We have examined the structural and thermoelectric properties of nanocomposites prepared by matrix-seeded growth, which consists of ion-beam-amorphization, followed by nanoscale recrystallization via annealing. We use a sputter-mask to increase the retained concentration of In+ ions in GaAs as a path towards the formation of nanoscale InAs crystals in an amorphous GaAs matrix. For the highest ion fluence, an enormous Seebeck coefficient of −12 mV/K is observed at 4 K. We discuss the temperature-dependence of the resistivity, Seebeck coefficient, and thermoelectric power factor in terms of the microstructure of the layers.
We have investigated the formation and coarsening of near-surface Ga nanoparticles (NPs) in SiNx using Ga+ focused-ion-beam-irradiation of SiNx, followed by rapid thermal annealing. For surfaces with minimal curvature, diffusive growth is apparent, leading to nearly close packed arrays with NP diameters as small as 3 nm and densities as high as ∼4 × 1012 cm−2. The diffusive flux increases with annealing temperature, leading to NP coarsening by Ostwald ripening. For surfaces with increased curvature, diffusion towards the valleys also increases during annealing, leading to Ga NP coalescence and a bi-modal distribution of NP sizes.
We have examined the influence of Bi on embedded nanocomposite formation and the resulting thermoelectric properties of GaAs. Bi implantation amorphizes the GaAs matrix, reducing both the free carrier concentration (n) and the electrical conductivity (σ). Following rapid thermal annealing, the matrix is transformed to single crystal GaAs with embedded Bi nanocrystals (NCs). In comparison to a GaAs reference, the Bi NC-containing films exhibit a sizeable reduction in thermal conductivity (κ), leading to a 30% increase in the thermoelectric figure-of-merit. We also present a universal trend for the influence of microstructure on the n-dependence of σ and κ.
We have examined the formation of embedded In nanocrystals (NCs) and their influence on the free carrier concentration, resistivity, thermal conductivity, and Seebeck coefficient (S) of GaAs. The In nanocrystals enhance the free carrier concentration, while electron and phonon scattering at crystallite boundaries increases the resistivity and reduces the thermal conductivity. Furthermore, the room temperature Seebeck coefficient exhibits a 25% increase due to carrier trapping. Application of this approach to more heavily doped GaAs layers will likely lead to further increases in S and reductions in resistivity.
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