| AVS 61st International Symposium & Exhibition | |
| Thin Film | Tuesday Sessions |
| Session TF+PS-TuM |
| Session: | ALD for Emerging Applications |
| Presenter: | Virginia Wheeler, Naval Research Laboratory |
| Authors: | V.D. Wheeler, Naval Research Laboratory M. Tadjer, Naval Research Laboratory N. Nepal, Naval Research Laboratory M. Currie, Naval Research Laboratory Z.R. Robinson, Naval Research Laboratory M.A. Mastro, Naval Research Laboratory K. Cheung, Naval Research Laboratory F. Kub, Naval Research Laboratory C.R. Eddy, Naval Research Laboratory |
| Correspondent: | Click to Email |
Vanadium oxides are thermochromic materials which have significant changes in thermal emittance, optical transmittance and reflectance, and intrinsic electrical properties due to a metal-insulator phase transition (MIT). These materials offer great advantages in a variety of applications including electrochemical applications, energy storage, thermoelectric devices, Mott transistors, and smart windows. In this work, atomic layer deposition (ALD) was used to produce thin, highly uniform, amorphous VO2 films which enabled the ability to investigate the impact structure (amorphous vs. crystalline) has on the rate of change of intrinsic properties due to the MIT.
Amorphous VOx films (5-45nm) were deposited by ALD at 150°C using tetrakis(ethylmethyl)amido vanadium and ozone precursors. X-ray photoelectron spectroscopy (XPS) was used to verify the quality, stoichiometry, and depth uniformity of the films. All as-grown films exhibited carbon surface contamination due to atmospheric transfer from the ALD to XPS chambers. Moreover, the top ~1nm of the film exhibited V2p peaks at 517.7 and 516.3eV correlating to V2O5 and VO2 components, respectively. At depths >1nm, XPS showed no residual carbon contamination and only a single VO2 peak with a FWHM from 2-2.7 eV, which is similar to crystalline films and indicative of the high uniformity and quality of these films. XPS depth profiles near the VOx/Si interface had a low binding energy shoulder at 513.5 eV, suggesting that initially the films are very oxygen deficient.
The influence of ex situ anneal temperature (200-550°C), time (0.17-2hr), and gas environment (forming gas, Ar, O2, and ozone) on the ability to obtain single phase, crystalline VO2 films was also examined. Initial results show that only O2 anneals produce crystalline VO2, but other factors such as gas flow, duration, and temperature require optimization to inhibit multiphase, polycrystalline films.
Electrical and optical performance of amorphous and crystalline ALD films was assessed from 77-500K and 300-380K, respectively. Unlike crystalline VO2 films that exhibit an abrupt, up to five orders of magnitude change in resistance around the MIT at 60°C (333K), amorphous VO2 films had an exponential change in resistance of ten orders of magnitude over the entire temperature range studied. Also, an average activation energy of -0.20eV and temperature coefficient of resistance of 2.39% at 310K was extracted. These results suggest that amorphous VO2 films, with less structural order, have the potential to induce larger, more gradual electrical changes that could be useful for bolometers or passive thermal management on spacecraft.