We have investigated a Ge δ-doped layer in Si(001) formed by solid phase epitaxy (SPE) using time-of-flight low-ener-gy ion scattering spectroscopy (TOF-LEIS). The Ge δ-doped layer was formed by annealing a Si buffer layer deposited on a Ge(1ML)/Si(001) surface. We found that Ge atoms diffuse from the δ-doped layer around the interface as a result of SPE growth of the Si buffer layer. Incident angle dependence of Ge and Si intensities at various annealing temperatures showed that the Ge atoms began to occupy Si lattice sites above 300°C; indicating Ge-Si alloying near the interface region, and that the diffusion of Ge atoms was promoted above 500°C accompanied by the crystallization process of the Si buffer layer. The thickness of the Si₁-_xGe_x layer at 600°C was estimated to be about 30 Å, where the Si buffer layer was nearly perfectly crystallized.