Competing tunneling conduction mechanisms in oxygen deficient Hf0.5Zr0.5O2

Abstract

The direct control of the tunneling current as a function of electric polarization in ferroelectric tunnel junctions has recently attracted noticeable attention through the availability of the CMOS compatible ferroelectric hafnium zirconium oxide (Hf0.5Zr0.5O2). While a lot of progress has been made in the understanding of ferroelectric tunnel junctions, the control and optimization of the volume fraction of the polar orthorhombic phase are still in its infancy and raise the question whether all observed resistive switching events are necessarily associated with polarization reversal. Trap-assisted tunneling is also able to modulate the current density through a field-induced variation of the oxygen vacancy density across the tunnel junction. The amplitude of the effect should depend on the pre-existing density of oxygen vacancies introduced during the fabrication. Here, by controlling the oxygen partial pressure during the RF magnetron sputtering deposition of the non-polar monoclinic phase of Hf0.5Zr0.5O2 thin films of approximately 3 nm, we demonstrate tuning of its transport mechanism due to the formation of oxygen vacancies. We show that two mechanisms dominate the current transport depending on the average distance between traps. For large oxygen content in the Hf0.5Zr0.5O2 thin film, direct tunneling (DT) is the dominant transport mechanism, while the electrical conductivity in the oxide can be described by the phonon-assisted tunneling between traps (PATTs) for the oxygen deficient Hf0.5Zr0.5O2 thin film. We derive a critical inter-trap distance and a critical thickness value that explains the transition from DT to PATT mechanism in Hf0.5Zr0.5O2 thin films.

Mechanism of Oxygen Vacancy Filament-Based Resistive Switching in Non-polar Hf0.5Zr0.5O2-δ Thin Films

“…I – V measurements of the device were performed by applying voltage sweeps with positive/negative polarity to the top electrode while the bottom electrode was kept as a grounded reference. In contrast to ref , the oxide layer of Hf 0.5 Zr 0.5 O 2 – δ at the present work was deposited at P O2 = 0, which promoted the formation of defects (oxygen vacancies) in the oxide layer . This paves the way for forming a conducting percolation path through the oxide layer without the need for electroforming.…”

“…The reactions between the electrode material and the oxide layer during the deposition process and possible post-deposition annealing processes are the main reasons for the practical existence of these oxygen vacancies at the oxide–electrode interfaces . In this case, the resistive switching (RS) process may result from the migration of oxygen vacancies (V o ), the formation and rupture of filamentary paths, and the trapping and de-trapping of charge carriers. , For these reasons, the effects of defect formation, particularly oxygen vacancies, need to be considered for a complete description of the underlying mechanisms involved in the switching process to achieve better performance of Hf 0.5 Zr 0.5 O 2 thin films in resistive random-access memories (ReRAM) …”