Colossal permittivity due to electron trapping behaviors at the edge of double Schottky barrier

Abstract

Achieving frequency- and temperature-independent colossal permittivity (CP) with low dielectric loss is a long-standing challenge for electronic materials, in which the basic issue is understanding the underlying relaxation mechanism. In this paper, taking CaCu3Ti4O12 ceramics as an example, CP was ascribed to electron-trapping behaviors at the edge of a double Schottky barrier (DSB). On the one hand, the widely reported origins of CP, i.e. Maxwell–Wagner relaxation and polaronic relaxation, were identified as two aspects of the same bulk conductivity. This caused the insights derived from the commonly employed impedance and admittance spectra to be revisited. On the other hand, hysteresis between CP and external voltages at low temperatures, which was caused by electron filling of interface states, was predicted and experimentally confirmed. This further supported the proposal that CP arose from electron trapping at the DSB. Moreover, multiple relaxations were foreseen when more than one kind of point defect existed in the depletion layers of a DSB. The establishment of intense ‘effective’ relaxation, which was related to shallow traps, was indispensable for achieving CP, while ‘redundant’ relaxation was induced by deep-level defects, resulting in relatively high dielectric loss. Therefore, proper manipulation of the DSB and its related defect structures was crucial for achieving stable CP with sufficiently low dielectric loss.

Evolution of Intrinsic and Extrinsic Electron Traps at Grain Boundary during Sintering ZnO Based Varistor Ceramics

“…They are equal to the trap depth of intrinsic point defects Zn i ¨ and V O ˙, respectively [ 24 ]. In consequence, the evolution of them could be possibly characterized via fitting the ε ″ spectra by using the Cole–Cole equation [ 23 , 24 , 35 , 36 ]: where k 0 represents the magnitude of DC conductivity while k i ( i = 1, 2, 3 ...) is the magnitude of permittivity contributed by the i th relaxation (∆ ε 0 ). τ i is the relaxation time and α i is the depression angle.…”

“…Relaxation D was reported to correlate with the electronic relaxation of interface states at GBs, whose activation energy is the corresponding trap depth [ 8 , 23 , 24 ]. The appearance of the relaxation E indicates the formation of two kinds of Schottky barriers during the cooling process.…”

“…They are equal to the trap depth of intrinsic point defects Zni¨ and VO˙, respectively [24]. In consequence, the evolution of them could be possibly characterized via fitting the ε″ spectra by using the Cole-Cole equation [23,24,35,36]:…”

“…The functional DSBs are attached to the morphology of ZnO varistor ceramics, whose electrical responses rely on the electron transport across DSBs. It is commonly accepted that interface states, which originated from absorbed oxygen, zinc vacancy, etc., are formed during the cooling process [ 8 , 23 ]. As a result, depletion layers are formed because free electrons nearby the surface of grains are exhausted by those negatively charged interface states, leaving only positively charged immoveable cations.…”

“…Intrinsic point defects like zinc interstitials and oxygen vacancies are the major ions in the depletion layers [ 24 , 25 ]. Either the short-term electrical properties or long-term degradation is inevitably modulated by electron trapping behaviors of interface states and intrinsic point defects in ZnO varistor ceramics [ 10 , 23 , 26 , 27 , 28 ]. Generally, a high barrier height is related with a high donor density in depletion layers.…”