Vanadium Dioxide (VO₂) is an interesting 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 convenient for many applications, it is desirable to modify the T_c to other values for either new applications or improved performance in the previously stated applications. The T_c of VO₂ is known to be strain-mediated.[1] Strain in VO₂ has been induced via deposition of thin films onto lattice mismatched epitaxial substrates, as well as by doping with other transition metal elements besides vandium.[2], [3]

For the first time, we are investigating digital doping of ALD VO₂ using other elements such as Al, Ti, Nb, and W by incorporation of their oxides into the binary ALD VO₂ process (TEMAV + O₃). We will discuss the fundamentals and limitations of doped ALD VO₂ growth and basic materials characterization, and will demonstrate the impact of dopant concentration and identity on the optical and electrical phase transition properties of the resulting films.

Generally, inclusion of dopants such as Ti and Al into the ALD VO₂ process in amounts between 1% and 10% results in surfactant-like film growth behavior, with increasing dopant concentrations reducing RMS roughness of the ALD films from ~2 nm to ~0.2 nm. Higher concentrations of dopants also interfere with film crystallization during annealing, inhibiting the magnitude of the metal-insulator transition. Smaller concentrations of dopants, between 1% and 3%, can lower the T_c of the VO₂ films while maintaining acceptable optical behavior, depending on dopant identity and concentration. In particular, we find that Nb doping at these low concentrations decreases T_c of VO₂ by -13.5˚C/Nb%.

[1] J. M. Atkin, S. Berweger, E. K. Chavez, M. B. Raschke, J. Cao, W. Fan, and J. Wu, “Strain and temperature dependence of the insulating phases of VO 2near the metal-insulator transition,” Phys. Rev. B, vol. 85, no. 2, pp. 020101–4, Jan. 2012.

[2] 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.

[3] 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.