Date/Time: 04-04-2018 - Wednesday - 05:00 PM - 07:00 PM

PCC North, 300 Level, Exhibit Hall C-E

The Hall effect is the primary method to measure carrier density, mobility and carrier type in materials. The most common method for measuring the Hall effect in semiconductors uses a DC magnetic field. The community has developed a well-defined protocol for removing spurious voltages in the measurement. This protocol depends are reversing the direction of the magnetic field. This protocol breaks down for materials with mobility < ~ 10 cm^{2} /(V s). One method to extend Hall measurements to lower mobility is to use AC magnetic fields. In practice, the frequency of the AC fields is very low ~0.1 Hz. A major limitation with this method is the length of time for the AC field Hall measurement and the effects of self-heating of the sample. We present a new measurement protocol based on the reverse-field reciprocity theorem. The reverse-field reciprocity theorem considers a four-port network with current inputs and voltage measurements and an applied magnetic field. If a current is applied to two of the inputs (say 1 and 3) and a positive field B a voltage (V) is measured on terminals 2 and 4. This voltage can depend of the magnetic field V_{1}(B). If the current and voltage leads are interchanged, current on terminals 2 and 4, voltage measured between 1 and 3, V_{2}(B). The theorem states that V_{2}(B) = V_{1}(-B). This is a very general result; the only requirement of the material is that it is electrically linear. In particular, this means that thermoelectric voltages require special treatment. In the above example, V_{1}(B)- V_{2}(B) removes the offset voltage without physically reversing the magnetic field. One consequence of this theorem is that measurements with electromagnets, superconducting magnets and permanent magnets can be made on the same time scale. Another new application is the ability to measure hall voltage on time scales not previously accessible. Although this method is the basis of the “spinning current” method often used with hall generators for field measurement, the method presented here has been extended for material characterization, in particular the measurement of low mobility materials based of the reverse-field reciprocity theorem. With this method, hall measurements can be completed in 1 to 10 second. For measurement of materials like organics and polymers this speed is an great advantage if the material properties can change during the measurement interval.