Control of the doping concentration, morphology and optoelectronic properties of vertically aligned chlorine-doped ZnO nanowires

Vertically aligned single-crystal and chlorine-doped ZnO nanowires (NWs) were grown by a low-cost, high-yield and seed-free electrochemical route. The effects of the applied potential and the concentration of ammonium chloride (NH4Cl) on the morphology, structural and optoelectronic properties of the ZnO:Cl NWs were comprehensively investigated. The amount of Cl ions introduced in the ZnO structure increased almost linearly with both the concentration of NH4Cl in solution, and the electrodeposition potential. As side-effects, the presence of NH4Cl in the growth solution slowed down the electrodeposition rate and resulted in closer packed and lower aspect ratio NWs, but having a higher degree of vertical alignment and less defective surfaces. The NW tip morphology also changed with the NH4Cl concentration, from pyramidal to flat tips. By changing the amount of NH4Cl in the growth solution, the carrier concentration of such ZnO:Cl NWs could be tuned in the range between 5 × 1017 and 4 × 1020 cm−3. The optical gap of the heavily doped NWs increased due to the Moss–Burstein effect. At the same time, a band gap narrowing was detected from photoluminescence measurements.