Before 2011, hafnium oxide was considered as a paraelectric material until the observation of the ferroelectric effect in doped thin HfO₂ films after high­temperature annealing [T.S. Böscke et al., Appl. Phys. Lett. 99, 102903 (2011); S. Mueller et al., Adv. Function. Mater. 22, 2412 (2012)]. Dopping by lantanum leads to the largest remanent polarization of HfO₂ films [U. Schroeder et al., Jpn. J. Appl. Phys. 53, 08LE02 (2014)]. Ferroelectricity in these materials is associated with the ability to stabilize noncentrosymmetric orthorhombic phase Pbc2₁ [J. Müller et al., Nano Lett. 12, 4318 (2012)]. It should be noted that hafnia-­based materials have a number of advantages over conventional ferroelectric regarding compatibility with the technological processes used in microelectronics, and they have already demonstrated their ability to provide a very high density of elements. Considering the advantages of ferroelectric random access memory (FRAM) as non­volatile, high­speed performance, high number of switching cycles, the discovery of ferroelectric effect in these materials gave an impetus for the development of the universal memory concept which may lead to a significant breakthrough in the development of memory devices [M.H. Park et al., Adv. Mater. 27, 1811 (2015)].

An unsolved problem in the way of development of FRAM­-based memory are is a reason of memory window of FRAM instability during set/reset cycling, i.e. endurance. One of the possible reasons for these effects is the presence of defects in HfO₂:La thin films. The purpose of the present work is to study the evolution of charge trap density in ferroelectric HfO₂:La after set/reset cycling.

Transport measurements were performed for TiN/HfO₂:La/TiN structures. Test structures were fabricated with the atomic layer deposition (ALD) technique. A 10-­nm­-thick TiN layer was deposited on the oxidized Si (100) substrate. Then the 10­-nm­-thickHfO₂:La films were deposited at 235°C from TEMAH (heated to 100°С) and La(iPrCp)₃⁺ (heated to 170°С) precursors using O₂+Ar plasma. Laser ellipsometry and Rutherford backscattering spectroscopy confirmed the thickness and stoichiometry of HfO₂:La films. TiN top layer (10 nm thick) was deposited using the ALD technique. For transport measurements, round (area is 7.854×10³ μm²) electrodes were formed by the photolithography process. The presence of ferroelectric properties ofHfO₂:La films is confirmed by observing the characteristic hysteresis on the polarization­voltage (P-­V) plate for TiN/HfO₂:La/TiN structures.

The P-­V and current-­voltage (I-­V) dependences were measured by the standard PUND technique at room temperature. The first “Positive” and the third “Negative” impulses aligned dipoles in the ferroelectric films (set and reset, respectively) and they were used to measure polarization, while, during the second “Up” and the forth “Down” impulses, the leakage currents were measured. The leakage currents were extracted from the current responses by removing displacement currents. The trap density was extracted from the leakage currents within phonon-assisted tunnelling between traps model [K. Nasyrov et al., J. Appl. Phys. 109 097705 (2011)].