Transmission electron microscopy analysis of reduction reactions and phase transformations in Nb2O5 films deposited by atomic layer deposition

• Published in: Journal of applied physics 2021-01, Vol.129 (2)
• Main Authors: Twigg, M. E., Kozen, A. C., Ruppalt, L. B., Prokes, S. M., Cho, H. S.
• Format: Article
• Language: English
• Subjects: Annealing; Applied physics; Atomic layer epitaxy; Chemical reduction; Composition; Crystallization; Membranes; Niobium oxides; Phase transitions; Silicon dioxide; Silicon substrates; Transmission electron microscopy
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/5.0035535

Amorphous films of Nb2O5 composition were deposited by thermal atomic layer deposition on (001) Si substrates and subsequently crystallized by annealing in forming gas at temperatures ranging from 550 °C to 1000 °C. After subjecting these films to an 850 °C anneal, cross-sectional transmission electron microscopy revealed the presence of B-Nb2O5 and T-Nb2O5 phases in the matrix, as well as reduced R-NbO2 in floret-shaped regions. Annealing at 1000 °C completed the reduction process, resulting in the insulator-to-metal transition (IMT) capable T-NbO2 phase throughout the film. ALD films of composition Nb2O5 were also deposited on electron-transparent SiN membranes and then subjected to 550 °C and 1000 °C anneals. Here, the 550 °C anneal induced the B-Nb2O5 and T-Nb2O5 phases without inducing the R-NbO2 phase. The 1000 °C anneal of the films deposited on SiN membranes retained B-Nb2O5 while inducing the R-NbO2 phase, but without bringing the process to completion and inducing the T-NbO2 phase. The effectiveness of the 1000 °C reducing annealing treatment to induce the T-NbO2 phase for Nb2O5 films deposited on (001) Si substrates, while stopping short of this transition for films deposited on SiN membranes, suggests the importance of the SiO2 layer on the Si substrate in contributing to the reduction reaction that results in the technologically important insulator-to-metal transition (IMT)-capable T-NbO2 phase.