Paper TF-MoA2
Digital Doping of ALD Nb:VO₂ Thin Films for Thermochromic Applications

Monday, October 30, 2017, 2:00 pm, Room 20

Vanadium Dioxide (VO₂) is a thermochromic material that undergoes a first order crystalline phase transition at a critical temperature (T_c) of 68˚C. This structural phase transition is accompanied by major changes in electrical and optical properties, particularly in the infrared. As such, VO₂ is suitable for many applications including microbolometers, adaptive thermal coatings, and passive spacecraft thermal shielding.

While the T_c of VO₂ is appropriate for many current applications, it is desirable to modify the T_c to other values for improved performance of current applications or to address new emerging application needs. The T_c of VO₂ has been shifted by doping with other transition metals on the order of 1-5%.[1], [2], however these doped VO₂ films were deposited by PLD, MBE, and sputtering, all line of sight physical vapor deposition techniques.

We demonstrate digital doping of ALD Nb:VO₂ by incorporation of ALD Nb₂O₅ (TBTDEN + O₃) into the conventional ALD VO₂ process (TEMAV + O₃). Inclusion of Nb into the ALD VO₂ process in amounts between 1% and 5% makes minimal impact on as-deposited film morphology. After annealing, the Nb:VO₂ films also exhibit minimal physical changes from their undoped analogues. We find that Nb doping can lower the T_c of the VO₂ films by -13.5˚C/Nb% while maintaining acceptable optical modulation behavior, critical for use of doped VO₂ in emerging applications.

[1] W. Burkhardt, T. Christmann, B. K. Meyer, and W. Niessner, “W-and F-doped VO 2 films studied by photoelectron spectrometry,” Thin Solid Films, vol. 345, no. 2, pp. 229–235, 1999.

[2] X. Wu, Z. Wu, C. Ji, H. Zhang, Y. Su, Z. Huang, J. Gou, X. Wei, J. Wang, and Y. Jiang, “THz Transmittance and Electrical Properties Tuning across IMT in Vanadium Dioxide Films by Al Doping,” ACS Appl. Mater. Interfaces, vol. 8, no. 18, pp. 11842–11850, May 2016.