Vacancies in CZ silicon crystals observed by low-temperature ultrasonic measurements

By means of the low-temperature ultrasonic measurements, we try to observe the single vacancies in the as-grown Czochralski (CZ) silicon crystal of high resistivity and commercial-base quality. The crystal ingot we adopt here is grown with the pulling rate gradually lowered to produce various regions of different point-defect states. The elastic constants CT2g(T) and CEg(T) measured for the samples taken from the Pv-region exhibit the softening of the type CΓ(T)=CΓ(0) [1−ΔJT/(T−Θ)] which was found in our previous study for the non-doped FZ silicon and attributed to the neutral vacancy. The response of the softening to the applied magnetic field is found to be the same as in the case of the non-doped FZ silicon. The samples in the Pi-region exhibit no such softening, confirming that the origin of the softening is due to the vacancies. A qualitative explanation is given to the measured distribution of the vacancy concentration. For the CZ silicon crystals, the vacancies can be well observed for the Pv-region only.