Role of the energy transfer in the optical properties of undoped and Er-doped interacting Si nanocrystals

In this article the luminescence properties of Si nanocrystals (nc) formed by plasma enhanced chemical vapor deposition and their interaction with Er ions introduced by ion implantation are investigated in detail. Si nc with different size distributions and densities were produced and all show quite intense room temperature luminescence (PL) in the range 700-1100 nm. It is shown that the time-decay of the luminescence follows a stretched exponent ial function whose shape tends towards a single exponential for almost isol ated nc. This suggests that stretched exponential decays are related to the energy transfer from smaller towards larger nc. Indeed, by comparing sampl es with similar nc size distributions, but with very different nc densities , it is demonstrated that the PL has a quite strong redshift in the high de nsity case, demonstrating a clear energy redistribution within the sample. Excitation cross sections have been measured in all samples yielding a valu e of similar to1.8 x 10(-16) cm(2) for isolated nc excited with 2.54 eV pho tons. This effective excitation cross section is shown to increase by a fac tor of 4 in interacting nc as a result of the energy transfer within the sa mple. When Er ions are introduced in these samples a strong nc-Er interacti on sets in and the energy is preferentially transferred from the nc to the Er ions. The nc-related luminescence is quenched and the Er-related lumines cence at 1.54 mum appears. The effective excitation cross section of Er ion s through Si nc has been determined to be similar to1.1 x 10(-16) cm(2). Th is number resembles the excitation cross section of nc themselves demonstra ting that the coupling is extremely strong. Moreover, by increasing the Er content the effective excitation cross section is seen to increase. In the same concentration range the Er lifetime decreases demonstrating that "conc entration quenching" effects, with the energy transferred among Er ions, ar e setting in. These Er-Er interactions are responsible for the effective in crease of the cross section. However, since the increase in the cross secti on is related to a simultaneous decrease in lifetime the net effect for the luminescence efficiency is negative. The best Er content to take advantage of the sensitizer action of Si nc avoiding the detrimental Er-Er interacti ons has been determined to be similar to2 x 10(20)/cm(3). These data are pr esented and their implications discussed. (C) 2001 American Institute of Ph ysics.