Control of diameter of ZnO nanorods grown by chemical vapor deposition with laser ablation of ZnO

We made a study of controlling diameters of well-aligned ZnO nanorods grown by low-pressure thermal chemical vapor deposition combined with laser ablation of a sintered ZnO target, which was developed by us. Until now, it has been impossible to control diameters of ZnO nanorods, while the growth orientation was maintained well-aligned. In this study we developed a multi-step growth method to fabricate well-aligned nanorods whose diameters could be controlled. Metal Zn vapor and O₂ are used as precursors to grow ZnO nanorods. N₂ is used as a carrier gas for the precursors. A substrate is an n-Si (111) wafer. A sintered ZnO target is placed near the substrate and ablated by a Nd–YAG pulsed laser during ZnO nanorod growth. The growth temperature is 530 C and the pressure is 66.5 Pa. A vertical growth orientation of ZnO nanorods to the substrate is realized in the first-step growth although the diameter cannot be controlled in this step. When an O₂ flow rate is 1.5 sccm, well-aligned nanorods with 100 nm diameter are grown. Next, the second-step nanorods are grown on only the flat tip of the first-step nanorods. The diameters of the second-step nanorods can be controlled by adjusting the O₂ flow rate, and the growth direction is kept the same as that of the first-step nanorods. When the O₂ flow rate in second-step growth is smaller than 0.6 sccm, the diameter of the second-step nanorods is 30–50 nm. When the O₂ flow rate is between 0.75 and 3.0 sccm, the diameter is almost same as that of the first-step nanorods. When the O₂ flow rate is larger than 4.5 sccm, the diameter is increased with increasing O₂ flow rate. Further, the third-step ZnO nanorods with gradually increased diameters can be grown on the second-step nanorods with 1.5 sccm O₂ flow rate and without laser ablation.