Adatom diffusion at high temperature on a seven-layer silicon slab with a one-atom layer step.

Chips and Steps

Where would we be without silicon? This abundant element -- it comprises 27 percent of the Earth's crust -- provides the starting material for nearly all the microelectronic "chips," from VCRs to the Space Shuttle, that have transformed the way we live. In computing and communications, 40 years of continuing innovation in smaller, faster circuitry, with silicon as the base material, is driving us headlong into the Information Age.

Silicon remains the choice for many applications, despite other materials that promise faster electronics, because it is durable and reliable, and over the years many problems associated with producing chip-quality crystals in commercial quantity have been worked out. That hardly means, however, that no challenges remain in silicon chip technology. Whether it is to solve important problems in science or for the rapidly evolving new world of communications and entertainment, we need faster circuitry, smaller, more perfect chips.

The Digital Equipment Corporation Alpha microprocessor, the core silicon "chip" used in the CRAY T3D, can do 150 million calculations a second.

With these needs in mind, Jerzy Bernholc and his colleagues are working to understand the atom-by-atom details of how silicon grows. The process of producing the relatively pure, defect-free material required for chips is called "growing" because it involves starting with a small "seed" crystal of the desired structure and adding to it one atom at a time. Bernholc, a solid-state physicist, has used the CRAY C90 and, recently, the CRAY T3D at the Pittsburgh Supercomputing Center to investigate "step-flow" growth, a prevalent silicon growth process by which layers build up, an atom at a time, into a stairstep-like formation of terraces.

This schematic shows three terraces separated by one-atom layer steps. Adatoms (magenta) arrive at a surface and travel to the step edges, where they fill out the crystal by extending the terrace. The bonds (green dashed lines) between paired surface atoms, called "dimers," alternate direction from one terrace to the next.

Using the C90, Bernholc' research group completed the first starting-from-scratch calculations to give the atomic and electronic structure of silicon "steps." Extending this work, further C90 calculations explored the pathways by which a deposited atom, an "adatom," moves across a silicon surface and fills in an empty space at the growing edge of a step. With availability of the CRAY T3D, Bernholc further expanded the project, adding complexity and realism to the calculations. The results are the beginnings of a detailed road map of the travels of silicon adatoms.

Researcher: Jerzy Bernholc, North Carolina State University, Raleigh.
Hardware: CRAY C90, CRAY T3D
Software: QMD (Quantum Molecular Dynamics)
Keywords: Silicon, circuitry, quantum molecular dynamics (QMD), molecular beam epitaxy (MBE), crystal growth, adatom, dimers, atomic structure, electronic structure, ab initio, atom-atom interactions, atomic resolution images, scanning tunneling microscopy (STM), steps, step structure, dimer buckling, materials science, solid state physics, Car-Parinello.

Related Material on the Web:
Projects in Scientific Computing, PSC's annual research report.

References, Acknowledgements & Credits