Isolated vacancies in crystalline Si were successfully observed by using low-temperature ultrasonic measurements. The float-zone Si exhibits softening in longitudinal elastic constants consisting of transverse C44 and (C11–C12)/2 in part with decreasing temperature below 20K down to 20mK. The applied magnetic fields of up to 16T at the base temperature 20mK revealed no effect on the low-temperature softening in non-doped float-zone Si, while the fields of up to 2T suppress the low-temperature softening in B-doped float-zone Si. This experiment suggested that isolated vacancies with non-magnetic charge state V0 in the non-doped float-zone Si and with magnetic charge state V+ in the B-doped float-zone Si lead to the low-temperature softening. Low-temperature softening in the Pv-region of non-doped Czochralski Si ingot was also observed, which indicated vacancy distribution. A coupling of electric quadrupoles of triplet state of vacancy orbital to the elastic strains of the sound waves was discussed for description of the low-temperature softening.

Observation of Vacancy in Crystalline Silicon Using Low-Temperature Ultrasonic Measurements. T.Goto, H.Yamada-Kaneta, K.Sato, M.Hikin, Y.Nemoto, S.Nakamura: Physica B, 2007, 401-402, 109-14