(Invited) Future Applications of SiGeSn and GeSn
Authors: Grützmacher Detlev 
Published: February 2020

Silicon photonics is considered to be an attractive pathway to drastically reduce power consumption in information technology based on classical Si microelectronics. Group IV alloys, namely Ge and SiGe on Si, are heavily investigated for the integration of modulators and detectors on Si [1,2]. However, higher integration levels of optical data transfer on the chip will require the integration of low power laser. The latter is approached by a wide range of technologies to deposit III/V structures on Si as well as by bonding III/V lasers Si (see ref. 3-7 in ref. [3]). Recently, a direct bandgap as well as optically pumped lasing was demonstrated for GeSn alloys deposited on Ge virtual substrates on Si (100) 8” wafers [3]. Here, efforts are reported of this ongoing endeavor towards an electrically pumped group IV laser. Moreover due to the predicted low effective mass for electrons in the Г-valley, resulting in high electron mobility as well as high carrier injection velocity, the material bears notable potential for high speed, low power circuitry.
A novel epitaxial growth technology, namely, reactive gas source epitaxy has been applied to grow GeSn and SiGeSn film and heterostructures in a commercial 8” LPCVD tool, using Ge2H6, Si2H6 and SnCl4 has source gases in an N2 ambient. PH3 and B2H6 are used as sources for n- and p-type doping. Hetero- and quantum well structures containing SiGeSn/GeSn and strained Ge / GeSn layer, respectively, have been deposited at low growth temperatures (down to 325°C) and Sn concentrations up to 14.5%. Careful structural investigation were performed using X-ray diffractometry and transmission electron microscopy. Thick GeSn films grown on Ge will release about 80% of the strain. Interestingly, misfit dislocations are found typically only at the lowest interface towards the virtual Ge substrate, the interface structure within Hetero- and quantum wells structures appears to be free from extended defects. Optical pumping was performed using a ND-YAG laser with pulse duration of 5 ns. PL spectra are taken using a FTIR spectrometer equipped with a InSb detector.
To improve the performance of optically pumped laser obtained from GeSn Fabry-Perot devices fabricated using simple mesa stripes [3], the GeSn mesa lines were undercut by RIE processing. This yields efficient strain relaxation in the undercut area. We have then an increase in the energy splitting between Г and L valley for electrons and a mode overlap of ~94% leading to an improved performance of these laser in comparison to the simple Fabry Perot devices previously reported [3]. These laser operate up to 135K, additionally the lasing efficiency is doubled and threshold pump power is cut by half. Similarly, micro-disc laser have been fabricated. Multi-mode lasing involving several TE and TM modes is observed up to 140K. The threshold is found at a pump power of ~225 kW/cm2. A minimum Sn concentration of ~ 8.5% is found to be required to obtain lasing in these relaxed GeSn structures.
Light emitting diodes have been processed from p-i-n GeSn homojunctions. Light emission was observed up to 300K. First SiGeSn layers exhibiting a direct band gap, verified by comparing PL data and a joint density of state model are discussed. SiGeSn/GeSn p-i-n double heterostructures were grown, labelling the next milestone on the path towards an electrically pumped group IV laser. Moreover, the potential of GeSn alloys for the fabrication of low power, high frequency electronics is discussed. In particular, Bandstructure calculation reveal low effective masses, high mobilities and a high carrier injection velocity for electrons in the Gamma valley of the conduction band, making this material most interesting for the monolithic integration of optical and electronic devices.
Acknowledgement
The staff at PGI-9 and the clean room team at FZ-Jülich is acknowledged for their support. Special thanks to continuous input of Jeremy Witzens (RWTH) and Zoran Ikonic (Univ. Leeds) in analysing the data and modelling GeSn devices.
References

S. Assefa, F. Xia, F. and Y.A. Vlasov, “Reinventing germanium avalanche photodetector for nano-photonic on-chip optical interconnects. Nature 464, 80–84 (2010).

Xu, Q., Schmidt, B., Pradhan, S. & Lipson, M. Micrometre-scale silicon electro-optic modulator. Nature 435, 325–327 (2005).

S. Wirths, R. Geiger, N. von den Driesch, G. Mussler, T. Stoica, S. Mantl, Z. Ikonic, M. Luysberg, S. Chiussi, J. M. Hartmann, H. Sigg, J. Faist, D. Buca and D. Grützmacher, Nature Photonics 9, 88–92 (2015)