Electrochemical monitoring of TiO₂ atomic layer deposition by chronoamperometry and scanning electrochemical microscopy

The scanning electrochemical microscope (SECM) was used to characterize the atomic layer deposition (ALD) of TiO₂ on indium-doped tin oxide (ITO) substrates by studying electron transfer through pores in the thin films (1-5 nm thickness). The extent of electron transfer, and thus the porosity of the films, was evaluated by transient electrochemistry. These studies show that ALD deposition of TiO₂ on ITO does not produce pinhole-free films but rather porous deposits with electrochemical behavior similar to that of microelectrode arrays up to about 30 ALD cycles. All the experimental results are explained in the context of a numerical model developed by finite element analysis and corroborated by complementary conductive atomic force microscopy (cAFM) results that directly reveal localized, nanoscale current conduction paths in thinner TiO₂ layers with a transition to more spatially uniform conduction in the thickest layers. SECM images demonstrate the existence of pinholes even on films that have been subjected to more than 100 ALD cycles (thicknesses larger than 4 nm).