THIN FILM THERMOELECTRIC CHARACTERIZATION TECHNIQUES

Abstract

The key thin film thermoelectric characterization techniques are described. Due to the small dimensions, a careful examination of electrical and thermal paths in the device is necessary. We describe both in-plane and cross-plane measurement methodologies. Sample requirements for four-probe and van der Pauw methods for in-plane electrical conductivity measurement are discussed. The in-plane Seebeck coefficient is characterized under a temperature gradient which generates a voltage. Precise measurements of temperature and voltage at the same location in the sample are very important. For the cross-plane electrical conductivity, the modified transmission line method is evaluated. To eliminate parasitic contact and substrate resistances, several samples with varying thicknesses are required. Two approaches, a DC method and the 3ω method, are described in detail for the cross-plane Seebeck coefficient characterization. Next, we focus on the transient Harman method to directly measure the cross-plane thermoelectric figure of merit of a thin film. The device requirements for reducing parasitic heat losses and current nonuniformity are presented. Thermoreflectance imaging can be used together with transient Harman in order to extract electrical and thermal conductivities and the Seebeck coefficient simultaneously. Finally, Z-meters are described for directly determining the figure of merit and efficiency of a thermoelectric element or module under a large temperature gradient. Recent developments have significantly reduced thermal and electrical parasitics, as well as radiation heat loss in the system, enabling ZT measurement of legs as thin as one hundred microns.