Next-Nearest Neighbor Interactions Matter in the Nanoscale World

When physicists form atomic surfaces using the technologically important semiconductor material gallium arsenide (GaAs), the surfaces appear in one of two kinds of states: flat or rough. But a team of physicists has shown experimentally that there's a third type of surface, which was predicted by a theoretical physicist--and that this state depends upon the interactions of distant neighbors, atomically speaking. This knowledge may help researchers better control the growth of nanoscale devices such as quantum dots and quantum wires for use in telecommunications devices and high-speed computers.

Physicists Zhao Ding, Dan Bullock and Paul Thibado of the University of Arkansas, Vincent LaBella of the University at Albany-SUNY and Kieran Mullen of the University of Oklahoma will present their findings in an upcoming issue of Physical Review Letters.

The researchers studied how gallium arsenide responds to changes in temperature and arsenic pressure. Previous studies have shown that, when heated, a GaAs surface will go from a flat surface to a disordered or rough surface with the formation of atomic islands. Theorists have predicted and explained the roughening with the nearest-neighbor interaction model: As the surface is heated, the energy required to keep an atom next to its neighbor becomes too high and the atoms detach, with one piling atop another.

Ding and his colleagues used scanning tunneling microscopy to examine the GaAs surface at different temperatures and pressures. They found that the surface goes through an in-between stage, where the disorder does not appear at the macroscopic level but can be seen as a series of up-down steps, or two-dimensional islands. The researchers call this state a "disordered flat" surface. Their finding gives an experimental proof to a theory postulated in the 1980s, which states that atoms also experience influence from other atoms they don't touch, in other words, from next-nearest neighbors.

In this case, an elevated temperature causes enough of an energy boost to loose some atoms from their moorings and create slight disorder, but atoms further away continue to keep the "loose" atoms from piling up on one another to form islands. The researchers also found that changing the pressure of arsenic within the system could drive the smooth to rough transitions.

"Usually people only think about the nearest neighbor interactions," said Ding. "But we need to consider more thoroughly the next-nearest neighbor interactions." Knowing that gallium arsenide exhibits this intermediate surface may help researchers build and better control the growth of quantum dots and quantum wires for use in technological applications.