Shrinking shapes give hope for silicon

17 Jun 2002

Cornell researchers have found a way of producing smaller silicon nanostructures that could lead the way to silicon LEDs.

Light-emitting silicon displays could be one of the outcomes of research underway at Cornell University in the US. Stephen Sass, professor of materials science and engineering, and Melissa Hines, associate professor of chemistry and chemical biology, have found a completely new way of fabricating silicon nanostructures using a process called controlled etching of dislocations (CED).

Unlike optical lithography - which uses a method akin to photographic reduction for etching devices and cannot produce features that are smaller than 150 nm in width - CED is not limited by the wavelength of light. The technique involves placing together two identical silicon wafers and twisting one against the other at a precisely controlled angle so that some of the atomic rows do not line up correctly. The wafers are then bonded together to form a bicrystal. Where the atoms are poorly aligned, and therefore have not formed strong bonds, chemical etching can be used to break the weak bonds between them, leaving an array of minute bumps that the researchers have called "nanobumps". (See http://www.chem.cornell.edu/mah11/Nanofab.html for an explanation.) Sass and Hines used a solution of chromium trioxide and hydrofluoric acid to fabricate a test structure with nanobumps that were approximately 25 nm in diameter and 38 nm apart.

The size of the bumps that are left and the space between them can be controlled by altering the angular displacement between the wafers. The scientists calculate that for a silicon bicrystal with a twist angle of 10 degrees, the distance between nanobumps would be just 2 nm. "We don't really know what the limit of the technique is," said Sass.

The method could enable manufacturing at biological dimensions and also the production of light-emitting silicon devices, because it would allow the construction of a dislocation structure for creating the local electric fields that trap electrons and positive holes, in order for them then to recombine and emit radiation. "That might mean that you could make a flat-panel display for a computer out of the same stuff that you use for the computer itself," said Hines.