Donor doping of Ga in ZnO varistor grain boundary
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The effect of Ga doping on the grain boundary properties of a ZnO varistor has been presented in this note. Within the range of doping level studied, it is shown that Ga acts as a donor at the grain boundary and behaves similar to that of Al. 1 The grain boundary doping behavior of both Ga and Al, when acting as donors, can therefore be represented by the same defect model.
In a recent article,1 a systematic approach to microstructural engineering of a polycrystalline semiconducting ceramic has been outlined in relation to the grain and grain boundary doping behaviors in a ZnO varistor. It is proposed that the microstructural engineering can be formalized into three hierarchical levels. In the first level, the primary objective is to identify the dopants that are grain specific, grain boundary specific, or both to a host polycrystalline semiconducting ceramic. In the second level, the important consideration is to determine whether the dopant, having been accepted by the grain or grain boundary, is acting as a donor, an acceptor, or both. In the third level, interactions between the dopants and the atomic defects at the grain and grain boundary lead to an alteration in local properties that are of major practical importance to users. Here again, the dopant can act as a donor, an acceptor, or both, depending on the size of the guest ion, crystal structure of the host lattice, and the relative valence state of the guest and the host ions. These ground rules for the microstructural engineering were hypothesized1"* by carefully examining the doping behaviors of Li, Al, and Na on the current-voltage (I-V) characteristics of a ZnO varistor, combined with the construction of atomic defect models1'4 for the grain and grain boundary for the specific dopant under investigation. In this paper, we further strengthen our hypothesis by citing the behavior of yet another dopant, namely Ga, on the ZnO varistor property, the data for which is again available from the literature.2'5 Since Ga has been previously found2 to behave similar to Al in the high current region (graincontrolled), it is only appropriate to inquire whether it also behaves similar to Al in the grain boundarycontrolled, low current region. Note that both Al and Ga belong top orbital elements and have corundum structure for the oxides. This paper examines the doping behavior of Ga at the grain boundary region.
To familiarize the reader with the effect of doping on the grain and grain boundary properties of a ZnO varistor, we briefly refer1 to the I-V curve of Fig. 1. In the ZnO varistor, since the "prebreakdown" current-voltage (I-V) characteristics are "grain boundary" controlled and the "upturn" I-V characteristics are "grain" controlled, it is argued that the effect of donor doping on the grain and grain boundary will be to shift the entire I-V curve to the right and that of acceptor doping to the left of the solid curve. This happens because donor doping causes a reduction in the impedances of both grain and grain boundary whereas an acceptor doping causes a
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