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 labo ratories which use the QHE to establish a reference standard of resist ance. Based on tests made on several samples taken randomly, the globa l metrological quality of the batch of unprotected samples is believed to be high. The resistance plateau corresponding to the quantum numbe r 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 resi stivity is lower than 5 muOMEGA for a current of 40 muA and lower than 300 muOMEGA for 200 muA. Results of quantized Hall resistance measure ments, 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 rathe r different high-quality QHE sample from another source. The quality o f the diffused contacts, allowing access to the two-dimensional electr on-gas (2DEG), is globally good. In particular, the access resistance to the 2DEG through the current contacts, as defined by a four-termina l method of measurement, is extremely low, lower than 30 muOMEGA for c urrents up to 150 muA.