Silicon surfaces having native oxides were implanted with 3 keV Cs+ ions and annealed. Implanted doses of 1, 1.7, and 3 X 10(16) Cs/cm(2) led to room-temperature work functions with stable, reproducible values of 2.3+/-0.15 eV after vacuum annealing at 100-560 degreesC. The resulting surfaces have been characterized with regard to the amount of Cs retained, the thermal and environmental stability of the work function, and the composition and chemistry of the implanted layer. The surface layers consisted of a compound of Si-Cs-O, which is compositionally stable to temperatures of similar to400 degreesC in vacuum. In addition, we found that these surfaces are stable with regard to exposures to background gases and ambient air.