The Structural Properties of Sputtered Zinc Oxide

Abstract

ABSTRACTX-ray diffraction, chemical etching and electron microscopy are useful in delineating the microstructural properties of c-axis normal oriented polycrystalline zinc oxide films, sputter deposited on amorphous substrates. Dense ZnO films with high crystalline content and low etch rates have a strong piezoelectric effect and low surface acoustic wave and guided optical wave propagation loss. The structural properties are dependent upon substrate conditions, deposition parameters and the type of sputtering system used. Scanning electron microscope examination of the surface and side fractures of high quality ZnO films show smooth continuous upper surface and side regions with no vertical fiber grain structures apparent. Light chemical etching reveals surface defects consisting of uniformly distributed conical shaped etch pit structures with a lateral to vertical etch ratio of 3 to 1. A correlation has been made between film structure and single crystal zinc oxide. The structural picture which emerges is that of a predominantly positive polarity (0001) upper surface layer with localized negative polarity (0001) fiber grain inversions. The sputter conditions which lead to low defect density films are a high substrate temperature, high deposition rate and low reactive gas pressures to promote the adatom mobility necessary for the preferred (0001) zinc face growth. Post deposition laser and thermal annealing can be used to reduce stress in the films and improve their guided optical wave properties.