Self-regulated charge transfer and band tilt in nm-scale polar GaN films

Using first-principles calculation for the electronic structures of nm-scale [0001] GaN freestanding films, it is found that the Ga-terminated surface (S_Ga) has a positive electrostatic potential, while the N-terminated surface has a negative electrostatic potential (S_N), so that the energy bands tilt upwards from S_Ga to S_N. Additionally, it is determined that an intrinsic self-regulated charge transfer across the film limits the electrostatic potential difference across the film, which renders the local conduction band energy minimum at S_Ga approximately equal to the local valence band energy maximum at S_N. This effect is found to occur in films thicker than ∼4 nm. If the dangling-bond/surface states at both S_Ga and S_N are passivated by pseudo-hydrogen atoms, the tilt of energy bands is similar, though the cross-film potential is reduced due to the extra H_5/4-Ga and N-H_3/4 dipole layers.