SELF-ORGANIZATION OF STEPS IN GROWTH OF STRAINED FILMS ON VICINAL SUBSTRATES

The continued miniaturization of electronic devices is leading us into a realm of nanostructures, which exhibit novel electronic and optical properties, and have a wide range of potential applications such as single-electron transistors, quantum-dot lasers, and quantum computing. Quantum well devices are already widespread. Recent effort has been devoted to fabricating quantum wires and quantum dots, respectively, oneand zero-dimensional analogs to the quantum well. Just as uniform thickness is a prerequisite for a film in a quantum well device, uniform size and spacing are prerequisites for applications of wires and dots. Different routes to fabricating wires and dots are possible. One can perform direct substrate patterning or lithography [1], or wirelike and dotlike structures can be induced to “self-assemble” during the growth of thin films. These self-assembled structures sometimes exhibit surprising uniformity [2–5], suggesting that their application in actual devices is a real possibility. An approach that is, in a sense, a hybrid between deliberate nanopatterning and self-assembly is the use of vicinal surfaces as templates for the growth of wires and dots. Such surfaces are cut at a small angle to the atomic planes, creating a staircase of atomic-height steps that can serve as preferential sites for the growth of wires [6,7] and the nucleation of clusters [8,9]. But single steps tend to meander [10], and their spacing is often irregular [7]. These drawbacks have caused difficulties in previous efforts to use steps to grow semiconductor and metallic wires [6,7] or vertical superlattices [6]. Step bunches, on the other hand, tend to be much straighter than individual steps, due to their greater “stiffness” [10]. They can also have different heights depending on the number of steps in the bunches, allowing the possibility of different thicknesses for quantum wires, or direct control of the size of quantum dots that nucleate on the step bunches [11]. Recently it was shown that the steps on a vicinal surface can be induced to bunch by introducing stress, through the deposition of a thin layer of a lattice mismatched film (e.g., a SiGe alloy on Si) [12,13]. The problem is to control the bunching, in order to achieve a useful template structure.