A first-principle study on the band structure of GePb alloys

Single crystal GePb alloys have been considered as potential direct bandgap materials for optoelectronics application. In this work, density-functional theory calculations were performed to investigate the crystalline and electronic structures of the GePb alloys. The lattice constants of the unstrained GePb alloys are found positively deviating from Vegard's law with a bowing coefficient of 0.587 & Aring;. GePb has a higher Poisson's ratios than GeSn with a similar alloying concentration. With the increasing Pb concentration x in Ge1-xPbx, a new alloying energy level brought by Pb appears at the bottom of the conduction band and continuously decreases. The new energy level is constructed to a new valley as compared to the initial Gamma valley and the new energy level is acquiring its higher spectra weights with increasing Pb concentration. An indirect-to-direct bandgap transition occurs with a Pb concentration of 3.3%. The effective masses of holes and electrons in the GePb Gamma valley are calculated to decrease with the increasing Pb concentration, while the effective masses of the electrons in the L valley only change slightly. The small effective masses of the electrons in the Gamma valley are favorable for high-speed GePb device application.