Defects in Irradiated Silicon: Electron Paramagnetic Resonance and Electron-Nuclear Double Resonance of the Si-ECenter

Abstract

The Si-$E$ center is one of the dominant defects produced by electron irradiation in phosphorus-doped vacuum floating zone silicon. It introduces an acceptor level at $\ensuremath{\sim}({E}_{c}\ensuremath{-}0.4)$ eV and gives rise to an electron paramagnetic resonance when this level does not contain an electron. As a result of electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) studies described in this paper, we conclude that the defect is a lattice vacancy trapped next to a substitutional phosphorus atom, with EPR arising from the neutral charge state. The observed hyperfine interactions with ${\mathrm{P}}^{31}$ and neighboring ${\mathrm{Si}}^{29}$ nuclei, as well as the observed $g$-tensor anisotropy, are discussed in terms of a simple linear combination of atomic orbitals (LCAO) molecular orbital treatment. In addition to the anisotropy associated with the phosphorus-vacancy direction in the lattice, an additional distortion of the defect occurs which is identified in the LCAO treatment as a manifestation of the Jahn-Teller effect. Thermally activated reorientation from one Jahn-Teller distortion to another causes motional broadening and narrowing effects upon the EPR spectrum in the temperature region 60-150\ifmmode^\circ\else\textdegree\fi{}K. The motion is also studied by stress-induced alignment at lower temperatures and the activation energy for this process is determined to be \ensuremath{\sim}0.06 eV. Alignment of the phosphorus-vacancy direction in the lattice is also achieved by stressing at elevated temperatures. The activation energy for this motion is 0.93\ifmmode\pm\else\textpm\fi{}0.05 eV. The magnitude and sense of the alignment in both kinds of stress experiments are consistent with the microscopic model of the defect. The role of the phosphorus-vacancy interaction in the diffusion of phosphorus in unirradiated silicon is discussed. Using the published value for the diffusion activation energy for phosphorus in silicon, we estimate the appropriate value for silicon self-diffusion to be 3.94\ifmmode\pm\else\textpm\fi{}0.33 eV and the formation energy for the lattice vacancy in silicon to be 3.6\ifmmode\pm\else\textpm\fi{}0.5 eV. These are quantities for which no direct experimental values are available. Also included is an appendix which gives estimates of ${|{\ensuremath{\psi}}_{3s}(0)|}^{2}$ and $〈{{r}_{3p}}^{\ensuremath{-}3}〉$ for the $3p$ atoms aluminum through chlorine.