Structure of thermal donors (NL8) in silicon: A study with electron-nuclear double resonance

Singly ionized thermal donors [(TD)⁺], which give rise to the NL8 ESR spectrum, were investigated with electron-nuclear double resonance (ENDOR) in B-doped Czochralski-grown silicon and float-zone silicon into which the magnetic isotope ¹⁷O was diffused. TD's were formed by annealing at 460 ^∘{}C for 2--8 h. ENDOR lines of two shells of ¹⁷O and seven shells of ²⁹Si were observed. Five different species of TD⁺'s were identified, which are formed one after another upon annealing. The ESR spectra of these TD's were superimposed in the NL8 spectrum. The smallest TD⁺ measured by ENDOR is probably identical with TD₃⁺ as identified from the infrared-absorption bands. The core of the TD's contains four oxygen atoms, two on each of the two (110) mirror planes. All the five TD⁺'s investigated showed C₂ᵥ point-group symmetry. A single Si atom is possibly also part of the TD core structure, while the other Si neighbors are ligands. The hyperfine interactions of all ²⁹Si and ¹⁷O nuclei are very small, representing only about 5% of the unpaired-electron spin density. The small size of the interactions cannot be explained by present theories. The compatibility of current TD models with the ENDOR results is discussed.