Optical Properties of Single Silicon Quantum Dots

For over 60 years silicon (Si) has dominated the semiconductor microelectronics industry mainly due to its abundance and good electrical and material properties. The advanced processing technology of Si has made it the workhorse for photovoltaics industry as well. However, this material has also a big drawback: it is an indirect-bandgap semiconductor in its bulk form, hence an inefficient light emitter. This has hindered the silicon photonics revolution envisioned in 1980s, where photons were thought to replace electrons inside computer chips. In parallel with the exponential scaling of Si transistor's size over the years, the discovery of quantum phenomena at the nanoscale raised new hopes for this semiconductor. In the 1990s bright luminescence from nanostructured porous Si was demonstrated claiming the quantum confinement effect as origin of the emission. Since then, an intense research activity has been focused on Si quantum dots (Si-QDs) due to their potential use as abundant and non-toxic light emitters. More precisely, they could be used as fluorescent biolabels in biomedicine, as light-emitting phosphors in e.g. TV screens or as down-converters in luminescent solar concentrators. Nevertheless, in order to realize such applications, it is necessary not only to improve the fabrication of Si-QDs but also to gain a better understanding of their photo-physics. Among different types of optical measurements, those performed at the single-dot level are free of sample inhomogeneities, hence more accurate for a correct physical description. This doctoral thesis presents a study of the optical properties of single Si-QDs of different type: encapsulated in an oxide matrix, capped with ligands or covered by a thin passivation layer. The homogeneous photoluminescence (PL) linewidth is found to strongly depend on the type of embedding matrix, being narrower for less rigid ones. A record resolution-limited linewidth of ~200 μeV is measured at low temperatures whereas room-temperature values can even compete with direct-bandgap QDs like CdSe. Such narrow PL lines exhibit intensity saturation at high excitation fluxes without any indication of emission from multiexciton states, suggesting the presence of fast non-radiative Auger recombination. Characteristic Auger-related lifetimes extracted from power-dependent decays show a variation from dot-to-dot and confirm the low biexciton quantum efficiency. For the first time, the absorption curve of single Si-QDs is probed by means of photoluminescence excitation in the range 2.0-3.5 eV. A step-like structure is found which depends on the nanocrystal shape considered and agrees well with simulations of the exciton level structure. Rod-like Si-QDs can exhibit ~50 times higher absorption than spherical-like ones due to local field effects and enhanced optical transitions. In contrast with previous studies, evidence of a direct-bandgap red-shift for small Si-QDs is missing at the single dot level, in agreement with atomistic calculations. Low-temperature PL decay measurements reveal no triplet-like emission lines, but two ~μs decay constants appearing at low temperatures. They suggest presence of a temperature-dependent fast blinking process based on trapping/detrapping of carriers in the oxide matrix, leading to delayed emission. The proposed model allows to extract characteristic trapping/de-trapping rates for Si-QDs featuring mono-exponential blinking statistics. From PL saturation curves, ligand-passivated Si-QDs do not exhibit such detrimental phenomenon, in agreement with the proposed model. Last, Si-QDs demonstrate to be very hard against ~10 keV X-ray radiation, in contrast with CdSe-QDs whose PL quenching is correlated with a change in the blinking parameters. This property could be exploited for example in space applications, where radiation-hard materials are required. To conclude, the results achieved in this thesis will help to understand and engineer the properties of Si-QDs whose application potential has increased after several years of research both at the ensemble and at the single-dot level.QC 20181114