Stabilizing mechanism of the dipolar structure and its effects on formation of carriers in wurtzite {0001} films: InN and ZnO

The stabilization of the dipolar structure and its effects on carrier formation in wurtzite films are investigated through a case study of InN and ZnO {0001} films by means of the highly precise all-electron full-potential linearized augmented plane-wave method. The calculated total energies of both the wurtzite and graphitic structures demonstrate that the structural phase transition from the graphitic to wurtzite occurs when the thickness is beyond six (ten) bilayers in InN (ZnO). Our analysis of the calculated energies reveals that the phase transition is due to the competition between the bulk energy and the extra energy originating from the macroscopic electric field of the dipole structure. Further, an analysis of the electronic structure and charge densities in InN {0001} films reveals the screening role of the electric field. The screening effect causes a broadening of N 2p states in the band structure by approximately 1.5 eV and charge redistribution in each polarizable unit. For InN and ZnO {0001} films, we have also found by using pseudohydrogens to passivate the opposite surfaces that the screening has little effect on the surface states and on the work functions. In contrast, the calculated formation energies of defects in InN {0001} films (N vacancy and substitutional Mg) using both pristine and pseudohydrogen passivated surfaces show that the internal electric field of the dipolar structure in wurtzite films may affect the formation of carriers near the surface. Thus, these results provide some fundamental characteristics for the polar surfaces of wurtzite films.