Research progress of silicon light source

Abstract

To meet the requirements for high speed, low cost, and more information capacity, silicon photonics has been booming in recent years. Silicon photonics covers a very wide field. For the silicon photonics, researchers have successfully achieved silicon-based optical waveguides, switches, modulators, and detectors. But the problem of silicon based light source has not been really resolved, which has become a primary bottleneck for further developing the silicon photonics. The momentum of a phonon is required to allow an electron to transit from the minimum of the conduction band to the maximum of the valence band in Si because of the indirect bandgap. This two-particle process with a low probability makes it difficult to achieve high-efficiency silicon-based light source by itself. However, much effort has been made to characterize and understand the light-emission phenomena of silicon-based devices. Also, more attempts were made to enhance the emission efficiency of silicon. Practical silicon lasers are very important for silicon photonics and have been a long goal for semiconductor scientists. A number of important breakthroughs in the past decade have focused on silicon as a photonic platform thanks to the efforts of scientists. In this review, we introduce the recent progress of silicon-based luminescence materials, silicon light emitting diodes and silicon lasers. In the first part of this paper, common types of silicon-based light emitting materials, including porous silicon, silicon nanocrystals, rare earth-doped silicon, silicon defect emission, germanium on silicon and semiconducting silicides are comprehensively reviewed. Among them, the quantum effects and surface effects of low-dimensional silicon can greatly enhance the light emission efficiency. The erbium atoms in silicon-based rare earth materials can produce the light emission at communication wavelength band independently of the host. The transition from the lowest excited state to the 4f ground state yields light at 1.54 m. Moreover the emission energy is independent of the temperature due to the inner atomic transition. Group IV materials grown on silicon such as Ge and GeSn alloy can change from indirect bandgap into direct bandgap by introducing mechanically strain and modifying the component. Strong enhancement of photoluminescence and net gain emerging from the direct transition are very significant for fabricating the devices. In the second part, different light emitting diodes (LEDs) fabricated with above luminescent materials are introduced. The Si PN diodes were once popular at the earlier research stage. One approach was to modify the effective surface on high-purity single crystal silicon and the other idea was to use optically active defects in silicon. Ten years later, silicon LEDs in which the dressed-photon-phonons assisted method is used, made the Si PN diode rejuvenated. LEDs fabricated on nano-structured Si and silicon-based film were limited in the optoelectronic integration since the luminescence wavelength is not corresponding to the low-loss communication region. Although erbium-doped and Er silicate LEDs emit suitable light, their high turn-on voltage and low luminescence efficiency block the practical application. The researches of Ge-on-Si LED mainly focus on modifying the band structure by introducing strain and n-doping. In the third part, firstly we summarize the basic rules of the silicon laser. Then, we review the most recent progress in the field. Nanometer Si with periodic array can only behave unambiguous laser action using optical pumping and at very low temperature. Low threshold silicon Raman lasers with racetrack ring resonator cavities can only stop on paper also due to the difficulty in electrical pumping. The Ge-on-Si lasers operating at room temperature by optical and electrical pumping were accomplished in the past 5 years. The GeSn laser that is CMOS-compatible also came into being this year. Although so far, lasing has been implemented only by using pulsed optical pumping and stopped working at 90 K, this first demonstration of lasing in a direct-gap group IV alloy grown on standard silicon substrates is potentially an important step on the way to a platform of fully integrated silicon-based photonics. Hybrid III-V-on-Si lasers are considered as one of the most practical means due to the excellent photoelectric properties and mature preparation technology. Finally, current problems and future development direction in the silicon light source are also presented briefly.