A phosphorus doping study was conducted in Si1−xSnx (x = 0.017–0.079) epitaxial layers grown on Si(001) by RF sputtering. First, we found high thermal stability of the Si1−xSnx layer even at 600 °C regardless of the ultra-low Sn solubility in Si. Thanks to this fact, we have successfully realized n-type Si1−xSnx layers with wide-ranging electron concentrations (6.4×1017–1.0×1020 cm–3) without the Sn precipitation by the ion-implantation and the activation annealing at 600 °C. Additionally, we found that pseudomorphic grown layers possess comparable electron mobility with that of Si bulk.