Detection and Characterization of Trap Levels in Semiconducting Ceramics

利用恒温电容暂态谱(ICTS)研究了氧化锌压敏电阻的退化现象，特别是陷阱能级。通过对退化前后ICTS信号的分析，发现界面陷阱能级向导带方向移动，能量分布变宽。这种退化现象与晶界的界面态有关。BaTiO_3中的PTCR效应是一种晶界效应，它是由二维受主层态吸引电子从晶界内部产生的。受主态密度和导带与受主态能级之间的能隙Es决定了PTCR的性质。在高PO2(10 Mpa)下退火的样品比在低PO2(0.1 Mpa)下退火的样品具有更高的ES，从而改善了PTCR的性能。基于这一发现和文献结果，提出了受主-施主-共掺杂BaTiO_3的唯象PTCR模型和相应的PTCR图。PTCR图表明，可以同时优化受主掺杂浓度、氧化时间和氧化或冷却过程中的氧分压，以获得最佳的PTCR陶瓷。

The degradation phenomenon in a ZnO varistor was investigated by means of isothermal capacitance transient spectroscopy (ICTS), paying special attention to the trap levels. From the analysis of ICTS signals before and after degradation, the interface trap levels were found to shift toward the conduction band, and their energetical distribution was broadened. The degradation phenomenon was related to the interface states of the grain boundaries.The PTCR effect in BaTiO_3 is a grain boundary effect caused by a two dimensional layr of acceptor states which attract electrons from the grain interior. The acceptor state density and the energy gap, Es, between the conduction band and the energy level of the acceptor states govern the PTCR-properties. The samples annealed at high Po_2 (10 MPa) had a higher Es than the samples annealed at lower Po_2 (0.1 MPa), leading to improved PTCR-properties. Based on this finding and results from the literature, a phenomenological PTCR model and an accompanying PTCR chart for acceptor-donor-codoped BaTiO_3 were proposed. The PTCR chart clarifies that acceptor dopant concentrations, oxidation time, and oxygen partial pressure during oxidation or cooling can be optimized simultaneously to obtain the best PTCR ceramics.