Introduction

Abstract

The purpose of this introduction is – after a few general words on ZnO – to inform the reader about the history of ZnO research, the contents of this book and the intentions of the authors. Zinc oxide (ZnO) is a II^b–VI compound semiconductor. This group comprises the binary compounds of Zn, Cd and Hg with O, S, Se, Te and their ternary and quaternary alloys. The band gaps of these compounds cover the whole band gap range from E _g ≈ 3. 94 eV for hexagonal ZnS down to semimetals (i.e., E _g = 0 eV) for most of the mercury compounds. ZnO itself is also a wide gap semiconductor with E _g ≈ 3. 436 eV at T = 0 K and (3. 37 ± 0. 01) eV at room temperature. For more details on the band structure, see Chaps. 4 and 6 or for a recent collection of data on ZnO, for example, [Rössler et al. (eds) Landolt-Börnstein, New Series, Group III, Vols. 17 B, 22, and 41B, 1999]. Like most of the compounds of groups IV, III–V, II^b–VI and I^b–VII, ZnO shows a tetrahedral coordination. In contrast to several other II^b–VI compounds, which occur both in the hexagonal wurtzite and the cubic zinc blende type structure such as ZnS, which gave the name to these two modifications, ZnO occurs almost exclusively in the wurtzite type structure. It has a relatively strong ionic binding (see Chap. 2). The exciton binding energy in ZnO is 60 meV [Thomas, J. Phys. Chem. Solids 15:86, 1960], the largest among the II^b–VI compounds, but by far not the largest for all semiconductors since, for example, CuCl and CuO have exciton binding energies around 190 and 150 meV, respectively. See, for example, [Rössler et al. (eds) Landolt-Börnstein, New Series, Group III, Vols. 17B, 22, and 41B, 1999; Thomas, J. Phys. Chem. Solids 15:86, 1960; Klingshirn and Haug, Phy. Rep. 70:315, 1981; Hönerlage et al., Phys. Rep. 124:161, 1985] and references therein. More details on excitons will be given in Chap. 6. ZnO has a density of about 5. 6 g ∕ cm³ corresponding to 4. 2 × 10²² ZnO molecules per cm³ [Hallwig and Mollwo, Verhandl. DPG (VI) 10, HL37, 1975]. ZnO occurs naturally under the name zinkit. Owing to the incorporation of impurity atoms such as Mn or Fe, zinkit looks usually yellow to red. Pure, synthetic ZnO is colourless and clear in agreement to the gap in the near UV. The growth of ZnO and ZnO-based nano-structures is treated in Chap. 3. ZnO is used by several 100,000 tons per year, for example, as additive to concrete or to the rubber of tires of cars. In smaller quantities, it is used in pharmaceutical industries, as an additive to human and animal food, as a material for sensors and for varistors or as transparent conducting oxide. For more details and aspects of present and forthcoming applications, see Chap. 13.