Controlling Losses With Gates and Shields to Perfect a Quantum Hall Impedance Standard

We report new results and insights concerning the effect of side gates, back gates, and ac losses on the ac properties of the quantum Hall resistance (QHR). Three different loss mechanisms have been identified, namely, dissipative losses in the 2-D electron gas (2DEG), relaxation losses in a surface layer above the 2DEG, and dielectric losses in the GaAs substrate. A novel shielding configuration reduces the effect of these loss mechanisms and makes the ac QHR independent of frequency and current to better than 1 part in 10⁸ at frequencies around 1 kHz.