Graphitic Nanofilms as Precursors to Wurtzite Films: Theory

Periodic ab initio density functional calculations on ultrathin films of AlN, BeO, GaN, SiC, ZnO, and ZnS demonstrate the stabilization of thicker films terminating with the polar $\{0001\}$ surface via charge transfer and metallization of the surface layers. In contrast thinner films remove the dipole by adopting a graphiticlike structure in which the atoms are threefold coordinate. This structure is thermodynamically the most favorable for these thinner films. Implications for the crystal growth of wurtzite materials are discussed.

Figure 1

Calculated band structure of GaN showing Fermi level (dashed line) (a) bulk wurtzite. (b) 16-layer GaN $(0001)/(000\overline{1})$ metallized film.

Figure 2

Structure of 6-layer ZnO film: (a) wurtzite structure, (b) graphitic structure, where the film has optimized to a 3-layer film.

Figure 3

Cleavage energies as a function of film thickness for GaN. For the graphitic surface the number of layers plotted is that before optimization. The $(0001)/(000\overline{1})$ films are unstable with respect to the graphitic structure for a small number of layers. These particular values are obtained by relaxation with a set of constraints that prevent the film optimizing to the graphitic structure and are plotted as open diamonds.