Influence of Embedded Metallic Nanocrystals on GaAs Thermoelectric Properties.

Thermoelectrics;Ion Implantation;GaAs;Materials Science and Engineering;Engineering;Materials Science and Engineering

Warren, Michael V.Clarke, Roy ;

University of Michigan

关键词: Thermoelectrics;    Ion Implantation;    GaAs;    Materials Science and Engineering;    Engineering;    Materials Science and Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/107186/mvwarren_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

For the past several years, there has been significant interest in low-dimensional structures, such as superlattices, nanocrystals, and nanowires, for thermoelectric applications due to their ability to enhance the figure-of-merit. These nanostructured materials must be optimized to maximize the Seebeck coefficient (S) and electrical conductivity (σ) while minimizing the thermal conductivity (κ). Due to the possibility of nucleating nanocrystals within an amorphous matrix, ion-beam-synthesized nanocomposites show promise as possible thermoelectric materials. To optimize these ion-beam-synthesized nanocomposites, an understanding of the microstructure and thermoelectric properties is essential. Here, we report on the formation of metallic In (Bi) nanocrystals (NCs) embedded in GaAs by In (Bi) ion implantation and rapid thermal annealing (RTA). The role of microstructure on the thermoelectric properties of ion-implanted GaAs is discussed via a comparison of GaAs:Bi, GaAs:In and GaAs:N films. We report on the relationship between microstructure and thermoelectric properties of ion-beam-synthesized In NCs in GaAs. We developed a sputter-mask method to enhance the retained ion dose. During annealing, In NCs are nucleated within a polycrystalline GaAs matrix. Electrons and phonons are scattered at interfaces, reducing σ, κ, and consequently the thermoelectric efficiency in comparison to that of unimplanted GaAs. We also report on the formation of Bi NCs embedded in GaAs and their influence on the thermoelectric properties. Implantation-induced defects reduce the free carrier concentration, n, and, consequently, σ, while annealing results in a partial recovery of n and σ. Phonon scattering at Bi NC boundaries serves to reduce κ by ~30% for all films. We discuss the role of microstructure on the electrical and thermal conductivity of the GaAs:Bi films through a comparison with GaAs:In and GaAs:N films, demonstrating a general trend of n and σ reduction following ion-implantation, while a partial recovery of n and σ, and a reduction in κ due to phonon scattering, follows RTA.This thesis reveals new insights into the structure-property relationship of ion-implanted GaAs. Embedded metallic NCs show promise for thermoelectric applications via κ reduction. Based on these results, it is suggested that epitaxial growth of embedded NCs will result in a reduction in κ while simultaneously preserving σ.