Electron emission from ferroelectrics

Electron emission from ferroelectrics (FEE) is an unconventional electron emission effect. Methods of FEE excitation are quite different compared to classic electron emission from solids. Two kinds of FEE have been observed, “weak” and “strong.” “Weak” electron emission (current density ${10}^{\text{−12}}{\text{–10}}^{\text{−7}} {\mathrm{A/cm}}^{\mathrm{2}})$ occurs from polar surfaces of ferroelectric materials in the ferroelectric phase only. A source of the electric field for “weak” FEE excitation is an uncompensated charge, generated by a deviation of macroscopic spontaneous polarization from its equilibrium state under a pyroelectric effect, piezoelectric effect, or polarization switching. The FEE is a tunneling emission current which screens uncompensated polarization charges. It is shown that the FEE is an effective tool for direct domain imaging and studies of electronic properties of ferroelectrics. “Strong” FEE, which is 10–12 orders of magnitude higher than “weak” FEE, achieves 100 A/cm² and is plasma-assisted electron emission. Two modes of the surface flashover plasma formation followed by strong electron emission have been studied. The plasma of ferroelectric origin has been observed only in the ferroelectric phase and it is induced by polarization switching or a field-enforced phase transition, such as antiferroelectric–ferroelectric or relaxor–ferroelectric. The second mode of plasma is conventional surface flashover which may be initiated by a high voltage application in any phase from any dielectric, including ferroelectrics. In this review paper we consider numerous experimental results, as well as mechanisms of both types of electron emission from ferroelectrics. The main stress is placed on the material aspect in order to clarify the influence of ferroelectricity (ferroelectric phase transitions, polarization switching, etc.) on electron emission. Another aspect which is broadly discussed is the potential applications of these unconventional FEE emitters in various devices for development of high density FEE cathodes for microwave devices, as well as FEE converters of IR irradiation into visible light, x-ray imaging, FEE flat panel displays, etc.