Focus: Surprising Atomic Layers
Using evaporation techniques, a layer of metal only a few atoms thick can be deposited on an ultrasmooth semiconducting surface, a process which is important for many applications. In the last few years, researchers have shown that unexpected electronic properties of the thin metal layer are explained by the confinement of the electrons to that narrow space, which creates electronic states akin to those of the quantum mechanical particle-in-a-box problem. In the 20 April PRL, they predict another surprising phenomenon resulting from electronic confinement: As a film of antimony (Sb) grows on gallium arsenide (GaAs), the layer should alternate between metallic and nonmetallic properties as each of the first several atomic layers is deposited.
When the first few “islands” of atoms are deposited on a surface, their electrons are localized within each cluster and unable to travel long distances, one of the hallmarks of a metal. So the film begins as a nonmetal and gradually becomes more metallic with the deposition of the first few layers of atoms, according to the conventional view. Recent work by Zhenyu Zhang of Oak Ridge National Laboratory (ORNL) in Oak Ridge, TN, and his collaborators, has shown that the confinement of electrons in the vertical direction can drastically affect the film’s properties as it grows. Zhang has termed this field “electronic growth,” to distinguish it from studies of the atomic stacking.
For their PRL paper, Jun-Hyung Cho of ORNL, Qian Niu of the University of Texas at Austin, and Zhang calculated the atomic positions and the full set of quantum mechanical states of the electrons in layers of Sb deposited on GaAs. They found that the energy change associated with each deposited Sb atom oscillates from low to high values with each atomic layer, for the first seven layers, and that the distances between adjacent layers follow a similar pattern. But the most dramatic oscillating property they found was that the film is nonmetallic with one or three monolayers of Sb atoms but metallic with two, four, or more layers.
To understand the oscillating behavior, imagine the evenly-spaced energy levels available for an electron in a box, or “quantum well,” in one dimension. In the simplest picture, each additional layer of atoms adds another free electron to the box, and two electrons fill each energy level. The highest occupied level is alternately filled (nonmetal) and half-filled (metal) with each added atomic layer, but the box also becomes progressively wider as the film thickens, which reduces the spacing between allowed energies. In the real system, each level actually has a finite spread of energies, or “band” available to its electrons, rather than a single energy, and when the inter-level spacing becomes small enough, the energy bands overlap. At that point, the system remains metallic with all additional atomic layers. The oscillating properties might be expected for any element that adds the equivalent of one free electron for each atomic layer, and indeed the authors expect other metals in the same family as Sb to show similar effects.
“It’s an elegant study of an important problem,” says John Weaver, of the University of Minnesota in Minneapolis. He says Zhang and his colleagues have convincingly demonstrated in a number of ways that the confinement of electrons is an essential element of any detailed theory of the growth of thin metallic films.