ECS Transactions, 80 (1) 279-281 (2017) 10.1149/08001.0279ecst ©The Electrochemical Society The Charge Trap Density Evolution in Wake-up and Fatigue Modes of FRAM Damir R. Islamova,b, O. M. Orlovc, V. A. Gritsenkoa,b,d, and G. Ja. Krasnikovc a Rzhanov Institute of Semiconductor Physics SB RAS, Novosibirsk 630090, Russian Federation b Novosibirsk State University, Novosibirsk 630090, Russian Federation c JSC Molecular Electronics Research Institute, Zelenograd 124460, Russian Federation d Novosibirsk State Technical University, Novosibirsk 630073, Russian Federation We study the transport properties in different modes of FRAM elements based on ferroelectric Hf0.5Zr0.5O2 thin films. The leakage currents in Hf0.5Zr0.5O2 are described by phonon-assisted tunneling between traps. Comparison experimental data with results of the simulations allows us to extract the evaluation of the charge trap density during endurance. A hypothesis about role of oxygen vacancies in fatigue is discussed. Introduction Hafnia (HfO2) was considered as a paraelectric material until observation of ferroelectric effect in thin films of doped HfO2 after high-temperature annealing (~1000 °C) (1, 2). Of particular interest is the fact that ferroelectricity was also demonstrated in thin of the solid solution Hf0.5Zr0.5O2 which requires annealing at significantly lower temperatures (3–6). Ferroelectricity in these materials is associated with the ability to stabilize noncentrosymmetric orthorhombic phase Pbc21 (4). It should be noted that hafnia-based materials have many 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 (7). 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 (endurance). Purpose of the present work is to identify the physical phenomena which cause FRAM degradation using the charge transport studies of ferroelectric Hf0.5Zr0.5O2. Experiment details Transport measurements were performed for TiN/Hf0.5Zr0.5O2/TiN metal/insulator/metal (MIM) structures. Test structure were fabricated with atomic layer deposition (ALD) technique. The 10-nm-thick TiN layer was deposited on oxidized Si (100) substrate. Then the 10-nm-thick Hf0.5Zr0.5O2 films we deposited at 240 °C from 279 Downloaded on 2017-10-25 to IP 80.82.77.83 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). ECS Transactions, 80 (1) 279-281 (2017) Tetrakis(EthylMethylAmino)Hafnium (TEMAHf), Tetrakis(EthylMethylAmino)Zirconium (TEMAZr), and H2O, as the Hf-precursor, Zr-precursor, and oxygen source, respectively. The TEMAHf and TEMAZr precursors were mixed in single-cocktail balloon. Laser ellipsometry and Rutherford backscattering spectroscopy confirmed the thickness and stoichiometry of as deposited Hf0.5Zr0.5O2 films. A part of samples was annealed at 400 °C in N2 environment during 30 sec (rapid thermal annealing, RTA). The crystalline structures of as deposited and annealed films were examined by symmetrical X-Ray diffraction (XRD) using ARL X'TRA tool (Thermo Scientific) utilizing CuKα radiation. The TiN top electrodes (thickness of 10 nm) were 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. Results and discussions The P-V measurements show that after 10 set/reset cycles, residual polarization 2Pr rises from 19 µC/cm2 to 20.6 µC/cm2 (wake-up mode). After 103 set/reset cycles, 2Pr 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 (8)–(10): eg , s2 æ 2s 2m *W æ Wopt - Wt ö 2p !Wt t ÷÷ expç expçç ç ! 2kT ø kT (Wopt - Wt ) è è J= g= m* s 2 ö ÷ sinhæç eFs ö÷ ÷ è 2kT ø ø [1] with the thermal (Wt = 1.25 eV) and optical (Wopt = 2.5 eV) trap energies related to the oxygen vacancy in Hf0.5Zr0.5O2 (11)–(13). Here J is the current density, e is the elementary charge, s is the mean distance between traps, γ is the probability rate of charge carrier tunneling between traps, ħ is the Planck constant, m* is the effective mass, k is the Boltzmann constant, T is the local temperatures, F is the local electric field in the dielectric medium. The slope of the initial I-V curves on the semi-log plot corresponds to the trap density of N = s-3 = 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. Despite that ab initio calculations predict amphoteric nature of the charge traps (13), the capacitance-voltage measurements on trapped charge accumulation demonstrate that 280 Downloaded on 2017-10-25 to IP 80.82.77.83 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract). ECS Transactions, 80 (1) 279-281 (2017) mostly holes are involving in transport processes (11). One can assume that holes, involved on the transport process, contribute to the formation of loosening Zr-O anf Hf-O bonds, which leads to forming more and more defects, namely oxygen vacancies. On the one hand, the trapped charge can pin dipoles and “switch them off” from the ferroelectric response (14). In the other hand, an oxygen vacancy might be formed by removing and oxygen atom, which is form a dipole by its movement in the lattice. This phenomenon leads to decreasing ferroelectricity of the films, i.e. degradation of the Hf0.5Zr0.5O2 film in the FRAM element (fatigue mode). Summary In conclusion, this study demonstrates that the fatigue of a Hf0.5Zr0.5O2-based FRAM element is caused by spasmodically formation of new defects, namely oxygen vacancies, in the active medium. The trap density can be extracted from the curve slope in the log(I)-V plate depicted for leakage currents, extracted from the standard PUND measurements. Models of the defects formation and fatigue were proposed. Acknowledgments The work was supported by the Russian Science Foundation, grant #14-19-00192. References 1. 2. 3. 4. 5. 6. T.S. Böscke, J. Müller, D. Bräuhaus et al., Appl. Phys. Lett. 99, 102903 (2011). S. Mueller, J. Mueller, A. Singh et al., Adv. Function. Mater. 22, 2412 (2012). J. Müller, T.S. 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Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).
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