Figure 1

(a) Dark field TEM image of sample A shows the defective GeSn (1^st layer) and low defect GeSn (2^nd layer), respectively. (b) SIMS of sample A indicates the spontaneous Sn gradient GeSn at both layers with different gradient rates. The 1^st layer was subdivided into region-I and II with boundary of critical thickness h _c . (c) In TEM Image of sample D, an additional GeSn (3^rd layer) is shown on top of two-layer structure. One bright line travels across the 2^nd and 3^rd layer indicates the penetration of threading dislocation. (d) SIMS of sample D shows the 3^rd layer with up to 17. 5% Sn content. (e) For TEM image of sample E, the boundaries between each Sn-enhanced step are indistinguishable in the 2^nd layer. (f) In SIMS of sample E, Sn incorporation increases continuously in the 2^nd layer. The Maximum Sn content of 22. 3% was achieved.