Metastable Ta2N3 with highly tunable electrical conductivity via oxygen incorporation.

Chang-Ming Jiang,Matthew K Horton,Kristin A Persson,Ian D Sharp,Tim Rieth,Viktoria F Kunzelmann,Yanbo Li,Max Kraut,Laura I Wagner,Johanna Eichhorn

doi:10.1039/d1mh00017a

Abstract

The binary Ta–N chemical system includes several compounds with notable prospects in microelectronics, solar energy harvesting, and catalysis. Among these, metallic TaN and semiconducting Ta3N5 have garnered significant interest, in part due to their synthetic accessibility. However, tantalum sesquinitride (Ta2N3) possesses an intermediate composition and largely unknown physical properties owing to its metastable nature. Herein, Ta2N3 is directly deposited by reactive magnetron sputtering and its optoelectronic properties are characterized. Combining these results with density functional theory provides insights into the critical role of oxygen in both synthesis and electronic structure. While the inclusion of oxygen in the process gas is critical to Ta2N3 formation, the resulting oxygen incorporation in structural vacancies drastically modifies the free electron concentration in the as-grown material, thus leading to a semiconducting character with a 1.9 eV bandgap. Reducing the oxygen impurity concentration via post-synthetic ammonia annealing increases the conductivity by seven orders of magnitude and yields the metallic characteristics of a degenerate semiconductor, consistent with theoretical predictions. Thus, this inverse oxygen doping approach – by which the carrier concentration is reduced by the oxygen impurity – offers a unique opportunity to tailor the optoelectronic properties of Ta2N3 for applications ranging from photochemical energy conversion to advanced photonics.

Highlights

We find that the formation of metastable Ta2N3 hinges upon the presence of a small amount (0.65%) of O2 in the process gas mixture, which provides an inductive effect to stabilize higher oxidation states of Ta and suppress formation of metallic tantalum mononitride (TaN)
Rather than attempting to achieve pure nitride synthesis, we suggest that nitride-related research should leverage the impact of oxygen impurities in phase formation and stabilization, such that the transition metal nitride chemical space can be more effectively explored
Comparison of computed phase stabilities with structural characteristics of these films provided significant insight into the metastable nature of Ta2N3, as well as the role of incorporated oxygen on the energetic window of metastability

Summary

Introduction

Transition metal nitrides represent a versatile chemical space for creating new functional materials with tunable properties for a variety of applications. This n-type semiconductor features a 2.1 eV direct bandgap and suitable band edge positions for driving water splitting reactions It has garnered extensive research interest for solar energy conversion.[6,20,21,22] The most common synthesis method of Ta3N5 is through nitridation of Ta2O5 in an ammonia flow at 850–1000 1C, though recently there are reports of direct Ta3N5 thin film deposition via atomic layer deposition[23] and reactive magnetron sputtering.[24] In contrast, tantalum mononitride (TaN) can form different polymorphs, including the d-, e-, and y-phases, which exhibit high electrical conductivities. Ta2N3 with controlled oxygen content represents a versatile system with tunable metallic and semiconducting character for potential applications ranging from advanced plasmonics to photoelectrochemical energy conversion

Results and discussion

Conclusions

Conflicts of interest