Photoexcited Electron Capture by Ionized and Neutral Shallow Impurities in Silicon at Liquid-Helium Temperatures

Abstract

The capture rates for photoexcited electrons due to ionized phosphorus, arsenic, antimony, and bismuth donors as well as neutral boron, aluminum, gallium, and indium acceptors in silicon have been determined in the liquid-helium temperature region, using combined optical and paramagnetic-resonance techniques. The ionized-donor capture rates exceed by about an order of magnitude the values predicted from the theories of Lax and of Ascarelli and Rodriguez for capture of thermalized electrons. The capture rates are independent of temperature between 1 and 4\ifmmode^\circ\else\textdegree\fi{}K, at variance with both theories, and their dependence on the ionization energy of the different donors also does not conform to the theoretical predictions. These results, as well as photoconductivity measurements as a function of charged-impurity concentration, demonstrate the inadequacy of any existing model of the low-temperature donor capture process for photoexcited electrons in silicon. Possible explanations of the results in terms of nonrandom impurity clusters or capture from hotelectron states are presented. The neutral-acceptor rates are about two orders of magnitude smaller than those of the ionized donors, are also independent of temperature between 1 and 4\ifmmode^\circ\else\textdegree\fi{}K, and show a small dependence of about ${I}^{\frac{1}{2}}$ on the hole ionization energy of the impurities, compared with expected ${T}^{\frac{1}{2}}$ and ${I}^{\ensuremath{-}\frac{3}{2}}$ dependences from the theory for thermalized-electron capture by neutral centers. As in the case of the ionized donors, no presently available model appears adequate to explain the results.