(Invited) Synergistic Use of Electrochemical Impedance Spectroscopy and Photoelectrochemical Measurements for Studying Solid State Properties of Anodic HfO2
Authors: Santamaria Monica , Tranchida Giada , Zaffora Andrea , Di Franco Francesco , Habazaki Hiroki , Di Quarto Francesco 
Published: February 2020

Within the past years, intense research has been carried out on HfO2 as high k material, promising candidate to replace SiO2 as gate dielectric in CMOS based devices (1), and as metal oxide for resistive random access memory (ReRAM) (2). For both technological applications compact, uniform and flat oxides are necessary, and a detailed understanding of their physical properties as a function of the fabrication conditions is strongly. Hafnia performance can be significantly influenced by carrier trapping taking place at pre-existing precursors states (induced by oxygen vacancies, interstitial ions, impurities acting as dopants), or by self-trapping in a perfect lattice, where the potential well is only created by the carrier induced lattice polarization (1). Barrier type anodic films can be grown on valve metals by anodizing, a simple and low cost process for preparing oxides, whose structure, thickness, composition and morphology can be strictly and easily tailored controlling the metal or alloy composition and the oxidation conditions. Since hafnium is a valve metal, anodizing can be an efficient technique to prepare HfO2 layers of controlled features. However, nowadays HfO2 is usually prepared by techniques such as atomic layer deposition, sputtering, pulsed laser deposition, chemical vapour deposition, e-beam evaporation, high energy ion beam assisted deposition and sol-gel methods. Only a few papers have been recently addressed on studying the anodizing behaviour of Hf and Hf alloys (3-4), thus there is need of further studies on this subject. This work is focused on the preparation and characterization of HfO2 films grown by anodizing hafnium in aqueous solutions with particular interest on the effect of anodizing conditions on the solid state properties of the oxides. Therefore, films were grown to different formation voltages (from 5 V to 100 V) potentiodynamically at several scan rates (ranging from 2 mV s-1 to 1 V s-1) or galvanostatically (at 5 mA cm-2) in 0.1 M NaOH. The relationship between current density and electric field strength during the anodizing process allowed to confirm that oxide growth is controlled by a high field mechanism with activation distance in the order of ~ 4 Å and activation energy of 650 meV. Electrochemical Impedance Spectroscopy (EIS) and differential capacitance measurements were employed to study the electrical properties of the oxides and to estimate their permittivity. Fitting of EIS spectra revealed the formation of insulating oxides, with very high resistance and, thus, low leakage current even under electric field close to those measured during the anodizing process (2.0 – 4.0 MV cm-1). Capacitance resulted to be almost independent on electrode potential, but slightly dependent on the frequency of the employed a.c. signal, thus suggesting the presence of allowed states inside the mobility gap of the oxide. These experimental findings were also confirmed by photoelectrochemical measurements, revealing the occurrence of optical transitions at photon energy lower than the band gap reported for HfO2 (ranging between 5.2 and 6.1 eV). We also prepared and characterized Nb doped HfO2 by anodizing sputter-deposited Hf-4at.%Nb in 0.1 M NaOH and N doped HfO2 by anodizing pure Hf in ammonium ions containing solution at pH higher than the pH of zero charge of the oxide, where N incorporation is expected to occur (5). Impedance and photoelectrochemical measurements allowed to evidence the effect of dopants on the electronic properties of the films, which were rationalized considering the influence of Nb or N doping on the density of states of the un-doped hafnia.
References
1 – D. Munoz Ramo et al., Phys. Rev. Lett., 99, 155504 (2007)
2 – A. Weding et al., Nat. Nanotechnology, 11, 67 (2016)
3 – A.I. Mardare et al., J. Electrochem. Soc., 162, E30 (2015)
4 – A.I. Mardare et al., Electrochim. Acta, 110, 539 (2013)
5 – F. Di Franco et al., Electrochim. Acta, 59, 382 (2012).