ABSTRACT
Light emitting diodes (LEDs) are one of the major components in realizing optoelectronic functions.
However, it is well known that Si is an indirect bandgap material, and therefore, its luminescence
efficiency is quite poor compared to III-V direct bandgap materials. Due to the nature of its indirect
bandgap, the radiative recombination in Si at the band edge needs phonon assistance to maintain the
momentum conservation. The transverse optical (TO) phonon-assisted peak dominates the emission
spectrum. Due to the multiparticle nature of the recombination process, the radiative lifetime of the
carriers is much longer compared to direct transition recombinations. Germanium bandgap is also
indirect, but the direct valley at G point is only 0.15 eV higher than the indirect valley. Thus, excited
electrons can enter both valleys, and recombination occurs via direct and indirect emission channels.
Thus, it is possible to observe direct transitions from thin samples [1]. However, the output is weak due
to reabsorption by the material, phonon scattering and low carrier density in direct valley. Nevertheless,
people were persistently exploring possible solutions for efficient radiation in Si-and Ge-based materials
on silicon substrate, such as porous silicon [2], Si or Ge quantum dots embedded in larger bandgap
matrix, e.g., SiOx [3,4] and a-Si:H [5]. The incorporation of rare-earth atoms into silicon has also been
studied. Erbium-based emitters take advantage of the intra-4f shell transitions at 1.54 mm [6]. Room
temperature electroluminescence (EL) of erbium-doped silicon LEDs has been reported [7].
