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A useful alloy of gold and silicon, called a eutectic, melts at a far lower temperature than either of its components. Until now, however, its odd behavior on the nanoscale has confounded researchers. By analyzing peculiar “nanoscale crop circles” formed from ultrathin layers of gold on silicon, Berkeley Lab scientists have discovered the eutectic alloy’s unique properties, including its special promise for engineering and processing nanoscale materials.

Solved: The Mystery of the Nanoscale Crop Circles Article When a thin layer of gold anneals on top of a silicon wafer coated with native silicon oxide, randomly distributed pools of eutectic alloy quickly form – and then go through a rapid series of strange changes, leaving behind bare silicon-dioxide circles surrounded by debris. Each denuded circle reveals a perfect square at its center. The area shown is about 107 by 155 micrometers (millionths of a meter). (Click on image for best resolution.) Almost three years ago a team of scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) was performing an experiment in which layers of gold mere nanometers (billionths of a meter) thick were being heated on a flat silicon surface and then allowed to cool. They watched in surprise as peculiar features expanded and changed on the screen of their electron microscope, finally settling into circles surrounded by irregular blisters. The circles varied in diameter up to a few millionths of a meter, and in the center of each was a perfect square. The mysterious patterns were reminiscent of nothing so much as so‑called “alien” crop circles. Until recently the cause of these strange formations remained a mystery. Now theoretical insights have explained what’s happening, and the results have been published online by Physical Review Letters at http://prl.aps.org/abstract/PRL/v108/i9/e096102. Eagerly melting alloys When two solids are combined in just the right proportions, changes in chemical bonding may produce an alloy that melts at a temperature far lower than either can melt by itself. Such an alloy is called eutectic, Greek for “good melting.” The eutectic alloy of gold and silicon – 81 percent gold and 19 percent silicon – is especially useful in processing nanoscale semiconductors such as nanowires, as well as for device interconnections in integrated circuits; it liquefies at a modest 363˚ Celsius, far lower than the melting point of either pure gold, 1064°C, or pure silicon, 1414°C. The rapid growth and evolution of the strange circles at 600˚C is caught in successive frames under an electron microscope (structures at right are already in place as the sequence begins). Underlying a thin layer of gold (mottled gray), a weak spot opens in the silicon dioxide barrier, allowing pure silicon in the substrate to react with the gold. A pool of molten eutectic quickly spreads (dark gray). When it becomes large enough, surface tension ruptures the liquid, pulling the eutectic aside to surround a cleared zone of silicon dioxide, now barren except for a central square of gold and silicon. Time from first to last frame is just 2.8 seconds; the area covered in each image is about 40 by 80 micrometers. “Gold-silicon eutectic liquid can safely solder chip layers together or form microscopic conducting wires, by flowing into channels in the substrate without burning up the surroundings,” says Berkeley Lab’s Junqiao Wu. “It’s particularly interesting for processing nanoscale materials and devices.” Wu cites the example of silicon nanowires, which can be grown from beads of eutectic liquid that form from droplets of gold. The beads catalyze the deposition of silicon from a chemical vapor and ride atop continually lengthening nanowire whiskers. Understanding just how and why this happens has been a challenge. Although eutectic alloys are well studied as solids, the liquid state presents more obstacles, which are particularly formidable at the nanoscale because of greatly increased surface tension – the same surface forces that make it difficult to form ultra-thin films of water, for example, because they pull the water into droplets. At smaller scales the ratio of surface area to bulk increases markedly, and nanoscale structures have been described as virtually “all surface.” These are the conditions that the team led by Wu, who is a faculty scientist in Berkeley Lab’s Materials Sciences Division and a professor in the Department of Materials Science and Engineering at the University of California at Berkeley, set out to examine, by creating the thinnest possible films of gold-silicon eutectic alloys. The researchers did so by starting with a substrate of pure silicon, on whose flat surface an extremely thin barrier layer (two nanometers thick) of silicon dioxide had formed. On this surface they laid layers of pure gold, varying the thickness from one trial to the next between just a few nanometers to a hefty 300 nanometers. The silicon dioxide barrier prevented the pure silicon from mixing with the gold. The next step was to heat the layered sample to 600 °C for several minutes – not hot enough to melt the gold or silicon but hot enough to cause naturally existing pinholes in the thin silicon dioxide layer to enlarge into small weak spots, through which pure silicon could come in contact with the overlying gold. At the high temperature, silicon atoms quickly diffused out of the substrate an...

Solved: The Mystery of the Nanoscale Crop Circles - Berkeley Lab