Crystal Growth Points to Defect-Free Thin Films
Cornell University researchers said colloidal research may lead to the discovery of principles that will enable the growth of defect-free thin films. The team led by Professor Itai Cohen tested conditions that lead to smooth crystal growth, and discovered that the random darting motion of the particles is a key factor affecting how crystals grow.
Alexander E. Braun, Senior Editor -- Semiconductor International, 2/1/2010
Cornell University (Ithaca, N.Y.) scientists are studying the challenges that arise from the fact that to make semiconductor thin films, layers of atoms must be grown in neat, crystalline sheets. While some materials usually grow smooth crystals, others develop bumps and defects, leading researchers to seek insights into how atoms arrange themselves into thin films.
Physics Professor Itai Cohen, who leads the research, indicated that the effort resulted from his colloidal suspension work. "Colloids are a good model system for investigating atomic-scale phenomena," he said. "The particles are small enough to behave like atoms but still large enough to be observed under a microscope and manipulated with optical tweezers. They're ideal because you set up the experiment and nature does the calculations."
Using an optical microscope, the Cornell scientists see exactly what these atom-mimicking particles — micron-sized silica particles suspended in fluid — do as they crystallize. By manipulating them individually they tested conditions that lead to smooth crystal growth, and discovered that the random darting motion of the particles is a key factor affecting how crystals grow.
Cornell researchers created a video of how micron-sized silica particles, suspended in fluid, move around as they crystallize. (Source: Cornell University) |
"A major challenge to growing thin films is that atoms often form mounds instead of crystallizing into thin sheets," Cohen said. "As atoms deposit onto a substrate, they initially form small crystals, called islands. When more atoms are dumped on top, they tend to stay atop the islands, rather than hopping off the edges, as if there were a barrier along the crystals' edges." The result is rough spots and imperfect thin films.
Cornell University researchers studied colloidal crystal freezing onto a square lattice. (Source: Cornell)
Conventional theory states that atoms landing on top of islands get an energetic "pull" from other atoms, keeping them from rolling off. In the colloidal simulation, the pull was eliminated by reducing particle bonds. However, the particles still hesitated at the islands' edges. Further analysis allowed researchers to measure how long it took for particles to move off the crystal islands. Because the particles were suspended in a fluid, they were knocked about by Brownian motion, replicating a random walk.
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There is nothing at all new here. Step edge barriers have been known and understood since the 1960s. This is a text book concept and step barrier heights have been measured for many metals, semiconductors, and ceramics.
Joe Greene - 3/3/2010 3:30:45 PM CST -
This isn't a new principle: so-called surfactant assisted growth of Ge, Si, and various metal systems has been explained for years via the step-edge barrier. And observed via an AFM! Camarero et al in the late 90s on metal systems, and a whole variety of authors on Sb assisted SiGe growth (Pianetta, Heen come to mind).
This concept has been in use commercially in the data storage industry for over a decade.
This is a neat experiment, but I really don't see anything new for thin film growth here. simlations of step edge efects are in every introductory thin film growth book (Venables, Barabasi, and many others)
regards,
Brennan Peterson - 2/2/2010 5:10:32 PM CST -
I see only limited value in this line of research. The typical epitaxial process uses molecular species instead of atomic. Thus, one has the interaction of the competitive processes of adsorption, surface migration, desorption and chemical dissociation. Also, it has been known for some time that the island edges are more energetic, in that the islands tend to grow from the edges. Film smoothing is accomplished by balancing the above processes through process variable control.
Michael Barger - 2/2/2010 1:21:45 PM CST
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