Atypical grain boundaries in ZnO layers deposited on sapphire by rf magnetron sputtering on (0001) sapphire

The crystalline quality of ZnO is investigated, in thin films prepared by RF magnetron sputtering on c-plane sapphire. The ZnO layers exhibit a columnar growth and the average column diameter depends on the deposition temperature. Along the [10 (1) over bar0] zone axis of sapphire, the diffraction pattern exhibits two zone axis patterns; the central rows belong to the two zones, with the 0002n spots clearly underlined by the overlap. On each part, the two inner rows belong to the [11 (2) over bar0] zone and the next two to the [10 (1) over bar0] zone. Therefore, the two main epitaxial relationships in mismatched growth of wurtzite structures coexist in these layers, and adjacent columns are rotated, with respect to one other, by 90 degrees around the [0001] direction. In cross section observations, the interface between two grains is abrupt, but it is not easy to determine its atomic structure. Observations in planar view show that although the long range rotation between grains is in agreement with the theoretical epitaxial relationships, the local angles oscillate between 27 degrees and 32 degrees. This discrepancy is explained by the formation of grain boundaries which are found to follow the coincidence site lattice rules which make them settle into minimal energy configurations. (c) 2007 Elsevier Ltd. All rights reserved.