Field-Effect Controlled Surface Superlattices

Abstract

Artificial semiconductor superlattices, initially proposed by Esaki and Tsu (1970), are now an essential part of modern semiconductor physics and technology. Usually such superlattices consist of alternating layers of semiconductor material with different composition or dopant concentration and are grown epitaxially. With increasing refinement of lithographic tools it has become possible in recent years to also create artificial surface superlattices in which the bandstructure, close to the surface of a semiconductor, is periodically modulated in the surface plane. Such surface superlattices have been proposed more than a decade ago by Sakaki et al.. (1976), Bate (1977), and Stiles (1978). In most cases they make use of the advantageous electronic properties of two-dimensional electron systems (2DES) confined at a semiconductor heterojunction or a semiconductor-oxide interface such as high mobility and easy tunability of carrier density. Presently the lateral modulation of the band structure is commonly achieved by a periodic potential modulation which modulates the carrier density of the electron system but essentially preserves the uniformity of the bandgap in the surface plane. The effect of the potential modulation on the conduction band electrons is indicated in Fig. 1 for two cases, namely weak potential modulation resulting in a relatively small periodic modulation of the electron density in a 2DES (Fig. la) and strong potential modulation resulting in the formation of isolated quasi-one-dimensional (ID) quantum wires or quasi-zero-dimensional (OD) quantum dots (Fig. 1b).