Silicon has become the most studied material in the past decades owing to its unique characteristics: 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.
