Fundamental insight into the formation of the zinc oxide crystal structure

Zinc oxide shows unique properties which are evident in a wide range of applications: as a transparent conducting oxide, wide-bandgap semiconductor, and piezoelectric device. The starting point for understanding the origin of these properties are the subtle details of the crystal structure of ZnO, and thus elucidating its formation process is essential. The in-situ characterization of films deposited at various temperatures provides an important contribution in this regard, especially since this study reports on ZnO film deposition below room temperature down to -240 °C. Systematic investigations on nanocrystalline ZnO films as function of the deposition temperature reveal structural disorders caused by the irregular occupation of oxygen tetrahedra forming dioxygen species in zinc oxide. Three distinct material ranges are identified in the range of deposition temperatures between -240 and 300 °C. The most surprising observations are the segregation of zinc next to ZnO particles for films deposited at room temperature and the disappearance of the Raman bands of the ZnO lattice for those deposited above 100 °C. On both ends of the investigated deposition temperature scale transparent colorless films are obtained, which form a random frozen solid at low temperatures as well as a highly disordered film at high temperatures. The deposits at -80 °C are yellow in color, indicating the presence of superoxide ions. This wide variety in the properties of ZnO is enabled by the high flexibility of the wurtzite structure, which tolerates huge distance variations. This observation and the results presented open up important insights into the behavior of zinc oxide.