Key words
(German)
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Internationales Einheitensystem, Einelektronen-Tunneleffekt, Stromnormal, Messung kleinster elektrischer Ströme
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Key words
(English)
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International system of units, single electron tunneling effect, current standard, measurement of small electric currents
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Short description
(German)
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Anwendung des Einelektronen-Tunneleffekts in der Metrologie
Seit der ersten Messung der Elektronenladung im Jahre 1911 ist es in der Zwischenzeit dank enormem technologischem Fortschritt auf dem Gebiet der Nano-Elektronik möglich geworden, einzelne Elektronen in Festkörperproben mit Ausdehnungen von lediglich einigen Nanometern zu manipulieren. Man spricht vom Einelektronen-Tunneleffekt (single electron tunneling SET). Zusätzlich zum fundamentalen Interesse an der Physik der entsprechenden Phänomene haben diese Entdeckungen eine grundlegende Bedeutung für die moderne Metrologie. Insbesondere wird damit die Realisierung eines Quanten-Primärnormals für den elektrischen Strom denkbar.
Zur Erreichung dieses Fernziels sind umfangreiche Vorarbeiten notwendig. Das SET-Projekt des EAM soll zu einem besseren Verständnis der physikalischen Phänomene beitragen, die für die metrologische Anwendung des Quanteneffekts von Bedeutung sind. Zudem sollen die notwendigen experimentellen Techniken für die Messung extrem kleiner Ströme entwickelt werden.
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Short description
(English)
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The Application of the Single-Electron Tunneling Effect (SET) in Metrology
Since the first measurement of the electron's charge in 1911, thanks to enormous technological progress in the field of nano-electronics, it has recently become possible to manipulate single electrons in solid state devices whose dimensions are on the order of a few tens of nanometers. In addition to the fundamental interest in the physics of such phenomena, these discoveries also have a profound impact on modern metrology. In particular, the realization of a quantum primary standard of current is now conceivable.
To reach this goal, extensive preparatory studies have to be carried out. The SET project at OFMET should lead to a better understanding of the physical phenomena which are of importance for the metrological application of the quantum effect. In addition, the experimental techniques needed for the measurement of extremely small currents will be developed
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Project aims
(German)
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Projektziele
• Entwicklung eines Stromnormals auf der Basis des Einelektronen-Tunneleffekts;
• Entwicklung der experimentellen Techniken für das Messen sehr kleiner Ströme;
• Erforschung der Rauschcharakteristiken von SET-Strukturen
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Project aims
(English)
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Objectives of the project
• Development of the techniques needed for the realization of a current standard based on the single electron tunneling effect (SET).
• Development of the experimental techniques needed for the measurement of very small electric currents.
• Characterization of the noise behaviour of SET structures.
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Abstract
(German)
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Abstract
(English)
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The work at METAS was carried out in the framework of an European collaboration preparing the future realization of a primary current standard. On one hand, the physical processes determining the performance of 'single electron devices' need to be investigated and understood. On the other hand, nanostructure device fabrication and characterization as well as the development of high-precision measurement techniques for low current levels require considerable research efforts. The work carried ou at METAS can be summarized as follows:
- With the setup of the cryogenic infrastructure, special attention was paid to a thorough shielding against environmental noise. Background (blackbody) radiation from parts warmer than the sample, or introduction of microwaves across the readout transmission lines can severely degrade the charge coherence in SET devices. Special coaxial cables with onset of strong dissipation above some 100 MHz were used for the attenuation of high energetic electromagnetic modes entering from outside. The measurement of the equivalent effective electron temperature, observed as thermal broadening of the electron occupation number in an electron box coupled to an SET electrometer, yielded a minimum value of about 45 mK. This low level proves the precautions against microwave radiation to be very effective and worth the effort.
- Characterization of Al-Al2O3-Al SET devices, fabricated by an other project partner (PTB), has shown a sensitivity of the electrometers on the order of 0.1 me/sqrt(Hz) at maximum SET transistor gain. The noise was identified as dominated by background charge fluctuations.
- Comparison of experimental IV and charge modulation characteristics with 'orthodox theory' calculations and numeric computer simulations have shown significant deviations due to the idealized assumptions of the theoretical model. Extension of the 'orthodox theory' with a 'horizon picture' (taking into account the coupling to the electromagnetic environment) and with a 'self-heating model' (non-equilibrium effects due to current flow) was found to provide a very satisfying description of the experimental data for devices with high resistance tunnel barriers.
