Plasma-deposited aluminum-doped zinc oxide : controlling nucleation, growth and electrical properties

In this work, the Expanding Thermal Plasma (ETP) deposition technique was employed to study the growth development of ZnO:Al thin film as a transparent conductive oxide layer for sustainable applications. Characteristic for ETP-grown ZnO:Al is a gradually reducing resistivity of the growing layer with increase in the layer thickness, so-called resistivity gradient, due to the gradually increasing grain size and mobility of the electrons. Therefore, structural development is studied in relationship with the electrical and optical properties of the films. Several possibilities are shown to improve the ZnO:Al properties. For example, passivation of grain boundary scattering centers is demonstrated upon capping ZnO:Al films with a-Si:H and subsequent high temperature treatment. Another approach achieved reduction of the resistivity gradient by formation of compact nucleation layer with a large grain size and multiple crystal orientations. That lead to achievement of resistivity as low as 3.9•10-4 O•cm at significantly lower thickness of 300 nm, compared to 700 nm for the reference film. Furthermore, the plasma processes leading to this improvement are studied in detail and a possible growth mechanism is proposed. Finally, the effect of improved nucleation of ZnO:Al grains is studied by using small (3-5 nm in diameter) silver nanoparticles deposited prior ZnO:Al growth. The silver nanoparticles act as an epitaxial template, also causing reduction of void inclusion throughout the film thickness and reduced as a factor of 2 resistivity in the thickness range up to 1100 nm.