Tin in the Photodiode: Next Step for Optical On-Chip Data Transmission

Scientists have long been looking for a suitable solution to integrate optical components on a computer chip. However, silicon and germanium alone - the material basis of chip production - are hardly suitable as a source of light. Together with international partners, Jülich physicists have now presented a diode that, in addition to silicon and germanium, also contains tin in order to improve the optical properties. The special thing about this is that since all elements of the fourth main group belong, they are fully compatible with the existing silicon technology.

optical on-chip
Sandwich-like design of the SiGeSn diode, which may lead to novel energy-efficient applications for on-chip optical data transmission. Copyright: Forschungszentrum Jülich

For long distances, optical signal paths have long been standard because they require only a fraction of the energy compared to electrical transmission.However, the existing solutions are hardly suitable for moving data on the chip directly between the processor and memory or between the processor cores of a server or a PC. The materials for semiconductor lasers and diodes, so-called III-V semiconductors, belong to chemical main groups other than silicon, from which computer chips are manufactured. This results in different grating structures which lead to the fact that such components can only be integrated very cost-effectively and inefficiently.

Silicon and germanium, on the other hand, belong to the so-called indirect semiconductors. Because of the energetic states of the electrons that are quantum-physically possible, they are hardly capable of emitting or absorbing light – or more precisely, photons. The addition of tin changes the electronic properties of the crystal. The resulting compound becomes a direct semiconductor that can absorb and emit photons directly – and therefore very efficiently.

optical on-chip
Tin-containing photodiode Copyright:  Forschungszentrum Jülich

Because tin, such as silicon and germanium, belongs to the fourth main group of the periodic system, the silicon-germanium-tin diode, in short: SiGeSn diode, can be applied to silicon directly during chip production. The scientists have produced the SiGeSn diode from a GeSn / SiGeSn layer system, as explained in the Small specialist journal. The sandwich design enhances the efficiency with which the injected stream is converted into light, as demonstrated in the specialist magazine Optica. In addition, the researchers were able to adjust the optical wavelength in a range of 2 to 2.6 micrometers by means of the gradual change in the silicon and tin content.

With the development of diodes, researchers from the Peter Grünberg Institute (PGI-9) at Forschungszentrum Jülich have come a step closer to the development of an infrared light source for on-chip data transmission. In addition, the material could allow further applications such as photodetectors. As early as January 2015, the Jülich physicists in Nature Photonics had demonstrated the basic suitability of the SiGeSn compound by means of a laser module, which can be applied directly to silicon chips. However, the laser at the time could not be excited electrically, but only optically for the generation of laser light. Its function was also limited to low temperatures of minus 183 degrees Celsius. The current SiGeSn photodiode, on the other hand, also functions at room temperature.

Video “Tin in the Photodiode”

Length: 2:30 min

Electromagnetic spectrum of the SiGeSn diode

At different electrical current densities (red = 239 A / cm 2, yellow = 84 A / cm 2, blue = 26 A / cm 2) and 300 K (about 27 ° C)

optical on-chip
Electromagnetic spectrum of the SiGeSn diode