Leakage Currents Mechanism in Thin Films of Ferroelectric Hf0.5Zr0.5O2
Authors: Islamov Damir R. , Chernikova Anna G. , Kozodaev Maxim G. , Perevalov Timofey V. , Gritsenko Vladimir A. , Orlov Oleg M. , Markeev Andrey M. 
Published: February 2020

Hafnia was considered as a paraelectric material until observation of ferroelectric effect in thin films of doped HfO2 after high-temperature annealing (~1000°C). [T.S. Böscke et al., Appl. Phys. Lett.
99, 102903 (2011); S. Mueller et al., Adv. Function. Mater.
22, 2412 (2012)]. 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 [J. Müller et al., Appl. Phys. Lett.
99, 112901 (2011); J. Müller et al., Nano Lett.
12, 4318 (2012); M.H. Park et al., Appl. Phys. Lett.
102, 112914 (2013); A. Chernikova et al., Microelectron. Eng.
147, 15 (2015)].

Ferroelectricity in these materials is associated with the ability to stabilize noncentrosymmetric orthorhombic phase Pbc2
1 [J. Müller et al., Nano Lett.
12, 4318 (2012)]. It should be noted that the hafnia-based materials have a number of advantages over conventional ferroelectric regarding compatibility with technological processes used in microelectronics, and has already demonstrated their ability to provide very high density of elements. Taking into account advantages of the ferroelectric random access memory (FeRAM) as non-volatile, high-speed performance, high number of switching cycles, the discovery of ferroelectric effect in these materials gave an impetus for development of 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 FeRAM-based universal memory is the spontaneous depolarization of the active medium, which leads to short data storage time (retention) that is observed in Ref. [C.-H. Cheng, A. Chin, IEEE Electron Dev. Lett.
35, 138 (2014)]. One of the possible reason of spontaneous depolarization is leakage currents through the ferroelectric thin films. High-κ Hf0.5Zr0.5O2 thin films (κ>16 depending on structure phase) can be used as sub-gate dielectrics in MOSFET and FinFET transistors instead of SiO2. Thus, it is very important to know charge transport mechanisms in thin dielectrics films to control the leakage currents for development of high quality memory devices. Purpose of the present work is to study charge transport mechanism in ferroelectric Hf0.5Zr0.5O2.

Transport measurements were performed for TiN/Hf0.5Zr0.5O2/Pt structures. Test structure were fabricated with atomic layer deposition (ALD) technique. 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 TEMAHf and TEMAZr precursors using H2O as 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 N2 environment during 30sec (rapid thermal annealing, RTA). The crystalline structures of as deposited and annealed films were examined by symmetrical X-Ray diffraction (XRD). Pt top electrodes (thickness of ~30nm) were deposited through shadow mask with round holes (area is 7.1×104μm2) by electron beam evaporation on as deposited Hf0.5Zr0.5O2films and after RTA.

The electronic structure of Hf0.5Zr0.5O2 was calculated within the spin polarized density functional theory using the ab initio simulation code Quantum ESPRESSO [P. Giannozzi et al., J. Phys.: Condens. Matter
21, 395502 (2009)] with B3LYP hybrid exchange-correlation functional. The oxygen vacancy was generated by the removal of an O atom, followed by relaxation of remaining atoms in 96-atom supercell.

The XRD spectra of Hf0.5Zr0.5O2 films show that as deposited films have amorphous structure (a-Hf0.5Zr0.5O2), while annealed films are polycrystalline with monoclinic (P2
1/c), tetragonal (P4
2/nmc) and noncentralsymmetric orthorhombic (Pbc2
1) phases. The presence of ferroelectric properties of f-Hf0.5Zr0.5O2 films after RTA is confirmed by observing characteristic hysteresis on the P-V plate for Pt/ Hf0.5Zr0.5O2/TiN structures.

Comparison of experimentally measured leakage currents-voltage dependencies at different temperatures (I-V-T) with Frenkel model of charged trap ionization returns overestimated value of dynamic permittivity with respect to predicted by ab initio calculations and underestimated value of frequency factor ν<<W/h (W is trap energy; h is the Planck constant). Phonon-assisted tunneling between traps describes experimental I-V-T with very good quantitative agreement with thermal and optical trap energies W
t=1.25eV and W
opt=2.5eV, respectively. The charge transport mechanism of leakage currents does not depend on crystal structure of Hf0.5Zr0.5O2 films. The only difference is charge trap density, which is lower in f-Hf0.5Zr0.5O2 (3×1019cm-3 with respect to 1×1020cm-3 in a-Hf0.5Zr0.5O2).

Despite that ab initio calculations predict amphoteric nature of traps, C-V measurements on trapped charge accumulation demonstrate that mostly holes are involving in transport processes.

The work was supported by the Ministry of Education and Science of the Russian Federation (project #RFMEFI57614X0065).

Figure 1