Oxygen-vacancy complex in silicon. II.O17electron-nuclear double resonance

An electron-nuclear double-resonance (ENDOR) study was performed on the negatively charged oxygen-vacancy complex in silicon. By introducing the isotope ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ (nuclear spin I=(5/2) to an enrichment of about 40%, it was possible to detect ENDOR transitions of this nucleus. In the experiment the magnetic hyperfine interaction with the unpaired defect electron and the nuclear quadrupole interaction were measured. The anisotropic hyperfine interaction could be described by a small allowed admixture of p orbitals. In such a simple one-electron description the Fermi-contact interaction could not satisfactorily be understood. The quadrupole interaction could be explained as arising mainly from unbalanced charges in the oxygen p orbital that points in the direction of the silicon neighbors with which a Si-O-Si ‘‘molecule’’ is formed. The technique of field-swept ENDOR was used to determine the relative signs of the hyperfine- and quadrupole-interaction constants. The strong interference of the two interactions gives rise to the occurrence of forbidden EPR transitions and a very complicated angular dependence of the ENDOR frequencies upon rotation of the magnetic field. As a consequence, for some orientations no ENDOR was observed at all.