Strain relaxation and Sn segregation in GeSn epilayers under thermal treatment

Abstract

We report the effects of thermal annealing on the characteristics of GeSn epilayers grown on Ge-buffered Si wafers with a high Sn content near a threshold value that affords a direct bandgap. On annealing at temperatures below 400 °C, the characteristics of the epilayer remain unchanged, compared to those of the as-grown samples. On annealing the samples at a temperature in the range of 440–540 °C, strain relaxation in the epilayer is observed, accompanied by generation of misfit dislocations at the GeSn/Ge interface. A further increase in annealing temperature beyond 580 °C causes not only a relaxation in strain but also a change in the microstructure of the epilayer. In addition, Sn forms clusters and segregates to the surface, resulting in a reduction in the Sn content of the epilayer. The present investigation shows changes in the characteristics of the film under thermal treatment, providing an insight into the physical properties of such devices.

The thermal stability of epitaxial GeSn layers

“…4(d)]. In agreement with the reports of Li et al 6 and Comrie et al, 8 we attribute this behavior to the beginning of the Sn out-diffusion, leading to the abrupt transition occurring at T C . As pointed out before, in the higher Sn content samples, the transition to the thermodynamically stable Ge 0.99 Sn 0.01 phase occurs at lower temperatures and this gradual out-diffusion process cannot be observed.…”

“…The weak peak observed at ∼25 • in REL9 is compatible with the presence of few clusters made of ordered Ge 0.5 Sn 0.5 zinc blende structure with a 0.62 nm lattice parameter, most likely located within the epi-layer. 11 Different from the report of Li et al, 6 we do not observe, in samples of comparable Sn content and relaxation degree, Sn clustering in the bulk of the epi-layer. This might be due to the different growth technique (molecular beam epitaxy) and the rather low deposition temperature (160 • C) used in Ref.…”

“…This might be due to the different growth technique (molecular beam epitaxy) and the rather low deposition temperature (160 • C) used in Ref. 6 that could have triggered a higher density of Ge vacancies in the epilayer. In fact, a high density of point defects could evolve in complex vacancy defects/clusters acting as seeds for the formation of non-substitutional segregated β-Sn defects, 12 as reported for a rather extreme case for the annealing of amorphous Ge 0.92 Sn 0.08 layers.…”

“…[6][7][8][9] Furthermore, the strain relaxation mechanisms in GeSn remain unclear and contradictory reports can be found in the literature. 10 This is due to the fact that several experiments were performed adopting an interpretative framework similar to that developed for SiGe alloys, a material system for which the strain relaxation and dislocation dynamics are well understood and often aprioristically considered valid for all the group IV alloys.…”

“…10 This is due to the fact that several experiments were performed adopting an interpretative framework similar to that developed for SiGe alloys, a material system for which the strain relaxation and dislocation dynamics are well understood and often aprioristically considered valid for all the group IV alloys. For example, Li et al 6 indicate that GeSn layers pseudormorphically grown on Ge buffers undergo plastic strain relaxation by thermal treatment above 420 • C, as evidenced by the formation of a strain relieving misfit dislocation (MD) network at the GeSn/Ge interface and of threading dislocations (TD) in the GeSn layers. On the other hand, Chen et al 7 studied the change of the surface morphology and Sn diffusion in pseudomorphic Ge 0.9 Sn 0.10 layers under annealing, not observing any plastic strain relaxation.…”