Atomic Layer Deposition of Al2O3 onto Sn-Doped In2O3: Absence of Self-Limited Adsorption during Initial Growth by Oxygen Diffusion from the Substrate and Band Offset Modification by Fermi Level Pinning in Al2O3

The growth of Al2O3 onto Sn-doped In2O3 (ITO) by atomic layer deposition (ALD) was studied in situ using X-ray photoelectron spectroscopy. Significant diffusion of oxygen from the substrate destroys the self-terminated monolayer adsorption of the metal precursor and results in a nominal initial growth per cycle of >1 nm. The observed mechanism precludes the preparation of monolayer thick Al2O3 films on ITO substrates by ALD. The energy band alignment at the ITO/Al2O3 interface is significantly different from that obtained when magnetron sputtering is used for the deposition of Al2O3 onto ITO [Gassenbauer et al., Phys. Chem. Chem. Phys. 2009, 11, 3049]. The difference is attributed to a pinning of the Fermi level in the ALD-Al2O3 layer close to midgap, which is attributed to the incorporation of hydrogen in the film during growth.