Contrasting conduction mechanisms of two internal barrier layer capacitors: (Mn, Nb)-doped SrTiO₃ and CaCu₃Ti₄O₁₂

The d.c. conduction is investigated in the two different types of internal barrier layer capacitors, namely, (Mn, Nb)-doped SrTiO₃ (STO) and CaCu₃Ti₄O₁₂ (CCTO). Scanning electron microscopy (SEM) and Capacitance - Voltage (C-V) analysis are performed to estimate the effective electric field at a grain boundary, E_GB. Then, the d.c. conduction mechanism is discussed based on the J (Current density)-E_GB characteristics. Three different conduction mechanisms are successively observed with the increase of E_GB in both systems. In (Mn, Nb)-doped STO, non-linear J-E_GB characteristics is temperature dependent at the intermediate E_GB and becomes relatively insensitive to the temperature at the higher E_GB. The J- E_GB at each regime is explained by the Schottky emission (SE) followed by Fowler-Nordheim (F-N) tunneling. Based on the F-N tunneling, the breakdown voltage is then scaled by the function of the depletion layer thickness and Schottky barrier height at the average grain boundary. The proposed function shows a clear linear relationship with the breakdown. On the other hand, F-N tunneling was not observed in CCTO in our measurement. Ohmic, Poole-Frenkel (P-F), and SE are successively observed in CCTO. The transition point from P-F and SE depends on E_GB and temperature. A charge-based deep level transient spectroscopy study reveals that 3 types of trap states exist in CCTO. The trap one with Et ∼ 0.65 eV below the conduction band is found to be responsible for the P-F conduction.