Adding optical functionality to a silicon microelectronic chip is one of the most challenging problems of materials research. Silicon is an indirect-bandgap semiconductor and so is an inefficient emitter of light. For this reason, integration of optically functional elements with silicon microelectronic circuitry has largely been achieved through the use of direct-bandgap compound semiconductors. For optoelectronic applications, the key device is the light source--a laser. Compound semiconductor lasers exploit low-dimensional electronic systems, such as quantum wells and quantum dots, as the active optical amplifying medium. Here we demonstrate that light amplification is possible using silicon itself, in the form of quantum dots dispersed in a silicon dioxide matrix. Net optical gain is seen in both waveguide and transmission configurations, with the material gain being of the same order as that of direct-bandgap quantum dots. We explain the observations using a model based on population inversion of radiative states associated with the Si/SiO2 interface. These findings open a route to the fabrication of a silicon laser.

Strong room-temperature photoluminescence (PL) in the wavelength range 650–950 nm has been observed in high temperature annealed (1000–1300 °C) substoichiometric silicon oxide (SiOx) thin films prepared by plasma enhanced chemical vapor deposition. A marked redshift of the luminescence peak has been detected by increasing the Si concentration of the SiOx films, as well as the annealing temperature. The integrated intensity of the PL peaks spans along two orders of magnitude, and, as a general trend, increases with the annealing temperature up to 1250 °C. Transmission electron microscopy analyses have demonstrated that Si nanocrystals (nc), having a mean radius ranging between 0.7 and 2.1 nm, are present in the annealed samples. Each sample is characterized by a peculiar Si nc size distribution that can be fitted with a Gaussian curve; by increasing the Si content and/or the annealing temperature of the SiOx samples, the distributions become wider and their mean value increases. The strong correlation between structural (nanocrystal radius and width of the size distributions) and optical (wavelength and width of the PL peaks) data indicates that light emission from the annealed SiOx films is due to carrier recombination in the Si nc, and it can be interpreted in terms of carrier quantum confinement. The possible reasons for the quantitative discrepancy between the experimentally measured luminescence energy values and the theoretical calculations for the enlargement of the band gap with decreasing the crystal size are also discussed.

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.

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In the present article, a detailed study of the optical properties of the Er-doped Si nanocrystals system, obtained through ion implantation of Er in samples containing Si nanocrystals formed by plasma enhanced chemical vapor deposition is reported. In particular, we present a phenomenological model based on an energy level scheme taking into account the strong coupling between each Si nanocrystal ͑NC͒ and the neighboring Er ions, and considering the interactions between pairs of Er ions too, such as the concentration quenching effect and the cooperative up-conversion mechanism. Based on this model, we wrote down a system of coupled first order differential rate equations describing the time evolution of the population of both the Si NC and the Er related excited levels. By studying the steady state and time resolved luminescence signals at both the 1.54 and 0.98 m Er lines and at the Si nanocrystals emission ͑at around 0.8 m͒, we were able to fit the experimental data in a wide range of Er concentration ͑between 3ϫ10 17 /cm 3 and 1.4ϫ10 21 /cm 3) and excitation pump power ͑in the range 1-10 3 mW), determining a value of 3ϫ10 Ϫ15 cm 3 s Ϫ1 for the coupling constant describing the interaction between Si NC and Er ions, and of 7ϫ10 Ϫ17 cm 3 s Ϫ1 for the cooperative up-conversion coefficient. Moreover, an energy transfer time of ϳ1 s has been estimated, confirming that Si nanocrystals can actually play a crucial role as efficient sensitizers for the rare earth. In addition, the role of Si nanocrystals and of strong gain limiting processes, such as cooperative up-conversion and confined carriers absorption from an excited NC, in determining positive gain at 1.54 m will be investigated in details. The impact of these results on the fabrication of optical amplifiers will be finally addressed.