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Department of Microsystems Engineering – IMTEK, University of Freiburg, 79110 Freiburg, Germany (Received 15 December 2010; accepted 26 April 2011)

This research investigates the combination of electrochemical deposition and postdeposition vapor annealing as a method for the fabrication of Bi2Te3 layers. The galvanostatic deposition of Bi2Te3 thin films is characterized as a function of electrolyte composition and deposition-current density. Material with near-stoichiometric composition can be synthesized from electrolytes containing 20 mM Te and 30 mM Bi ions and a deposition-current density of 3.75 mA/cm2. All deposited samples show n-type behavior with Seebeck coefficients around
55 lV/K. An equilibrium annealing process in Te atmosphere is used to readjust the composition of the material after the deposition, consistently leading to tellurium-rich Bi2Te3 with a Te content of 60.4 6 0.4 at%. At a temperature of 250 °C, an annealing duration of 60 h is sufficient for the material properties to reach a steady state, with a Seebeck coefficient of
130 lV/K. I. INTRODUCTION

Bi2Te3-based materials are known as the best thermoelectric materials for applications around room temperature, typically reaching a thermoelectric figure of merit, ZT, close to 1. Thermoelectric generators based on these compounds can potentially be used for energy harvesting applications, for example, as independent powers sources for sensor networks or in consumer electronics. To integrate the material into microscale devices, it is necessary to use synthesis methods that are compatible with microfabrication processes. Various synthesis methods can be used in the fabrication of microscale thermoelectric devices, including electrochemical deposition (ECD).1 However, the thermoelectric properties of electrochemically deposited Bi2Te3 are typically low when compared with state-ofthe-art bulk materials. This can partly be attributed to the high sensitivity of the material properties to small changes in the composition, a parameter that, in electrochemical processes, is difficult to control with the necessary precision.2 Additionally, the carrier density in electrodeposited materials is typically significantly higher than predicted by the commonly applied antistructure model, even if the desired composition is achieved.3 Postdeposition annealing in oxygen-free environments has been investigated as a means of reducing the defect density in the material and can lead to an improvement in the thermoelectric properties.4 However, because of the high partial pressure of tellurium and its associated Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2011.141 J. Mater. Res., Vol. 26, No. 15, Aug 14, 2011

preferred evaporation at elevated temperatures, this treatment is again very sensitive to the initial composition.5 One possible method to avoid the dependence of the annealing results on the starting composition of the material is the use of an equilibrium annealing process. The process is based o