Growth and characterization of lanthanide pnictide nanoparticles by pulsed laser ablation for incorporation into III-V semiconductor nanocomposites
Date
2018
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Lanthanide monopnictide (Ln-V) nanoparticles embedded in III-V semiconductor thin films have shown tremendous potential in device applications ranging from terahertz detection to thermoelectrics. However, the primary technique used to produce these materials, molecular beam epitaxy (MBE), necessarily has a low growth rate and is limited in the range of nanoparticle morphology and compositions that can be produced. In this dissertation, a new growth technique for these semiconductor nanocomposites is proposed, using a combination of inert gas condensation (IGC) and liquid phase epitaxy (LPE). Such an approach takes advantage of the flexibility in nanoparticle composition and morphology that is achievable with IGC, and the high growth rates demonstrated by LPE. This potentially allows for the growth of thick films required for thermoelectric applications, as well as the investigation of Ln-V:III-V combinations that would be difficult to grow by MBE. As an initial step toward developing this growth technique, nanoparticle synthesis by IGC using a pulsed laser ablation source is investigated. IGC is not only able to synthesize a wide variety of Ln-V compounds, but also allows for nanoparticle quantities sufficient for LPE film growth to be produced. The ablation process will be described, along with methods for characterizing and optimizing nanoparticle powder properties for thermoelectric applications. ☐ The dominant ablation mechanism associated with nanosecond pulsed laser ablation, at fluences below the plasma threshold, is thermal vaporization. This results from the rapid, localized heating of the target material. However, the different ablation thresholds of the target components leads to the mechanical removal of material at low fluences. This can be reduced by increasing laser fluence above the threshold fluence of each component, while the spot size is kept large enough to promote vapor interaction. Incongruent ablation of the pressed powder targets is observed for short ablation times, with As:Er ratios reaching 22:1 within the first 10 minutes of growth. This is determined to be the result to differences in vaporization enthalpies of the two elemental species, and is reduced as the growth proceeds and the target becomes more Er-rich. It is demonstrated that powder composition and morphology can be controlled through parameters related to laser-target interactions such as laser fluence and target composition, and parameters related to vapor interactions such as inert gas pressure and collection distance. Vapor interaction is increased with inert gas pressure, resulting in increased ErAs composition. After the optimization of these growth parameters, an ErAs powder composition of 76.9 at.% has been achieved, with Er2O3 as the primary impurity. Control over nanoparticle size has been demonstrated by varying the collection distance, therefore controlling the interaction time between nucleated particles and the vapor species. Variation of the collection distance from 2-7 cm results in a minimum grain size of 28-38 nm, respectively. ☐ Finally, the thermal stability of ErAs nanoparticle powders under thermal processing conditions expected for LPE film growth is investigated. Despite bulk-like ErAs demonstrating thermal stability at temperatures over 2500 C, degradation of nanoparticle powders is observed at baking temperatures as low as 350 C. A comparison of TGA results from nanoparticles grown by IGC and larger agglomerates grown by direct reaction demonstrates a clear difference related to the particle size, potentially the result of increased vapor pressure with decreasing nanoparticle size. Alternative strategies for overcoming the low degradation temperature of ErAs nanoparticle powders will also be discussed.