Growth parameters of the ZnO nanowire

4.1.1 Pressures dependence on optical properties of ZnO nanowire

Fig.4-1~4-2 show PL spectral patterns of ZnO nanowires grown on silicon substrates pored with J=10mA/cm² by VLS method under the tube pressures of (a) 1 (b) 5 (c) 10 (d) 30 Torr at a fixed flux rate of argon gas 150 sccm. From the PL pattern, we can observe the three main photon emission categories: the near band edge emission (NBE) with peak position of 3.263 eV, the low energy tail extending from the near band edge emission, and the deep level emission with peak position of 2.48 eV at room temperature. We used the least square method to decouple the NBE and low energy tail extending from the NBE shown in the insert of Fig.4-1. Under the growth pressure of 10 Torr the FWHM of the NBE feature is corresponding to 98meV.

From Fig4-2 we find the FWHM of NBE becomes larger when the growth pressure is higher or lower than 10 Torr. The concentration of Zn vapor generated with the thermal reduction of ZnO powder by carbon depends on the tube pressure. When the concentration of Zn vapor is too high or low, the growth of ZnO nanowires is not proper. Because too much Zn vapor will make the Au-Zn alloys quickly to be saturated, the ZnO nucleation will be completed too fast to be a good single crystalline nanowire. And the appropriate growth rate is necessary for the ZnO nanowires growth. As the pressure is too low the ZnO nucleation will be completed too slowly to be a good single crystalline nanowire. So the concentration of Zn vapor must be appropriate for growing ZnO nanowires. Besides, we also observe ZnO nanowires on the porous silicon substrates with different current density show the

different FWHM of NBE. The reason is that the lattice mismatch between ZnO and PS substrate is bigger as the current density is larger. As increasing current density, the surface roughness of PS is more obvious. So the surface roughness perhaps is another reason.

4.1.2 Flow dependence on optical properties of ZnO nanowire

As indicated above, although optimal growth pressure can make ZnO nanowires have the better performance, the highly textured ZnO nanowires was still hardly obtained by using appropriate argon gas flow. So we used a sequence of different argon gas flow to grow better quality ZnO nanowires. Fig.4-3 shows the PL spectra obtained from ZnO nanowires grown under the tube pressure of 10 Torr at argon flow of 125, 150, 200, 250 sccm. The NBE intensity and the full width at half maximum both increase with the increasing argon gas flow as shown in Fig.4-4.

When argon flow is higher than 150sccm, the FWHM increases and the crystalline structure of nanowire becomes poor. Because when argon gas flow is higher than 150sccm, too much Zn vapor carried by argon gas is making the nucleation fast. The crystalline structure with fast nucleation in ZnO nanowires does not give a good optical property. The FWHM of NBE is broader than that of lower argon gas flow. We also found the ZnO nanowires did not grow on the PS substrate as the argon gas flow is lower than 100sccm. We speculate that Zn vapor is too low to make Zn in the Au-Zn alloy to be saturated, and there for Zn can not nucleate on the Au-Zn alloy surface to form ZnO.

4.1.3 Growth temperature dependence on optical properties of ZnO nanowire

The substrate temperature is one of the important factors during the growth

of ZnO nanowires. When the substrate temperature is higher than 750⁰C, Au still forms Au-Zn alloy with Zn. But it doses not form ZnO nanowires. Fig.4-5 shows only Au-Zn droplets- like alloy on the substrate (indicated by arrow) and there is no ZnO nanowires . Because the solubility of Zn in Au-Zn droplets is higher at high temperature, it is not supersaturated and Zn vapor does not nucleate at the solid- liquid interface. But when the substrate temperature is decreased to lower temperature (~650⁰C), the solubility of Zn in the Au-Zn alloys decrease and then supersaturated Zn alloys forms. It makes Zn start to oxidize and forms ZnO nanowires. When the temperature is much lower than 650⁰C, Zn separates fast from the Au-Zn alloys due to the low solubility of Zn at low temperature. That produces the poor crystalline structure of ZnO nanowires and not only nanowires but also other blocks or films exist.

4.1.4 Thickness of Au thin film dependence on optical properties of ZnO nanowire

According to the above optimal growth condition: tube pressure 10 Torr, argon flow rate 150sccm, substrate temperature 700⁰C, different thickness of Au thin film was changed to grow ZnO nanowires. The SEM images for a series of Au thickness in growing nanowires were shown in Fig.4-6~4-8. From SEM images we can observe the thicker thickness of Au film, the smaller diameter ZnO nanowires were obtained by CVD. The experiment results are analogous to other group ’s results.³¹⁾ It is strange that the ZnO belts and nanowires were grown together on the thicker thickness of Au thin film shown in Fig.4-8. We speculate that the thicker Au thin film forms bigger Au clusters or islands closely with each others at high temperature, that makes each Au-Zn alloys to connect together. Consequently, the piece of Zn reacts with oxides to form the ZnO belts.