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 (11 (2) over bar0) 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 degrees 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 nm(1/2) are obtained. (C) 2008 Elsevier B.V. All rights reserved.