Yuta Nagai

Quadrupole Effects of Vacancy Orbital in Boron-Doped Silicon

We have investigated low-temperature properties at a longitudinal elastic constant C L[111] of a boron-doped silicon crystal grown by a floating zone (FZ) method. The softening of C L[111] depending on magnetic fields indicates that a magnetic charge state V + of a vacancy orbital accommodating three electrons is stable. Appreciable anisotropy in the softening of C L[111] at magnetic fields along the [111] and [1\bar10] axes up to 10 T is described in terms of a quadrupole susceptibility for the vacancy orbital consisting of a Γ 8 quartet ground state and Γ 6 doublet excited state due to the spin–orbit interaction.

Ultrasonic study of vacancy in single crystal silicon at low temperatures

We have performed ultrasonic measurements at low temperatures in order to investigate vacancy in single crystal silicon. The longitudinal elastic constants of non-doped and boron-doped silicon grown by a floating zone method exhibit appreciable softening with decreasing temperature down to 20 mK. The softening of boron-doped silicon is easily suppressed in applied magnetic field up to 2 T, while the softening of non-doped silicon is robust in fields even up to 16 T. The softening of elastic constants in high-purity crystalline silicon is certainly caused by the coupling of elastic strains of the ultrasonic waves to electric quadrupoles of the vacancy orbital.

Growth of Czochralski Silicon Crystals Having Ultralow Carbon Concentrations

Reducing the carbon concentration in Czochralski (CZ) silicon crystals is crucial in order to improve the properties of high-power devices, such as on-resistance and carrier lifetime. To achieve carbon concentration reduction, it is necessary to reduce carbon monoxide (CO) contamination from the CZ furnace graphite components and to remove the carbon impurities originating from the starting material. In this study, suppressing the chemical reaction between silicon monoxide (SiO) and the graphite heater effectively reduced the CO contamination rate. Furthermore, we attempted to promote CO evaporation during the CZ process in order to remove carbon impurities from the melt. Increasing the Ar gas flow velocity above the melt surface was found to be effective in increasing the CO evaporation rate during both the melting and growth processes. The CO evaporation rate during the melting process of 8-inch CZ silicon was calculated as being of the order of 10-2 μg/s. Owing to the effects of the CO evaporation, 8-inch CZ silicon crystals with carbon concentrations lower than 2.0 × 1014 atoms/cm3 at a solidified fraction of 0.85 were grown.

Effect of Nitrogen Doping on Vacancy State in Silicon Crystals Observed by Low-Temperature Ultrasonic Measurements

For the B-doped silicon crystals grown with and without N-doping, we measured the temperature dependence of the elastic constant in low-temperature region, to examine whether the N-doping annihilates the elastic softening caused by the gap-states of the isolated single vacancy. We have found that the elastic softening clearly observed for the N-free crystals is not observed for the N-doped ones, suggesting that the gap-states of the vacancies causing the elastic softening are destroyed by the N-doping. This is consistent with the model of Abe [T. Abe, J. Crystal Growth, 327 (2011) 1] in which the nitrogen molecule (N-N pair) occupies the vacancy to destroy its original gap-states. We have further observed that the N-doped silicon, which exhibits no softening in its as-grown state, exhibits the softening after the short-time annealing. This suggests that during the annealing the N-N pair is thermally activated to jump off the lattice site leaving the vacancy.

Low-Temperature Elastic Softening due to Vacancies in Boron-Doped FZ Silicon Crystals

We confirm the following findings obtained in our previous experiment for the low-temperature elastic softening by the vacancies in boron-doped silicon crystals: (1) the steep softening that suddenly starts at 2-4 K in the cooling process, and (2) the complete disappearance of the softening by a weak magnetic field of 4 T applied along [111] direction. We further investigate in detail how the low-temperature softening at a fixed temperature responds to the applied magnetic field, to find the following characteristic anisotropy: The manner of disappearance of the softening strongly depends on the direction of the magnetic field. For the magnetic field imposed along [1-10] direction, nearly 60 % of the full softening still remains even at a strong magnetic field of 8 T, in contrast to the case of magnetic field applied along [111] direction.