A CMOS compatible, direct bandgap material for optical interconnects can be obtained by alloying Ge with Sn ¹, applying tensile stress to Ge ² or both ³. Lasing in GeSn was demonstrated in 2015 ⁴ by Wirths et al., followed in 2020 by electrically pumped lasing up to 100K ⁵ and, in 2022, optically pumped lasing at room temperature ^6,7. In-situ doped SiGeSn might offer high dopant incorporation, while delivering good electronic confinement, improving thereby the performances of devices. Such doped layers can be used in photodetectors ^8–10, light-emitting diodes ^11–13 and modulators ^14,15 operating at wavelengths higher than 1.55 µm, enabling their use in future CMOS compatible lab-on-a-chip devices with integrated light sources ^5,16,17.

The in-situ doping of SiGeSn was compared to that of GeSn. All layers were grown at 349 °C, 100 Torr in a 200 mm Epi Centura 5200 RP-CVD tool from Applied Materials. Ge strain relaxed buffers were used to accommodate the lattice mismatch between (Si)GeSn and the Si substrates ¹⁸. The F(Ge₂H₆)/F(H₂), F(Si₂H₆)/F(H₂) and the F(SnCl₄)/F(H₂) Mass-Flow Ratios (MFRs) were constant at 7.92x10^-4, 1.25x10^-3, and 4.69x10^-5, respectively.

In-situ doped SiGeSn Growth Rates (GRs), shown in Figure 1 (a), were around 30 nm min.^-1. They were below that of GeSn:B and GeSn:P (40 nm min.^-1). The latter significantly increased for high dopant flows. Meanwhile, SiGeSn:B GR slightly increased and SiGeSn:P GR decreased as the dopant flow increased. B₂H₆ and PH₃ might have opened surface sites for GeSn and SiGeSn:B, while the formation of gas phase intermediates might have reduced the SiGeSn:P GR.

Interestingly, the surface quality improved significantly for in-situ doped SiGeSn, reaching the same quality as that of GeSn for high dopant flows, as shown in Figure 1 (b). Surfaces had RMS roughness values below 0.40 nm, close to that of GeSn, with a full surface cross-hatch recovery.

There was, for in-situ doped GeSn, a Sn content reduction for high dopant flows (Figure 1 (c)+(d)) most likely because SnCl₄ was mass-transport limited¹⁹ and not impacted by a larger amount of open surface sites. The influence of dopants on the layer composition was even more pronounced in SiGeSn:B. Si/Sn ratios of 3.5, with Si contents of up to 25%, were obtained, which should result in improved electrical confinement. The formation of Si and Ge gas phase intermediates might explain why Sn contents were higher, in SiGeSn:P, for high PH₃ flows. Such insights should yield better control of Si and Sn contents in stacks for optical or electronic purposes.

Electrically active carrier concentrations c_active of the order of 2x10²⁰ cm^-3 were achieved in SiGeSn:B (Figure 1 (e)). These were seven times higher than the 3x10¹⁹ cm^-3 obtained for GeSn:B. For GeSn:P, c_active was likely limited by the formation of Sn_mP_nV nanoclusters. It was at most 7x10¹⁹ cm^-3 (Figure 1 (f)). Four times higher c_active values were obtained for SiGeSn:P with at most 3x10²⁰ cm^-3. No decrease of c_active was observed for high PH₃ flows in SiGeSn:P. This might have been due to the formation of fewer Sn_mP_nV nanoclusters.

Such c_active values and better electrical confinement were used to fabricate (Si)GeSn based photodiodes with improved electroluminescent integrated intensity compared to photodiodes with doped Ge contact layers.

1. Gassenq, A. et al. Appl. Phys. Lett. 109, 242107 (2016).
2. Elbaz, A. et al. Nat. Photonics 14, 375–382 (2020).
3. Chrétien, J. et al. ACS Photonics 6, 2462–2469 (2019).
4. Wirths, S. et al. Nat. Photonics 9, 88–92 (2015).
5. Zhou, Y. et al. Optica 7, 924 (2020).
6. Chrétien, J. et al. Appl. Phys. Lett. 120, 051107 (2022).
7. Bjelajac, A. et al. Opt. Express 30, 3954 (2022).
8. Li, X. et al. Photonics Res. 9, 494 (2021).
9. Zhou, H. et al. Opt. Express 28, 10280 (2020).
10. Wu, S. et al. IEEE J. Sel. Top. Quantum Electron. 28, 1–9 (2022).
11. Stange, D. et al. Optica 4, 185 (2017).
12. Oehme, M. et al. IEEE Photonics Technol. Lett. 26, 187–189 (2014).
13. Schwartz, B. et al. Opt. Lett. 40, 3209 (2015).
14. Bertrand, M. et al. 2020 IEEE Photonics Conference (IPC) 1–2 (IEEE, 2020).
15. Zhou, H. et al. Opt. Express 28, 34772 (2020).
16. Casiez, L. et al. 2020 IEEE Photonics Conference (IPC) 1–2 (IEEE, 2020).
17. Soref, R. Nat. Photonics 4, 495–497 (2010).
18. Hartmann, J. M. & Aubin, J. J. Cryst. Growth 488, 43–50 (2018).
19. Margetis, J. et al. Vac. Sci. Technol. A 37, 021508 (2019).