Structural analysis of ZnO nanorods grown on a ZnO homo-buffer layer by using MOCVD

Vertically well-aligned ZnO nanorods were fabricated on Al ₂O₃ with ZnO homo-buffer layers or GaN interlayers by using a catalyst-free metal-organic chemical vapor deposition (MOCVD). X-ray diffraction (XRD) measurements demonstrated that, compared with the nanorods grown on the GaN interlayer, a substantial amount of structural disorders existed in the ZnO nanorods grown on the ZnO homo-buffer layer. Field-emission transmission electron microscope (FE-TEM) measurements from the interfaces of the nanorods and the substrates also exhibited structural disorders existing in the bottom part of the ZnO nanorods grown on the sapphire and on the ZnO homo-buffer layer while no distinguishable disorder was observed at the ZnO/GaN interface. However, extended X-ray absorption fine structure measurements at the Zn K edge revealed that a small, but distinguishable, amount of structural disorder existed in the Zn-O pairs in the beginning of the nanorod growth. Our observation strongly suggests that structural strain due to the surface roughness of the ZnO homo-buffer layer and to the lattice mismatch between the ZnO and the GaN mainly contributed to island growth in the beginning of the ZnO nanorod growth.