Effects of Electrodes and Gd Doping on the Ferroelectric Properties of HfO2 Thin Films رسالة دكتوراة

Abstract

Ferroelectrics have a wide range of applications relevant properties, such as sponta neous polarization that can be reversed by applying of electric field. This class of materials is represented by perovskites (ABX3, which are used in sensors, actuators and non-volatile memories. The non-volatile memory application has crucial technological challenges; since perovskites are not easy to integrate into CMOS fabrication. The high annealing budget and instability of ferroelectric phase at ultrathin scale are two main issues that prevent aggressive scaling. The discovery of ferroelectricity in hafnia-based films 13 years ago get the ferroelectric random-access memories (FeRAMs) back to the race. They are employed in CMOS tech nology and were evidenced to retain their ferroelectric properties down to few nanometers (5 nm or lower), controlling the limitations of perovskites class and bringing them as the ideal candidate for low power ferroelectric applications. Afterwards, research focused on the physics behind the polarization response, improving a reliable fabrication process for integration in memory devices. This work addresses intriguing and partially controversial points and contributes to CMOS-compatible processing of ferroelectric HfO2. In this thesis, the hafnia films are grown by plasma enhanced atomic layer deposition (PEALD) technique. The fabrication and characterizations of TiN/(Gd):HfO2/SiO2/Si metal/oxide/semiconductor (MOS) and TiN/Gd:HfO2/TiN/SiO2/Si metal/ferroelectric/ metal (MFM) are considered. The layers need high thermal budget to get ferroelectric properties. The three deposited layers and interfaces are studied by means of x-ray reflectometry (XRR), grazing incidence x-ray diffraction (GIXRD), in-plane and out-of plane Bragg-Brentano x-ray diffraction, leakage current, positive up negative down and endurance measurements. v The ferroelectric properties evidenced on undoped hafnium oxide which are similar to those of doped HfO2 are rather unexpected since it is believed that doping is a prerequisite to stabilize the HfO2 ferroelectric behavior. Two different origins were invoked to account for this behavior. The first is related to thermodynamically HfO2 orthorhombic/ferroelectric phase favored upon the monoclinic/non ferroelectric one by its lower surface energy in very small crystallites, i.e. in very thin layers . The second one is related to the existence of mechanical stress within the HfO2 layer that allows the stabilization of non-centrosymmetric orthorhombic/ferroelectric phase. The mechanical stress is due either to doping or to top and bottom metal layers in the metal/HfO2/metal stack. The origin of undoped HfO2 ferroelectric behavior is still under debate. In this work, additional evidence of the mechanical stress role in stabilizing the HfO2 ferroelectric phase is presented. The effect of top and bottom TiN electrodes on the crystallization of Gd-doped and undoped HfO2 layers were studied as a function of the layer thickness. The results show that the HfO2 orthorhombic phase of undoped HfO2 disappears when the bottom TiN electrode is removed. In contrast, Gd-doped HfO2 layers show intense orthorhombic phase even without the TiN bottom layer. The mechanical stress role is further evidenced by atomic planes interplanar distance measurements which show a large deformation between in-plane and out-of-plane x-ray diffraction geometries, only in the case of Gd-doped HfO2. In addition to, the effect of annealing before and after capping TiN layer on TiN/Gd:HfO2 /TiN/SiO2/Si (MFM) stacks were presented structurally and electrically. Structurally, the orthorhombic/tetragonal phase peak can be seen in thickness range of 5-42 nm. Electrically, it was observed that the total remnant polarization value (2Pr) shrinks via over 24.8 and 29.7 nm-thick of Gd:HfO2 layers due to orthorhombic phase relaxation into tetragonal phase, for MFM capacitor devices encapsulated with top TiN electrode before and after annealing, respectively. Furthermore, an evident gradual increase and decrease in total remnant polarization value (2Pr) and coercive field (Ec) values, in respective, upon cycling were registered. Thus, such MFM devices can be employed as non-volatile memory applications.