Silicon is the second most common element found in the earth's surface. It has a wide variety of commercial uses, including in electronics and semiconductors, in metallurgical materials, and in photovoltaics. Silicon has a large impact on the world economy, and much of modern technology depends on it.
“Normal” silicon has a diamond structure, d-Si. Despite the prevalence of “normal” silicon in the photovoltaic industry, silicon is actually a relatively poor absorber of sunlight. This is because d-Si has an indirect band gap of 1.1 eV and a direct bandgap of 3.2 eV. That is, d-Si cannot directly absorb photons with an energy level less than 3.2 eV to promote electrons into the conduction band; it requires assistance from lattice phonons to transfer momentum of electrons to the conduction band, which are excited by photons with an energy level greater than 1.1 eV. This large disparity between the indirect and direct gaps means that thick layers of silicon are required to absorb light. As a consequence, efficiency decreases and cost increases.
For at least the above reasons, there is a need for new phases of silicon with direct or quasi-direct band gaps (nearly degenerate indirect and direct gaps) to improve the light absorption efficiency and lower manufacturing costs. In addition, optically active silicon (silicon capable of readily absorbing and emitting light) is desired in many optical applications (for example, diodes, lasers, sensors, etc.).