| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Thin Films | Monday Sessions |
| Session TF-MoM |
| Session: | Advances/Innovation in Synthesis & Characterization |
| Presenter: | Virginia Wheeler, US Naval Research Laboratory, USA |
| Authors: | V.D. Wheeler, US Naval Research Laboratory, USA B.P. Downey, US Naval Research Laboratory, USA M. Currie, US Naval Research Laboratory, USA M. Tadjer, US Naval Research Laboratory, USA A. Kozen, US Naval Research Laboratory, USA R. Bell, Cornell University, USA M. Thompson, Cornell University, USA D.J. Meyer, US Naval Research Laboratory, USA F.J. Kub, US Naval Research Laboratory, USA C. Eddy Jr., US Naval Research Laboratory, USA |
| Correspondent: | Click to Email |
VO2 is a phase change material that exhibits significant changes in thermal emittance, optical transmittance and reflectance, and intrinsic electrical properties due to a metal-insulator phase transition (MIT) at a critical temperature (Tc). Electrical and optical properties across the MIT are strongly reliant on microstructural characteristics which are dependent on deposition technique. While several different methods have been explored to deposit VO2 films, atomic layer deposition (ALD) provides a way to obtain large area film uniformity, abrupt interfaces and angstrom-scale control of thickness conformally across planar and high surface area nanostructures. Thus, this method could be used to integrate VO2 films into complex electronic and optical devices for additional functionality. Moreover, as-deposited films are amorphous which can be subsequently annealed to tailor the crystallinity and thus the MIT properties. Here, we will show the properties of amorphous and crystalline ALD VO2 films and discuss their integration with other materials to achieve functional optical and electrical coatings.
Amorphous ALD VO2 films were deposited at 150°C using TEMAV and ozone precursors. XPS revealed that beneath ~1nm of atmospheric contamination, there was no residual carbon impurities and only a single VO2 peak was evident. Electrical measurements showed an exponential change in resistance of ten orders of magnitude from 77-500K, though no characteristic sharp transition was detected. This resulted in an average activation energy of -0.20eV and temperature coefficient of resistance of 2.39% at 310K.
To realize sharp MIT transitions, ALD VO2 films were crystallized with an optimized ex-situ anneal at 650°C in 1.5x10-5 Torr of oxygen for 1-4hrs depending on film thickness. Electrical measurements showed that the Roff/Ron ratio increased from 3.4 to 7083 as VO2 thickness increased from 12 to 92nm. The Tc also increased with increasing thickness, while all films exhibited relatively low hysteresis (<8°C). Temperature dependent near-IR measurements also show an increase in the change in reflectivity (up to 80%) and decrease in transmission (up to 60%) with film thickness.
ALD is a preferred method to deposit on soft materials due to its low deposition temperature. However, for VO2 films, this benefit is negated by the higher anneal temperatures required for crystallization. To overcome this problem, laser annealing was investigated as an alternative crystallization approach. Initial results show that by varying the laser power one can tailor the local temperature within the VO2 film to attain the required crystallinity without affecting the underlying substrate.