Silicon Photonics: Fundamentals and Devices

2.8 Direct Gap: Ge/SiGeSn Heterojunctions

In this section, some of the recent developments in heterostructures made of group IV elements and their alloys will be mentioned. The most interesting features of the heterostructures are that direct gap can be obtained in the active material, which is mainly Ge, and that the band alignments are type I. Further, the multilayered structures may be grown on silicon platform.

2.8.1 Structures

The conventional heterostructures combining Ge, Si_1−xGe_x, and Si_1−x−yGe_xC_y have the limitations that the active layers are indirect gap in nature, and in addition the Si–Si_1−xGe_x heterostructures mostly show type II band alignment. These heterostructures are not useful as light emitters. SiGeC grown on Si is believed to show type I alignment 10; however, the band gap is still indirect.

The band gap variation in SiGe alloys with incorporation of Sn has been investigated by Soref and coworkers [14, 16], who predicted that the direct band gap may be achieved in SiGeSn ternary alloy for some compositions of Sn.

Recently, two significant achievements have been made in the quest for direct-gap group IV materials. The first is the growth of device-quality Ge_1−ySn_y alloy, in which an indirect-to-direct gap crossover is expected for y 0.2 17. The second is the development of Ge_1−x−ySi_xSn_y ternary alloy 18, the band gap of which has been theoretically studied by ...