Li-Defect Interactions in Electron-Irradiatedn-Type Silicon

Abstract

Single-crystal silicon, both with and without oxygen, has been diffused with lithium to concentrations \ensuremath{\sim} ${10}^{17}$/${\mathrm{cm}}^{3}$, irradiated with 1-1.5-MeV electrons, and the ensuing defects studied by EPR and electrical measurements. The presence of oxygen strongly affects the properties of these defects. In O-containing material, the room-temperature carrier-removal rate is 0.25 ${\mathrm{cm}}^{\ensuremath{-}1}$, and the carrier concentration continues to decrease after the cessation of the irradiation; in O-free material, the carrier-removal rate is 0.39 ${\mathrm{cm}}^{\ensuremath{-}1}$, but here the carrier concentration can either increase or decrease after cessation of the irradiation. EPR measurements have indicated the presence of two new defects which involve Li --- one in O-containing material and one in O-free material. Their introduction rates are much smaller than the carrier-removal rates. The defect in O-containing Si exhibits a strong hyperfine interaction with the ${\mathrm{Li}}^{7}$ nucleus, while that in O-free Si exhibits no hyperfine structure. Their $g$ values are 2.0046 and 2.0090, respectively, and are isotropic. The defects are observed in their electron-filled state, and indicate a net electron spin of \textonehalf{}. The defect spectra disappear (with time) at room temperature, and this, together with the carrier-concentration changes, can be explained by the formation of other Li-involved defects which lie deeper in the energy band gap and are not visible by EPR. Electron irradiation at 40 \ifmmode^\circ\else\textdegree\fi{}K followed by annealing at higher temperatures show that both EPR defects described above begin to form at about 200 \ifmmode^\circ\else\textdegree\fi{}K and begin to decrease at about 275 \ifmmode^\circ\else\textdegree\fi{}K --- just as does the 250 \ifmmode^\circ\else\textdegree\fi{}K reverse annealing observed generally for $n$-type Si. Based on these data, and the work of others, it is suggested that both defects form as a result of the motion of Si interstitials which produce a (Li-O-interstitial) complex in O-containing Si, and a (Li-interstitial) complex in O-free Si.