  The photoconductions of ultrafast InGaAs:ErAs nanocomposites at low
temperatures were investigated. The parabolic Tauc edge as well as the
exponential Urbach tail are identified in the absorption spectrum. The Tauc
edge supports that the density of states at the bottom of conduction band is
proportional to the square root of energy. The Urbach edge is attributed to
interband transition caused by smooth microscopic internal fields. The square
root of mean-squared internal fields, whose distribution is Gaussian, is found
in the order of $10^{5}$V/cm, agreeing very well with the theoretical
predictions by Esser (B. ~ Esser, Phys. stat. sol. (b), vol. 51, 735 (1972)).
