Morphology-controlled growth of ZnO nanorods by chemical bath deposition and seed layer dependence on their structural and optical properties

Zinc oxide (ZnO) nanorods (NRs) were grown on ion-plated ZnO:Ga (GZO)/glass, sputtered Au/SiO2/Si(100) and commercial Au/Ti/Si(100) substrates by chemical bath deposition (CBD) using the aqueous solution of zinc nitrate hexahydrate [Zn(NO3)(2)center dot 6H(2)O] and hexamethylenetetramine (C6H12N4) at the different growth times (t(g)). Regardless of the difference in the substrate materials, ZnO NRs were highly oriented in the direction perpendicular to the substrate surface. Up to a t(g) of 30 min, for all the NRs grown on the three different types of substrates, both the average width and length of the NRs increased rapidly with increasing t(g). With further increasing t(g), the average widths of the NRs grown on the GZO/glass, Au/SiO2/Si(100) and Au/Ti/Si(100) substrates tended to be saturated at the widths of similar to 200 nm, similar to 400 nm and similar to 1000 nm, respectively. The maximum average lengths grown on the GZO/glass, Au/SiO2/Si(100) and Au/Ti/Si(100) substrates were similar to 1100, similar to 1600 and similar to 2000 nm, respectively. The t(g) dependence of the residual stress acting on the NRs grown on the GZO/glass exhibited a typical compressive-tensile-compressive evolution, which is characteristic to the films grown by the Volmer-Weber type growth mechanism. For the NRs grown on the Au/SiO2/Si(100) or Au/Ti/Si (100) substrates, however, the stresses acting on the NRs were tensile over the entire t(g) range. Photoluminescence (PL) spectra of the NRs grown on the Au/Ti/Si(100) substrates were dominated by an orange band (OB) emission associated with oxygen interstitials over the entire t(g) range. The NRs grown on the GZO/ glass or Au/SiO2/Si(100) substrates at longer t(g)s exhibited a near-band-edge (NBE) emission at a wavelength of similar to 380 nm, and the PL intensity of the NBE emission relative to that of the OB emission became larger with increasing t(g), indicating a decrease in surface state density with increasing t(g). PL excitation spectra revealed that the OB emission is effectively excited by the generation of the excitons and/or the electron-hole pairs associated with the NBE emission. Photoacoustic spectra results yield that the selection of the substrate material or the precursor should be effective in the suppression of the formation of the deep-level defect associated with the ingap photoacoustic band.