Strain suppressed Sn incorporation in GeSn epitaxially grown on Ge/Si(001) substrate

The effects of lattice misfit strain in epitaxial GeSn/Ge/Si(001) heterostructures on Sn incorporation, misfit dislocations (MDs), and the critical thickness were investigated using high-resolution x-ray diffraction. By performing a simulation of the x-ray reciprocal space maps measured in the vicinity of an asymmetrical reflection, we determined the strong correlation between the strain relaxation, density of MDs, and Sn content in the GeSn alloy for a compositional range of 4 to 15 at. %. Herein, we quantitatively describe the phenomenon of strain-suppressed Sn incorporation in GeSn, leading to the formation of top Sn-rich, middle compositionally graded, and bottom Sn-poor regions at fixed growth conditions. It is shown that the thicknesses of the bottom Sn-poor region at which the composition spontaneously changes are correlated with the theoretically predicted critical thickness for the nucleation of MDs. Depending on the degree of strain relaxation in the bottom GeSn layer, the density of MDs at the GeSn/Ge interface varies in the range of ∼2–5 × 105 cm−1 as the Sn content increases from 4 to 12 at. %. The thickness of the compositionally graded region, 160 ± 25 nm, corresponds to the thickness at which the GeSn layer fully relaxes through the interaction/multiplication of MDs. These results contribute to the fundamental understanding that strain engineering is not only crucial to tune the bandgap of Sn-containing group-IV semiconductors but also a key factor for the growth of Sn-rich metastable GeSn layers for mid-infrared photonic devices.