Cheng Li

Silicon Thin Film


Xiamen University

422 Simin South Road, Xiamen University, Xiamen, Fujian 361005, China




  Professor Li Cheng systematically studies the epitaxial growth of silicon-based IV group heterostructures and photonic devices towards monolithically photonic integrated circuits. One of the key challenges is to tailor the electronic band structure of IV group materials to make high-efficient integrated light sources. By growing Ge and Ge-Sn epilayers and quantum wells on silicon substrates using ultra-high vacuum chemical vapor deposition and molecular beam epitaxy, his group has successfully developed silicon-based light emitting diodes and photodetectors.

Abstract for Presentation

Strain relaxation and photoluminescence of GeSn thin films grown by molecular beam epitaxy




  Germanium tin (GeSn) alloy has been the promising candidate for Si- based light emitting devices due to its direct band gap transition with Sn fraction of above 8% and compatible with CMOS technology. However, the complex behavior of strain relaxation and Sn segregation of GeSn epilayers makes it difficult to obtain strain relaxed GeSn films with high Sn fraction, especially with molecular beam epitaxy technique. In this talk, we reported on the observation of self- delamination of GeSn epilayer grown on Ge substrate at low temperature (~150oC) by molecular beam epitaxy. The GeSn film delaminates into a fully strained layer at bottom and a fully relaxed layer at top spontaneously, which is believed to be initiated by kinetic roughening and localized amorphization at low temperature.

  The strain relaxation and Sn segregation of fully strained Ge1-xSnx (x=0.097) films during rapid thermal annealing were investigated. We found that when the GeSn film was thinner than the critical thickness, GeSn epilayers would hardly be relaxed before Sn segregation during rapid thermal annealing. While for GeSn film thicker than the critical thickness, GeSn epilayers will undergo the process of strain relaxation before Sn segregation. A semi-quantitative model is proposed to explain the priority of Sn segregation or strain relaxation during rapid thermal annealing in terms of temperature-dependent critical thickness, which can be used to guide the design of strain-relaxed GeSn epilayer without Sn segregation for optoelectronic device application.

  GeSn films grown by molecular beam epitaxy were thought lack of luminescence because of its indirect band gap nature under compressive strain and rich non- radiative centers due to Sn agglomeration. In this talk, the strong photoluminescence is demonstrated with the strained GeSn and relaxed GeSn thin films after post- growth treatments. Those results indicate that GeSn grown by molecular beam epitaxy can also be applied for Si- based light emitting devices. 








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