Polarization induced switching in PZT back gated multilayer MoS2 FETs for low power non-volatile memory

Abstract

We demonstrated polarization induced switching in lead zirconium titanate (PZT) back gated multilayer MoS2 ferroelectric field effect transistors (Fe-FETs) for low power non-volatile memory devices. In this work, we investigate the influence of the interface between ferroelectric and 2D (MoS2) materials on the transistor characteristics for non-volatile memory applications. We fabricated Fe-FET devices with multilayer MoS2 as a channel material and polycrystalline and single crystal PZT thin films as the ferroelectric gate materials. The Fe-FET with polycrystalline PZT as a gate shows good memory characteristics; however, the hysteresis in transfer characteristics of the Fe-FET is in the clockwise direction (anti-hysteresis), which indicates that the transistor characteristics are not purely controlled by the ferroelectric charge. On the other hand, the Fe-FET devices with a single crystal PZT as the gate shows stable memory characteristics with a clear anti-clockwise hysteresis, which shows that the transistor characteristics are purely controlled by the ferroelectric charge. To understand the source of the anti-hysteresis and the interface between the ferroelectric and 2D materials, we have performed polarization switching at nanoscale using piezo force microscopy (PFM). The local piezoelectric hysteresis loop measured by PFM reveal that the polarization reversal in polycrystalline MoS2/PZT heterostructures is due to inhomogeneous polarization distribution, oxygen vacancies and high surface roughness of the polycrystalline PZT thin films. Our MoS2 Fe-FET devices with epitaxial PZT as a gate exhibit low switching voltages ≤2 V (much lower than the reported 2D-FET devices (8–20 V)), high ON-OFF ratios ≥104 (comparable to state of the art Fe-FET devices) and reproducible hysteresis behaviour with good sustainability over 200 cycles of switching operations. This study helps with the understanding of the fundamental phenomenon for anti-hysteresis, and the realization of low power and reliable non-volatile memory devices.