The technological basis for quantum circuits to realise decade resistance values is expanded through the vertical integration of two conducting layers positioned one over the other in a novel semiconductor structure.
The quantum Hall effect in a twin pack
Head news 2008.2
PTB News 2/2008 (4.7 MB) PTB News 2/2008, English edition, Issue August 2008
Like the volt, the resistance unit ohm is reproduced by a macroscopic quantum effect which is only dependent on the fundamental constants h and e. For the ohm, this is the quantum Hall effect (QHE) which occurs in ultra-thin conductive layers, so-called two-dimensional electron gases (2DEG) at high magnetic fields. The quantum standards developed to date made of gallium arsenide semiconductors now furnish the precise resistance value of h/2e2 (at 12 906 ohm), but this has to be transferred with difficulty to the decade resistance scale necessary for practical applications by means of measuring bridges. Work is therefore being undertaken throughout the world to establish a direct decade quantum ohm scale made of parallel and series circuits of quantum Hall elements on one chip.
In the Clean Room Centre of PTB, a double 2DEG structure has now been developed, with which the integration of parallel circuits is now simpler. A novel crystal contains, instead of the usual single conductive layer, two layers positioned one over the other. In order to get a certain resistance value, only half the number of parallel Hall elements have to be integrated. The challenge during development was controlling the growth of the crystal in layers in such a way that both layers have almost the same electrical characteristics. Only then do they have the same working point, and the necessary accuracy of the quantised resistance value is also achieved for the parallel circuit.
Precision measurements at the new double 2DEG structures have impressively shown that this goal has been reached. The measured resistance value agreed to the theoretical value within an accuracy of 10–9. A double quantum Hall circuit is, as such, just as precise as a conventional quantum standard with only one layer.
In future, integrated parallel circuits are to be made from the double layers, which will expand the quantum resistance scale below 12 906 ohm. Through the double 2DEG, the number of individual elements will be halved, the size of the chips will fall to the same extent, and the frequency of errors in producing circuits will be reduced.