REPORT ON A JOINT BIPM-EUROMET PROJECT FOR THE FABRICATION OF QHE SAMPLES BY THE LEP

The BNM/LCIE, acting in the framework of the EUROMET organization, and BIPM asked LEP to produce a large number of protected and unprotected GaAs-based quantum Hall effect (QHE) samples. Part of those produced by LEP (more than 350 first choice samples) was distributed among laboratories which use the QHE to establish a reference standard of resistance. Based on tests made on several samples taken randomly, the global metrological quality of the batch of unprotected samples is believed to be high. The resistance plateau corresponding to the quantum number i = 2 is obtained for a magnetic flux density on the order of 10 T. On this plateau, at 1.2 K, the residual value of the longitudinal resistivity is lower than 5 muOMEGA for a current of 40 muA and lower than 300 muOMEGA for 200 muA. Results of quantized Hall resistance measurements, carried out at 0.5 K on the i = 2 and i = 4 plateaus, are self-consistent to within the measurement resolution of a few parts in 10(9), and agree, to within the same resolution, with results from a rather different high-quality QHE sample from another source. The quality of the diffused contacts, allowing access to the two-dimensional electron-gas (2DEG), is globally good. In particular, the access resistance to the 2DEG through the current contacts, as defined by a four-terminal method of measurement, is extremely low, lower than 30 muOMEGA for currents up to 150 muA.