Phenomenological scaling of optical absorption in amorphous semiconductors

The concept of a universal absorption edge spectrum, L(E), is proposed for amorphous semiconductors. It follows from the renormalization group treatment and perturbative calculations that the Urbach regime, L(E) proportional to exp[E/E(u)], is continuously connected to the Tauc regime, L(E) proportional to (E - E(o))(nT), with E(u) and E(o) being closely correlated, while the gap, E(o), should be determined with the Tauc exponent, n(T) congruent to 3. The plausibility of the theoretical prediction is argued in light of experimental results on hydrogenated amorphous silicon alloys.