Growth of β-NaGaO2 thin films using ultrasonic spray pyrolysis

beta-NaGaO2 thin films used as a precursor for thin films of beta-CuGaO2, which is a safe and environment-friendly thin-film solar cell material, are grown using the ultrasonic spray pyrolysis technique employing precursor aqueous solutions dissolving oxide and hydroxide. While the morphology of the obtained films was dependent on the preheating temperature of the precursor solution droplet, [00 l]-oriented beta-NaGaO2 thin films were obtained at a high growth rate of 300 - 500 nm min(-1). The atomic ratios of Ga and Na in the films are the same as those in the precursor solution; this is in contrast with the thin films grown using sputtering or evaporation where the concentration of Na is significantly smaller than that in the source material. This indicates that the spray pyrolysis method can overcome the lack of the high-vapor-pressure sodium component in films owing to its lower deposition temperature and higher pressure of the growth atmosphere compared to the sputtering and evaporation. As the chemical composition of the resultant film is controlled by that of the precursor solution, the spray pyrolysis technique is suitable for growing thin films, particularly for materials that include components with high vapor pressure such as Na and Na2O.