Residual strain in ZnO nanowires grown by catalyst-free chemical vapor deposition on GaN/sapphire (0001)

ZnO nanowires were grown on 2 - μ m -thick GaN templates by chemical vapor deposition without employing any metal catalysts. The GaN template was deposited by metal-organic chemical vapor deposition on a c -plane sapphire substrate. The diameters of the resulting nanowires were in the range of 40 - 250 nm depending on growth time. The ZnO nanowires were vertically well aligned with uniform length, diameter, and distribution density as revealed by electron microscopy. X-ray diffraction spectra showed that ZnO grew in single c -axis orientation with the c axis normal to the GaN basal plane, indicating a heteroepitaxial relationship of ( 0002 ) Zn O || ( 0002 ) Ga N . The lattice constant of the c axis of the ZnO nanowires with diameter of 40 nm was 5.211 Å , which is larger than that of bulk ZnO ( 5.207 Å ) . The ZnO nanowires exhibit a residual tensile strain along the c axis, which decreases with increasing diameter.