Conventional theory says when films are being formed at the atomic scale, atoms land on top of each other and form mounds or "islands" and feel an energetic "pull" from other atoms that prevents them from hopping off the island's edges and crystallizing into smooth sheets. The result is rough spots on the thin films used to produce semiconductors. Cornell University-led researchers eliminated this pull by shortening the bonds between their particles. But they still saw particles hesitate at the island's edges.

In this image, green particles are the ones that encounter a step edge or corner barrier. The orange particle encounters smaller barriers as it moves from site to site. The #1 indicates the bond being broken. The #2 indicates the bond that is forming. Near a step edge or corner the atoms do not have a new neighbor to form a bond with (so no #2 particle). This is what sets up the barrier.

Credit: Rajesh Ganapathy, Sharon Gerbode, Mark Buckley, and Itai Cohen

Using a solution of tiny plastic spheres 50 times smaller than a human hair, scientists at Cornell University discovered the thin, smooth crystalline sheets needed to make semiconductors can be grown more smoothly by managing the random darting motions of the atomic particles that affect how the crystals grow. Researchers reproduced the conditions that lead to crystallization on the atomic scale by using particles much bigger than atoms, but still small enough that they behave like atoms to watch how particles crystallize. Additionally, with special laser beams known as "optical tweezers," researchers placed an individual particle (atom) on top of a growing crystal island and determined how easy it was for the particle to hop off that island. They found the random darting motions of a particle are a key factor that determines how long it spends on the island. When particles can hop off islands more easily, smooth crystals can be grown. Here a colloidal crystal freezes onto a square lattice template. The video is sped up by a factor of about 20.

Credit: John Savage, Rajesh Ganapathy, and Itai Cohen

The time an atom spends on top of a mound or "island" often determines whether a rough spot will form during a thin film's crystallization process. Rough spots, bumps and defects are a serious problem for thin-film manufacturing. This image shows how long on average a particle remains in the same location. The bars correspond to how long (on average) a particle placed on top of this triangular island resided at each lattice location.

Credit: Rajesh Ganapathy, Sharon Gerbode, Mark Buckley, and Itai Cohen

The researchers' finding appears in the January 22, 2010, online issue of the journal Science.

Credit: Copyright 2010 AAAS