Expanding Thermal Plasma Deposition of Al-Doped ZnO: On the Effect of the Plasma Chemistry on Film Growth Mechanisms

This work presents a review of expanding thermal plasma – chemical vapour deposition (ETP-CVD) of Al-doped ZnOtransparent conducting oxides (TCOs), alongside new results providing insights into the role of the plasma chemistry on film microstructure. Standard growth conditions generate high resistivities (>10−3ohm cm) at low film thicknesses (<300 nm) as a result of a high grain boundary and void density. Microscopy studies of the early growth stage reveal that a high nucleation probability and strong <0002>-texture are the causes of this microstructure. We investigate how the precursor feed composition (diethylzinc-to-O2 flow rate ratio) can be utilised to modify the growth mechanism and consequently reduce film resistivity (∼10−4ohm cm), focussing on the role that this flow rate ratio has on the plasma chemistry developing in the downstream region of the expanding plasma (as supported by Langmuir probe and mass spectrometry measurements).