Solid State Properties of Anodic Hf-Nb Mixed Oxides

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Abstract

In last decades, HfO2 and Nb2O5 have been extensively studied due to their manypotential 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 physicalproperties as a function of the fabrication conditions is needful.Scientific community gradually shifted its interest from pure metal oxides to mixedmetal oxides trying to exploit the advantages of pure oxides and to suppress theirdisadvantages. Mixed oxides can be grown on valve metals alloys by anodizing, asimple and low cost electrochemical process for preparing oxides, whose structure,thickness, composition and morphology can be easily tailored by changing substratecomposition or some growth parameters. Few papers have been recently addressed onstudying the anodizing behavior of Hf-Nb alloys [5], thus there is need of further studieson this subject.This work is aimed to study the solid state properties of anodic films on sputteringdeposited Hf-Nb alloys. Mixed oxides were prepared by anodizing alloys with differentcomposition (from pure Hf to Hf-76at.%Nb) to several formation voltages in neutral oralkaline solutions. Morphological and structural features were analyzed by ex-situ XRayDiffraction, Glow Discharge Optical Emission Spectroscopy, RutherfordBackscattering, Transmission and Scanning Electron Microscopy. Aphotoelectrochemical 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. Inorder to get information on films dielectric properties, differential capacitance andelectrochemical impedance measurements were also carried out for all the specimens.The experimental findings provide evidence that crystalline oxide are formed on Hf richalloys with insulating behavior, whilst Nb-rich anodic films resulted to be amorphousand showed n-type semiconducting properties. Occurrence of sub-band gap opticaltransitions was detected, which has been correlated to structural defects in the anodicfilms (namely oxygen vacancies), whose presence can be detrimental in view ofpossible 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.
Lingua originaleEnglish
Numero di pagine1
Stato di pubblicazionePublished - 2016

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