Combining reactive sputtering and rapid thermal processing for synthesis and discovery of metal oxynitrides
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Recent efforts have demonstrated enhanced tailoring of material functionality with mixed anion materials, yet exploratory research with mixed anion chemistries is limited by the sensitivity of these materials to synthesis conditions. Synthesis of a particular metal oxynitride compound by traditional reactive annealing requires specific, limited ranges of both oxygen and nitrogen chemical potentials to establish equilibrium between the solid-state material and a reactive atmosphere. Using Ta–O–N as an example system, we describe a combination of reactive sputter deposition and rapid thermal processing (RTP) for synthesis of mixed anion inorganic materials. Heuristic optimization of reactive gas pressures to attain a desired anion stoichiometry is discussed, and the ability of RTP to enable amorphous to crystalline transitions without preferential anion loss is demonstrated through the controlled synthesis of nitride, oxide, and oxynitride phases.
I. INTRODUCTION
Contributing Editor: Rong-Jun Xie a) Address all correspondence to this author. e-mail: [email protected] b) Equal contribution authors DOI: 10.1557/jmr.2015.140
exhibit desirable band energetics for solar water oxidation with valence band energies more positive than metal oxide analogs, yet the chemical instability of metal nitrides in aqueous conditions has largely rendered them unsuitable for aqueous photoelectrocatalysis. Initial investigations into metal oxynitride compounds and oxide–nitride alloys have demonstrated the combined desirable properties of aqueous stability and band energetics necessary for a photoanode in a solar fuel device.14 To enable efficient exploration of metal oxynitride phase space, we describe a methodology for synthesis of oxynitride thin films using a combination of physical vapor deposition (PVD) and rapid thermal processing (RTP). We note that the technique may be expanded for the general synthesis of mixed anion materials. By regulating the partial pressure (PP) of O2 and N2 gases during reactive magnetron sputtering, anion stoichiometry can be systematically controlled and is unchanged during RTP crystallization of the film into high-temperature phases. A variety of techniques,15 such as ammonolysis,1,16 chemical vapor deposition,17 intercalation,18 reactive cathodic arc evaporation,19 wet-chemical approaches,2 and PVD20,21 have been used for the synthesis of metal oxynitride materials. While ammonothermal treatments have been widely used for the synthesis of oxynitrides,1 high-temperature processing of oxides in NH3 to form oxynitrides requires careful control of reaction kinetics, as the formation of nitrides is thermodynamically favored over oxynitrides in the oxygen-free and high-nitrogen chemical potential environment. RTP of metal films in O2 and then in NH3 (Ref. 22) or directly in mixed O2/N2
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Ó Materials Research Society 2015
While metal oxides, nitrides, carbides, sulfides, etc. are widely used in numerous applications, such as electronics, optoelectronics, photocatalysis, and energy technologies, superior m
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