Characterization of electrical and thermoelectric properties

The electric conductivity of thin layers is measured by means of the ”4-point-method“. After using the 4-point-resistance-measuring unit to determine the sheet-resistance, the conductivity can be calculated by including the known layer thickness. A further advantage is the fact that the contact resistance of the electrodes does not affect the result. Hence measurements of thin layers in micro or nanometer range produce reliable results.

The Seebeck-coefficient is a material constant which indicates the amount of electrical voltage a certain material generates within a temperature gradient of 1 K; α = dU/dT. This coefficient largely depends on carrier density and temperature. To measure the Seebeck-coefficient the material is applied on two heating bars as bulk or thin film. One bar gets heated (T1) while the other remains cold (usually room temperature; T2). The result is a temperature gradient within the material. Thermocouples selectively measure T1 and T2 and the Seebeck-voltage. By changing T1 and thus the T-gradient a U-T-curve can be recorded to calculate the Seebeck-coefficient.

The power factor (PF) is a material constant which characterizes thermoelectric materials. The power factor is the product of the electric conductivity (σ) and the Seebeck-coefficient (α) squared; PF = α² · σ.

A further way to characterize materials  the group is the IWS test site for measuring carrier density and carrier mobility in layers. Therefore the Hall voltage is orthogonally measured to the current flow in the layer resulting from a magnetic field (flux density 0,7 T) with vertical alignment to the layer and a constant current flow.