Evidence of thermally activated transfer of excited carriers between CdSe/ZnSe quantum dots

Temperature dependent photoluminescence and cathodoluminescence of self-ass embled CdSe/ZnSe quantum dots grown by metalorganic vapor phase deposition were investigated. We found an unusual large red shift and a narrowing of t he photoluminescence peak with temperature increases. Cathodoluminescence s tudies of a small number of quantum dots showed that the broad peak observe d in the photoluminescence spectra is, in fact, made up of a series of narr ower peaks, coming from quantum dots of different sizes. While the intensit y of luminescence from small dots drops monotonously with temperature rises , that from the large dots displays a peculiar behavior. It actually increa ses within the temperature range of 140-170 K, the same range in which the photoluminescence peak shows narrowing. The simultaneous increase of lumine scence from some quantum dots and decrease from others are believed to be r esponsible for the red shift and narrowing of the observed photoluminescenc e peak. A simple analytically solvable rate equation model was used to unde rstand the spectral data. We suggest that the unusual behaviors observed ca n be understood as resulting from a transfer of thermally activated carrier s from small to large quantum dots.