Atomic and electronic structures of silicene and germanene on GaAs(111)

By using first-principles method in the density-functional theory, we clarify the atomic and electronic structures of silicene and germanene on 1×1 GaAs(111). We find stable structures for silicene and germanene on both the As-terminated and Ga-terminated GaAs surfaces. The structures of silicene and germanene are similar to those of the free-standing ones, which present a honeycomb-hexagonal geometry. The cohesive energies of silicene and germanene on both As and Ga sides of GaAs surfaces are comparable to those of their bulk structures and/or those on Ag(111) substrates which have been widely observed in experiment, showing the possibility of synthesizing them on both sides of GaAs surfaces in experiment. The corresponding binding energies are in a range of 0.56-1.37 eV per Si (Ge) atom, 10 times larger than the usual van der Waals interaction, showing the covalent interaction between silicene (germanene) and GaAs surfaces. The band structure calculations show that such a covalent interaction induces the absence of Dirac electrons for silicene and germanene on GaAs surfaces. We then explore the method of recovering the Dirac electrons by using hydrogen (H) intercalation. It is found that the intercalated H atoms are chemically bonded to GaAs surface, and the silicene (germanene) shifts upward distance from GaAs surface increasing from 2.50-2.58 Å to 3.49-3.86 Å, where a covalent van-der-Waals interaction transition happens between silicene (germanene) and GaAs surface. Moreover, the distances between silicene (germanene) and H atoms are 30% and 8% larger than the atomic-radius sum of Si (Ge) and H on As-terminated and Ga-terminated GaAs surfaces, respectively. This shows that the interaction between silicene (germanene) and H on the As-terminated GaAs surface is obviously weaker than the typical covalent interaction, while on the Ga-terminated GaAs surface, it is comparable to the typical covalent interaction. This difference is induced by the difference in electronegativity between As and Ga atoms. We further find that the H intercalation recovers the Dirac electrons well on the As-terminated GaAs(111) due to the weaker Si (Ge)-H interaction, while it does not on the Ga-terminated GaAs(111) due to the stronger Si (Ge)-H interaction. The results are confirmed by performing calculations for silicene (germanene) on larger GaAs(111) surfaces, i.e., the 3×3 GaAs surface. Our study provides the theoretical basis for the preparation and application of silicene and germanene on semiconductor surfaces.