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Subsurface lattice defects in silicon induced by ion implantation were studied by the use of the photo‐acoustic displacement (PAD) method based on the sensitive measurements of the surface displacement due to the absorption of laser‐light energy. A definite correlation between PAD and displaced atoms density (DAD) was found because PAD reflects the change in thermal conductivity associated with the net amount of displaced atoms in the crystal lattice beneath the surface. According to the linear dependence of 1/PAD on DAD, defects below a DAD of 1014/cm2 (corresponding to implant doses of 2×1011, 8×1010, and 6×1010 ions/cm2 for 100 keV B+, P+, and As+, respectively) were concluded to be point defects. After the DAD reached 1014/cm2, the PAD showed a gentle increase, and this can be attributed to the growth of point‐defect clusters. A marked dependence of the PAD on the DAD was not observed beyond a DAD of 1016/cm2. In this region, the presence of an amorphous layer was observed by cross‐sectional transmission electron microscopy. Annealing behavior due to low‐temperature heating was studied by the change in temperature dependence curves of the PAD, and the results reflected the characteristics of the defects described above.