Nonvolatile Memory Using Epitaxially Grown Composite-Oxide-Film Technology

Abstract

We have developed a ferroelectric-gate field-effect transistor (FeFET) composed of heteroepitaxially stacked oxide materials. A semiconductor film of ZnO, a ferroelectric film of Pb(Zr,Ti)O3 (PZT), and a bottom gate electrode of SrRuO3 (SRO) are grown on a SrTiO3 (STO) substrate. Structural characterization shows a heteroepitaxy of the fabricated ZnO/PZT/SRO/STO structure with good crystalline quality and the absence of an interface reaction layer. When the polarization direction of the PZT film is downward, all the electrons across the ZnO film are depleted. When the polarization direction is upward, on the other hand, quasi-two-dimensional electron gas is accumulated at the ZnO/PZT interface. The switching of the quasi-two-dimensional electron gas between the two states is confirmed by capacitance–voltage measurements. Drain and source electrodes of Pt/Ti are formed on the ZnO film and in-plane conduction at the ZnO/PZT interface is probed. When gate voltages applied to the bottom electrode are swept between -10 and 10 V, the on/off ratio of drain currents is higher than 105. Such a high on/off ratio is preserved even after 105 s and the extrapolation of the retention behavior predicts a definite memory window of more than 10 years.