Investigation of Resistive Switching phenomenon in acceptor-doped oxides: Ca-doped BiFeO3 and Y-doped CeO2

Abstract

The effect of application of a small voltage, above the theoretical decomposition voltage, to inorganic materials has become increasingly important in the development of two new emerging areas, resistive switching of memristors and flash sintering of ceramics. Recently, many examples, in bulk, of acceptor-doped fluorite- and perovskite-structured materials show increasing p-type conductivity with increasing voltage. Among these materials, perovskite-structured Ca-doped BiFeO3 showed a dramatic increase in conductivity on application of a small dc bias, and was reversible on removal of the bias. It was the first example of low field resistive switching, OFF-ON, of bulk ceramics. Fluorite-structured, Y-doped HfO2 is the second material to show low field resistive switching in bulk. Oxygen exchange between the sample and surrounding atmosphere plays an important role in the switching mechanism. It is attributed to break down of p-i-n junction in which at low field, underbonded oxygen ions undergo single ionisation creating holes on those oxygens at the positive electrode and at higher voltage oxygen ions undergo double ionisation liberating oxygen molecules at the positive electrode and ionised electrons are re-injected at the negative electrode creating an n-type region. This thesis investigates resistive switching behaviour in Ca-doped BiFeO3, with the aim of understanding the switching mechanism and whether any of the induced p- and n-type behaviour occurs at different or both electrodes and which dominates the sample conductivity during switching. Also, it investigates possible low field resistive switching in another acceptor-doped ceramic, Y-doped CeO2. The materials were synthesised by solid state reaction and characterised using x-ray diffraction. The effect of application of a small dc bias on the electrical properties were analysed using impedance spectroscopy as function of different variables such as temperature and pO2. Since the electrical properties of Ca-doped BiFeO3 were sensitive to pO2 during synthesis, three sets of samples, Bi1-xCaxFeO3-x/2 (where: x =0.23, 0.3 and 0.4) were processed in N2, air and O2. The conductivities increased by a few orders of magnitude and the properties changed from mainly oxide ion conductors to p-type semiconductors with increasing pO2. Circuit fitting was carried out in order to identify electrical microstructure of differently processed samples. On application of a small dc bias, Ca-doped BiFeO3 samples processed in different atmospheres reversibly switch their resistances, ON-OFF, by 2-5 orders of magnitude with some hysteresis, on removal of the dc bias. This was attributed to a dramatic increase in hole v concentration by internal ionisation of redox active elements, which are believed to be underbonded oxygen ions, and trapping of electrons. Fluorite-structured, Ce0.84Y0.16O1.92 showed mixed n-type and oxide ion conductivity in which oxide-ion conductivity attributed to oxygen vacancies introduced to maintain charge neutrality as a result of lower valence doping, Y3+. The n-type conductivity was attributed to the presence of a transition metal that can easily be reduced, Ce4+ to Ce3+. On application of a dc bias, the sample conductivity increased by about two orders of magnitude and was reversible, with hysteresis, on removal of the bias. The increase in conductivity was attributed to an increase in the concentration of n-type carriers as result of oxygen loss. Ce0.84Y0.16O1.92 is the third example to show this kind of low field resistive switching, OFF-ON, in bulk ceramics. Bi0.77Ca0.23FeO3 interestingly showed flash electroluminescence, due to electron-hole recombination, under certain conditions after switching the sample to the highly conductive, ON state. The flash mechanism was sensitive to the level of p-type conductivity and when the sample loses its p-type conductivity by heat treatment close to the sintering temperature (~945 ℃) or in low pO2 just below the sintering temperature, the luminescence effect was suppressed.