All the contact pads of a quantized Hall resistance (QHR) device should have a low contact resistance (similar to 1 Omega) for the application of DC resistance standard devices at cryogenic temperatures. Corrosion of a GaAs/AlGaAs substrate occurs along the edge of the photoresist before the deposition of Ni/AuGe, and we assume that this corrosion degrades the contact resistance. A SiO2 protective layer is effective in preventing the corrosion of and protecting the surface of GaAs/AlGaAs substrate. With the use of this SiO2 protective layer, the yield ratio of the contact resistance to the two-dimensional electron gas (2DEG) layer at low temperature (similar to 0.5 K) and high magnetic field (similar to 9 T) improves to nearly 100%.
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Cross-sectional SEM image of the contact area of the QHR device. The SiO2 layer was formed before the deposition of Ni/AuGe, and it protected the GaAs/AlGaAs substrate. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim