The optical functions of silcon heavily doped with Ge, P, As, and B ar
e determined using spectroscopic ellipsometry measurements from 240 to
840 nm (5.16 to 1.47 eV). Below the direct band gap, there is a resid
ual enhancement of the optical-adsorption coefficient for silicon heav
ily doped with n-type dopants, which cannot be explained by surface ro
ughness. In the low-energy region of the observed spectrum, it is foun
d that both free-carrier and strain effects alter the complex dielectr
ic function.