Final Report Summary - NEWIRES (Next Generation Semiconductor Nanowires) The rapid advancement of computing and other technologies that depend on semiconductor components has depended for many years on the continuous downscaling (shrinking) of the semiconductor structures. However, over the last decade limits have been reached on how small the components can be made, and the industry has had to look for new ways to improve performance in order to continue the advancement of computing and other high-technology devices. In the last few years this has depended on finding novel ways to construct the semiconductor components, but further advancements will depend on incorporating new materials and new types of structures. One potentially very important technology is III-V semiconductor nanowires, which combine a new class of materials (III-V semiconductors) with a new and important geometry (one-dimensional or wire-like geometry). Today, however, these structures are typically fabricated with a process that relies on gold nanoparticles as seeds. Gold is entirely incompatible with semiconductor industry as it degrades the properties of semiconductor components, and so it will be very difficult to make use of these interesting structures unless a new fabrication process can be developed that does not depend on gold.In this project we developed new fabrication processes for III-V semiconductor nanowires that use other types of metal nanoparticles in place of gold. The potential to use these structures in future devices will be greatly improved by avoiding the use of this industry-forbidden material. However, we also aimed to develop the fabrication much farther than has been done to date, by using the new foreign metal nanoparticles to steer the properties of the semiconductor nanowires. We aimed to control structural properties (such as crystal structure), geometrical properties (such as shape and size) and electrical properties (such as the density of electric charges and their motion within the semiconductor).We have developed fabrication of a wide variety of metal nanoparticles, which include Sn, Pd, Ag, Cu, Fe, Ni, Cr, Bi, Pd, Co, Zn, Pt and Rh. The fabrication process occurs with the metal nanoparticles suspended in the gas phase, which allows for clean fabrication relatively free of chemical impurities, as well as a high control of the nanoparticle size and a simple process for depositing them in a controlled way onto a semiconductor surface. Within the project we have further developed this method with the inclusion of small amounts of hydrogen in the carrier gas, which prevents oxidation and improves compaction of the nanoparticles into uniform (quasi-spherical) shapes.For the growth of III-V semiconductor nanowires, we found that for instance Sn, Pd and Pb are very successful for obtaining nanowire growth with high yield and uniformity. Sn for example yields nanowires with high crystal purity (free of stacking defects, unlike nanowires fabricated by most processes shown in the past). Sn is also useful for controlling the electrical properties as it incorporates into the nanowire during growth in trace amounts, allowing us to increase the density of electrons within the material. Pb, by contrast, does not incorporate at all into the nanowire as it grows, making it a more useful metal to use for applications in which high electron density is not desired. Based on the observations from a wide range of materials, we have also developed a fundamental understanding of how the properties of the metal nanoparticle determine the properties of the grown nanowires. We have determined for instance that the surface energy of the metal nanoparticle plays an important role in the resulting nanowire crystal structure. We have also found that the phase of the nanoparticle (especially whether it is liquid or solid) strongly influences the nanowire nucleation as well as the crystal orientation. The understanding that we have developed today allows us to predict what type of nanoparticle to choose based on what type of nanowire properties we want to achieve, allowing us to design nanowire for specific purposes.
