Electron paramagnetic resonance study of silicon-28 single crystal for realization of the kilogram

Abstract

To realize the kilogram with high accuracy using the x-ray crystal density method, mass deficit correction to account for the number of vacancy defects in silicon-28 crystals is necessary. In this paper, we present the results of electron paramagnetic resonance (EPR) spectroscopy experiments performed on a silicon-28 crystal. The crystal was cut from near the radial center of the Si28–23Pr11 crystal boule where a relatively high vacancy defect concentration was expected within the boule, which was grown by the floating zone method. We obtained both the in-phase and out-of-phase EPR spectra, which focused on the fast- and slow-spin relaxation centers, respectively. Based on EPR measurements both in the dark and under illumination, the concentrations of phosphorus impurities with unpaired electrons were found to be between 1.6 × 1012 cm−3 and 3.4 × 1012 cm−3. In contrast, EPR-active vacancy defects in the silicon-28 crystal were not detected at the detection sensitivity of the present measurements (around 1 × 1012 cm−3), although at the Si/SiO2 interface, dangling-bond defects (P b0 centers) were detected with an areal density between 0.4 × 1012 cm−2 and 1.7 × 1012 cm−2. The experimental detection of a larger amount of phosphorus donors under illumination indicates the presence of compensation centers (electronic levels acting as acceptors) with a concentration of at most 1.6 × 1012 cm−3. We determined that such a concentration is mainly compensated by the boron acceptors, and that there is little contribution of electrically active vacancy defects, 0.0(6) × 1012 cm−3. This value is much smaller than the previously reported concentrations of 1 × 1014 cm−3 to 4 × 1014 cm−3 for vacancy defects determined by positron annihilation spectroscopy.