Schönau, S.; Ruske, F.; Neubert, S.; Rech, B.: Analysis of Urbach-like absorption tails in thermally treated ZnO:Al thin films. Applied Physics Letters 103 (2013), p. 192108/1-4
10.1063/1.4829999

Abstract:
The sub bandgap absorption of as deposited and thermally treated ZnO:Al has been investigated and described applying Urbach tail theory. In the as deposited state Urbach energies show a decreasing trend for higher deposition temperatures. Annealing leads to much less band tailing and resulting Urbach energies are the same as for undoped ZnO. It has been concluded, that structural disorder is the main contribution to the bandtails in the as deposited state. In summary films with excellent electrical properties due to very high carrier mobilities and excellent optical properties due to the strong reduction in sub bandgap absorption were obtained.