Epitaxy of Pseudomorphic GeSn Layers with Germane (GeH4) or Digermane (Ge2H6) as Ge Precursors and Tin Tetrachloride (SnCl4) as the Sn Precursor
Authors: Hartmann Jean-Michel , Frauenrath Marvin , Richy Jérôme 
Published: December 2020

In CEA-LETI, GeSn/SiGeSn stacks are typically grown at 100 Torr with Ge2H6, Si2H6 and SnCl4 and temperatures in the 301°C-363°C range. Ge2H6 has however a reputation of being unstable for high concentrations (10% in H2). The short shelf time can be adressed by switching over to lower concentrations and regularly changing bottles, which is complex as Ge2H6 is costly and complicated to order. One solution could be to switch from digermane to germane, which was shown by several R&D institutes to yield high quality GeSn layers. The epitaxy tools used, then, were capable of Atmospheric Pressure CVD (i. e. @ 760 Torr), while we have Reduced Pressure CVD chambers that can operate only up to a few hundreds of Torr.
We have thus investigated the growth of tens of nm thick GeSn layers on 2.5 µm thick Ge Strain-Relaxed Buffers, themselves on Si(001). First GeSn growth trials at 349°C, 100 Torr with the same GeH4 flow than Ge2H6 did not yield any peaks in X-Ray Diffraction (XRD). Changing the Mass-Flow Controller and using a GeH4 flow 4 times higher resulted in modest Sn content layers, with some Sn surface segregation, however. Increasing, for fixed GeH4 and SnCl4 flows, the precursor partial pressures by (i) reducing the H2 carrier flow or (ii) increasing the chamber pressure resulted in linear GeSn Growth Rate (GR) and Sn content increases. Sn droplets were present on the surface up to 300 Torr, however. By contrast, GeSn layers grown at 400 Torr were high quality in XRD (well defined, intense peaks with thickness fringes, as shown in Fig. 1), with cross-hatched surfaces as the Ge SRBs underneath (Fig. 2).
For a fixed GeH4 partial pressure (1.28 Torr), we have then evidenced a linear increase of the GeSn GR with the SnCl4 flow at 349°C. This increase was sub-linear at 325°C, with a slope change for P(SnCl4) = 0.008 Torr (Fig. 3). By contrast, the Sn content x was almost steady with the SnCl4 flow at 349°C. At 325°C, we had first of all an increase of the Sn content up to P(SnCl4) = 0.008 Torr, then a decrease, as shown in Fig. 4. Two SnCl4 partial pressures were therefore selected for the investigation of the impact of temperature on GeSn growth kinetics: 0.008 and 0.016 Torr (i.e. F(SnCl4)/F(H2) = 2E-05 or 4E-05).
We had exponential increases, over the 301°C - 349°C range, of the GeSn GR with the temperature, with similar activation energies for both chemistries: 8.6 – 11.8 kcal mol.-1 for GeH4 ó 10.4 kcal. mol.-1 for Ge2H6 (Fig. 5). Meanwhile, the Sn content linearly decreased as the growth temperature increased, with a slope which was less for GeH4 than for Ge2H6 (- 1.0 or -1.1 % / 10°C, to be compared with - 1.7% / 10°C) (Fig. 6). Although the GeH4 flow was 4 times the Ge2H6 one and the chamber pressure 4 times higher (400 Torr ó 100 Torr), the GeSn GR was almost the same for a given SnCl4 flow. Sn contents were by contrast significantly lower with GeH4 than with Ge2H6. Halving, for the same GeH4 flow, the SnCl4 flow resulted in GeSn GR 40% lower but slighly higher Sn contents, which was once again counterintuitive!
The Ge GR component (= (1-x)*GeSn GR) increased exponentially with the temperature, with similar values and activation energies for both precursors. Meanwhile, we had, over the 301°C-349°C range, a slight decrease of the Sn GR component (= x*GeSn GR) as the temperature increased. This was likely due to Sn sublimation. Sn GR components were 50% higher, for the same SnCl4 flow, with Ge2H6 than with GeH4. Complex interplays between precursors thus govern the GeSn growh kinetics with both precursors. Higher Sn contents and GeSn GR are nevertheless feasible with Ge2H6 + SnCl4 instead of GeH4 + SnCl4. Finally, while the GeSn GR and Sn contents were rather uniform over the 200mm wafer surface with Ge2H6 + SnCl4, the situation was definitely worse with GeH4 + SnCl4. GR were then significantly higher at the wafer edges than close to the center (although the Sn content was fairly uniform).

Figure 1