Comparative study of highly dense aluminium- and gallium-doped zinc oxide transparent conducting sol-gel thin films

Transparent conducting aluminium- and gallium-doped zinc oxide (AZO and GZO) layers have been deposited by spin coating on glass substrates. The coatings have been sintered in air at 450 °C for 30 min and then post-annealed at 350 °C in a reducing atmosphere for 30 min. The electrical, optical and morphological properties of both coatings have been studied and compared. The conventional sols lead to very thin coating, typically 24 nm for a single layer of AZO and 32 nm of GZO with electrical resistivity of 0.72 and 0.35 Ω cm, respectively. The value however, drastically decreases down to a minimum of 2.6 × 10−2 Ω cm for AZO and 1.76 × 10−2 Ω cm for GZO, when five multilayer coatings are made. The origin of these differences is due to the different morphology of the coatings showing different electron scattering process. The GZO sol leads to denser smoother structure (porosity of 5%) layers with an average roughness of 2.76 A, while the AZO coating is formed by a more porous assembly (porosity of 20%) with an average roughness of 3.46 A. Both coatings exhibit high transparency (T > 85%) in the visible spectrum range with a slight shift of the absorption energy gap.