Control of the surface morphology of ZnO thin films for solar cells by novel two-step MOCVD process

Polycrystalline ZnO films with various sizes and shapes of grain are required for their further applications as transparent conducting material in solar cell in order not only to scatter sunlight in wider wavelength range but also to avoid the shunting due to strongly rough surface morphology. In this study, two-step growth method of metalorganic chemical vapor deposition (MOCVD) process is proposed to modify the surface morphology of the films. In the proposed method, the film was grown in two steps successively. For the first layer, the tetrapodlike surface morphology of ZnO film with a [110] grain orientation was grown using the diethylzinc/H₂O reactant system at a substrate temperature of 150°C. The film having smoother surface morphology with [002] and/or (100) grain orientations were then grown on the top of the first layer using diethylzinc/ethanol/H₂O reactant system. By varying the thickness of both layers, ZnO film with various shapes of textured and various grain sizes could be successfully deposited. B₂H₆ was also employed as n-type dopant gas for increasing the conductivity of the film. The obtained films are proper for being used as transparent conducting electrode.