SET-circuits for standard of current

The effect of pumping of electrons can be observed with the help of the current steps I = ef on the voltage-current characteristics. The width of the steps characterizes the stability of pumping regime against the unwanted cotunneling processes. The results (Fig.3) of our first experiments [3] with the 3-junction R-pump on the basis of the junctions of type Al/AlO_x/Al and the on-chip resistors of Cr (Fig.4), fabricated by means of shadow angle evaporation technique, indicate that the metrological level of accuracy of 10 ppb, which otherwise can be realized with the multijunction pump only, is achievable even with the 3- or 4-junction R-Pump.

The forthcoming applications for the SET-pump of high accuracy involve the closing of the metrological triangle and the standard of electric capacitance.

In the framework of cooperation with Working Group 2.61 towards the development of the single-electron capacitance standard, we developed a 5-junction R-pump to be fabricated on a dielectric quartz glass substrate, due to its substantially smaller dielectric constant. The importance of a low dielectric constant is related to a single-electron resolution of an SET electrometer connected to a 0.1mm-large on-chip needle pad which is used to contact the cryocapacitor. The technological steps (for example, PMMA/Ge/Copolymer-three layer mask, E-beam lithography, three-angle deposition) have been implemented in a similar way as on standard Si-wafer and must be adjusted to the conditions of an insulating substrate. In particular, we could avoid a destructive effect of substrate electrostatics by using a 30nm-thin semiconductor Si layer evaporated on top of the wafer.

Figure 5:
Equivalent circuit of a long array with high-ohmic resistors
and a shape of a Cooper-pair soliton.

We investigate electric transport properties of a long array (Fig.5) of small Al/AlO_x/Al tunnel junctions, biased through high-ohmic on-chip Cr-resistors and driven by an external high-frequency signal using capacitive gates. In order to be able to tune the Josephson coupling strength, we fabricated an array of small-junction SQUIDs (see an SEM-Image in Fig.6). The IV-curves, Fig.7, measured at low temperatures show a region of a negative differential resistance, which, in such structures, indicates the presence and dominance of so-called Bloch-oscillations [4]. Due to a fundamental current-frequency relation I = 2ef, the use of these oscillations can be of metrological relevance, opening a possibility of a quantised current source. As compared to single tunnel junctions [5], the long arrays have an advantage of substantially larger oscillation amplitudes in voltage, and they are featured by similarly wide frequency range up to several GHz. A series connection of small Josephson junctions gives rise to a specific physical problem: the dynamics of charge in form of Cooper pair solitons with variable mass. In the framework of electro-mechanic analogy, the role of the mass is played by the quasicharge-dependent so-called Bloch inductance of the junctions [6].

Figure 6: SEM-Image of an array of ultrasmall SQUIDs and on-chip high-ohmic resistors.	Figure 7: IV-characteristics shows a region of the negative differential resistance (see also Inset) and indicates a presence of so-called Bloch oscillations.

We successfully tried to reduce the rate of leakage with the help of compact high-ohmic microstripline resistors of Cr. This improves the turnstile performance at the typical frequencies ~10-100MHz. However, due to the resistor, the current plateaus were found to be shifted to a higher bias-voltage range (see Fig. 9). On the other hand, the high-bias extension of the plateaus was limited by the heating effects, increasing with the driving frequency. Both mentioned effects result in a practical frequency limitation of ~100MHz. The work is supported by the EU through the project SCOPE.

References