Sammendrag
Transparent conducting materials such as tin-doped indium oxide (ITO) and doped ZnO based films are widely used in photovoltaic and electro-optical devices due to their high optical transmission and excellent electrical conductivity. It is well established that depending on growth or post-deposition treatment conditions of TCO layers, a nanostructure formation in the bulk and on the surface of such materials occurs. In particular exposure of ITO layers to hydrogen-containing plasma, typically generated by plasma-enhanced chemical vapour deposition during the fabrication of hydrogenated amorphous (a-Si:H) based transistors or solar cells, causes a chemical reduction of indium and tin oxides to their corresponding metallic elements and the formation of metallic nanoparticles. The goal of this work is to investigate conditions at which metallic nanoparticles can be formed upon the growth and post-deposition treatments of TCO materials. Magnetron sputtering of thin ITO and ZnO layers on different substrates (Si, a-Si:H/Si, glass) was studied in this work to investigate peculiarities of the morphology of these materials on a nano-scale. It is found that these peculiarities depend on the type of substrate used and on post deposition treatments. It is shown that various types of compositional and structural changes on ITO thin film surfaces induced by hydrogen occur: formation of nano-structures with sizes around 20-40 nm, metallic spots with sizes starting at 200 nm, and large balls (up to 5 micron). Similar peculiarities were observed also at the ITO/a-Si:H interfaces for Si based heterojunction solar cells. It is suggested that such metallic nano-particles can form conductive nano-channels in the case of ITO/Si heterojunction solar cells and thereby enhance the efficiency of these solar cells. It is established that the ZnO based materials are more resistant under hydrogen plasma treatments. It is found also that metallic nano-structures on the surface of ITO films can