Study and validation of Quantum Hall Array Resistance Standards (QHARS)
Project Description
The development of Quantum Hall Array Resistance Standards (QHARS) is likely to open new prospects in metrology of the resistance unit: (i) the realization of accurate and stable traveling standards for international comparisons, (ii) the improvement of the calibration of low and high resistance standards, (iii) a new way to verify or calibrate resistance bridges, for example. QHARS realization is based on the application of the multiple connection technique which allows, in principle, to make artificial macroscopic quantum Hall standards, composed of many Hall bars, while keeping the fundamental properties of the Integer Quantum Hall Effect. The feasibility of QHARS has been firstly verified by interconnecting a few Hall bars [1-2]. Then, it has been shown that quantum standards constituted of a large number of Hall bars, up to 145 fully integrated on chip, and based on heterostructures with one or two 2DEGs, could be accurate within some parts in 109 and behave as quantum standards [3-6]. Indeed, the exactness of these QHARS was observed in coïncidence with the checking of a set of quick technical criteria valid for the whole sample and similar to that used to validate single Hall bars.
The aim of this project is thus to test the universal nature of a set of QHARS by organizing their exchange between several NMI
Final Report 2009-12-08
The development of Quantum Hall Array Resistance Standards (QHARS) is likely to open new prospects in resistance metrology: (i) the realization of accurate and stable traveling standards for international comparisons, (ii) the improvement of the calibration of low and high resistance standards, (iii) a new way to verify or calibrate resistance bridges, for example. QHARS realization is based on the application of the multiple connection technique which enables, in principle, to define a composite macroscopic Hall resistance, with many single Hall bars combined in series/parallel arrays, while keeping the fundamental properties of the Integer Quantum Hall Effect, and particularly the quantization of the Hall resistance. The feasibility of QHARS has been firstly verified by interconnecting a few Hall bars. Then, it has been shown that quantum standards constituted of a large number of Hall bars, up to 145 fully integrated on chip, and based on heterostructures with one or two vertically stacked 2DEGs, can be accurate at the level of some parts in 10^9 and behave as quantum standards. Indeed, the exactness of these QHARS was observed in coincidence with the checking of a set of quick technical criteria valid for the whole sample and similar to that used to validate single Hall bars.
The aim of this project was thus to test the universal nature of a set of QHARS and demonstrate their potential for the metrological purposes listed in the beginning by organizing their exchange between several NMIs. The set of QHARS was constituted of height LNE/OMMIC samples and two PTB samples. QHARS Participants were invited to measure the exactness of these quantum standards and their dependence on several external parameters: magnetic field, temperature, current. It is also desirable to link the exactness observed with the checking of the technical criteria.
In the framework of this project, first results have been reported by BIPM, METAS, MIKES, LNE and PTB: Five different types of QHARS with nominal values close to 100 ohm (LNE), 129 ohm (LNE), 1290 ohm (LNE and PTB), 129 kohm (PTB) and 1.29 Mohm (LNE), respectively, on the i =2 plateau have been characterized by comparison with the quantized resistance of a single Hall bar.
For the first three arrays, the relative differences between measured and nominal values do not exceed a few parts in 10^9. For the last two high value arrays which combine single Hall devices using exclusively series connection, the results are not of so homogeneous high quality as for first three arrays. Bad quality contacts on some of these arrays prevent from performing high quality measurements. Nevertheless the accuracy of the quantization of other better quality high resistance samples was demonstrated to range from 1 part to 3 parts in 10^8. Those very low uncertainty are remarkable when dealing with measurements of megohm range resistances. 1.29 Mohm arrays have also been used in commercial bridges with very high bias voltage (up to 14 V) for accuracy checks on high value resistance ratios with uncertainty as low as one part in 10^7. As a preliminary conclusion, it appears that high value resistance series arrays because of the lack of determining criteria to be measured enabling to asses their quality (concerning the contacts particularly) imposed by the series configuration, should not be considered as primary standards but could be very useful as transfer standards. The QHARS based on parallel connection offer the possibility to asses very simply the mean value of the contacts resistances and of the dissipative state of the quantum devices involved and thereby could to be the base of the next generation of primary quantum resistance standards.
This study of QHARS as high precision resistance standards is still going, but as a workpackage of the iMERA+ JRP "ULQHE".