Introduction

Abstract

This book deals with charge transport in semiconductor structures whose spatial dimensions are on a nanometre scale. With the advent of modern semiconductor growth technologies such as molecular beam epitaxy (MBE) or metal-organic chemical vapour deposition (MOCVD), artificial structures composed of different materials with layer widths of only a few nanometres have been grown and additional lateral patterning by electron-beam lithography or other lithographic or etching techniques (ion beam, x-ray, scanning probe microscopies) can impose lateral dimensions of quantum confinement in the 10 nm regime. Thus, it has become possible to design and fabricate semiconductor structures whose vertical and lateral dimensions are controlled on an atomic length scale. This has given us the unprecedented capability to tailor devices with extraordinary electrical and optical properties. If the geometrical dimensions of the semicon-dutur structures reach the order of the characteristic physical length scales of transport which will be discussed below, the transport and optical properties are no longer determined by the bare material constants but will heavily depend upon the size and geometry of the device. This has opened up a vast field of research activity which is, on one hand, of fundamental interest since it pushes the border of physically accessible phenomena in semiconductors to the ultimate quantum limit, but which is also, on the other hand, of utmost importance with respect to applications since the miniaturization and ultralarge-scale integration of electronic components is still in progress. For example, channel lengths of field-effect transistors (FETs) of 100 nm are nowadays standard in mass-production Megabit chips, and memory chips with 10⁹ transistors on a single chip are anticipated in a few years. While these are based on silicon technology, the class of GaAs-AlGaAs materials is essential for high-speed electronic and optoelectronic nanometre devices such as for example the high electron mobility transistor (HEMT), the modulation doped field effect transistor (MODFET) and injection laser diodes. Even single-electron tunnelling has been realized in GaAs nanostructures. A whole new class of promising optoelectronic devices are quantum dot lasers based on InAs or InGaAs grown on strained GaAs substrates. Under appropriate growth conditions the InAs or InGaAs spontaneously forms self-organized regular arrays of islands (‘quantum dots’) with a diameter on the nanometre scale, leading to highly efficient laser emission at low threshold currents with high temperature stability. Those few examples may serve to demonstrate the applicative and technological relevance of the subject.