GaN nanowire is a type of III-nitride nanostructures that are widely used in optical and optoelectronics applications due to their large optical band gap and waveguiding properties. The high demand for GaN nanowires and discovery of other areas of applications has led to more research aimed at developing precise, scalable and cost-effective GaN nanowire fabrication methods.
Researchers at the University of New Mexico led by Dr. Tito Busani developed a two-step top-down method for fabricating large area GaN nanowires arrays. This was in a bid to develop a high controllability and cost-effective method for fabricating nanowires with high aspect ratio minimum crystal defects and high optical properties. Their work is currently published in the research journal, ACS Nano.
Top-down fabrication technique provides more control over aspect ratio, uniformity and diameter of the nanostructure thereby allowing fabrication of inexpensive photonic devices. During the top-down process, the dimensions and morphology of the GaN nanowires were controlled through dry and wet etch processes after patterning. Generally, this is an improvement of bottom-up epitaxial growth of nanowires on a sapphire/Si substrate. However, various challenges are experienced in nanowire fabrication such as achieving nanowires with high aspect ratios and uniform diameters and also sidewall roughness.
In this study, Behzadirad et al commenced their study by using interferometric lithography for fabricating high-aspect-ratio GaN nanowire array lasers. The experiment was performed at room temperature and on sapphire substrates with accurate control of the aspect ratio and minimal surface roughness, however, the same results can be achive on other substrates like silicon. The lithography process is maskless to enabled cost-effective and large area and faster fabrication of 1D, 2D and 3D nanostructures. Furthermore, the authors also studied the influence of Si doping on the etching mechanism and fabrication process. Finite difference time domain (FDTD) mode was essential for investigating reflectivity and modal properties of the interface between GaN-air and GaN-Sapphire.
The researchers successfully fabricated uniform nanowires arrays over a large area with a diameter of sub-50nm, the aspect ratio of 50 and sidewall roughness less than 1nm. Consequently, the high reactivity of the silicon atoms resulted in an increasing etch rate with a concentration of the doping thus its ability to obtain etch rate of m-plane facets in the wet solution. In a radius less than 100nm, HE11 was found to be the dominant transverse mode that allows propagation in the vertical cavity during the nanowire fabrication process.
Generally, the presented two-step top-down approach with interferometric lithography enables effective control of the parameters to fabricate high-quality GaN nanowire arrays with the desired large area uniformity, smooth sidewalls and aspect ratios. The method is not limited to sapphire substrate and the same quality nanowires can be achieved on silicon substrate too. The demonstrated approach minimizes the cavity loss due to the sidewall roughness as well as enabling single-mode lasing in vertical cavity GaN nanowire lasers, thereby a significant advantage as compared to the other conventional methods. The authors are optimistic that this study will help advance the future fabrication of III-nitride nanophotonic devices.
Behzadirad, M., Nami, M., Wostbrock, N., Zamani Kouhpanji, M., Feezell, D., Brueck, S., & Busani, T. (2018). Scalable Top-Down Approach Tailored by Interferometric Lithography to Achieve Large-Area Single-Mode GaN Nanowire Laser Arrays on Sapphire Substrate. ACS Nano, 12(3), 2373-2380.Go To ACS Nano