(Invited) Epitaxial Growth of GeSn Layers on (001), (110), and (111) Si and Ge Substrates

Ge_1-xSn_x alloy thin films are much attractive for not only improving in Si nanoelectronics but also introducing new functional applications. High Sn content Ge_1-xSn_x promises to be direct semiconductor which promises the optoelectronic applications. We need to control the Sn precipitation, strain, dislocations, and point defects in Ge_1-xSn_x epitaxial layers for practical electronic and optoelectronic applications. We have been developing the growth technique of Ge_1-xSn_x epitaxial layers on various Si, Ge, Si on insulator (SOI), and Ge on insulator (GOI) substrates with molecular beam epitaxy (MBE) method [1-3]. Recently, we achieved high crystallinity Ge_1-xSn_x layers with a high Sn content up to 12% by lowering the growth temperature and controlling the lattice mismatch and strain [4]. In addition, the orientation of substrate has to be considered to improve on the crystalline quality and performance of Ge_1-xSn_x devices, because the substrate orientation significantly influence on the strain structure, defect structure, propagation of dislocations, energy band structures, and effective mass. The development of global- and local-growth technology of Ge_1-xSn_xepitaxial layers on not only Si(001) but also various substrates with (110) and/or (111) orientations is essentially required for various applications.

Ge_1-xSn_x epitaxial growth on (110) and (111) substrates

We examined the epitaxial growth of Ge_1-xSn_x layers on (110) and (111) Ge substrates [5-8]. We found that introducing Sn with a content as low as 2% into Ge is effective to reduce staking faults and twin defects in Ge epitaxial layers on (110) substrates even with the low-temperature growth of 150°C. We also found that the strain relaxation of Ge_1-xSn_xon (110) and (111) substrates takes place preferentially with a smaller Sn content than that grown on (001) substrate, that is attributed to the surface orientation dependence of the elastic modulus of epitaxial layers.

The Sn incorporation into Ge also improves on the crystalline quality of epitaxial layers prepared with low-temperature MBE on Si(110) substrate [9]. Formation of twin defects in Ge epitaxial layers on Si(110) can be drastically suppressed by the incorporation of Sn with a content of 2%. The propagation of misfit dislocations can be effectively enhanced during the post deposition annealing by suppressing the formation of twin defects.

Local growth of Ge_1-xSn_x epitaxial layers

We examined the formation of locally strained Ge_1-xSn_x/Ge fine structure and characterized the microscopic local strain structures by using X-ray microdiffraction method [10]. The local growth of Ge_1-xSn_x with a Sn content of 6.5% on fine-structure-patterned-Ge is achieved with low-temperature MBE. The concentration of local stress in embedded Ge_1-xSn_x layer causes the degradation of crystallinity with tilting of the lattice plane and Sn precipitation. We can directly measure the individual local strain in Ge sandwiched with Ge_1-xSn_xlocal stressor by microdiffraction. We demonstrate increasing in the in-plane strain with shrinking the Ge width and increasing the Sn content. The maximum in-plane compressive strain is estimated to be 1.4% for a width of 25 nm.

In the presentation, we will also talk about some electronic and optical properties of high Sn content Ge_1-xSn_x epitaxial layers and introduce some recent achievement in the epitaxial growth of Sn-related ternary alloys such as Ge_1-x-ySi_xSn_y.

Acknowledgements

This work was partially supported by a Grant-in-Aid for Specially Promoted Research of MEXT, the JSPS through the FIRST Program, and the ALCA program from JST in Japan. The microdiffraction measurement was performed at SPring-8 with the approval of general proposals (No. 2013A1682 and No. 2013B1779/BL13XU).

References

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