Microstructural study of epitaxial Zn1−xMgxO composition spreads

We have fabricated Zn1−xMgxO epitaxial thin-film composition spreads by layer-by-layer pulsed laser deposition (PLD) method where the composition across the chip is linearly varied from ZnO to MgO. In this paper we discuss the cross-sectional transmission electron microscopy study of the combinatorial spreads. The full orientation relationships between substrate and the ZnO and MgO phases were established. Formation of twin-related domain structure was found for the [111]-oriented MgO. Formation of the [100]-oriented cubic MgO in [0001]-oriented ZnO was observed in the mixed region in the middle of the spread. This relationship can potentially be used to grow [100]-oriented cubic structures epitaxially on c-axis-oriented hexagonal substrates. For the extended solid solution of ZnO, a high density of defects was found. Analysis of the defects established that they are translational domains formed by three-dimensional island growth of PLD and specific to the crystallography of ZnO. The ZnO phase appears without precipitation of the MgO phase, which supports the idea of extended substitutional solid solution.