GaN Nanowire Review
Nanowires are one of the fastest growing areas of study in the last decade due to their unique electronic, physical, and chemical properties. Gallium nitride is a semiconducting material that has been used in industry and research for its large bandgap, piezoelectric properties, and strength. By utilizing nanofabrication techniques, gallium nitride nanowires have been utilized to develop next generation catalysts, probes, and electronics. This paper reviews the most recent developments in both efficient synthesis of gallium nitride nanowires as well as novel and optimized devices that utilize gallium nitride. The current trend in devices is the incorporation of various organic and inorganic materials for their synergistic effects.
Hundreds of various synthetic methods and devices have been developed for nanowires in the last decade. Gallium nitride (GaN) has also garnered attention due to its simple synthesis as well as functionality as a wide-bandgap semiconductor. As the most basic synthesis research has been completed and streamlined for GaN nanowires, functional devices on the nanoscale have also been developed: sensors, photovoltaics, probes, LEDs, and lasers. The most recent research done on GaN nanowires focuses on the utilization of the nanowire’s physical structure as well as augmentation with other materials in order to create both unique and efficient devices. Of course, these devices would not be possible if it were not for newer synthetic methods that simply the production of high-quality GaN nanowires.
One of the most commonly utilized and well-developed synthesis methods used to create nanowires is VLS, a method that has proven to be both simple and efficient in not only the growth of singular nanowires, but also large arrays of nanowires.(Suo 2014) VLS utilizes a unique chemical reaction between the two feed gases that introduce gallium and nitrogen (usually ammonia) into the reaction chamber. At the same time, a nonreactive carrier gas, such as nitrogen or argon, is pumped into the reaction chamber along with a small amount of hydrogen gas. Once at a nucleation point designated by a metal powder catalyst, the gallium-containing vapor forms a liquid droplet of gallium and partially oxidizes into Ga2O3. The equilibrium between liquid gallium and Ga2O3 is controlled by the hydrogen gas. At the same time, the liquid gallium layer also cracks the ammonia gas to produce more hydrogen and to allow the nitrogen to react with the liquid gallium and allow GaN nanowires to crystallize.(Chen 2000)
In 2000, Chen et al. report a method they utilized to create a large mass of nanowires by using an iridium powder catalyst to accelerate the reaction between gallium and ammonia gas. A liquid droplet containing iridium, gallium, and nitrogen served as the nucleation point for the nanowires. These sites were produced while the reaction chamber was heated, and the nanowires grew outwards as the liquid droplet remained on top of the nanowire itself. Although initially growing independently from one another, the nanowires became entangled into a large mesh as they grew longer.
Though highly efficient, this method resulted in the growth of nanowires with diameters from 20 – 50 nm. This is not the best specificity, especially since the synthesis does not allow for diameter control: at the quantum level, the effects of size discrepancies may result in highly different properties. Another major issue with VLS is the usage of metal powder catalysts, as they may become infused into or coated upon the nanowire, changing the physical and electronic characteristics of the final product. Therefore, though VLS may be able to create a large array of entangled GaN nanowires, its low specificity prevents it from being used for more delicate applications such as computer chips. The major challenges with VLS synthesis include creating aligned nanowires as well as prevention of unintended doping of the nanowire by leftover catalysts. (Chen 2000)
On the other hand, the purposeful doping of GaN nanowires with magnesium has also been a unique alternative explored in a synthetic technique published by the Patsha group in 2014. The amount of magnesium that was deposited upon the nanowires was controlled by varying the distance of the Mg3N2 source from the substrate. The nanowires had diameters of around 60 nanometers as well as lengths of hundreds of nanometers. Further analysis by x-ray spectrometry confirmed the successful integration of magnesium into the nanowires. (Patsha 2014)