- Statistics of Coulomb blockade conductance peaks give information on energy level distributions and interaction effects detected by the SET device. We have found significantly non-Gaussian nearest-neighbour conductance peak spacings for metallic SET devices, in strong contrast to an expected equidistant peak spacing for an isolated SET device with a large number of electrons (averaging over many thermally excited electron states). The experimental results are explained by parametric redistribution in the background charge configuration as a consequence of gate potential variation. Reproducibility of the charge trapping mechanisms with a hysteretic behaviour has been observed. The deeper insight gained into the processes of the crucial background charge fluctuations is as important as the impact on a puzzling discrepancy between theory and experimental peak spacing distributions found in semiconducting quantum dots.
- Alternatively to standard e-beam lithography, we have modified or fabricated nanostructures by means of AFM anodic oxidation. Reduction of the size and increase of the resistance of small sized high ohmic resistors in a controlled way was shown to be feasible in principle. The purpose of such shunts close to a SET device would be a decoupling of the sample from the electromagnetic environment. Moreover, we have succeeded to fabricate a semiconducting quantum dot with in-plane gates structured with an AFM.
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Transfer and application
(German)
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Die Arbeiten führten zu einem grundlegenden Verständnis der physikalischen Eigenschaften von Einelektronen-Tunneleffekt-Transistoren (SET-Transistoren). Die Forschungsresultate bilden zusammen mit der zusätzlich entwickelten rauscharmen experimentellen Einrichtung die Basis für die Verwirklichung von komplexeren SET-Strukturen ("Elektronenpumpen") und die Realisierung der Kapazitätseinheit Farad mit Hilfe quantenelektrischer Effekte.
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Transfer and application
(English)
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The work performed gives a profound understanding of the physics in metallic single electron tunneling transistors and together with the developed low-noise experimental setup provides the basis for the implementation of a multi-junction SET device ('electron pump') for the realization of a current or capacitance standard based on quantum-electrical effects and related to elementary constants only.
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Publications / Results
(German)
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Publikationen in Fachzeitschriften und Konferenzbeiträge
• M. Furlan und B. Jeanneret, Ein Elektron kommt selten allein, OFMETInfo, vol. 6/2, 1999.
• H.-O. Müller, M. Furlan, T. Heinzel, K. Ensslin; Modelling background charge rearrangements near single-electron transistors as a Poisson process; Europhys. Lett., 55(2), pp. 253-259, 2001.
• M. Furlan, T. Heinzel, B. Jeanneret and S.V. Lotkhov; Coulomb blockade peak statistics influenced by background charge configuration; J. Low. Temp. Phys.; 118; 297; 2000.
• M. Furlan, T. Heinzel, B. Jeanneret, S.V. Lotkhov and K. Ensslin; Non-Gaussian distribution of nearest-neighbour Coulomb peak spacings in metallic single-electron transitors; Europhysics Letters; 49(3); 369-375; 2000.
• M. Furlan, A.L. Eichenberger, T. Heinzel, B. Jeanneret and S.V. Lotkhov; Realistic and relevant models for the description of SET transistors; Physica B; 284; 1798; 2000.
• M. Furlan, A.L. Eichenberger, E. Käch, B, Jeanneret and B. Jeckelmann; Single-electron tunneling devices as a possible dc current standard; Helv. Phys. Acta ; 71; 5; 1998.
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Publications / Results
(English)
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Publications and Conference Contributions
• H.-O. Müller, M. Furlan, T. Heinzel, K. Ensslin; Modelling background charge rearrangements near single-electron transistors as a Poisson process; Europhys. Lett., 55(2), pp. 253-259, 2001.
• M. Furlan, T. Heinzel, B. Jeanneret and S.V. Lotkhov; Coulomb blockade peak statistics influenced by background charge configuration; J. Low. Temp. Phys.; 118; 297; 2000.
• M. Furlan, T. Heinzel, B. Jeanneret, S.V. Lotkhov and K. Ensslin; Non-Gaussian distribution of nearest-neighbour Coulomb peak spacings in metallic single-electron transitors; Europhysics Letters; 49(3); 369-375; 2000.
• M. Furlan, A.L. Eichenberger, T. Heinzel, B. Jeanneret and S.V. Lotkhov; Realistic and relevant models for the description of SET transistors; Physica B; 284; 1798; 2000.
• M. Furlan, A.L. Eichenberger, E. Käch, B, Jeanneret and B. Jeckelmann; Single-electron tunneling devices as a possible dc current standard; Helv. Phys. Acta ; 71; 5; 1998.
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