Optical Gain From Silicon Nanocrystals A critical perspectives

It has generally been considered impossible to fabricate a silicon laser. The reason being that -due to its indirect bandgap- silicon has a small cross-section for stimulated emission. As a result, optical losses due to free carrier absorption are dominant. It has been proposed that Si nanocrystals offer a solution to this problem. The optical properties of silicon nanocrystals are quite well understood. This is the result of extensive research over the past ten years on porous Si as well as on Si nanocrystals embedded in an SiO2 matrix. It is generally found that well-prepared and passivated Si nanocrystals exhibit photoluminescence in the wavelength range between 500 and 1100 nm. The luminescence is attributed to the recombination of quantum-confined excitons, the emission energy thus being strongly dependent on the nanocrystal size. SiO2 is the ideal matrix for Si nanocrystals as it can passivate dangling bonds that may cause non-radiative quenching. Indeed, many of the optical characteristics of Si nanocrystals in SiO2 prepared by different methods, as well as oxidized porous Si, are very similar. The radiative recombination process can be entirely understood assuming a “classical” model of recombination of excitonic singlet and triplet states of the excited Si nanocrystals. In this model, the optical transition is indirect in nature, and has a relatively small cross- section. In recent years, several applications of Si nanocrystals have been explored, and include light-emitting diodes,1,2 non-volatile memories,3,4 and sensitized optical amplifiers.5,6 The fabrication of an optical amplifier or laser based on interband transitions has been considered impossible because -by analogy with bulk Si- the cross-section for free carrier absorption was thought to be higher than that for stimulated emission. Yet, in an article published in 2000, Pavesi et al. 7 claimed that optical gain could be achieved using Si nanocrystals, contrary to earlier predictions. Central in this claim is the presumption that the observed light emission from silicon nanocrystals is not due to the recombination of “free” excitons but rather to the recombination of electron-hole pairs trapped at an interface state. This could, according to the authors, reduce the deleterious effect of free carrier absorption. A three-level model was introduced to explain the observed optical gain, with the intermediate level attributed to a Si=O double bond at the interface between the nanocrystal and the surrounding silicon oxide matrix.