A recombination center at Si surfaces and Si/SiO2 interfaces was identified using highly sensitive spin-dependent recombination (SDR) detection of electron paramagnetic resonance (EPR). The defect (Si-KU1) has a spin-triplet (S = 1) state owing to excitation by band-edge light employed in measurement. The Si-KU1 SDR-EPR spectrum exhibits orthorhombic symmetry along two equivalent 110 axes running parallel to the interface, which disappears upon mechanical lapping of the surface oxide or heat treatment above 400 °C. However, removal of surface oxide with diluted HF solution sharpens the Si-KU1 SDR-EPR lines. A plausible structural model of the center responsible for Si-KU1 is presented.
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