Phase Transformation Driven Wake-Up in Ferroelectric Hafnium-Zirconium Oxide Films Observed Using Nano-FTIR

Ferroelectric hafnium zirconium oxide (HZO) is being pursued as a near-memory computing technology as it can be densely scaled and is compatible with silicon. The orthorhombic phase making HZO ferroelectric is metastable, however. Being metastable, the formation of this technology-enabling orthorhombic phase can be affected by several factors including dopants, stress, crystal size, and oxygen vacancies. HZO devices are typically multi-phase in their as fabricated state and require wake-up to realize a stable ferroelectric polarization. The mechanisms driving this wake-up are under debate. Maturation of ferroelectric HZO technology, therefore, requires means of observing, understanding, and ultimately controlling the wake-up process.<br/><br/>Here, we observe an electric-field driven transformation from the non-ferroelectric tetragonal to ferroelectric orthorhombic phase in 20 nm thick atomic layer deposited (ALD) HZO films using a combination of piezoresponse force microscopy (PFM) and tip-enhanced nanoscale Fourier transform infrared spectroscopy (nano-FTIR). HZO films were subject to varying magnitudes and polarities of electric field over 20x20 µm areas as applied by PFM within a glovebox filled with inert gas. The ferroelectric response was then measured in each of these regions with PFM, whereupon the local phase fraction was assessed with nano-FTIR. IR-spectra show that the orthorhombic phase becomes more predominant relative to its non-ferroelectric tetragonal form in areas exposed to larger electric field magnitudes. The field driven nature of the phase-transformation, in turn, suggests the central role of the charged oxygen vacancies in orthorhombic phase formation, wake-up, and ultimately device performance.