Wide range dielectric spectroscopy of ZnO-based varistors as a function of sintering time

1. Plasma Application LAB http://plasma.cbnu.ac.kr Wide range dielectric spectroscopy of ZnO-based varistors as a function of sintering time 발표자 : 권득철

2. Plasma Application LAB http://plasma.cbnu.ac.kr Abstract Broadband electrical response analysis and charge transport theory through double Schottky barriers in ceramic semiconductors, are both used in order to separately study the grain boundary, depletion layer, and bulk grain regions of ZnO-based varistor samples sintered at 1180 °C for 0 h (no significant time at the sintering temperature), 2, 4, and 8 h. It is found that increased sintering times: (1) do not sensitively affect the bulk grain region; (2) broaden and flatten the space-charge-related dielectric loss term; and (3) make disappear a particular interface trap, deep below the equilibrium Fermi level, hence modifying the grain boundary density of states.

3. Plasma Application LAB http://plasma.cbnu.ac.kr Introduction The presence of a liquid Bi2O3 phase is known to enhance densification and grain growth, and there exists empirical evidence about the prominent role of Bi2O3 in enhancing low-field resistivity and non-linearity. However, very little is known about the actual impact of bismuth on the double Schottky barrier (DSB) electronic structure that describes the varistor electrical response.

4. ZnO n-type Bi2O3 p-type ZnO n-type Plasma Application LAB http://plasma.cbnu.ac.kr ZnO structure Fig 2. Ideal artificial lattice structure of ZnO varistor

5. cb cb Ef Ef vb vb p-type n-type after contact cb Ef vb p-type n-type Plasma Application LAB http://plasma.cbnu.ac.kr ZnO IGL ZnO Bottom of Conduction band Fermi level Filled IGL acceptor states Top of valence band No voltage applied Forward Reverse Thermionic activation Tunnelling Voltage applied

6. Plasma Application LAB http://plasma.cbnu.ac.kr Bismuth is known to segregate to the grain boundaries (GBs), where it is believed to contribute to the interface density of states that originates the DSB, but the extent to which segregated bismuth supplies interface electronic traps remains unclear. Several papers have recently studied the process of Bi2O3 loss during sintering, and it has been shown by Metz et al. that final bismuth content is different from initial bismuth content to a degree that depends upon sintering temperature and time. This Bi-loss process leads to very different electrical responses within materials with similar density, grain size distribution, and secondary phase distribution.

7. Plasma Application LAB http://plasma.cbnu.ac.kr Experimental procedure Discs were uniaxially pressed at 80 Mpa, and then sintered at 1180 °C for 0 h (the pellets were taken up to 1180 °C and then cooled without being held at that temperature for any significant time), 2, 4, and 8 h. Results

8. Plasma Application LAB http://plasma.cbnu.ac.kr Grain size distribution Fig. 2 reveals a close similarity between the grain size distributions of the samples sintered for 2, 4, and 8 h, while the 0 h distribution is markedly narrower.

9. Plasma Application LAB http://plasma.cbnu.ac.kr Frequency evolution of the loss term Re[Y()]/  Fig. 3 shows that the loss term broadens and flattens for the 0 and 8 h samples, with respect to that of the 2 h sample. The dielectric behavior of the 0 and 8 h samples approaches the constant-loss property, characteristic of the dielectric response in highly disordered systems.

10. Plasma Application LAB http://plasma.cbnu.ac.kr Frequency evolution of the conductance Re[Y()] Fig. 4 shows the real part of the admittance under zero-bias for the 0, 2, 4 and 8 h samples.

11. Plasma Application LAB http://plasma.cbnu.ac.kr Bias evolution of the dc conductance

12. Plasma Application LAB http://plasma.cbnu.ac.kr Conclusion We have found that increased sintering time: (1) induces no change on the bulk free carrier density, indicating that defect equilibrium within this region is mainly controlled by the peak sintering temperature (2) broadens the dielectric response of the depletion layer, approaching the constant-loss property that characterizes highly disordered electronic structures (3) makes disappear a deep, essentially monoenergetic, interface trap, which can be associated with the presence of segregated bismuth at the grain boundaries, hence indicating that changes in the grain boundary electronic structure are consistent with Bi-loss during sintering.