Investigation of polycrystalline ZnO film growth by quantitative TEM orientation mapping

Polycrystalline chemical vapour deposited ZnO films are commonly used as transparent electrodes for photovoltaic devices. Over the past years much effort has been put into improving their properties. Even though these films are now well optimised for specific device applications, a detailed understanding of their microstructure and formation mechanisms has not yet been established. Up to now the microstructure formation of these films was qualitatively described by a competitive overgrowth of neighbouring grains that leads to the formation of columnar V-shaped grains with a fibre texture. The lack in a more detailed and quantitative knowledge about the microstructure of these films is in part due to their complex nanometre sized grain structure, where previously applied standard characterisation techniques have provided only limited insights. To overcome this limitation, the present work used an automated crystal orientation mapping (ACOM) technique in transmission electron microscopy (TEM) to quantitatively characterise the grain structure of ZnO films. This technique allows the microstructure to be characterised down to lateral resolutions of a few nanometres. This work presents a methodology using ACOM for the analysis of the evolution of grain size, texture and grain boundary character of films, from the substrate up. The extensive data generated by this approach has, on the one hand, led to the identification of previously unnoticed growth mechanisms, such as renucleation of grains with a preferred orientation and frequent (0 1 -1 3) twinning, and on the other hand provided the necessary input for comparison with predictions from polycrystalline film growth simulations. Polycrystalline film growth simulations based on the van der Drift growth model were performed and a good agreement between simulations and experiments was found for films with a minimal presence of renucleation during growth. In both, the average grain size in function of the distance to the substrate h of the film are described by a power-law of the form proportional to h^a, with a=0.4. Furthermore, it has been found that the experimental grain size distribution of the film is self-similar. Both of these findings are consistent with previous predictions from 3D simulations based on the van der Drift model. Further, the origin of renucleation has been studied in more detail. Renucleating grains were identified by ACOM, in order to guide targeted complementary high-resolution TEM studies of the interface between renucleating grains and their neighbouring grains. The formation of nanometre sized regions with zinc blende (ZB) structure was identified in between stable grains with a wurtzite (WZ) structure, a polytypism which has so far not been thought to occur in polycrystalline ZnO films. By analysing grain orientations, it was found that the ZB leads to a tetrahedral coordination of surrounding WZ grains, where the misorientation between the WZ grains is characterised by [2 -1 -1 0] | 71°. Furthermore, it is shown that renucleating grains change in their orientation by several degrees with increasing distance to the ZB region. This structural distortion rotates the grains into a low-energy (0 1 -1 3) twinning relationship with neighbouring grains. The ZB-WZ polytypism is thus provides an explanation for the high frequency of (0 1 -1 3) twinning observed in the films.