Expanding Thermal Plasma Deposition of Al‐Doped ZnO: On the Effect of the Plasma Chemistry on Film Growth Mechanisms. Issue 1 (30th November 2015) - Williams, Benjamin L.
Abstract : 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 −3 Ω · 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 (∼10−4 Ω · 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). Abstract : A review of expanding thermal plasma—chemical vapour deposition (ETP‐CVD) of Al‐doped ZnO (ZnO:Al) transparent conducting oxides (TCOs), alongside new results. Insights into the roles of: (a) plasma chemistry; and (b) surface growth mechanisms on the microstructure and eventual opto‐electronic properties of ZnO:Al films are provided. Poor electrical properties (resistivity, electron mobility) at low film thicknesses is attributed to a growth‐mode driven by high nucleation probability and strong <0002>‐texture which results in small‐grained, voided material. By increasing Zn‐precursor flow rate, the atomic‐O flux arriving at the substrate is reduced, thereby reducing nucleation probability and the dominance of <0002>‐texture, ultimately yielding larger grained, more compact films with significantly improved electrical properties. The plasma chemistry reaction routes are investigated to provide the link between plasma conditions and film properties.