Growth of ZnO nanowires by MOCVD on sputter deposited metallic underlayers

Synopsis

ZnO is a II-VI semiconductor with a direct bandgap of 3.37 eV. Its high electron mobility, wide bandgap, and low cost make it attractive as a material for use as an active layer in multijunction solar cells. Additionally, ZnO nanostructures have found use as electron transport layers in extremely thin absorber cells. Despite ZnO nanostructures having wide application in optoelectronics, the self-catalysed growth of ZnO nanowires remains poorly understood. In the following, thermodynamic and mass transport kinetics are explored as mechanisms for the large aspect ratio observed in self-catalysed ZnO nanowires grown by metalorganic chemical vapour deposition (MOCVD). It is shown that the surface energy anisotropy alone is insufficient for the formation of observed nanowire morphology and that the diffusion length of ZnO adatoms along the nanowire sidewalls play a major role in their growth. In addition, the growth of self-catalysed ZnO nanowires is explored on sputter deposited Ta/Ru/Ti and Ta/Ru/TiN seed layers. This technique is proposed as a substrate independent fabrication method for self-catalysed ZnO nanowires. Resultant nanowires show growth along the [0001] axis with good vertical orientation indicated by scanning electron microscopy (SEM)and X-ray diffraction (XRD) rocking curves of 3.1° (Ti) and 6.6° (TiN) while photoluminescence (PL) measurements show the growth process results in defect-free wires. Finally, Ti films with thicknesses of 10, 200 and 500 nm are nitrided in situ at 605° C as a possible alternative to reactively sputtered TiN. After 60 mins of annealing, N content in the 500 nm Ti film is shown by X-ray photoelectron spectroscopy (XPS) to reach a maximum atomic concentration of 38%. This concentration decreases with etching depth and is found to be 28% at a depth of between 40 and 60 nm. ZnO wires grown on the 10, 200, and 500 nm films are shown by SEM to have good alignment with XRD rocking curves of 2.0°, 1.7°, and 1.2° respectively. Nanowire morphology on the 10 nm nitrided Ti film is similar to that seen on sapphire substrates while the morphology on the 200 and 500 nm nitrided Ti films exhibits significant sidewall growth and shorter overall nanowires.