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

Abstract

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 exponential function whose shape tends towards a single exponential for almost isolated nc. This suggests that stretched exponential decays are related to the energy transfer from smaller towards larger nc. Indeed, by comparing samples 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 density case, demonstrating a clear energy redistribution within the sample. Excitation cross sections have been measured in all samples yielding a value of ∼1.8×10−16 cm2 for isolated nc excited with 2.54 eV photons. This effective excitation cross section is shown to increase by a factor of 4 in interacting nc as a result of the energy transfer within the sample. When Er ions are introduced in these samples a strong nc–Er interaction 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 luminescence at 1.54 μm appears. The effective excitation cross section of Er ions through Si nc has been determined to be ∼1.1×10−16 cm2. This number resembles the excitation cross section of nc themselves demonstrating 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 “concentration quenching” effects, with the energy transferred among Er ions, are setting in. These Er–Er interactions are responsible for the effective increase of the cross section. However, since the increase in the cross section 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 interactions has been determined to be ∼2×1020/cm3. These data are presented and their implications discussed.