Direct observation of vacancy in silicon using sub-Kelvin ultrasonic measurements

We carried out sub-Kelvin ultrasonic measurements for observation of vacancies in crystalline silicon. The longitudinal elastic constants of non-doped and B-doped floating zone (FZ) silicon crystals in commercial base revealed low-temperature elastic softening below 20 K. The applied magnetic fields turns the softening of the B-doped FZ silicon to a temperature-independent behavior, while the fields up to 16 T at base temperature 20 mK make no effect on the softening of the non-doped FZ silicon. This result means that the vacancy accompanying the non-magnetic charge state V⁰ in the non-doped silicon and the magnetic V⁺ in the B-doped silicon is responsible for the low-temperature softening through the Jahn-Teller effect. The direct observation of the vacancy using the sub-Kelvin ultrasonic measurements advances point defects controlling in silicon wafers and semiconductor devices.