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\ifmmode^\circ\else\textdegree\fi{}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 \ensuremath{\sim}70\ifmmode^\circ\else\textdegree\fi{}-200\ifmmode^\circ\else\textdegree\fi{}C; above 200\ifmmode^\circ\else\textdegree\fi{}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\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ at 1200\ifmmode^\circ\else\textdegree\fi{}C, is higher than the radiotracer solubility but the former was measured in more rapidly quenched samples. The distribution coefficient is 8\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$. 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.Copper introduced a donor level at 0.24 ev and an acceptor level at 0.49 ev, both as measured from the valence band. The maximum electrical activity in quenched samples was 5\ifmmode\times\else\texttimes\fi{}${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ out of a total concentration of ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ at 1200\ifmmode^\circ\else\textdegree\fi{}C. Infrared photoconductivity spectra support the position of the deep levels due to iron and copper.The apparent lack of electrical activity or inconsistency in producing a level is discussed for a number of other elements in silicon. Precipitation while cooling from high temperature is believed to reduce the soluble component of most of these elements below the observable limit of \ensuremath{\sim}${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$.