Role of ultra-thin Ti and Al interfacial layers in HfZrO2 ferroelectric tunnel junctions

International audience; In this work, the Hf0.5Zr0.5O2 (HZO) layer was realized by room temperature magnetron sputtering of a HZO ceramic target and subsequently crystallized by rapid thermal annealing [1]. The titanium nitride bottom and top electrodes were grown by reactive magnetron sputtering of a titanium target. We explored the impact of the insertion of an ultra-thin buffer layer at the HZO/top electrode interface on the stabilized crystalline phase, microstructure and electrical properties of thin HZO films. We investigated two materials, Ti and Al. Behind the annealing process Ti and Al turned into TiO2 and Al2O3 respectively, following the creation of oxygen vacancies inside the HZO barrier. The higher concentration of oxygen vacancies promoted by the addition of the buffer layer plays a significant role in the stabilisation of the orthorhombic phase for decreasing HZO thickness. This allowed us to synthesise very thin HZO films with ferroelectric properties. Furthermore we observed a clear improvement of the electrical performances of the n^ Si(001)/TiN/HZO/TiN/Ti/Pt structure. We exploited transmission electron microscopy to investigate the structure and the morphology of the electrode/HZO interfaces. X-ray reflectometry and grazing incidence X-ray diffraction were used to probe the thickness and structural characteristics of HZO layers. X-ray photoemission spectroscopy was used to analyse the chemistry and the electronic state of the HZO/electrode interface. We will discuss our results in the framework of structural, chemical and physical properties of the ferroelectric/electrode interfaces and their effect on the electrical properties of thin HZO-based tunnel junctions. The present optimized stack will eventually be considered for the demonstration of synaptic learning mechanisms for neuromorphic applications. References: [1] J. Bouaziz, P. R. Romeo, N. Baboux, and B. Vilquin, “Huge Reduction of the Wake-Up Effect in Ferroelectric HZO Thin Films,” ACS Appl. Electron. Mater., vol. 1, no. 9, pp. 1740–1745, 2019, doi: 10.1021/acsaelm.9b00367.