Strain in ZnO Epilayers: Elastic and Inelastic

What is it about?

The study analyzed the structural characteristics and strain state of ZnO epitaxial films grown on r-sapphire using MOCVD with thicknesses ranging from 12 to 770 nm. The research found that a critical thickness of about 150-200 nm marks a point of change below which important structural changes take place. For thin films, the calculated shape of crystallites is shown to be more or less spherical, while thicker ones become columnar, beginning with the elongation of crystallites at this critical thickness and giving rise to the stable columnar structure. The study also found that the plastic strain in the films is mainly responsible for the orthorhombic distortion of the unit cell, and that the relaxation of plastic strained ZnO films proceeds predominantly by nucleation of misfit dislocations, which is a consequence of a three-dimensional surface morphology.

Why is it important?

This research is important because it provides a detailed study of the structural and microstructural characteristics of sub-micron a-ZnO epilayers grown on r-sapphire by metal-organic chemical vapour deposition. The knowledge of stress levels is crucial to optimize the growth process of optoelectronic devices, which have high technological interest due to their applications in the blue and ultraviolet regions. Key Takeaways: 1. The study analyzed the structural and elastic properties of ZnO epitaxial films as a function of film thickness. 2. A critical thickness of about 150-200 nm marks a point of change below which important structural changes take place. 3. For thin films, the calculated shape of crystallites is shown to be more or less spherical, while thicker ones become columnar, beginning with the elongation of crystallites at this critical thickness and giving rise to stable columnar structure. 4. The study found that a plastic behavior is responsible for the orthorhombic distortion of the unit cell, and this deformation is confirmed from HRTEM images.