Barium strontium oxide [(BaSr)O] thin films approximately 1 mu m in thickness were deposited on tungsten substrates using rf magnetron sputter deposition for thermionic cathode applications. Three substrate temperatures ranging from 25 to 700 degrees C were used in the deposition processes to create oxide films with different surface morphologies and crystalline structures. The films were characterized with scanning electron microscopy and their surface morphologies were correlated to their thermionic emission properties. The results showed that the surface morphology and crystalline structure of the oxide films strongly affected the emission properties. The oxide film deposited at the lowest substrate temperature of 25 degrees C showed a rough surface and a crystalline structure consisting of nanograins. At higher substrate temperatures, the oxide films exhibited smooth surfaces and close-packed crystalline structures with larger grains. The work function of the oxide films was reduced and the emission current density increased as a result of the increase in the growth temperature. The (BaSr)O film made at 700 degrees C exhibited the lowest work function of 1.57 eV and the largest emission current density of 1.60 A/cm(2) at 1198 K under an electrical field of 0.88 V/mu m. The emission current density and the work function of the (BaSr)O thin film cathodes were stable over the testing period of 8 h. Compared to the traditional cathode fabrication process, which involves the coating of carbonates followed by an activation process, rf magnetron sputtering has a greater ability to control the deposition parameters, which makes it a valuable alternative technique to fabricate oxide cathodes. (c) 2006 American Vacuum Society.