Intrinsic linear optical properties close to the fundamental absorption edge

Abstract

In this chapter, we review the intrinsic linear optical properties of ZnO close to the fundamental absorption edge. This comprises band-to-band transitions and free excitons and polaritons in bulk samples and epitaxial layers; free and localized excitons and polaritons in quantum wells and wires, including nanorods; also localized excitons in alloys and in quantum dots (or nano crystals) and finally cavity polaritons. By the term “free excitons”, we mean the quanta of the intrinsic electronic excitation in semiconductors (and insulators), which can move freely through the sample and which are described by a plane wave factor exp(i Kr) in d dimensions (d = 3, 2 or 1), where K is the wave vector of the centre of mass motion described by r, multiplied by the envelope function of the relative (hydrogen-like) motion of electron and hole around their common centre of gravity. By the terms “bound exciton complexes” or “bound excitons” [(BEC) and (BE), respectively], we understand excitons that are bound to some centres like neutral or ionized donors or neutral acceptors but also to more complex centres. They will be treated in Chap. 7. In contrast, by the term “localized excitons”, we mean electron–hole pairs, which are localized by disorder like intrinsic alloy disorder, for example, in Mg1−xZnxO and/or fluctuations of well (or wire) width in quantum structures. These phenomena are inherent to alloys and to structures of reduced dimensionality and are therefore included in this chapter. The influence of external fields on both free and bound excitons is then covered in Chap. 8.