(Invited) Optimization of Pulsed Electrodeposited Bi2Te3-Based Thin Films from DMSO Solution: Influence of Deposition Parameters, Electrolytic Bath Composition and Annealing

Bismuth telluride based alloys have been extensively investigated for thermoelectric power generation (TEG) and cooling near room temperature applications. To employ materials in TEG, requires materials with large Seebeck coefficient (S), high electrical conductivity (s) and low thermal conductivity (k) to achieve a high thermoelectric efficiency defined by zT = S²σT/k, where T is the absolute temperature at which properties are measured. Nanostructured materials, specifically mesoscale composites showed improved zT values due to the reduction in thermal conductivity without reducing the electrical conductivity [1]. However, most of these materials are being fabricated through alloying using high temperature and pressure [2, 3], and thereby not scalable in terms of Si based fabrication processes. The growing demand of miniaturized room temperature thermoelectric devices based on bismuth telluride alloys has led to research focused on synthesizing thin films, in particular, using electrochemical deposition. Electrodeposition of thin films is a facile, low cost process without the need for sophisticated high-vacuum equipment in contrast to the physical deposition methods [4-6], which is also Si-Fab compatible. The electrodeposition of bismuth telluride alloy films has been reported in the literature [2-4]. The challenge, however, is to achieve bulk-like thermoelectric properties in the electroplated thin films. A comprehensive analysis of the literature demonstrates that the composition of bismuth telluride alloys plays a pivotal role in determining their thermoelectric properties [3, 4]. Schumacher et al., reported that the Seebeck coefficient of (Bi_xSb_1-x)₂Te₃ thin film increases after annealing the films in tellurium (Te) atmosphere for 60 h which is attributed to the improved Te content in the annealed films [4].

      The objective of the present work is to synthesize electrodeposited p and n-type bismuth telluride alloy based thin films and to optimize the thermoelectric properties similar to the bulk by tuning the composition of the films. Here, we studied comprehensively the influence of electrolytic bath content, deposition potential and annealing on the composition of as-deposited films and in turn on the thermoelectric properties. The impact of annealing temperature on the films composition, structure and the Seebeck coefficient is discussed. The annealed p- and n-type thin films exhibit a room temperature Seebeck coefficient of about 175 and -160 μVK^-1 respectively, which is similar to the bulk materials [3]. Structural and chemical investigations of the as-deposited and annealed thin films are performed with X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. Thus, we synthesize the electroplated thin films based on bismuth telluride and optimize its composition and micro-structure by adjusting the deposition parameter and annealing temperature to enhance the thermoelectric properties.

Acknowledgements

This work is financially supported by Enterprise Ireland and Analog Devices, Limerick, Ireland under Innovation Partnership project IP-2012-0197.

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