Vacancies in defect-free zone of point-defect-controlled CZ silicon observed by low-temperature ultrasonic measurements

The low-temperature ultrasonic measurements are performed for the direct observation of the vacancies in Czochralski-grown (CZ-grown) silicon crystal. The elastic softening similar to that we recently found for the floating-zone-grown (FZ-grown) silicon crystals is observed also for the vacancy-rich region of the defect-free zone (DFZ) in the CZ silicon crystal. We further uncover that both of the interstitial-rich region in the DFZ and the region of the ring-like oxidation-induced stacking faults of the same crystal ingot exhibit no such elastic softening of detectable magnitude, confirming our previous conclusion that the defects responsible for the low-temperature softening are the vacancies. We observe how the vacancy concentration in the DFZ varies along the pulling direction.