Hydrothermal synthesis of zinc oxide nanowire arrays for photovoltaic applications

Abstract

Zinc oxide (ZnO) nanowires with excellent crystal quality can be grown vertically aligned from a substrate using hydrothermal synthesis, a low-cost, scalable process that is compatible with many semiconductor processing techniques. However, precise control over nanowire array dimensions such as nanowire spacing, diameter, length, and alignment, which is important for optoelectronic device applications, has proven elusive due to lack of understanding regarding fundamental aqueous growth mechanisms at the nanoscale. Here, we utilize electron-beam lithography to template ZnO seed layers, demonstrating that seed layer engineering via judicious choice of seed deposition conditions and annealing can yield well-aligned nanowire arrays with single nanowire spatial precision on a variety of device relevant substrates. Subsequently, we use bottom-up patterning techniques and investigate the competition between diffusive transport and surface reaction in hydrothermal growth to achieve control over nanowire spacing and enhanced nanowire array uniformity over length scales suitable for photovoltaic (PV) device fabrication. By analyzing the role of temperature, concentration, and areal seed density on the balance between diffusion vs. reaction rates at the solution-nanowire interface, we show that the c-facet grows via the direct incorporation mechanism. With this knowledge, we use additives to shift the nanowire growth system into a reaction-limited regime, making nanowire growth rate independent of the patterned template. As a consequence, we achieve ZnO nanowire array uniformity that is critical for device applications.