ABSTRACT
Vertically aligned ZnO rods were uniformly grown on two different substrates: 1 × 1.5 cm2 n-type (111)-oriented Si (with an electron concentration of ca. 1017 cm−3) and fluorine-doped tin oxide (FTO, resistance: ~8 Ω/square)-coated sapphire substrates by VPT method in a horizontal tube furnace.22 In our work, the Si substrate facilitates conventional top-emitting configuration, while a bottom-emitting configuration is adopted for transparent FTO/sapphire substrate. The as-grown ZnO rod arrays were subsequently implanted using a VARIAN (E-220) ion implanter. The conditions for As+ and P+ ion implantation and thermal activation are tabulated in Table 4.1. Since the rods are intentionally grown slightly thicker at top and thinner at bottom, ion implantation into the side walls of the rods could be effectively avoided (see Fig. 4.2 (b)]. Dopant activation annealing of the As-implanted ZnO rods were performed for 2 h at 750°C under vacuum (base vacuum ~5 × 10−2 Torr), with an O2 flow of 50 sccm, while for P-implanted samples, the annealing were performed at 900°C for 1 h under O2 flow of 100 sccm. Figures 4.1 (a) and (b) show the schematic diagrams of the p-ZnO:As (or P) rods/n-ZnO homojunction LEDs on Si-and FTO-coated sapphire substrates, respectively, where the p-type top layer of the vertically aligned ZnO is indicated. To prepare top contact on the ZnO rods, an insulating supporting layer such as hydrophobic poly(methyl methacrylate) (PMMA) or spin-on-glass (SOG, Honeywell) was spin-coated onto the rod array.23 After spin-coating of insulating layer, either oxygen plasma etching or reactive ion etching was performed to expose ZnO NR tops for contact formation.21, 24 Finally, a thin layer of Au was fabricated by sputtering through a shadow mask, and the device was ready for test.
