Embodiments of the present invention are directed to fabrication method and resulting structures for vertical tunneling field effect transistors (VFETs) having an oxygen vacancy passivating bottom spacer. In a non-limiting embodiment of the invention, a first semiconductor fin is formed in a first region of a substrate and a second semiconductor fin is formed in a second region of the substrate. A bilayer bottom spacer is formed in direct contact with sidewalls of the semiconductor fins. The bilayer bottom spacer includes a first layer and an oxygen-donating second layer positioned on the first layer. A first dielectric film is formed on the sidewalls of the first semiconductor fin. The first dielectric film terminates on the first layer. A second dielectric film is formed on the sidewalls of the second semiconductor fin. The second dielectric film extends onto a surface of the oxygen-donating second layer.