Growth of ZnO thin films - experiment and theory

Many recent studies of ZnO thin film growth have highlighted a propensity for forming c-axis aligned material, with the crystal morphology dominated by the polar {0001} surface. This is illustrated here for ZnO thin films grown by pulsed laser deposition methods, and put to advantage by using such films as templates for aligned growth of ZnO nanorods. Complementary to such experiments, we report results of periodic ab initio density functional theory calculations on thin films of ZnO which terminate with the (0001), (000(1) over bar), (10(1) over bar 0) and (11(2) over bar 0) surfaces. Thin (<18 layer) films which terminate with the polar (0001) and (<000(1)over bar>) surfaces are found to be higher in energy than corresponding films in which these polar surfaces flatten out forming a new 'graphitic'-like structure in which the Zn and O atoms are coplanar and the dipole is removed. For thinner (<10 layer) slab sizes this coplanar surface is found to be lower in energy than the non-polar (<10(1)over bar 0>) and (11(2) over bar 0) surfaces also. The transition between the lowest energy geometries as the ZnO film thickness increases is investigated, and possible consequences for the growth mechanism discussed.