Isotopic effects on the dielectric response of Si around the E-1 gap

The effect of isotopic composition on the dielectric function of silicon fr om 3.1 to 3.7 eV has been investigated using spectroscopic ellipsometric da ta obtained on Si-28, natural Si (Si-nat, M-nat = 28.09 amu), and Si-30 cry stals. At low temperatures, the energies of the E-0' and E-1 interband tran sitions, which occur in the energy range under study, become mass dependent through the dependence of the electron-phonon interaction and the lattice parameter on the average isotopic mass. We determine the mass dependence of critical point energies and other optical parameters as accurately as poss ible by analyzing the ellipsometric data in reciprocal (Fourier-inverse) ra ther than direct (frequency) space. The obtained dependence of the critical point energy versus isotope mass [Delta E-1/Delta M= +1.9(4) meV/amu] is i n reasonable agreement with estimated values obtained from the temperature dependence of E-1 in natural silicon.