structures to demonstrate the feasibility of the approach.