Thin (∼18 nm) polyoxide films have been grown on phosphorus‐implanted polycrystalline silicon (poly‐Si) by rapid thermal oxdiation (RTO). With an emphasis on the bias polarity dependence, we have studied the electrical characteristics of polyoxides, such as leakage current, breakdown field, and charge trapping. In comparison with polyoxides grown in conventional furnace, RTO polyoxides exhibit a significantly reduced leakage current. In addition, the dielectric breakdown strength and breakdown field distribution have been improved. When electrons were injected from the top poly‐Si/SiO2 interface, RTO polyoxide shows a reduced trapping rate. However, when electrons were injected from the SiO2/bottom poly‐Si interface, RTO polyoxide shows an increased trapping rate.