Evaluation of ZnO Varistors by Photoacoustic Spectroscopy

Abstract

ZnO varistors degraded under various conditions were evaluated by photoacoustic spectroscopy (PAS). The degradation where the grain boundary is damaged by DC bias stress is more than that by AC bias stress. PAS, however, reveals that the interior of the grains of the sample degraded by AC bias stress is much more damaged than that by the DC bias stress. The PA signal intensity at a wavelength of more than 500 nm increases and the dispersion of the spectrum decreases throughout the wide wavelength range considered with the degradation time. In particular, the decrease of the spectral dispersion below 500 nm is caused by the change of the electronic states at the interface, that is, the increase of the recombination center of the space charge. The annealing effect on the degradation of ZnO varistors was also studied. The PA spectrum of the sample annealed in N2 gas corresponds to those of the sample degraded by DC and AC bias stress for a long time. This suggests that the degradation of ZnO varistors is closely related to the release of oxygen from both ZnO grain interior and grain boundaries.

Evaluation of Co- and Pr-Doped Zinc Silicate Powders by Photoacoustic Spectroscopy

“…3(a), the large change in the PA signal intensity of ZSP6 indicates the degeneration of the ZSO lattice, which includes many nonradiative transition centers originating in defects. 7) The difference in the valence state of additives might cause the production of defects with non-radiative transitions in forbidden bands in host materials. Hence, we should be vary careful when we select activator agents, which can easily transform to the other valence state, in Zn 2 SiO 4 -based phosphors for desired luminescent properties.…”

“…To understand nonradiative transition processes, photoacoustic (PA) spectroscopy is a powerful tool compared to PL measurements. [7][8][9][10][11][12] PA spectroscopy with a microphone enables the detection of acoustic waves generated on a sample surface through heat propagation due to incident light absorption; thus we can obtain information regarding net light absorption. We have applied PA technique to study the inner state of Pr-ion in ZnO:Pr ceramic powders 13) and to monitor sintering processes on ZnO:Co powders, which have internal light scattering.…”

Photoacoustic Spectra of Heavily Co-Doped ZnO Powders

“…With this technique, for example, the band gap variation in lightly Co-doped ZnO can be estimated, 8) and the grain growth in the sintering process can also be monitored. 9) CoO is also used for ZnO varistors and very small amounts of Co-doped ZnO was also studied for use as varistors 10,11) and sintered semiconductors. 12) In this study, the PA techniques are applied to heavily Codoped ZnO powders.…”

Nonradiative Transition Processes Observed from Photoacoustic Spectra of ZnO Thin Films Fabricated by Pulsed Laser Deposition

“…Hence, the PA technique can allow sensitive and direct evaluation of nonradiative transition processes related to defects and optical absorption in materials, whereas PL and cathode luminescence can enable the detection of radiative transition processes. 19,20) It is well known that an a-axisoriented or a c-axis-oriented epitaxial ZnO film can be obtained on an R-sapphire or a C-sapphire substrate under appropriate growth conditions, that is, (11 2 20) ZnO/Rsapphire or (0001) ZnO/C-sapphire. The former is promising for polarization-sensitive optoelectronic devices operating in the UV region due to the unique crystalline orientation.…”