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 non-volatile, high-speed 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)].
The unsolved problems in the way of development of FRAM-based universal memory are the reason for Wake-up and Fatigue modes of FRAM elements. One of the possible reasons for these effects is the presence of defects in Hf0.5Zr0.5O2 thin films. The purpose of the present work is to study the evolution of charge trap density in ferroelectric Hf0.5Zr0.5O2 after set/reset cycling.
Transport measurements were performed for TiN/Hf0.5Zr0.5O2/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-thick Hf0.5Zr0.5O2 films were deposited at 240°C from TEMAHf and TEMAZr precursors using H2O as an oxygen source. Laser ellipsometry and Rutherford backscattering spectroscopy confirmed the thickness and stoichiometry of as deposited Hf0.5Zr0.5O2 films. Some samples were annealed at 400°C in the N2 environment during 30 sec. The crystalline structures of as deposited and annealed films were examined by symmetrical X-Ray diffraction. 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 of Hf0.5Zr0.5O2 films is confirmed by observing the characteristic hysteresis on the polarization-voltage (P-V) plate for TiN/Hf0.5Zr0.5O2/TiN structures.
The P-V and currents-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 P-V measurements show that after 10 set/reset cycles, residual polarization Pr rises from 19 μC/cm2 to 20.6 μC/cm2 (wake-up mode). After 103 set/reset cycles, Pr began to decrease (Fatigue mode) and, after 106 cycles, it reached the value of 13 μC/cm2. Note that, at a ~104 cycle, the residual polarization exhibits a local maximum.
The leakage current depends on the voltage exponentially and can be described by the phonon-assisted tunneling between traps with the thermal (Wt=1.25 eV) and optical (Wopt=2.5 eV) trap energies related to the oxygen vacancy in Hf0.5Zr0.5O2 [D.R. Islamov et al., JETP Letters 102, 544 (2015); D.R. Islamov et al., ECS Transactions 75, 123 (2017)]. The slope of the initial I-V curves on the semi-log plot corresponds to the trap density of N=1.3×1020 cm-3. After 104 set/reset cycles, the current increases spasmodically and the slope of the log I-V curves corresponds to N=1.5×1020 cm-3. During further cycles the current rises at a particular voltage on the test structure. After 106 cycles, the leakage current is described by the phonon-assisted tunneling between the traps with trap density N=1.8×1020 cm-3.
The work was supported by the Russian Science Foundation, grant #14-19-00192.