To make thin films for semiconductors in electronic devices, layers of atoms must be grown in neat, crystalline sheets. But while some materials grow smooth crystals, others tend to develop bumps and defects a serious problem for thin-film manufacturing.
In the online edition of the journal Science (Jan. 22, 2010), Cornell researchers shed new light on how atoms arrange themselves into thin films. Led by assistant professor of physics Itai Cohen, they recreated conditions of layer-by-layer crystalline growth using particles much bigger than atoms, but still small enough that they behave like atoms.
"These particles are big and slow enough that you can see what's going on in real time," explained graduate student Mark Buckley. Using an optical microscope, the scientists could watch exactly what their "atoms" actually, micron-sized silica particles suspended in fluid did as they crystallized. What's more, they were able to manipulate single particles one at a time and test conditions that lead to smooth crystal growth. In doing so, they discovered that the random darting motion of the particles is a key factor that affects how the crystals grow.
A major challenge to growing thin films with atoms is that the atoms often form mounds, rather than crystallizing into thin sheets. This happens because as atoms are deposited onto a substrate, they initially form small crystals, called islands. When more atoms are dumped on top of these crystals, the atoms tend to stay atop the islands, rather than hopping off the edges - as though there were a barrier on the crystals' edges. This creates the pesky rough spots, "and it's game over" for a perfect thin film, Cohen said.
Conventional theory says that atoms that land on top of islands feel an energetic "pull" from other atoms that keeps them from rolling off. In their colloidal system, the researchers eliminated this pull by shortening the bonds between their particles. B
|Contact: Blaine Friedlander|