The Urbach–Martienssen absorption tails in the optical spectra of semiconducting variable-sized zinc selenide and cadmium selenide quantum dots in thin film form

Abstract

The sub-bandgap exponential absorption tails of Urbach–Martienssen type in chemically deposited variable-sized ZnSe and CdSe quantum dots in thin film form were studied. The Urbach energy, characterizing the steepness of the exponential absorption tails, was found to decrease upon particle size enlargement due to thermal annealing of the as-deposited ZnSe and CdSe quantum dots in thin film form. Such decrease of the Urbach energy was attributed to the decrease of the degree of structural disorder upon annealing of the semiconducting quantum dot thin films, manifested through lattice strain relaxation, average crystal size and lattice constant increase and dislocation density decrease. This behavior is in line with the predictions of the Cody model, relating the Urbach energy to the degree of structural disorder for a given material. In this way, it is shown that semiconducting quantum dots deposited in thin film form have a certain non-thermal component to the exponential absorption tails of the Urbach–Martienssen type. This non-thermal component is due to the inherent nanocrystalline character of the semiconducting quantum dots, characterized with a rather pronounced structural disorder, manifested through a certain degree of lattice strain and the rather large values for the dislocation densities.