INVESTIGATION AND ENGINEERING OF HfZrO2 INTERFACES FOR FERROELECTRIC BASED NEUROMORPHIC DEVICES

Abstract

This dissertation describes the study of ferroelectric hafnium zirconium oxide (HZO) and its integration into ferroelectric field effect transistors (FeFET). Ferroelectric HZO is uniquely situated for energy efficient, non-volatile memory applications such as FeFETs due to its CMOS compatibility and ferroelectricity at scaled thicknesses less than 10 nm[1]. This work covers material growth of HZO via atomic layer deposition (ALD), as well as electrode metallization (W and Pt) via sputtering and electron beam physical vapor deposition, to optimize ferroelectricity in capacitive structures. Preliminary results show Pt-based devices were sufficient in producing ferroelectric HZO, but had issues in electrode degradation at high thermal processing > 450 °C. In contrast, HZO capacitors in W devices showed drastic improvement in the ferroelectric response reaching remnant polarization values > 40 μC/cm2. To integrate into a FET structure, gate dielectrics (Al2O3 and HfO2) and the 2D semiconductor tungsten diselenide (WSe2) are introduced to the HZO stack. Material and electrical characterization was performed and gave indication of challenges such as: low remnant polarization (<10 μC/cm2), surface roughness (> 20 nm), and high trap characteristics in FeFET modulation. Electrical characterization was performed via variable pulsing, high frequency cycling, current vs voltage, capacitance vs voltage, and polarization vs voltage testing. Challenges such as low remnant polarization, leaky dielectrics, and surface roughness are identified through transmission electron microscopy, atomic force microscopy, and electrical characterization. These challenges were addressed by altering the growth conditions, scaling the thickness of each material, and thermally processing within the bounds of material stability. Upon integration of these various materials into FETs, the challenges of reliability, stochasticity, and consistency were evaluated on through various means of electrically testing such as, variable pulsing, high frequency cycling, current vs voltage, capacitance vs voltage, and polarization vs voltage. A greater depth of understanding of fundamental aspects of these device architectures is required to untangle the complex electrical characteristics of the fabricated devices. Characterization of material properties is performed by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Throughout the studies performed in this dissertation, the phase landscape of HZO was investigated on inert Ti/Pt electrodes. While the ferroelectric nature of the HZO was sufficiently explored at CMOS compatible temperatures, yielding remanent polarization values of 20 μC/cm2 and demonstrating multi state memory within Ferroelectric field effect devices (3.5 order of magnitude conductivity change), due to the phase landscape evolution under thermal processing. Higher temperatures were found to be incompatible with the electrode choice as the interdiffusion and breakdown resulted in poor device performance. W electrode HZO capacitors were then used to study the higher temperature ferroelectric devices as well as incorporate scaling of the ferroelectric films to better match the needs of modern device architectures. The optimal ferroelectric films were found to have remanent polarization values > 40 μC/cm2 and when implemented in a FEFET were able to demonstrate a memory window of 6.3 volts, allowing for a large range of modulation for neuromorphic devices.