Static and Dynamic Conductivity of Inversion Electrons in Lateral Superlattices

Extended Abstract

Two-dimensional (2D) electron inversion layers at semiconductor heterojunction interfaces can be transformed into lateral superlattices via field effect. The fabrication of laterally periodic field-effect devices with submicrometer periodicities on GaAs-AlGaAs heterojunctions containing a high-mobility 2D electron system as well as metal-oxide-semicon-ductor (MOS) structures on Si and InSb is discussed (Kotthaus, 1987). The effective lateral width of the confining potential in such devices can approach the Fermi wavelength of the inversion electrons and be much smaller than their elastic or inelastic mean free path. Hence they are well suited to investigate confinement to quantum wires (Hansen et al., 1987) and quantum dots (Sikorski and Merkt, 1989). On GaAs-AlGaAs heterojunctions the electronic mean free paths can also be much larger than the lateral periodicity such that lateral superlattice effects become observable. Here, recent low-temperature studies of the static and high frequency conductivity of laterally periodic field-effect devices are summarized that demonstrate the importance of quantum confinement and the large tunability of electronic properties in these devices (Kotthaus, 1989; Kotthaus and Merkt, 1989; Merkt et al., 1989).

Novel magnetoresistance oscillations observed in GaAs-AlGaAs heterojunctions with a one-dimensional (ID) modulation of the inversion electron density (Weiss et al., 1989) are shown to manifest ID superlattice phenomena in a magnetic field applied perpendicularly to the sample plane (Winkler et al., 1989; Gerhardts et al., 1989; Beenakker, 1989). Magnetoresistance studies are also employed to analyse field-effect confinement to arrays of quantum wires via observation of magnetic depopulation of ID subbands (Alsmeier et al., 1988; Brinkop et al., 1988).

The dynamic conductivity of laterally periodic structures is studied with far-infrared transmission experiments. In arrays of quantum wires strong infrared resonances are observed with resonance frequencies determined by the joint action of classical depolarization and quantum confinement. GaAs-AlGaAs heterojunctions are found to be particularly useful to investigate the transition from 2D to 1D confinement and to exhibit infrared excitations with dominantly collective character (Hansen, 1988). On InSb the infrared resonances are more single-particle-like and essentially reflect lateral quantization (Alsmeier et al., 1988). Using 2D periodic field-electrode arrays one can also electrostatically create and tune quantum dot arrays. The characteristic infrared excitations of such dot arrays realized in MOS-devices on InSb (Sikorski and Merkt, 1989) and Si (Alsmeier et al., 1989) are discussed in dependence on electron density, confinement size, and magnetic field strength.