Thermally activated diffusion and lattice relaxation in (Si)GeSn materials

Abstract

Germanium-Tin (GeSn) alloys have emerged as a promising material for future optoelectronics, energy harvesting and nanoelectronics owing to their direct bandgap and compatibility with existing Si-based electronics. Yet, their metastability poses significant challenges calling for indepth investigations of their thermal behavior. With this perspective, this work addresses the interdiffusion processes throughout thermal annealing of pseudomorphic GeSn binary and SiGeSn ternary alloys. In both systems, the initially pseudomorphic layers are relaxed upon annealing exclusively via thermally induced diffusional mass transfer of Sn. Systematic post-growth annealing experiments reveal enhanced Sn and Si diffusion regimes that manifest at temperatures below 600°C. The amplified low-temperature diffusion and the observation of only subtle differences between binary and ternary hint at the unique metastability of the Si-Ge-Sn material system as the most important driving force for phase separation.

Quantitative Correlation Study of Dislocation Generation, Strain Relief, and Sn Outdiffusion in Thermally Annealed GeSn Epilayers

“…As we discussed above, this strain relaxation is related to the elongation of pre-existing MDs and not to the generation of new MDs/TDs. The thermal stability of the GeSn BL layer for sample S1 correlates with the high critical temperature of 500 °C for Sn diffusion reported for pseudomorphic GeSn layers . It should be noted that the results in ref may be influenced by a 300 nm thick Ge cap layer.…”

“…The thermal stability of the GeSn BL layer for sample S1 correlates with the high critical temperature of 500 °C for Sn diffusion reported for pseudomorphic GeSn layers. 16 It should be noted that the results in ref 16 may be influenced by a 300 nm thick Ge cap layer. Consequently, the diffusion of Sn in the pseudomorphic layers is characterized by a rather gradual decrease of the Sn content with annealing temperature and time.…”

“…Similarly, Li et al 14 reported on strong strain relaxation by MDs in GeSn layers at a The occurrence of Sn precipitations was also shown by Takeuchi et al 15 after the annealing of GeSn layers at 600 °C, which was correlated with a critical misfit strain in the GeSn layer. In contrast, compressive strain relaxation exclusively via Sn outdiffusion was shown for low-dislocation-density Ge 0.92 Sn 0.08 layers on Ge(001) by Von Den Driesch et al 16 for annealing temperatures above 450 °C. Considering that little attention has been paid toward the quantitative study of the process of strain relaxation, the relationship among (1) strain, (2) Sn outdiffusion, and (3) density of dislocations, under postgrowth annealing at temperatures lower than for Sn precipitation, is of interest for further study.…”

Dislocation Pipe Diffusion and Solute Segregation during the Growth of Metastable GeSn

“…In recent annealing experiments realized on GeSn layers, the authors attributed the observed fast and brutal segregation of Sn in relaxed layers to the presence of dislocations, as opposed to diffusional mass transport in pseudomorphic layers. 23,40 The transport of Sn from bulk to surface leads to the formation of self-propelled Sn droplets. The motion of these droplets is likely to be a result of a gradient in the surface Sn content, whereas the directionality of their guided motion can be a consequence of the surface symmetry or atomic steps that are characteristic to the epitaxial growth of strained layers.…”

“…This efficient defects engineering allows the TL to grow well above its theoretical critical thickness, estimated around 20 nm for a direct growth on the Ge VS. − However, when the total thickness of the GeSn stacking increases above 400 nm, the defective area extends, and dislocations propagate toward the surface. , Whereas the diffusion in the bulk is very slow, the nucleation of threading dislocations and their propagation across the TL to reach the surface provide Sn atoms with a much higher mobility and a preferential path to diffuse and segregate on the surface. ,− It is noteworthy that similar atomistic and microstructural features are also observed when the phase separation is activated by annealing GeSn monocrystalline layers above the growth temperature. In recent annealing experiments realized on GeSn layers, the authors attributed the observed fast and brutal segregation of Sn in relaxed layers to the presence of dislocations, as opposed to diffusional mass transport in pseudomorphic layers. , …”