Comparative studies of band structures for biaxial (100)-, (110)-, and (111)-strained GeSn: A first-principles calculation with GGA+U approach

• Published in: Journal of applied physics 2015-10, Vol.118 (16)
• Main Authors: Huang, Wenqi, Cheng, Buwen, Xue, Chunlai, Liu, Zhi
• Format: Article
• Language: English
• Subjects: Applied physics; Buffers; Comparative studies; Compressive properties; Crossovers; Deformation; Elastic properties; Energy gap; First principles; Germanium; Lattice parameters; Tensile strain; Tin; Valleys
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.4933394

Experiments and calculations performed in previous studies indicate that compressive strain will increase (100)-strained GeSn's need for Sn to realize a direct bandgap when it is pseudomorphically grown on Ge buffers. To eliminate this negative effect, we systematically investigate the band structures of biaxial (100)-, (110)-, and (111)-strained GeSn using a first-principle calculation combined with supercell models and the GGA+U approach. This method has proven to be efficient and accurate for calculating the properties of GeSn. The calculated lattice constants and elastic constants of Ge and Sn are in good agreement with the experimental results. The crossover value of Sn concentration which is required to change the bandgap of unstrained GeSn from indirect to direct is found to be 8.5%, which is very close to the recent experimental result of 9%. The calculated bandgaps of strained GeSn show that the moving rate of the Γ valley is higher than those of the L and X valleys in (100)- and (110)-strained GeSn. However, the moving rate of the L valley is higher than those of Γ and X valleys in (111)-strained GeSn. Tensile strain has a positive effect on the transition of (100)- and (110)-strained GeSn, changing the bandgap from indirect to direct, whereas compressive strain has a positive effect for (111)-strained GeSn. The use of the (111) orientation can reduce GeSn's need for Sn and greatly increase the energy difference between the L valley and Γ valley. Thus, for strained GeSn grown on Ge buffers, the (111) orientation is a good choice to take advantage of compressive strain.