Theoretical prediction of two-dimensional II-V compounds

Graphene has attracted significant attention as a pioneer of two-dimensional zero gap semiconductors, but the development of new two-dimensional materials with a finite band gap has been actively pursued. In this study, the structural stability of double bilayers (DBs) of group II-V compounds (II: Be, Zn, and Cd; V: P, As, and Sb) has been systematically investigated using first-principles calculations based on density functional theory. The thermodynamic calculations have confirmed that BeP, BeAs, ZnP, and ZnAs can be produced through exothermic reactions from their constituent bulk systems. It has also been confirmed that all the compounds have phonon dynamical stabilities. Only CdP and CdAs have been found to have an AB-stacked DB structure with threefold symmetry, while the other compounds have ${\text{AB}}^{\prime }$-stacked DB structure with broken symmetry. The difference in atomic radii between group II and group V results in the so-called size effect, which determines the stacking pattern. The structural stability of II-V DB thin films is explained by analogy with the surface structural stability of compound semiconductors: The change in the atomic arrangement of the DB structure alters the electronegativity of the surface orbitals of the II-V thin film, which does not result in any unsaturated bonds, i.e., no metallic bands across the Fermi level appear. The various DB II-V compounds proposed in this study will join the ranks of atomic-level 2D semiconductor materials.

Figure 1

Top and side views of initial structures of DB-stacked III-VI and II-V compounds. (a) AA-stacking for III-VI compounds, (b) AA-stacking for II-V compounds, and (c) AB-stacking for II-V compounds. Parallel quadrilaterals indicate primitive cells. Crystal orientation is shown in a triaxial representation, assuming a hexagonal crystal structure.

Figure 2

(a) Band structure of the DB GaSe compound. The dashed line shows the top of the valence band. (b) Electron localization functions for the GaSe in the cross section including the Se-Ga-Ga-Se bonding network.

Figure 3

(a) Top and (b) side views of the ${\text{AB}}^{\prime }$ configuration.

Figure 4

Formation energy, $\mathrm{\Delta }{H}_{\mathrm{f}}$, of II-V DB compounds for each group II series. Red, blue, and green indicate that the group V elements are P, As, and Sb, respectively.

Figure 5

(a) Band structure and (b) ELF of DB ZnAs. (c) Band structure and (d) the isosurface of the probability density for DB CdAs at the $\mathrm{\Gamma }$ point for the metallic band marked with a circle in Fig. 5. The top of the valence band and the Fermi energy were set to 0 eV for DB ZnAs and DB CdAs, respectively.

Figure 6

Relationship between the roughness of BL, $\mathrm{\Delta }h$(V-II), and the difference in atomic radius, $\mathrm{\Delta }r$(V-II), of DB group II-V compounds (see the text). Group V elements P, As, and Sb are indicated by red, blue, and green, respectively. The circles, triangles, and squares identify the group II atoms as Be, Zn, and Cd, respectively.