Abstract
The coupling between size and surface effects on the mechanical and piezoelectric properties of wurtzite-type piezoelectric materials at the nanoscale is still not entirely exploited. The influence of surface orientation and thickness of wurtzite GaN nanofilms on their electromechanical properties (mechanical, piezoelectric and dielectric properties) is therefore investigated in this study by means of a multiscale analysis. The density functional theory (DFT) is utilized to study the atoms reconstruction phenomenon near surface region. Molecular dynamics (MD) simulation is employed to examine the effects of different thicknesses and surface orientations on the electromechanical properties. A continuum mechanics model incorporating the effects of both thicknesses and surface orientations is then developed and can be helpful for investigating the structure–property of GaN nanofilms. It is found through MD simulations that the elastic constant decreases in the out-of-plane direction but increases in the in-plane direction, regardless of surface orientations. This can be justified by DFT-based calculations, which show that the change of surface electron density of the nanofilm causes the reconstruction of surface atoms, leading to the extension of bond length in the out-of-plane direction and the shortening of bond length in the in-plane direction and thereby the difference between surface elasticity and bulk elasticity. The change of surface elasticity further affects the surface piezoelectric and dielectric properties. Also, the developed continuum mechanics model can reproduce the thickness- and orientation-dependent behaviors of the electromechanical parameters by fitting MD results. Finally, the developed continuum mechanics model is applied for predicting the thickness dependence of the piezopotential coefficient, showing that the piezopotential increases by 10% when the nanofilm thickness is 3 times the crystal constant.
Published Version
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