Recent measurements of the quantum hall effect in AlGaAs/GaAs heterostructures to obtain a resistance standard

In the last few years we have been working in order to obtain a Resistance Standard using the Quantum Hall Effect (QHE). Several AlGaAs/GaAs structures and a variety of' ohmic contacts procedures have been evaluated to achieve devices with metrological quality. Recently we have obtained devices with the following characteristics: The resistance plateau corresponding to the quantum number i = 2 is obtained for a magnetic flux density in the order of 7 T at 1.3 K, the residual value of the longitudinal resistance is 107 mu Omega for a current of 50 mu A. These characteristics are comparable with those obtained in QHE devices used at Centro Nacional de Metrologia (CENAM). Such results show that the developed devices have the quality required for reference standard dc resistance measurements.