MICROSTRUCTURAL ENGINEERING THROUGH DONOR AND ACCEPTOR DOPING IN THE GRAIN AND GRAIN-BOUNDARY OF A POLYCRYSTALLINE SEMICONDUCTING CERAMIC

This paper deals with the concept of microstructural engineering through donor and acceptor dopings within the grain and at the grain boundary of a polycrystalline semiconducting ceramic. These concepts are derived from an analysis of the "prebreakdown" and the "upturn" current-voltage characteristics of a ZnO varistor and from the construction of corresponding defect models as a function of donor and acceptor dopants at the grain and grain boundary. By using Li, Al, and Na as dopants, it is shown that the dopants can be grain or grain boundary specific in the ZnO microstructure and that they can act as donors, acceptors, or both, depending on the nature and concentration of dopants and their location on the host crystal lattice structure. In the case of the ZnO varistor, the grain and grain boundary properties can thus be tuned independently or concurrently by systematic engineering of the entire microstructure through defect dopings that are specific to the grain, grain boundary, or both. Following a detailed analysis of the defect models thus developed for the ZnO varistor, a set of ground rules are proposed for applying these concepts of donor and acceptor dopings at the grain and grain boundary to the general case of microstructural engineering in a polycrystalline semiconducting ceramic.