Properties of lanthanide monopnictide nanocomposite and study of liquid phase epitaxial-grown thin films
Date
2019
Authors
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Publisher
University of Delaware
Abstract
Lanthanide monopnictide (Ln-V) nanoparticles embedded within III-V semiconductors are of great interest for thermoelectric applications: nanoparticles act as electrons donors, maintaining high electrical conductivity while reducing thermal conductivity. TbAs:InGaAs shows unusually high electrical conductivity, Seebeck coefficient, and power factor at high temperatures, possibly due to energy dependent electron scattering introduced by TbAs nanoparticles. 0.78% TbAs:InGaAs demonstrated ZT of power factors as high as 7.1x10^(-3)WK^(-2)m^(-1) and ZT as high as 1.6 at 650K. ☐ Unfortunately, the high cost and material inflexibility exhibited by molecular beam epitaxy, a commonly used nanocomposite growth technique, prevent Ln-V:III-V from being widely used, especially when thick films are needed. This thesis proposes a 2-step growth processes, which uses inert gas condensation to synthesize nanoparticles and liquid phase epitaxy (LPE) to incorporate them into thin films. Our attempts at growing ErAs:GaAs contribute to the understanding of the Ga-Er-As ternary phase diagram at high temperatures, and established that nanoparticles have to be thermally stable and thermodynamically compatible in the growth solution at high temperatures for LPE growth. We also showed that TbAs nanoparticles grown by inert gas condensation exhibit similar thermal disintegration at temperatures as low as 240-degree C, contrary to their bulk TbAs counterpart. ☐ Another source of ErAs particles, produced by direct reaction, was found to be thermally stable at high temperatures, but the resulting thin film grown by LPE did not show incorporation of ErAs. One possible explanation is that the lattice mismatch between ErAs and GaAs inhibits growth. The search for an optimum nanoparticle for the growth of nanocomposites by LPE led to ZnSe. Even though ZnSe quantum dots showed thermal disintegration by 450-degree C (unlike previously reported results), ZnSe seemed to incorporate into GaAs thin films. However, it was difficult to study the nature of ZnSe in GaAs, and we were unable to determine whether ZnSe incorporated as nanoparticles. The resulting thin films showed an increase in sheet resistivity combined with a reduction in carrier mobility. Our attempts to incorporate nanoparticles in semiconductors by LPE have yielded much needed background understanding on nanoparticle material properties. The insights gleaned from these observed growth requirements point toward the growth of Ln-Sb:III-V and II-VI:III-V nanocomposites as the natural next step for future LPE growths.