Automated Hall facility for characterisation of electric transport properties
Fundamental semiconductor properties like concentration and mobility of charge carriers (electrons, holes) provide information for application-oriented materials optimisation. An optimal magnitude of the carrier density has to be adjusted to achieve the maximal thermoelectric figure of merit of semiconductors. In the range beneath ambient temperature essential solid-state physical processes become detectable – mechanism of charge carrier scattering, temperature dependent excitation processes of conduction electrons, the shape of the energy distribution of carriers, as well as special phenomena such as hopping conduction, phonon drag or polaron conductivity. They substantially govern charge and heat transport in highly doped semiconductors and are thus highly meaningful for thermoelectric transport processes.
The carrier mobility is a central quantity for the estimation of a materials’ maximal possible thermoelectric figure of merit and thus for its potential in a thermoelectric application. It can be determined directly by the measurement of the Hall coefficient and the electrical conductivity and allows – based on a single measurement on a non-optimised sample – for a prognosis on the limits of the application-relevant properties of a thermoelectric material, already prior to a laborious experimental optimisation of carrier concentration by variation of doping concentration. The Hall equipment of the institute allows for determination of transport properties versus temperature (Hall coefficient, electrical conductivity, Seebeck coefficient) in a low magnetic field (up to 1 T) ranging from lowest up to very high temperatures (10 K–1200 K). It is equipped with a movable magnet unit which can be positioned to either of two vacuum recipients with measuring heads developed at DLR: a module for cryogenic measurement of Hall coefficient and electrical conductivity (10 K–330 K) and a new unit for high temperature measurement of Hall and Seebeck coefficients as well as of electric conductivity (ambient temperature–1200 K; under completion).
Sample holder for cryogenic temperatures (10 K–330 K)
Metallic contacting areas are evaporated to thin platelet-shaped specimens of rectangular geometry. Alternatively, contact leads are attached by focussed pulse laser welding. By means of lock-in technique accuracy down to the nanovolt range is achieved, which also enables very low Hall voltage, as is typical for heavily doped semiconductors, to be determined.
High temperature measurement
Coplanar cylindrical specimens (d = 12.5 mm) in van der Pauw geometry are contacted by four spring-forced measuring probes,e designed as shielded thermocouples. The specimen is pressed against a slot sample desk, both halves of which are equipped with gradient heaters. Thus a temperature gradient inside the specimen can be formed by asymmetrical heating. On both sides of the holder the thermo-voltages for determination of the Seebeck coefficient are measured by the thermocouples.