Depolarization of few-layer III-V and II-VI materials through symmetric rumpling

It is generally believed that few-layer films of wurtzite materials remove the destabilizing dipole by converting to a flat hexagonal structure. However, using first-principles calculations, we demonstrate that contrary to the existing consensus these few-layer hexagonal films exhibit a small symmetric rumpling and are not perfectly flat. We then perform a systematic study of the rumpling behavior of a range of few-layer III-V and II-VI films. The symmetric rumpled configuration enables such films to cancel out the dipole and thereby to avoid the polar instability. This stabilization mechanism is quite distinct from those known for bulk and few-layer polar materials. Compared to the perfectly flat films, the rumpled films exhibit lower electrostatic potential energy, lower total energy, higher bonding strength, and thus greater stability and larger band gaps. We also discuss the relationship between rumpling behavior, interlayer interactions, and ionicity through electrostatic analysis.