氧化鋅變阻器之缺陷研究

Abstract

本研究利用深能階暫態量測系統研究分析氧化鋅變阻器之缺陷。在本研究 中，量測到三個多數載子缺陷分別定義為L1、L2、L3。L1與L2之活化能分 別是0.12 eV及0.22 eV，在不同添加物系統中缺陷皆穩定存在，根據深能 階暫態量測與電腦模擬可推論其缺陷模式屬於本質單能階塊材缺陷，L1對 電性影響不大，而L2可能和氧空缺有關。L3之活化能介於0.75-0.95 eV之 間，缺陷能階受燒結溫度及添加劑之影響，在以鐠為主添加物的氧化鋅系 統中L3呈單能階分佈，而在玻璃添加物系統中呈能帶分佈，推測其應屬於 結構複雜且與添加物有關的異質介面缺陷，而且經本研究發現氧有助於L3 的形成。L3在劣化之後缺陷濃度有明顯變化，所以L3應該與劣化反應有密 切之關連，而蕭基能障高在劣化後亦明顯下降，推論L3缺陷濃度的降低為 導致蕭基能障下降的主要原因之一。劣化的發生據推論可能由於離子的遷 移致介面處與介面的氧離子發生反應導致介面氧離子的數目減少，因此而 使得L3的濃度降低並造成蕭基能障的下降。 Three deep levels are detected in bulk ZnO varistors by means of deep leveltransient spectroscopy (DLTS). Two electron traps, L1 and L2, located at 0.12 and 0.22 eV below the conduction band are believed to be native defects. Another electron trap L3 lying at 0.85 eV below the conductionband is related to impurities. The results of computer simulation suggestthat L1, L2, and L3 in Pr-doped ZnO varistors are simple single- energylevels. Trap L3 in Glass-doped ZnO varistors are distributed as a befect band because of the complexity of additives exist in Glass-ZnO samples.According to zero bias DLTS, trap L1 and L2 are bulk defects, while L3 exists at the grain boundary interface. DLTS results suggest that trap L1does not affect device properties and the first ionized state of oxygenvacancy may be the origin of trap L2. L3 is believed to be complex defects composed of chemisorbed oxygen at the grain boundaries and additives such as Bi, Pr, Pb, Si or B. The degradation of non-Ohmic electrical characteristics of ZnO varistors is observed by applying high-intensityimpulse currents. The potential barrier height of ZnO and the density ofinterface states decrease after current impulse test. Interface state L3 is strongly correlated to the formation of Schottky barriers and to theelectrical degradation in the varistors. Barrier deformation after current impulse is owing to the reduction of oxygen ions and L3 trap densities atthe grain boundaries.