Impact of Oxygen on the Conduction Mechanism Through Hf0.5zr0.5o2 For Reram Memory Applications

• Main Author: Gonzalez Hernandez, Yoandris Lazaro
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01.01.2022
• Subjects: Analytical chemistry; Annealing; Chemistry; CMOS; Computer science; Condensed matter physics; Electrical engineering; Electrodes; Energy; Information storage; Information Technology; Integrated circuits; Materials science; Metal fatigue; Microscopy; Optics; Physics; Random access memory; Silicon; Smartphones; Spectrum analysis; Thin films; Transistors; Universal Serial Bus
• ISBN: 9798379645908

Ferroelectric tunnel junctions (FTJs) are emerging non-volatile resistive memory holding the promise of combining the versatility of resistive random-access memory (ReRAM) in neuromorphic computing with the well-established fatigue resistance of ferroelectric devices. An FTJ comprises two metallic electrodes surrounding an ultrathin ferroelectric layer (metal/ferroelectric/metal). The development and introduction of FTJs into industrial processes were initially limited, impeded by the incompatibility of available ferroelectric materials with CMOS technology and the difficulty of scaling down to a few nanometers. The recent discovery of ferroelectricity in doped-HfO2 paves the way for fully CMOS-compatible FTJ memory devices. In addition, it is possible to achieve spike-time-dependent-plasticity (STDP) in FTJs, highlighting its potential for neuromorphic computing. The orthorhombic phase (Pca21) is widely accepted as responsible for ferroelectricity in doped-HfO2. However, this ferroelectric phase is a metastable phase. Therefore extensive research to stabilize this non-centrosymmetric phase is ongoing, including doping, strain, surface/interface/grain boundary energy, annealing temperature and intentionally introduced oxygen vacancies.Nevertheless, while these mechanisms improve the ferroelectric properties of HfO2-based films, they also adversely affect the performance of an HfO2-based FTJ. In this thesis, we focus on the influence of oxygen vacancies on the formation of the ferroelectric orthorhombic phase in Hf0.5Zr0.5O2 and on the mechanisms of charge conduction through this film. This study uses radio frequency magnetron sputtering to fabricate TiN/Hf0.5Zr0.5O2/Au structures, in which the oxygen partial pressure during Hf0.5Zr0.5O2 film deposition was variable. The diffraction peak (111) intensity of Hf0.5Zr0.5O2 corresponding to the orthorhombic phase increased when the oxygen concentration was reduced in the sputtering chamber. This enhancement of the orthorhombic phase is associated with the generation of defects in the Hf0.5Zr0.5O2 film as a consequence of oxygen-deficient deposition. Further investigation of the implications of such defects in the tunneling mechanism through the Hf0.5Zr0.5O2 is provided. Samples deposited in an oxygen-rich atmosphere (pO2 = 67%) were demonstrated to be suitable for direct tunneling. In contrast, those obtained in a semi-oxygen-deficient atmosphere (pO2 = 50%) were favourable for trap-assisted tunneling. It was also demonstrated that the formation of an oxygen vacancy filament is possible for samples deposited in a fully oxygen-depleted atmosphere (pO2 = 0%). Our results corroborate the rapid thermal annealing (RTA) process as a straightforward pathway for forming the ferroelectric orthorhombic phase (Pca21) in Hf0.5Zr0.5O2. Furthermore, an enhancement of the ferroelectric phase in Hf0.5Zr0.5O2 films deposited in an oxygen-free atmosphere was demonstrated. The endurance test demonstrates the stability of direct tunneling devices over those based on trap-mediated tunneling. Using the phonon-assisted tunneling between traps (PATT) model was possible to obtain the average distance between traps. A critical thickness for a dominant direct tunneling mechanism was determined from a comparative analysis of the PATT and direct tunneling (Wentzel-Kramers-Brillouin approximation) models. Likewise, comparing different trap densities (average distance between traps) allows us to determine the average distance between traps threshold for the prevalence of the tunneling mechanisms. This work, therefore, provides a window of thin film thicknesses and trap densities for optimal FTJ tunneling performance.