Edge Absorption Stimulated by Disorder in Mixed Crystals of Thallous Halides

Low level absorption spectra at the edges of TlCl(I), TlBr(I), TlCl(Br) and TlBr(Cl) mixed crystals are measured. At the low concentration of minority halogen, the absorption peak orginated in the localized level due to iodine is found in TlCl(I) while the dicorder-assisted zerophonon X 6 + ×R 6 - exciton absorptions are observed in TlBr(I),TlCl(Br) and TlBr(Cl). Above the critical concentration of the minority halogen, a long tail associated with the bound states due to the cluster formation appears. The absorption is analyzed and the growth of the long tail is discussed.