Ultrahigh Carrier Mobility in the Two-Dimensional Semiconductors B8Si4, B8Ge4, and B8Sn4

Based on evolutionary search and first-principles calculations, we predict for B8Si4 structural stability in terms of cohesive energy, phonon spectrum, and melting point. The size of the indirect band gap is similar to that of bulk Si, and the electronic transport turns out to be highly anisotropic for both holes and electrons. The predicted structure prototype is shared by B8Ge4, B8Sn4, and B8Pb4. B8Ge4 is an indirect band gap semiconductor, with the hole mobility similar to that of B8Si4. B8Sn4 is an indirect band gap semiconductor with the gap size similar to that of bulk Ge. The hole mobility of B8Sn4 turns out to be as high as similar to 10(6) cm(2) V-1 s(-1) and the electron mobility as high as similar to 10(5) cm(2) V-1 s(-1), exceeding the performance of graphene (2 x 10(5) cm(2) V-1 s(-1)). B8Pb4 is found to be metallic.