(G03 - Best Student Presentation Award) Advances in In-situ Boron and Phosphorous Doping of SiGeSn
Authors: Frauenrath Marvin , Casiez Lara , Concepción Díaz Omar , Coudurier Nicolas , Gauthier Nicolas , N'hari Sidi-Mohammed , Nolot Emmanuel , Rodriguez Philippe , Buca Dan , Pauc Nicolas , Reboud Vincent , Hartmann Jean-Michel 
Published: November 2022

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 SnmPnV 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 SnmPnV 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.

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Figure 1