SETamp: Application of single electron devices for the development of a current standard

• Research unit: EU RFP
• Project number: 96.0223

Key words (English)

Current standard; single electron tunneling; device characterisation

Alternative project number (English)

EU project number: SMT4-CT96-2049

Research programs (English)

EU-programme: 4. Frame Research Programme - 2.2 Measurements and testing

Short description (English)

See abstract

Further information (English)

Full name of research-institution/enterprise:
Bundesamt für Metrologie und Akkreditierung METAS

Partners and International Organizations (English)

NMi (Nederlands Meetinstituut, Delft (NL), CTH (Chalmers University of Technology, Gothenburg (S), PTB (Physikalisch-Technische Bundesanstalt, Braunschweig (D), DFM (Danish Institute for Fundamental Metrology, Lyngby (DK), NPL (National Physical Laboratory, Teddington (UK), LCIE (Laboratoire Central des Industries Electriques, Fontenay-aux-Roses (F), SP (Swedish National Testing and Research Institute, Borås) (S)

Abstract (English)

Formation of a strong European basis for the future realization of a current standard was the main object of the SETamp project. 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. Testing and characterization of single electron tunneling (SET) transistors was OFMET's contribution according to the project agreement. However, we have finally contributed on a much broader range of tasks, beyond the initial project agreement.
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/vHz 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. Recently, we have succeeded to fabricate a semiconducting quantum dot with in-plane gates structured with an AFM. In a next step we plan to fabricate more complicated structures like multi-junction or coupled devices.
We have strongly benefited from the collaboration among different Metrology institutes. Knowledge exchange and the advantages derived from complementary individual experimental and technical strength have significantly motivated the partners and stimulated the success of the project.
The work performed gives a profound understanding of the physics in metallic SET 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.

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 96.0223