Temperature Evolution of Exciton and Impurity Emission Spectra of CdSe Single Crystals

Photoluminescence (PL) spectra of CdSe single crystals grown by Bridgman technique are investigated in the wavelength range from 640 (1.938 eV) to 760 nm (1.632 eV) at temperatures between 15 and 200 K. PL spectra consist of a series of narrow bands of short-wavelength exciton emission and wider bands of impurity emission, the energy position and intensity of which are determined by the sample temperature. It is shown that the total exciton energy, including its kinetic energy, weakly depends on temperature and is equal to 29–30 meV for temperatures below 70 K, and then increases linearly to the value of 38 meV at 200 K. At low temperatures, the impurity PL band with phonon replica with the energy of LO-phonon equal to (25.6 ± 0.8) meV is attributed to donor–acceptor (D–A) pairs and is of self-activated nature. With increasing temperature, as shallow donors are ionized, this band is attributed to recombination of free electrons with the holes localized at acceptor level with the energy depth of 0.117 eV. In the short-wavelength range of PL spectra for CdSe samples annealed in the melt of Cd and CrCl3 salt, a new PL band is observed at 649 nm (1.9106 eV), with localization energy exceeding the band gap energy. This band is supposed to be caused by intracenter transitions within the background impurity ions, probably, of manganese. The possible structure of the radiative center is discussed.