Absorption edge and urbach's rule in ZnO

Measurements of the absorption edge of ZnO up to 4000 cm −1 for 11 K < T < 878 K are presented. The results are analyzed in terms of Urbach's rule. For T < 350 K, Urbach's rule is reasonably well fulfilled, and the parameters of the rule are derived. Both LO‐phonons and acoustic phonons seem to be involved in the absorption process. For T > 350 K, Urbach's rule in its usual form is no longer valid.