(99c) Transparent Conductive Oxide Nanocrystals Coated with Insulators By Atomic Layer Deposition

Thin films comprised of transparent conducting oxide (TCO) nanocrystals are attractive for a number of optoelectronic applications. However, it is often observed that the conductivity of such films is very low when in contact with air. It has recently been demonstrated, somewhat surprisingly, that filling in films comprised of initially insulating TCO nanocrystals with another insulator by atomic layer deposition (ALD) dramatically increases the conductivity by many orders of magnitude. This work aims to elucidate the mechanism by which the ALD coating increases the conductivity. We used an in situ electrical probe to examine the effect on sheet resistance of removing two adsorbed oxygen species (physisorbed molecular water and chemisorbed hydroxide) and compared this result to thin films comprised of ZnO nanocrystals coated with Al₂O₃ and also HfO₂ by ALD. Although both insulating infills decrease the sheet resistance and increase the stability of the films, there is a stark discrepancy between the two. From the in situ measurements, it was found that coating with Al₂O₃ removes both physisorbed water and chemisorbed hydroxide resulting in a net reduction of the ZnO nanocrystals. Coating with HfO₂ only removes physisorbed water, which was confirmed by Fourier transform infrared spectroscopy. A similar phenomenon was observed when coating thin films comprised of Sn-doped In₂O₃ nanocrystals with Al₂O₃ by ALD; suggesting Al₂O₃ can be used to reduce and stabilize metal-oxide nanocrystals in general.