Anisotropic Semiconducting Thin Films: Synthesis, In-Plane and Cross-Plane Characterization, and Thermoelectric Application

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Anisotropic semiconducting thin films have attracted attention in recent years for important applications such as electrical interconnects, electronic sensors, field-emission devices and thermoelectric devices. However, the characterization of the full conductivity tensor, especially the cross-plane conductivity, remains a great challenge for anisotropic thin films. In addition, the synthesis of large-area thin-film conductors with controllable in-plane conductivity anisotropy is not thoroughly studied. To address these problems, this work explores new directions in the characterization, synthesis, and applications of anisotropic conductors. A new method to characterize the in-plane and cross-plane electrical conductivity of anisotropic semiconductor thin films is proposed. In this method, a "triple stripline" device structure is created by adding three narrow stripline contacts to the top of the anisotropic thin-film layer of interest. The substrate is assumed to be highly conductive, as is typical for optical devices, and serves as a grounding back-plane. The electrostatic potential distribution of this device is proved to have no analytical solution and solved numerically. It is shown that experimental measurements of the potential can uniquely determine all components of the conductivity tensor. Initial progress towards an experimental demonstration is presented. Three semiconducting thin films with artificial in-plane conductivity anisotropy are synthesized: the carbon nanotube (CNT) film, the graphene-polymer film, and the AlGaAs thin film. The conduction channels for these three films are aligned by evaporation-driven self-assembly, three-dimensional printing, and ion-implantation isolation, respectively. Anisotropy ratios above 1E4 are observed for both the CNT and AlGaAs thin films. While the CNT and graphene-polymer films are p-type, the AlGaAs film can be doped to be either n or p-type. "Cross-hatched" n and p-type anisotropic thin-film conductors with conducting axes orthogonal to each other can open up new applications in p x n-type transverse thermoelectrics, in which the induced heat flow is perpendicular to the applied electric current or conversely, the generated electric field is perpendicular to the applied temperature gradient. The theory of transverse thermoelectrics is studied with normalized electric field and heat flux scales, and the maximum cooling performance is presented here for the first time. The theoretical model of the p x n-type transverse thermoelectrics is reviewed, and candidate materials are proposed. Among the candidate materials, artificially structured anisotropic thin films are shown to enable the cross-hatched p x n structures, and development towards a p x n AlGaAs structure is reported.

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  • 02/13/2018
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