GaN nanowires are promising for optical and optoelectronic applications because of their waveguiding properties and large optical band gap. However, developing a precise, scalable, and cost-effective fabrication method with a high degree of controllability to obtain high-aspect-ratio nanowires with high optical properties and minimum crystal defects remains a challenge. Here, we present a scalable two-step top-down approach using interferometric lithography, for which parameters can be controlled precisely to achieve highly ordered arrays of nanowires with excellent quality and desired aspect ratios. The wet-etch mechanism is investigated, and the etch rates of m-planes {11̅00} (sidewalls) were measured to be 2.5 to 70 nm/h depending on the Si doping concentration. Using this method, uniform nanowire arrays were achieved over a large area (>10 μm) with an spect ratio as large as 50, a radius as small as 17 nm, and atomic-scale sidewall roughness (<1 nm). FDTD modeling demonstrated HE is the dominant transverse mode in the nanowires with a radius of sub-100 nm, and single-mode lasing from vertical cavity nanowire arrays with different doping concentrations on a sapphire substrate was interestingly observed in photoluminescence measurements. High Q-factors of ∼1139-2443 were obtained in nanowire array lasers with a radius and length of 65 nm and 2 μm, respectively, corresponding to a line width of 0.32-0.15 nm (minimum threshold of 3.31 MW/cm). Our results show that fabrication of high-quality GaN nanowire arrays with adaptable aspect ratio and large-area uniformity is feasible through a top-down approach using interferometric lithography and is promising for fabrication of III-nitride-based nanophotonic devices (radial/axial) on the original substrate.
Imaging of high-aspect-ratio nanostructures with sharp edges and straight walls in nanoscale metrology by atomic force microscopy (AFM) has been challenging due to the mechanical properties and conical geometry of the majority of available commercial tips. Here we report on the fabrication of GaN probes for nanoscale metrology of high-aspect-ratio structures to enhance the resolution of AFM imaging and improve the durability of AFM tips. GaN nanowires were fabricated using bottom-up and top-down techniques and bonded to Si cantilevers to scan vertical trenches on Si substrates. Over several scans, the GaN probes demonstrated excellent durability while scanning uneven structures and showed resolution enhancements in topography images, independent of scan direction, compared to commercial Si tips.
The patterning process in field-emission scanning probe lithography (FE-SPL), a high-resolution and cost-effective method for nanofabrication, is based on the field emission of electrons from ultrasharp tips in close proximity to a sample (distances below 100 nm). Thereby, the emitted electrons expose directly an ultrathin resist film. The field enhancement at the tip apex is crucial for the field emission current, which follows the Fowler–Nordheim theory. Despite the success of FE-SPL in nanofabrication, systematic experimental studies of the field-emission process, including the determination of the tip radius and tip-to-sample distance during the measurement, for these small tip-to-sample distances and different tip materials are lacking. To resolve this issue, experimental measurements of the field-emission current for tip–sample proximity distances below 100 nm were performed. For this purpose, the developed AFM in SEM system was modified,1,2 which enables one to monitor the tip–sample distance with a high accuracy using SEM while simultaneously recording the field-emission current. The authors present experimental results of the dependence of the field-emission current on the tip shape, tip material, applied voltage, and tip–sample distance. Therefore, the emission characteristics of silicon, diamond, GaN, and tungsten tips are shown. The knowledge about the field-emission process for small tip-to-sample distances will help to understand and improve the current FE-SPL, regarding also the choice of tip material. Furthermore, these measurements enable the detailed comparison with current FE models beyond state-of-the-art since all necessary parameters (voltage, current, tip diameter, and tip-to-sample distance) could be measured and controlled during the FE experiment due to the unique experimental system.
Lithium/sulfur batteries are a promising candidate for energy storage as they are capable of providing higher energy density in comparison to conventional Li-ion batteries.
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