Orthorhombic Ta3-xN5-yOy thin films grown by unbalanced magnetron sputtering: The role of oxygen on structure, composition, and optical properties

Jui-Che Chang,Fredrik Eriksson,Mauricio A Sortica,Grzegorz Greczynski,Babak Bakhit,Zhangjun Hu,Daniel Primetzhofer,Lars Hultman,Jens Birch,Ching-Lien Hsiao

doi:10.1016/j.surfcoat.2020.126665

Jui-Che Chang, Fredrik Eriksson + Show 8 more

Open Access

https://doi.org/10.1016/j.surfcoat.2020.126665

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Abstract

Direct growth of orthorhombic Ta3N5-type Ta-O-N compound thin films, specifically Ta3-xN5-yOy, on Si and sapphire substrates with various atomic fractions is realized by unbalanced magnetron sputtering. Low-degree fiber-textural Ta3-xN5-yOy films were grown through reactive sputtering of Ta in a gas mixture of N2, Ar, and O2 with keeping a partial pressure ratio of 3:2:0.1 in a total working pressure range of 5–30 mTorr. With increasing total pressure from 5 to 30 mTorr, the atomic fraction of O in the as-grown Ta3-xN5-yOy films was found to increase from 0.02 to 0.15 while that of N and Ta decrease from 0.66 to 0.54 and 0.33 to 0.31, respectively, leading to a decrease in b lattice constant up to around 1.3%. Metallic TaNx phases were formed without oxygen. For a working pressure of 40 mTorr, an amorphous, O-rich Ta-N-O compound film with a high O fraction of ~0.48, was formed, mixed with non-stoichiometric TaON and Ta2O5. By analyzing the plasma discharge, the increasing O incorporation is associated with oxide formation on top of the Ta target due to a higher reactivity of Ta with O than with N. The increase of O incorporation in the films also leads to a optical bandgap widening from ~2.22 to ~2.96 eV, which is in agreement with the compositional and structural changes from a crystalline Ta3-xN5-yOy to an amorphous O-rich Ta-O-N compound.

Highlights

Photoelectrolyzing water into hydrogen and oxygen by renewable electricity is a possible method to produce clean hydrogen
From films deposited without introducing oxygen in the reactive gas, the diffractogram showed one high intensity peak located at 2θ = 35.89◦, and one low intensity peak at 2θ = 41.67◦, corresponding to the 111 and 200 diffraction peaks of cubic δ-TaN according to the inorganic crystal structure database (ICSD) pattern no.07-6456, in addition to the 111 and 222 Si substrate peaks at 2θ = 28.56 and 58.86◦, see Fig. 1(a)
Multiple diffraction peaks located at 2θ = 17.35, 31.45, 35.02, 36.04, 39.33, and 53.82◦ were measured, as shown in Fig. 1(b), revealing a polycrystalline structure. These peaks correspond to 020/002, 023, 040/004, 113, 042/ 024, and 060/006 planes of orthorhombic Ta3N5, respectively, plotted in the bottom of the figure according to ICSD pattern no

Summary

Introduction

Photoelectrolyzing water into hydrogen and oxygen by renewable electricity is a possible method to produce clean hydrogen. Other oxide semi conductors, such as Fe2O3, Cu2O [2], and WO3 [3] have been studied as photoanodes because of their stability. These materials suffer from various limitations such as large bandgaps and improper band edge positions for water splitting. Monoclinic and bixbyite tantalum oxynitrides (TaOxNy) and with a bandgap range of 2.4–2.8 eV [5,6,7,11] and ~2 eV [12], respectively, were demonstrated to have a good response for water splitting its valence band energy is lower than Ta3N5. The Ta-O-N compounds are promising materials with tunable bandgap and band positions to fit a desired value for the water splitting

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