(Invited) The Influence of Defects on the Electronic Properties of Hafnia
Authors: Islamov Damir R. , Gritsenko Vladimir A. , Perevalov Timofey V. 
Published: February 2020

Currently, high-κ dielectrics, such as hafnium oxide (hafnia) HfO2, zirconia, alumina etc. are replacing SiO2 - traditional dielectric in silicon devices. Among high-κ dielectrics the hafnia causes the greatest interest because it combines a wide band gap of E
g=5.6-5.8eV, high permittivity κ=16-40, high thermal stability (melting point ≈2780°С), high thermodynamic stability in contact with silicon, and large energy barriers for electrons (2.0eV) and holes (2.5eV) with respect to the silicon [Robertson J, Wallace R M, Mater. Sci. Eng.: R: Reports
88, 1 (2015)]. Now, HfO2 is under intensively studied mainly due to its prospective application as high-κ gate dielectric in metal-oxide-semiconductor (MOS) field-effect transistors [Lee S J et al., J. Appl. Phys.
92, 2807 (2002)].

In 2007, Intel Corporation implemented high-κ HfO2 as the gate dielectric in silicon devices of Intel Core family instead of SiO2. This phenomenon was called the revolution in microelectronics. Currently, hafnia replaces SiO2 in flash memory devices. Hafnia (pure, doped, and oxygen-deficient) attracts additional interest as a perspective candidate for nonvolatile resistive random access memory (ReRAM) and ferroelectric random access memory (FeRAM). Hafnia is widely used in optical and protective coatings in the production of special types of glass for fiber optic products.

Hafnia has defects. The most thermodynamically favorable defects are oxygen vacancies (VO). The defects act as electron and hole traps and play a dual role in silicon devices. If HfO2 is used as a gate dielectric layer and the blocking elements in a flash memory, then the electrons capture on the oxygen vacancies (traps) and conductivity via traps will play negative roles, devices will degrade, reliability will reduce. At the same time, using the hafnia as the storage medium in the flash memory cell requires localization of electrons (and holes) at the traps to store information. Hence, understanding the nature of traps responsible for the electrons (and holes) localization and the charge transport is required to control the properties of HfO2-based electronic devices.

According to quantum-chemical simulations, there are energy levels in the band gap for neutral and charged oxygen vacancies in HfO2 as shown in Fig.1: (a) [Xiong K et al., Appl. Phys. Lett.
87, 18350 (2005)], (b) [Gavartin J L et al., Appl. Phys. Lett.
89, 36 (082908)], and (c) [Broqvist P, Pasquarello A, Appl. Phys. Lett.
89, 262904 (2006)]. Indeed, XPS experiments and ab initio simulations show electron states at 3.3eV above the valence band ceiling (Fig.2) [Perevalov T et al., Microelectron. Eng.
109, 21 (2013)]. Optical transitions on the VO accord to blue photoluminescence (PL) band at 2.7eV [Ito T et al., J. Appl. Phys.
97, 054104 (2005)], which is excited by photons with energy near 5.2eV (PLE) [Perevalov T V et al., Appl. Phys. Lett.
104, 071904 (2014)] (Fig.3). The configuration coordination energy diagrams of optical transition on a neutral oxygen vacancy in hafnia is shown in Fig.4.

The current through HfO2 in hafnia-based structures grows exponentially with increasing of the gate voltage and temperature. So, Fowler-Nordheim emission does not limit the current in HfO2. Many studies of the charge transport in HfO2 described experiment results by Poole-Frenkel (PF) mechanism. However, the fitting parameters, obtained by experimental data analysis in terms of PF model, are far from realistic values. E.g., either the frequency factor in 5-7 order lower than expected, or trap density in 13-16 orders lower than values obtained from CV measurements. It was shown [Islamov D R et al., Appl. Phys. Lett.
105, 222901 (2014)] that the charge transport in HfO2 is described by phonon-assisted tunneling between traps (PATT) in the bulk of HfO2 [Nasyrov K A, Gritsenko V A, J. App. Phys.
109, 093705 (2011)]. Found thermal trap energy of 1.25eV and optical trap energy of 2.5eV, obtained from analysis of experiment data with PATT model, do not depend on technique of HfO2 synthesis. Fig.5 shows experiment I-V characteristics with results of PATT calculations. Fig.6 shows configuration coordination energy diagram of trap ionization process on negative charged oxygen vacancy in hafnia. Despite the fact that obtained trap density is ~1019-1020cm-3, CV measurements shows that trapped charge density is ~5×1018cm-3 [Islamov D R et al., Appl. Phys. Lett.
105, 222901 (2014); Lu W-T et al., Appl. Phys. Lett.
85, 3525 (2004)]. This phenomenon can be explained by the Coulomb repulsion of charge carriers, leading to the Wigner crystallization of trapped electrons (holes) to form a hexagonal glassy structure [Shaimeev S S, Gritsenko V A, Wong H, Appl. Phys. Lett.
96, 263510 (2010)].