Paper TF-FrM2
X-ray Absorption Spectroscopy Study of Nanocomposite Thin Films Grown by Atomic Layer Deposition

Friday, November 11, 2016, 8:40 am, Room 105A

We have established an ALD approach to synthesize nanocomposite coatings comprised of conducting, metallic nanoparticles embedded in an amorphous dielectric matrix. These films are nominally composed of M:Al₂O₃ where (M= W and Mo) and are prepared using alternating exposures to trimethyl aluminum (TMA) and H₂O for the Al₂O₃ ALD and alternating MF₆/Si₂H₆ exposures for the metal ALD. By varying the ratio of ALD cycles for the metal and the Al₂O₃ components during material growth, we can tune precisely the various material properties such as microstructure, electrical, optical and chemical properties. We have exploited these nanocomposite coatings in several applications such as resistive coatings for large-area microchannel plates suitable for large area photodetectors, charge drain coatings for electron-optic MEMS devices (Digital Pattern Generation chips) for maskless reflection electron beam lithography, protective barrier coatings for Li-ion battery cathodes and solar selective absorber coating for high temperature concentrated solar power (CSP) components.

The ALD surface chemistry for these M:Al₂O₃ nanocomposite films is complex, particularly during the transitions between the Al₂O₃ and the metal ALD since the surface functional groups are completely different for these two types of processes. To better understand the relationship between the ALD surface chemistry and the resulting microstructure and composition of these nanocomposite materials, we used a suite of analytical methods including transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and synchrotron X-ray absorption spectroscopy (XAS) performed at the Argonne Advanced Photon Source to characterize W:Al₂O₃ films while varying the W cycle ratio, W%=(W cycles)/(total cycles)*100. A key result was that for W% < 50, W is present in both metallic and sub-oxide states whereas for W% ≥50, only metallic W is seen. This transition from dielectric to metallic character at W% ~50 is accompanied by an increase in the electrical conductivity and the disappearance of a clear bandgap in the absorption spectrum. TEM revealed that the conducting phase is composed of 1-2 nm metallic nanoparticles embedded in an amorphous matrix. We believe that these nanoparticles form spontaneously during the TMA exposure following a W ALD cycle, and that the TMA acts as a reducing agent.