Nanoindentation on a-plane ZnO thin films

The structural and mechanical properties of radio-frequency sputtering derived a-plane ZnO epilayers on r  -plane sapphire substrates developed at the various deposited temperatures was systematically studied by means of X-ray diffraction (XRD), atomic force microscopy (AFM) and nanoindentation techniques. The result of XRD analysis revealed that the angle of (1 1 2¯ 0) orientation increased with the deposited temperature. Both of the surface roughness and grain size of the a-plane ZnO thin films, which investigated by AFM and XRD, exhibits an increasing trend with the deposition temperature. In addition, the hardness and Young's modulus of a-plane ZnO thin films calculated from the load–displacement curves varied with the deposition temperature accordingly. As the deposition temperature increases from 300 to 500 °C, the grain size increases from 52 to 84 nm. The hardness of a-plane ZnO thin films dropped from 8.6 to 11.3 GPa due to the increase of the grain size. Moreover, by fitting experimental data with the Hall–Petch equation, a probable lattice friction stress of 1.64 GPa and Hall–Petch constant of 66.27 GPa nm1/2 are obtained.