EPR and ENDOR study of thePbcenter in porous silicon

The ${\mathit{P}}_{\mathit{b}}$ center at the (111) Si-${\mathrm{SiO}}_2$ interface has been studied with electron-nuclear-double-resonance (ENDOR) spectroscopy taking advantage of the high specific interface area of oxidized porous Si samples. The ENDOR spectrum consists only of two structureless lines at the Larmor frequencies of ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$. No hyperfine interaction with ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ could be detected. The ${\mathit{P}}_{\mathit{b}}$-center${–}^1$H${,}^{19}$F distances have been estimated in the dipole-dipole approximation to ≊13 Å, which corresponds to the average ${\mathit{P}}_{\mathit{b}}$-center separation. The intensity ratio of the ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ ENDOR lines is stable under high-temperature vacuum annealings and hydrogen treatments. Quantitative electron-paramagnetic-resonance measurements of the central hyperfine-structure intensity confirm the assignment of the ${\mathit{P}}_{\mathit{b}}$ center to the Si dangling-bond defect, a model recently questioned by the suggestion of a hydrogen-molecule defect.