ALRIFAI Liliane's PhD thesis defence

Elaboration and characterization of ferroelectric hafnium oxide thin films for non-volatile memory applications integrated on silicon

This thesis explores the ferroelectric properties of HfO₂-based thin films in view to their potential applications in ferroelectric random access memory (FeRAM). It highlights the growing interest in hafnium oxide (HfO₂) due to its compatibility with CMOS technology and potential for ultra-scaled ferroelectric devices. The research investigates both Gd-doped and undoped HfO₂ thin films deposited by plasma-enhanced atomic layer deposition (PEALD) in a metal/insulator/metal (MIM) structure (TiN/HfO2/TiN), evaluating their structural, electrical, and endurance properties. This work investigates the properties of sub-10 nm Gd-doped HfO₂ films deposited by PEALD with 1.8% doping and annealed at 650 °C in an N₂ atmosphere. The films demonstrated strong ferroelectric properties even at ultra-thin dimensions, confirmed by X-ray diffraction and electrical measurements, revealing a clear polarization hysteresis behavior. The orthorhombic phase, crucial for ferroelectricity, was stabilized at this doping level, while the non-ferroelectric monoclinic phase was suppressed. As film thickness increased to 8.8 nm, the orthorhombic phase grew, and polarization values increased accordingly. The coercive field remained constant around 2 MV/cm. However, films thinner than 4 nm were found to be non-ferroelectric, which was attributed to the presence of a non-ferroelectric phase close to the interfacial layer. Despite this, Gd-doped HfO₂ films showed excellent endurance, withstanding 10¹⁰ switching cycles without fatigue and switching polarization at low voltages (as low as 0.9 V).

To gain deeper insights into the origin of ferroelectric orthorhombic phase stabilization Gd-doping in stabilizing the ferroelectric phase, a comparison between Gd-doped HfO2 and undoped HfO₂, processed under the same conditions, was conducted. While undoped HfO₂ exhibited ferroelectric behavior in films under 14 nm, with polarization levels comparable to Gd-doped HfO₂ below a 7 nm thickness, Gd-doping showed superior performance in films thicker than 7 nm. Gd-doping not only enhanced polarization but also introduced a pronounced wake-up effect with cycling. In undoped HfO₂, mechanical stress from TiN electrodes was sufficient to induce ferroelectricity, whereas Gd-doping stabilized the orthorhombic ferroelectric phase even in the absence of electrodes. Although undoped HfO₂ is more suitable for ultra-thin layers due to its simpler fabrication process, Gd-doped HfO₂ offers better performance in thicker films and applications requiring greater polarization.

Further analysis explored the effects of annealing temperatures and HfO₂ thickness on the ferroelectric properties, to study the compatibility of these devices with CMOS systems. Gd-doping significantly lowered the crystallization temperature for the orthorhombic phase, enabling ferroelectricity at temperatures as low as 450 °C, compared to undoped films requiring temperatures above 550 °C. In parallel, increasing the HfO₂ thickness further promoted orthorhombic phase crystallization at lower temperatures in both cases.

Key words: Ferroelectricity, HfO2, thin films, polarization, doping, ALD, mechanical stress, annealing temperature.