Spin-1 Centers in Neutron-Irradiated Silicon

Electron paramagnetic resonance was used to study a number of fast-neutron-induced defects formed in pile-irradiated silicon and to follow their concentrations as a function of annealing. Measurements were made at 300, 77, and 4.2°K on samples which had attained intrinsic resistivity during irradiation, using superheterodyne spectrometers operating at 24 and 9.4 kMc/sec. Aside from the previously reported Si-$N$ center, the most prominent lines of the spectrum arise from the $m_s=0\mathrm{} \mathrm{to} \pm 1$ transitions of four spin-1 systems. The distinct symmetry and small production rate (≈0.05 center per fast neutron collision) indicate a class of well-defined but relatively rare defects. Their $g$ tensors, zero-field splitting tensors, and hfs are compatible with systems having two weakly interacting $〈111〉$ dangling bonds separated by about 5 Å, giving the $S=1\mathrm{}$ Hamiltonians in the triplet levels formed by the weak exchange interaction. Low-temperature measurements suggest that the singlet-triplet splitting lies between 3 and 50 ${\mathrm{cm}}^{-1\mathrm{}}$. Comparison with floating-zone silicon shows Center (II, III), which is dominant in unannealed samples, to be independent of impurity. The remaining three $S=1\mathrm{}$ centers, which grow and decay rapidly at higher temperatures, involve oxygen. Precise measurements of the parameters of the spin Hamiltonians are given to permit reproducible identification of the centers.