Properties of Silicon Doped with Iron or Copper

Iron introduces a donor level into silicon at 0.40 ev from the valence band observed both in crystals doped in the melt and in crystals into which iron was diffused at 1200°C. This level converts anomalously to a level 0.55 ev from the conduction band on standing at room temperature. The conversion is reversible in the range ∼70°-200°C; above 200°C, the electrical activity of iron irreversibly disappears. No evidence for acceptor action of iron was found. The electrically active solubility of iron, 1.5×${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$ at 1200°C, is higher than the radiotracer solubility but the former was measured in more rapidly quenched samples. The distribution coefficient is 8×${10}^{-6\mathrm{}}$. Preferential trapping of electrons by iron centers was shown by Hall mobility measurements on optically-excited charge carriers. Lifetime studies by the photoconductive decay method indicated a larger capture cross section for electrons than for holes.