Development of ferroelectric tunnel junction devices based on hafnia zirconia films for neuromorphic applications

Abstract

This thesis focuses on the development of FTJ devices using a Hf0.5Zr0.5O2 (HZO) ferroelectric layer, demonstrating their compatibility with CMOS integration and their applicability in neuromorphic hardware. A bilayer structure comprising metal- ferroelectric-dielectric-metal layers with a ∼ 10 nm HZO ferroelectric layer and a thin tunneling Al2O3 layer has been investigated.

We examine the impact of dielectric positioning, metal electrode placement (with W and TiN), and dielectric thickness on device performance. Additionally, we explore the role of charge traps in the dielectric or at the dielectric-ferroelectric interface, and the influence of the fabrication process on charge trap density and polarization switching behavior. W bottom electrode is found to give optimized device performance, and positioning Al2O3 next to the bottom electrode further enhances the device performance in terms of ON current and ON/OFF ratio. Furthermore, longer pulses are necessary to stabilize higher remnant polarization due to charge trap dynamics.

We also analyze various electrical parameters affecting FTJ device performance, demonstrating that the cycling waveform significantly influences the wake-up process and the resulting remnant polarization in TiN-Al2O3-HZO-W FTJ devices. Square waveforms outperform triangular waveforms, yielding higher remnant polarization (PR) post-wake-up. By employing an asymmetric waveform for field cycling and adjusting the pulse width, the PR and the ON/OFF ratio after wake-up are significantly improved.

Finally, we explore the integration of bilayer FTJ devices into CMOS back-end-of- line (BEOL) processes, demonstrating a 1T1C circuit by connecting an FTJ in the BEOL with an nMOS transistor in the front-end-of-line. Measurements on standalone FTJ devices in the BEOL reveal an ON/OFF ratio of 18 and an ON current density of 24.5 μA/cm2. Crucially, BEOL fabrication has negligible impact on transistor characteristics, and the 1T1C circuit exhibits a 2.6-fold amplification of the FTJ ON current. The FTJ devices integrated on the CMOS-BEOL demonstrate multiple resistance states with the application of partial switching Reset and Set pulses. These FTJ devices have the potential to be utilized in neuromorphic hardware systems.