Thin films of würtzite materials-AlN vs. AlP

Ab initio periodic density functional calculations are reported of the polar (0 0 0 1)/(0 0 0 1¯) and non-polar (1 0 1¯ 0) surfaces of AlN and AlP thin films. We analyse the optimised structures of the films in detail. These demonstrate the stabilisation of thicker films terminating with the polar (0 0 0 1) surface via charge transfer and surface metallisation. The nature of the charge transfer is different in the two compounds and influences the observed relaxations of the surface atoms. For the (0 0 0 1)/(0 0 0 1¯) ultra-thin film of AlN, as for ZnO, the dipole is removed by the formation of a graphitic-like structure in which both Al and N are threefold coordinate. In contrast, the (0 0 0 1)/(0 0 0 1¯) ultra-thin film of AlP forms a “snake”-like structure with P present at both surfaces of the film. We discuss the possible implications of our results for the growth of the two materials.