Structure and fundamental optical absorption of CdSe films

The absorption spectra at the fundamental absorption edge of as-deposited and recrystallized CdSe films have been measured at low temperatures. The large number of crystal imperfections in as-deposited films leads to the appearance of band tails which give rise to a flat fundamental edge whose steepness is a function of the substrate temperature. After recrystallization of the films in selenium vapour the absorption edge becomes steeper and intrinsic exciton lines are observed. Below a characteristic temperature the line form of the A_{n = 1} exciton, which is of Lorentzian type, does not depend on temperature. This is related to the dominant mechanism of line broadening, which is due to the interaction with charged impurities. Above the characteristic temperature the A_n=1 exciton line can be fitted by a symmetrical Lorentzian with a linewidth that is determined by weak exciton-phonon coupling and intraband scattering.