Microstructural engineering through donor and acceptor doping in the grain and grain boundary of a polycrystalline semic
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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.
I. INTRODUCTION The electronic properties of the polycrystalline semiconducting ceramics such as BaTiO3, SrTiO3, and ZnO are known to be greatly influenced by the defect chemistry of the ceramics. In these and related materials, there is a growing realization that the properties that are unique to these materials are indeed derived from the defect structures that prevail both in the grain and grain boundary of the microstructure. Thus understanding the defect chemistry of the entire microstructure is gaining credence with many researchers. Although a great deal of knowledge exists regarding the defect chemistry of the bulk grain, very little is presently known with a high degree of certainty about the defect chemistry of the grain boundary. Balluffi1 in a recent article states that "a vacancy in a grain boundary is essentially a defect in a defect and is, therefore, doubly difficult to isolate and study." While the direct observations of grain boundary defects remain elusive, there is mounting evidence that supports their existence2'3 as well as their importance in the art of designing, controlling, and optimizing the performance of many electronic devices such as barrier layer capacitors,4"6 PTC thermistors,4"6 and ZnO varistors.7 From these studies, it is now possible to isolate the grain boundary effects from grain effects in a semiconducting ceramic, and make important conclusions about the defect states that exist in these regions. Because of the familiarity of the present author 3280 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 12, Dec 1992 Downloaded: 11 Nov 2014
with the ZnO varistor technology, this paper will be restricted to the discussion of grain and grain boundary defect structures of the ZnO var
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