HfO2-based FeFETs: A Re-Emerging Technology Poised to Take over DRAM or Flash Memory

摘要

The ferroelectric-gated field-effect transistor (FeFET)-based technology offers numerous theoretical advantages compared to the dominating nonvolatile memory technology (flash memory). However, this promising technology has not yet succeeded in commercialization, due to the lack of suitable ferroelectric materials that enable fabrication of FeFETs to fulfill the requirements for a viable memory technology. including memory retention, CMOS compatibility, and scalability. The recent advent of HfO2-based ferroelectric has totally changed the outlook. To accelerate the implementation of this promising technology, the following four issues of HfO2-based ferroelectric technology are addressed in this thesis, including the switching kinetics, the retention characteristics, the endurance failure mechanisms, and the possibility for a "versatile memory" technology.;The switching kinetics reveals the dominating mechanism to control the polarization switching times of HfO2-based ferroelectrics. In order to characterize the switching kinetics of the HfO2-based ferroelectrics, samples based on the metal-ferroelectric-metal (MFM) structure are studied. The experimental results of these MFM devices are found to agree well with the results of the nucleation-limited-switching (NLS) model, and the extracted parameters from the analysis of the switching kinetics are consistent with the results from the density-functional-theory (DFT) analysis, which helps to reveal the dominating mechanism of the switching kinetics of HfO2 -based ferroelectrics.;The retention characteristic of a non-volatile memory is used to characterize its capability to store information over a period of time without any power supply. The FeFETs using conventional ferroelectric materials (i.e., PZT, or SBT) suffer from their short retention times, which impede the commercialization of the FeFET technology. The HfO2-based FeFET, on the other hand, has shown 10-year extrapolated retention time experimentally, making it very promising for a non-volatile memory technology. In this thesis, the retention loss mechanisms of a FeFET are discussed, followed by systematic comparisons among HfO2, PZT, and SBT-based FeFETs, from which we attempt to make it understandable why HfO2-based FeFETs are able to yield longer retention times.;The endurance is used to characterize the robustness of the memory cell after accumulated program/erase pulses. The HfO2-based FeFETs have shown much earlier endurance failure before the fatigue of polarization, which requires a clear understanding of its underlying mechanisms. In this thesis, the root causes that lead to the endurance failure in the HfO2-based FeFET are revealed by various approaches, including investigating the evolution of the Id-Vg curves, the analysis from band diagrams, and the 1/f noise measurement results. The associated impacts of the two major effects on the endurance failure of HfO2-based FeFETs are discussed in detail, and the right approach to enhance the endurance is proposed.;Armed with the understanding of the retention characteristics and the endurance failure mechanisms, we performed a systematic study of the correlation between these two attributes as functions of the programming voltage. By breaking down the associated contributing effects to the retention and the endurance characteristics, we arrived at a methodology to tune the retention and the endurance characteristics via the modulation of programming voltage, which paves the pathway for potential implementation of the futuristic "versatile memory" technology by the use of HfO2-based FeFETs.