Thermoelectric Characterization of Electrochemically Deposited Bi₂Te₃Films Accounting for the Presence of Conductive Seed Layers

Abstract

This work presents a method for measuring the electrical resistivity, carrier concentration and Seebeck coefficient of electroplated thermoelectric layers in the presence of electrically conducting seed layers, and suitable for in-line characterization. The method is based on measuring the initial properties of the seed layer, and of the combination of seed layer and deposited film after the electroplating process. Using only these values and the individual layer thicknesses, the properties of the deposited film can then be calculated based on a theoretical model of the two-layer system. Additionally, it is possible to use seed layers with different geometry and size when compared to the deposited film. The model is experimentally verified by a full characterization of electroplated Bi 2 Te 3 on Pt seed layers, with varying thickness for both layers. Reference values for the properties of the deposited films are obtained by removing the Bi 2 Te 3 layers from the seed layers and their subsequent characterization. By using the proposed method, the thermoelectric properties of the deposited film can be determined with an accuracy of 15% or better as long as the sheet resistance ratio of deposited and seed layer is smaller than 0.4. Electrodeposition of thermoelectric thin film materials is an attractive synthetic approach, offering the advantages of a relatively simple setup, low associated costs, efficient use of material and high growth rates. 1 The deposited materials are typically evaluated with regards to their thermoelectric power factor PF, which includes the Seebeck coefficient S of the material and its electrical resistivity ρ:Additionally, measurements of the carrier concentration are a useful tool for the further optimization of the thermoelectric materials. 2 The measurement of these properties of electroplated films presents a fundamental challenge, as the electroplating process necessitates the existence of an electrically conductive seed layer underneath the deposited film.The most common approach to avoid this problem is the removal of the deposited film from the seed layer after the electroplating process. 3-6 This allows the direct measurement of the thermoelectric properties without the interference of the seed layer. However, the disadvantage of this method is that the deposited material is potentially damaged during the release process by introducing cracks or other defects. Furthermore, this requires a rather poor adhesion between the deposited film and the seed layer, which is the opposite of the typical requirement for the fabrication of thermoelectric devices. Another possible approach is the deposition of thin films on seed layers which are composed of similar or ideally identical materials, and will therefore have no influence on the measured properties. 7,8 While these methods generally yield good results and are useful in a research environment, they are not necessarily applicable for actual fabrication processes, or any environment where high throughput, inline monitoring or non-destruc...

Electrodeposition of stoichiometric bismuth telluride Bi2Te3 using a piperidinium ionic liquid binary mixture

“…The electrical resistivity measured is 133 µΩ m, a higher value than what is commonly obtained for coatings synthesized from aqueous electrolytes (around 10 µΩ m [16,17,46,47] A c c e p t e d M a n u s c r i p t 13 impurities…). Moreover it should be noted that the used lift-off method required for the transport properties measurements could have damaged the electroplated films during the release process by introducing cracks or other defects that increase the electrical resistivity, as suggested by Rostek et al [48].…”

Direct measurement of the thermoelectric properties of electrochemically deposited Bi2Te3 thin films

“…Many electrolytes have been utilised for the electrodeposition of bismuth telluride but undoubtedly, the electrodeposition from aqueous-acidic electrolytes is the most widespread 14 . However, TE measurement of electrodeposited thin films has been extremely challenging due to the metallic seed layer underneath the electrodeposited film 15,16 . Two approaches have been traditionally employed and are summarised in Supplementary Table I.…”