Hollow-cathode plasma deposited vanadium oxide films: Metal precursor influence on growth and material properties

Due to its different polymorphs, including vanadium pentoxide (V₂O₅) and vanadium dioxide (VO₂), the vanadium oxide (VO_X) compound is an immensely interesting material with many important applications. While atomic layer deposition (ALD) is among the possible VO_X film synthesis methods, literature reports have majorly utilized thermal-ALD, which reveals as-grown amorphous VO_X films. Further post-deposition annealing process is needed to crystallize these films. High-temperature crystallization indeed limits the use of low-temperature compatible materials, processes, and substrates. In this work, we report on the low-temperature crystalline VO_X film growth in a hollow-cathode plasma-enhanced atomic layer deposition reactor using two different vanadium precursors, tetrakis(ethylmethylamino)vanadium and vanadium(V) oxytriisopropoxide. Oxygen plasmas were used as co-reactants at a substrate temperature of 150 °C. Along with the purpose of investing in the impact of metal precursors on VO_X film growth, we also studied Ar-plasma in situ and thermal ex situ annealing to investigate possible structural enhancement and phase transformation. In situ Ar-plasma annealing was performed with 20 s, 20 SCCM Ar-plasma, while post-deposition ex situ annealing was carried out at 500 °C and 0.5 mTorr O₂ pressure. In situ ellipsometry was performed to record instant film thickness variation and several ex situ characterizations were performed to extract the optical, structural, and electrical properties of the films. The outcomes of the study confirm that both metal precursors result in as-grown crystalline V₂O₅ films at 150 °C. On the other hand, post-deposition annealing converted the as-grown crystalline V₂O₅ film to VO₂ film. Finally, we have also successfully confirmed the metal-to-insulator transition property of the annealed VO₂ films via temperature-dependent structural and electrical measurements.