High-resolution magnetic-resonance spectroscopy of thermal donors in silicon

Abstract

Oxygen-related thermal donors in silicon form a group of defects with very similar properties. In a magnetic resonance experiment carried out under the normally applied conditions the individual species, up to 16 have been reported, are not distinguished. Due to lack of resolution only an overall integrated signal covering the whole group is recorded. In this paper two independent ways of improving the resolution in magnetic resonance are reported and applied to the Si-NL8 spectrum of the thermal double donors and to the structurally related Si-NL10 center. Line widths of the resonances caused by unresolved hyperfine interactions with the magnetic nuclei of isotope 29Si, nuclear spin I = 1 / 2 and natural abundance 4.7%, are near 0.3 mT. Monoisotopic silicon with the 29Si concentration reduced to 0.09% has been used to observe the Si:P, Si-NL8 and Si-NL10 resonances. In this material a line width as low as 0.045 mT has been observed. The second improvement of resolution is achieved using high-frequency high-field EPR spectrometers at 92 GHz and at 140 GHz for the Si-NL8 spectrum and at 349 GHz for Si-NL10. At these higher frequencies an increase of line structure is achieved by, e.g., a factor 15 when comparing a D band (140 GHz) with a classical X band (9 GHz) experiment. Due to ‘ g strain’ an increase of line width may occur. Under the high-resolution conditions individual species of thermal donors are separated.