Magnetoresistance Technique for Determining Cross-Plane Mobility in Superlattice Devices

The cross-plane mobility, in the direction perpendicular to the planes of a superlattice, is critical for the computation of the figure of merit (ZT) in a thermoelectric device. The measurement of cross-plane mobilities in thermoelectric superlattice structures cannot be performed by conventional techniques such as the van der Pauw method. Therefore, alternative techniques must be used to obtain this important parameter. Magnetoresistance is the increase in material resistivity due to a lengthened path for charge carriers in a perpendicular magnetic field. The magnetoresistance is related to the magnetic field strength as (μB)² in the standard configuration, but the field dependence is also influenced by device geometry. This work focuses on measuring superlattice samples of composition Bi₂Te₃/Sb₂Te₃ that are removed from their growth substrate and mounted on metal-coated substrates. This resulting mesa structure has a 100 μm-square contact metallization. Technical issues related to the sample preparation for the measurement are discussed. The magnetoresistance effect is expected to be small due to the anticipated low mobilities in Bi₂Te₃-based materials. Magnetoresistance studies with such superlattice thermo-elements were attempted using a dc magnetic field, but the sensitivity was insufficient. An ac magnetoresistance with lock-in detection can yield improved sensitivity.