Structural and photoluminescence properties of zinc oxide nanorods grown on various transparent conducting oxide seed layers by chemical bath deposition

Zinc oxide (ZnO) nanorods (NRs) were grown directly on gallium doped ZnO (GZO), tin doped indium (III) oxide (ITO) and fluorine doped tin (IV) oxide (FTO) seed layers by chemical bath deposition using the mixed aqueous solutions of Zn(NO3)2 center dot 6H2O and C6H12N4 for the different growth times (tgs). Almost all of the NRs grown on the GZO seed layers, denoted by "NRs/GZO", were vertically aligned to the substrate surface independently of tg. Some of the NRs grown on the ITO and FTO seed layers, denoted by "NRs/ITO" and "NRs/FTO", respectively, were vertically aligned to the substrate surface and others were inclined. For both the NRs/ITO and NRs/FTO, moreover, the relative portion of the inclined NRs tended to increase with increasing tg. Both the average width (Wav) and average length (Lav) became larger in the order of the NRs/GZO, NRs/ITO and NRs/FTO, corresponding to the ascending order of the average diameter of the grains forming the seed layer. For the NRs/GZO, the Wav, Lav and density of NRs (nNRs) were almost unchanged at tg > 60 min. For the NRs/ITO and NRs/FTO, however, the increase in tg resulted in the increases in Wav and Lav and the decrease in nNRs. The opposite tg dependences between Wav and nNRs observed for the NRs/ITO and NRs/FTO indicated that the coalescence among the neighboring NRs contributed to the enhancement of Wav. For the NRs/GZO, the strains acting along the a- and c-axis directions were compressive and tensile strains, respectively. For both the NRs/ITO and NRs/ FTO, however, both the strains along the a- and c-axis directions were compressive strains. All the photoluminescence (PL) spectra of the NRs exhibited a near-band-edge (NBE) emission at -380 nm and an orange band (OB) emission at -600 nm. Regardless of the kind of the seed layer, PL intensity ratio of the OB emission to the NBE emission (IOB/INBE) became smaller with the increase in Wav. The decrease in IOB/INBE with increasing Wav is probably due to the decrease in the volume ratio of the surface region with high concentration of interstitial oxygen atoms (Ois) to the bulk region with few Ois and the decrease in the band bending in the surface region.