Thickness-dependent physical and nanomechanical properties of AlxGa1−xN thin films

A set of undoped AlxGa1−xN epilayers with different thicknesses were grown on (0001) sapphire substrates using metal-organic chemical vapor deposition (MOCVD) technique, using the same gas-phase composition of trimethylaluminum (TMAl) and trimethylgallium (TMGa). Giving that the first growth stage is crucial for the successful fabrication of AlxGa1-xN epilayers; thus, SiN nano-mask, as an alternative method to nanoimprint lithography (NIL) which is used for patterning substrates, was deposited on sapphire substrate. Indeed, SiN treatment initiates the three-dimensional (3D) growth mode to efficiently decrease the threading dislocation density. It was found that depending on the film surface coalescence degree, the physical and nanomechanical properties are thoroughly dependent. As the non-coalescent surface is present, the porosity effects are dominant. As soon as the film thickness attains the surface coalescence, the Al incorporation is enhanced and convoyed with it point defects. Despite the decrease of the threading dislocations along with the film thickness evolution, it was observed that the physical properties are slightly decreased and the nanomechanical properties are improved. This is likely related to point defect interaction with nucleated dislocations. Moreover, the defects did not severally affect the plastic deformation capacity (ductility) of AlxGa1-xN films, where high indentation force was applied and no failure was revealed. Therefore, the balance between AlxGa1−xN physical and nanomechanical performances devotes to manufacture a wider range of efficient UV devices.