ABSTRACT
Silicon has become the most studied material in the past decades owing to its unique charac teristics: Si is the second most abundant element (after oxygen) in the Earth’s crust, making up 25.7% of its mass; it can be produced with impurity levels of less than 10−9; it remains a semiconductor at higher temperatures than germanium; its native oxide is easily grown in a furnace and forms a better semiconductor/insulator interface than any other material. These properties have made Si the widest used material for electronic devices, such as photovoltaic (PV) cells, light emitters, lasers, environmental probes, and so on. Nevertheless, the use of Si in photonic applications remains highly limited because the
indirect gap of the Si band structure-radiative interband transitions from the conduction-band minimum (Δ-point) to the top of the valence band (Γ-point)—requires electron-phonon coupling in order to satisfy the momentum conservation rule. Besides, such coupling is quite weak, and consequently the emission of a phononassisted photon results in a very unfavorable process compared with the direct no-phonon Γ-Γ radiative transitions. Some works in the early 1990s suggested that the problems related to the indirect bandgap of bulk Si might be overcome in highly conϐined systems, such as porous-Si (PS) [1-3] and Si nanocrystals (Si NCs) [4], in which the exciton is constrained in a narrow region of space while the momentum distribution spreads due to the Heisenberg uncertainty relation. In this case the momentum conservation law is not violated, allowing the Δ-Γ radiative transitions even in the absence of phonons. More recent works demonstrated the possibility to achieve efϐicient photoluminescence (PL) and optical gain from Si NCs [5].
