Self-regulated charge transfer and band tilt in nm-scale polar GaN films
  To date, the generic polarization of Bernardini, Fiorentini and Vanderbilt (PBFV) has been widely used to address the issue of polarity in III-V nitride semiconductors, but improvements in nitride materials and the performance of optoelectronic devices have been limited. The current first-principles calculation for the electronic structures of nm-scale [0001] GaN films show that the internal electric fields and the band tilt of these films are in opposite direction to those predicted by PBFV. 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 the Ga-terminated surface) approximately equal to the local valence band energy maximum (at the N-terminated surface). This effect is found to occur in films thicker than ~4nm. 
M. -H Tsai and S. K. Dey