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 (SN), 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 similar to 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.