Introduction

When an electron is excited from a bond, for example, an Si-Si bond in silicon, it is attracted by a hole left in the bond due to Coulomb interaction; such an electron-hole pair is called geminate electron-hole pair. On the contrary, distant electron-hole pairs such as those mentioned above are called nongeminate electron-hole pairs. For the geminate electron-hole pair, ESR of either an electron or a hole of the geminate electron-hole pair causes the PL intensity to be decreased, because ESR changes the antiparallel spin state of the geminate electron-hole pair to its parallel spin state; then the PL due to recombination of geminate electronhole pairs decreases in intensity [Biegelsen et al., 1978]. Thus, this change is opposite to that expected for nongeminate electron-hole pairs. Spin-dependent recombination can be observed through ESR of recombination centers [Lepine, 1972; Lepine et al., 1976; Solomon et al., 1977; Solomon, 1979]. This phenomenon is generally used for observation of the ESR signal of recombination center, monitoring

photocurrents under illumination, that is, this is sometimes called EDMR [Lips et al., 1996]. See also [Morigaki, 1999]. At low temperatures, neutral impurity scattering affects photoconductivity, for example, in lightly doped crystalline silicon, so that ESR of impurities changes their spin polarization so as to affect the photoconductivity [Honig, 1966; Schmidt and Solomon, 1966]. This effect has been observed by Schmidt and Solomon [1966] and Maxwell and Honig [1966]. Such spin-dependent effect has been reviewed by Solomon [1972].