Impact of strain on Si and Sn incorporation in (Si)GeSn alloys by STEM analyses
Résumé
The impact of device structure on the properties of CVD-grown (Si)GeSn heterostructures was assessed thanks to scanning transmission electron microscopy (STEM) at the nanometer scale. Quantitative energy dispersive x-ray spectroscopy (EDX) together with precession electron diffraction (PED) and geometrical phase analysis (GPA) were performed to probe the chemical and structural properties of the different layers. The results presented in this paper demonstrated the advantages of a multilayer structure, with successive layers grown at decreasing temperature in order to gradually accommodate the in-plane lattice parameter and incorporate more and more Sn into the stack. It was shown how the GeSn emissive layer could be manufactured with low plastic deformation and high relaxation rate, necessary for better emissive performances. SiGeSn alloys used as confinement barriers around the emissive layer were also investigated. For such thin layers, we showed the importance of the starting lattice parameter (SLP) before the growth on their composition. Indeed, higher SLPs resulted, for the very same process conditions, into higher Sn contents and lower Si contents. The interest of combining EDX, which was accurate enough to detect slight chemical concentration variations, and GPA, for local strain analyses, was clearly demonstrated. Present results will be very useful to predict and control the bandgap and structural quality of (Si)GeSn materials and in turn the device properties.