Speciation and the nature of ZnO thin films from chemical bath deposition

Thin films of monophasic crystalline hexagonal ZnO were deposited from solutions of zinc acetate in presence of ethane-1,2-diamine and sodium hydroxide onto glass microscope slides and tin oxide coated glass. At least two distinct morphologies of ZnO were observed, depending on the deposition conditions, by scanning electron microscopy. The films deposited on glass slides were generally more adherent and of better morphology than those on the tin oxide substrate. Calculations on the speciation of zinc in the solution indicate that supersaturation of hydroxy species of zinc is an important factor controlling bath deposition; solutions which are kinetically unstable with respect to the formation of zinc hydroxide deposit ZnO films of the best quality. The bandgaps of these films were estimated to be 3.15 eV, on either substrate, by optical absorption.