Theory and Experiment in “Step” on Semiconductors

February 8th, 2010 by George Fitzgerald, PhD

A recent news article by the University of Texas at Dallas (UTD)  highlighted recent joint work by the Department of Materials Science and Engineering and Accelrys on critical surface reactions of Silicon. The research points the way to ”improve semiconductor devices’ performance in health care and solar power applications in particular.”

Who cares? Anybody who uses chips, solar cells, or any other device containing semiconductors (in other words, all of us.)  

Insertion of Nitrogen atom is predicted to occur preferentially at the step edge of Si(111)

 How does the latest research help? A typical semiconductor device consists of a metal oxide semiconductor layer (e.g., HfO2) deposited on a silicon substrate. As explained by co-author Dr. Mat Halls, formation of an SiO2interlayer between the silicon substrate and metal oxide can decrease semiconductor performance. One approach to solving this is to introduce a nitride barrier to prevent the growth of interfacial SiO2. The ability to introduce such heteroatoms into the topmost layers of Si affords additional opportunities to tune the surface properties by enhancing chemical reactivity at these sites to form functional surfaces. But how do you get the nitrogen to stick to the surface?     

In the latest research, published in Nature Materials, used infra-red spectroscopy  to explore the possible formation mechanisms of nitride on silicon surfaces terminated by hydrogen. Calculations using density functional theory (DFT) demonstrated how stepped edges are important to formation of the nitride layers. The reaction mechanism on the stepped surface provides a means of controlling the reaction. As the authors wrote: “The ability to control the reaction … enables the realization of applications … including sensing, electrical and thermal transport, and molecular computing.” This is a beautiful demonstration of the complementarity of theory and experiment. One can deal with facts, but requires interpretation. The other provides detailed explanations at the atomic level, but sometime requires an anchor to the “real world.” Together they can do more. Wouldn’t it be great if all viewpoints could be reconciled this well?

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Take the leap: Materials Studio 5.0

October 16th, 2009 by Gerhard Goldbeck-Wood, PhD

Just back from the EUGM and Nanotech Consortium Meeting, a week of lively discussions (and foosball matches ;-) and of course our announcement of the release of Materials Studio 5.0. It’s been great finally to talk about and demo all the new features, which we are all so excited about. Getting the requests in for shipment of the new version already … well, it won’t be long.

You can read more about Materials Studio 5.0 at a high level in our Press Release, or in more detail in our ‘What’s New’ document. Perhaps you have read the ‘Transforming Materials Modeling’ tag line in there: imagine the discussions we’ve had about that: “Is it really?” “What is transforming…” and so on. But honestly it is what we are aiming to do with Materials Studio, and there are many things in the 5.0 release that make a real difference.

My take right now from the discussions at the Consortium and User Group Meetings is that the efficiency you gain because of the integration and flexibility this new release provides is quite a step change. The new Amorphous Cell for example got some wows from Materials Science and Life Science folks alike. It’s really a kind of universal structure builder. Want to build a nanocomposite, for example with nanotubes and polymers around them: not a problem. And perhaps there is some small molecule inside the tube: easy.  And what about a protein soaked in a solution: consider it done!

For the second ‘transforming’ example, for me it’s Kinetix, the new Kinetic Monte Carlo module we built for the Nanotech Consortium. I alluded to Kinetic Monte Carlo development earlier, and thanks to a great collaboration with Tonek Jansen and Johan Lukkien from TU Eindhoven, you can now simulate processes such as a Fuel Cell cathode reaction in Materials Studio, over real time scales of minutes. Considering we start at femtoseconds, that’s quite a leap anyway.

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Driven by Multiscale Simulation: from Carbon atoms to car engines

July 14th, 2009 by Gerhard Goldbeck-Wood, PhD

Multiscale has been a buzzword for such a long time now, most of us must be genuinely tired of it. Nevertheless, when you see actual applications, and the fruits of a lot of hard work come together, I find it still exciting.

A great example I encountered last week is the work by Prof Markus Kraft and his group at Cambridge University’s Chemical Engineering Department. He was over at our Cambridge office for an Accelrys Science and Technology Seminar, talking about soot particles, the black stuff of course that’s actually used to good effect in dyes, and that engineers try and avoid in combustion engines.

The formation of these nano-particles is really and truly a multiscale process. Kraft’s research team starts the long multiscale journey at the quantum level, using DMol3 in Materials Studio to calculate transition states for oxidation reactions of polycyclic aromatic hydrocarbons (PCAH)

This information then enters into rate constant calculations, which then in turn go into Kinetic Monte Carlo simulations (see some cool and funny examples). With KMC you can see the PCAH structures grow. They are then analysed to give input to a population balance model for particles at the next scale, finally entering into engine models.

You can obviously read up the whole story much better in the Kraft group publications. The point here is that it’s a great example of how the different simulation tools through the scales fit together to solve a complex engineering problem.

Developing such a multiscale toolset is what the Nanotechnology Consortium is all about. Already its 14 Members access a module (also tested at Markus Kraft’s lab), to determine rate constants on the basis of transition state calculations. The tool was developed by Struan Robertson, Accelrys’ Simulations group manager. Incidentally he’s just got another great publication out on the topic: “Detailed balance in multiple-well chemical reactions” with guys from Sandia, Argonne, Leeds and Oxford. Great stuff about how you get a handle on calculating rate constants for complex reactions such as in combustion and atmospheric chemistry.

Transition state calculations themselves become more realistic as a result of another Consortium development, i.e. hybrid QM/MM calculations with MS QMERA, based on the well-known ChemShell environment.

In many cases, a detailed understanding of reactive processes, especially at interfaces, is required. The challenge is that quantum methods can only provide a very limited range of dynamics, while forcefield methods cannot adaequately describe reactions.

So we got together with Prof Frauenheim’s group at Bremen University and collaborators to integrate DFTB+ into the Materials Studio toolset .

Last not least of course there is Kinetic Monte Carlo. As in the work by Kraft I described above, KMC really makes the leap in scale, especially time scale, and connects the ‘science into engineering’ world. The Nanotech Consortium is moving forward in this field as well. Watch this space for more on Kinetic Monte Carlo in the Accelrys toolset.

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