Solid State Properties of Anodic Hf-Nb Mixed Oxides

Abstract

In last decades, HfO2 and Nb2O5 have been extensively studied due to their many potential applications, from corrosion protection to CMOS (as high-k gate dielectrics) [1-2] and ReRAM technologies [3-4]. For these 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 needful. Scientific community gradually shifted its interest from pure metal oxides to mixed metal oxides trying to exploit the advantages of pure oxides and to suppress their disadvantages. Mixed oxides can be grown on valve metals alloys by anodizing, a simple and low cost electrochemical process for preparing oxides, whose structure, thickness, composition and morphology can be easily tailored by changing substrate composition or some growth parameters. Few papers have been recently addressed on studying the anodizing behavior of Hf-Nb alloys [5], thus there is need of further studies on this subject. This work is aimed to study the solid state properties of anodic films on sputtering deposited Hf-Nb alloys. Mixed oxides were prepared by anodizing alloys with different composition (from pure Hf to Hf-76at.%Nb) to several formation voltages in neutral or alkaline solutions. Morphological and structural features were analyzed by ex-situ XRay Diffraction, Glow Discharge Optical Emission Spectroscopy, Rutherford Backscattering, Transmission and Scanning Electron Microscopy. A photoelectrochemical investigation was performed in order to estimate band gap, Eg, and flat band potential, UFB, of the oxides as a function of the growing conditions. In order to get information on films dielectric properties, differential capacitance and electrochemical impedance measurements were also carried out for all the specimens. The experimental findings provide evidence that crystalline oxide are formed on Hf rich alloys with insulating behavior, whilst Nb-rich anodic films resulted to be amorphous and showed n-type semiconducting properties. Occurrence of sub-band gap optical transitions was detected, which has been correlated to structural defects in the anodic films (namely oxygen vacancies), whose presence can be detrimental in view of possible application of the investigated oxides in MOS based devices. References [1] D. Munoz Ramo et al., Phys. Rev. Lett., 99, (2007), 155504. [2] M.D. Pickett et al., Nanotechnology, 23, (2012), 215202. [3] A. Wedig et al., Nat. Nanotechnology, 11, (2016), 67-74. [4] S.K. Nandi et al., J. Phys. D. Appl. Phys., 48, (2015), 195105. [5] A.I. Mardare et al., Electrochim. Acta, 110, (2013), 539-549.