The relentless scaling of semiconductor devices pushes the doping level far above the equilibrium solubility, yet the doped material must be sufficiently stable for subsequent device fabrication and operation. For example, in epitaxial silicon doped above the solubility of phosphorus, most phosphorus dopants are compensated by vacancies, and some of the phosphorus-vacancy clusters can become mobile around 700 °C to further cluster with isolated phosphorus ions. For efficient and stable doping, we use microwave annealing to selectively activate metastable phosphorus-vacancy clusters by interacting with their dipole moments, while keeping lattice heating below 700 °C. In a 30-nm-thick Si nanosheet doped with 3 × 1021 cm-3 phosphorus, a microwave power of 12 kW at 2.45 GHz for 6 min resulted in a free-electron concentration of 4 × 1020 cm-3 and a junction more abrupt than 4 decades/nm. The doping profile is stable with less than 4% variation upon thermal annealing around 700 °C for 5 min. Thus, microwave annealing can result in not only efficient activation and abrupt profile in epitaxial silicon but also thermal stability. In comparison, conventional rapid thermal annealing can generate a junction as abrupt as microwave annealing but 25% higher sheet resistance and six times higher instability at 700 °C.