Growth of ZnO thin films—experiment and theory

TLDR

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. The study investigates ZnO thin films grown by pulsed laser deposition, using them as templates for aligned nanorods and examining how lowest‑energy geometries change with film thickness. The authors grow ZnO films by pulsed laser deposition and perform periodic ab initio density functional theory calculations on films terminated with (0001), (000), (100), and (110) surfaces. Thin films (<18 layers) with polar (0001) or (000) terminations are higher in energy unless the surfaces flatten into a coplanar graphitic‑like structure, and for slabs thinner than 10 layers this coplanar surface is even lower in energy than the non‑polar (100) and (110) surfaces.

Abstract

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), (100) and (110) surfaces. Thin (<18 layer) films which terminate with the polar (0001) and (000) 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 (100) and (110) 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.