Boron and Phosphorous Doping of GeSn for Photodetectors and Light Emitting Diodes
Authors: Frauenrath Marvin , Hartmann Jean-Michel , Nolot Emmanuel 
Published: December 2020

The holy grail of group-IV light emission would be electrically pumped lasing at Room Temperature (RT). In CEA-LETI, the performance of p-i-n light emitting diodes is limited by the use of in-situ doped Ge layers beneath and above the active layer stack [1]. To overcome Sn segregation during the growth of the top electrode and, in thick GeSn/Ge stacks, avoid plastic relaxation, it would be ideal to switch over to in-situ doped GeSn grown at temperatures similar to that of the active region. We have therefore explored the in-situ boron and phosphorous doping of GeSn on Ge strain relaxed buffers with Ge2H6+SnCl4+ B2H6 and Ge2H6+SnCl4+PH3 chemistries. Growth pressure, temperature, F(Ge2H6)/F(H2) and F(SnCl4)/F(H2) Mass Flow Ratio (MFR) were 100 Torr, 349 °C, 7.92x10-4 and 4.78x10-5.
X-Ray Diffraction (XRD) measurements were conducted to gain access to the GeSn composition and layer thickness, as shown in Fig. 1 and Fig. 2. As the GeSn peak position was determined by the Sn content and also the B or P substitutional concentrations, the actual layer composition could not be unambiguously determined. Therefore, the Sn content in the following is an apparent Sn content. Thickness fringes outline the high GeSn crystalline quality.
As the F(B2H6)/F(Ge2H6) or the F(PH3)/2 F(Ge2H6) MFRs increased, the apparent Sn content stayed at first roughly around 6.5 %, as shown in Fig. 3. This value was the same for i-GeSn, GeSn:B and GeSn:P, in contrast to previous literature findings [2]. As the MFRs reached the highest values, the apparent Sn content reduced significantly for GeSn:B (from 6.5 % down to 4.6 %) and less significantly GeSn:P (from 6.6 % down to 5.6 %). The significantly smaller size of B compared to P (aB = 3.852 Å <=> aP = 5.014 Å) reduced the compressive strain in GeSn:B 2.8 times more than in GeSn:P for the same dopant concentrations, explaining the more pronounced effect. We quantified the substitutional B and P incorporation by assuming that the Sn content was not impacted by B or P incorporation. The estimated B and P substitutional concentrations then increased from 2.8x1019 cm-3 up to 3.8x1020 cm-3 and from 2.5x1019 cm-3 up to 5.4x1020 cm-3 (highest four MFRs for GeSn:B and GeSn:P), shown in Fig. 4. Those values are close indeed to the highest substitutional B or P concentrations obtained in Ge:B or Ge:P layers grown at 350°C, 100 Torr with Ge2H6: 4.8x1020 cm-3 [3] and 4.1x1020 cm-3 [4], respectively.
We determined elemental growth rates by multiplying the GR by the Ge or apparent Sn content to have a more detailed look at the growth mechanics (Fig. 5). The introduction of SnCl4 catalyzed the growth rate of GeSn compared to that of pure Ge, in line with the literature [4]. As the F(B2H6)/F(Ge2H6) or the F(PH3)/2F(Ge2H6) MFR increased, the Sn GR component stayed constant at around 2.5 nm min.-1. Meanwhile, the Ge GR component was constant at around 37 nm min.-1 for lower MFRs. For high dopant flows, the Ge GR component increased significantly for GeSn:B and reached 48 nm min.-1. The increase was less significant for GeSn:P, with at most 40 nm min.-1. This is to our knowledge, the first time an increase of the GeSn:P GR is reported for high dopant flows. Similar increases were found for Ge:P and Ge:B grown at 350°C, however [3-4]. The constant Sn GR component and the increase of the Ge GR component at high MFRs are likely due to an increased incorporation of Ge thanks to B and P opening surface sites (Indeed, the Ge GR component is surface rate limited, while the Sn GR component is reaction rate limited [5]).
A cross-hatch along <110> was found by Atomic Force Microscopy for all layers, which were otherwise smooth and featureless, as shown in Fig. 6 top for GeSn:B and Fig. 6 bottom for GeSn:P. The only exceptions were the GeSn:B samples grown at the three highest MFRs. There were then some islands, which were likely due to B and/or Sn surface segregation. Other studies will be performed to gain access to the Sn, P and B contents.
[1] M. Bertrand et al., 2018 IEEE International Conference on Group-IV Photonics, pp. 45-46.
[2] J. Margetis et al., Mater. Sci. Semicond. Process. 70, 38 (2017).
[3] J.M. Hartmann et al., ECS PRIME 2020 Meeting Abstract ID 138196, Symposium G03.
[4] J. Aubin et al., ECS J. Solid State Sci. Technol. 6, 1 (2017).
[5] J. Margetis et al., J. Vac. Sci. Technol. A 37, 021508 (2019).

Figure 1