Zinc oxide (ZnO) nanorods (NRs) have attracted lots of attention as a potential material for light-emitting diode (LED) applications, due to its outstanding characteristics such as large direct band gap and high exciton binding energy. However, despite these advantages, ZnO NRs are not used in LEDs due to several problems. Since ZnO NRs have naturally n-type conductivity owing to donor type defects in the lattice, the achievement of p-type conductivity is difficult. As a result, most studies have employed heterojunctions which use alternative p-type materials to fabricate LEDs using ZnO NRs. But, heterojunction LEDs generally exhibit poor luminescent efficiency due to lattice mismatch at the p-n junction interface. Another difficulty in fabricating ZnO NRs-based LEDs is the initial growth control of vertical ZnO NR on conventional electrodes, which is due to different crystal structure between the ZnO NR and the electrodes.
In this report, to overcome the problems of ZnO NRs, an Ag bottom electrode was employed as a multiple role implementing layer, which acted as a dopant source for p-type ZnO NRs and a growth template for ZnO NRs. During growth of ZnO NRs on the Ag bottom electrode without seed layer, Ag+ ions are dissolved from the Ag bottom electrode and then incorporated into the ZnO lattice. Then, the n-type ZnO NRs were homoepitaxially grown on these Ag-doped p-type ZnO NRs to fabricate a highly efficient p-n homojunction LED with minimum interface defects. These ZnO NRs p-n homojunction LEDs showed a typical rectifying behavior with a turn-on voltage of 3.5 V and a high rectifying ratio of 1.5 × 105 at 5 V. Furthermore, under a forward bias of 9 V, the LED exhibited a wide yellow EL emission centered at 645 nm, which was attributed to the various emission sites of ZnO deep-level defects.
Keywords: Ag doping, p-type ZnO NRs, Homoepitaxial p-n junction, Solution process, Light-emitting diode.