A long tail in the absorption spectra is a symptom of indirectness, which was observed in this case accompanied by a few regions showing feeble hyperchromic shifts (increase in intensity) as well. These shifts are associated with excitonic defect levels which trap electrons and prohibit a fast e -h recombination. The first excitonic level can be found at 1.55 eV (600 nm) and 0.9 eV (1300 nm). A certain amount of indirectness, therefore, was found to be persistent even in direct transitions. The absorption coefficient for indirect transitions is given by eq 2