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IEEE Transactions on Applied Superconductivity

Advantages of superconducting Coulomb-blockade electrometers

D. Song, A. Amar, Christopher J. Lobb, F. C. Wellstood

Published Jun 1, 1995• 28 citations
Publisher

Abstract

We have measured the current-voltage gate charge characteristics and noise in superconducting Coulomb-blockade electrometers from 50 mK to 1.05 K. The Al-AlO/sub x/-Al junctions in our devices have capacitances which range from 0.3 fF to 1 fF and tunneling resistances from 20 k/spl Omega/ to 200 k/spl Omega/. We find that our normal and superconducting electrometers have comparable charge noise but that the latter have much larger modulation which persists to significantly higher temperatures. The maximum operating temperature in the superconducting state is close to T/sub c/. This behavior Is consistent with a simple model and makes the superconducting electrometer preferable for many applications.>

DOI
10.1109/77.403244

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This thesis presents results focused on fabrication and electrical characterization of superconducting tunnel junctions and Josephson junctions, e.g. measuring the current–voltage response and 1/f noise from different samples at different temperatures. Junctions were fabricated inside a vacuum chamber where metal evaporation through a resist mask was used to create submicron-sized junctions. The tunneling barrier was common between all different samples, i.e. in-situ oxidized aluminium between the metal depositions. Different material combinations were used, e.g. Al–AlOₓ–Al, Al–AlOₓ–Al:Mn and Al–AlOₓ–Nb. The manganese doping in aluminium (Al:Mn) suppresses the superconductivity making the Al:Mn parts normal, while pure aluminium is superconducting, creating a normal metal–insulator–superconductor (NIS) device. The qualities of junctions were improved, if possible, by vacuum thermal annealing at 400 °C, which was found both to stabilize aluminium and aluminium–manganese based junctions and to reduce the characteristic 1/f noise in them. These results are desired, since for most applications it is important to have a junction with stable parameters over long time and 1/f noise is limiting the sensitivity of many devices. However, the procedure is not for granted, since most other NIS material combinations didn't survive the annealing treatment. We studied Al–AlOₓ–Nb junctions at cryogenic temperatures (≈ 100 mK) either as an SIS' device, or as an NIS device by suppressing the superconductivity of aluminium with magnetic field, and observed excess sub-gap current, which could not be explained by standard tunneling theory with the Dynes model.

•Jan 1, 2016•0 citations
We have measured the current-voltage gate charge characteristics and noise in superconducting Coulomb-blockade electrometers from 50 mK to 1.05 K. The Al-AlO/sub x/-Al junctions in our devices have capacitances which range from 0.3 fF to 1 fF and tunneling resistances from 20 k/spl Omega/ to 200 k/spl Omega/. We find that our normal and superconducting electrometers have comparable charge noise but that the latter have much larger modulation which persists to significantly higher temperatures. The maximum operating temperature in the superconducting state is close to T/sub c/. This behavior Is consistent with a simple model and makes the superconducting electrometer preferable for many applications.>
We report measurements and quantum mechanical simulations of the behavior of superconducting two-junction Coulomb-blockade electrometers biased near the superconducting gap. Our quantum model allows coherent superpositions of states with different number of charges on the island. It predicts maximum current modulation /spl delta/I for devices with tunneling resistance R/sub /spl Sigma///spl ap/64/spl pi//spl Delta//spl planck//sup 2/C/sub /spl Sigma///sup 2//(e/sup 6//spl tau/), where /spl tau/ is the time for which the system maintains quantum coherence, C/sub /spl Sigma// is the total island capacitance and /spl Delta/ is the superconducting energy gap. For comparison, we built Al-AlO/sub x/-Al electrometers with R/sub /spl Sigma// from 61 k/spl Omega/ to 1.7 M/spl Omega/ and C/sub /spl Sigma// ranging from 0.9 fF to 8 fF. We find good agreement for /spl tau//spl ap/0.35 nsec.
In view of future macroscopic quantum experiments, we have fabricated prototypes of charge qubits (with tunable Josephson coupling and capacitive gate) provided with capacitively coupled readout single-electron transistor electrometers. The devices are prepared by electron beam direct-writing in conjunction with material deposition by sputtering. All leads are made of Al, and the AlO/sub x/ tunnel barriers are generated by local thermal oxidizing of the surface of the base striplines. The island electrodes are made of Al or Nb in a subsequent deposition step. In the case of Nb it is suitable to protect the barriers by a thin sputtered Al film first. The small-area (100 nm/spl times/100 nm) tunnel junctions have capacitances in the order of a few 10/sup -16/ F; their resistances can be adjusted in a wide range from k/spl Omega/ to M/spl Omega/ with an on-chip spread of less than 10%. The (double) superconducting gap energy was determined to be about 0.4 meV for Al and 1.4 meV for Nb, respectively. Measuring the so-called Coulomb staircase, we have demonstrated the operation of both qubit boxes and sub-e electrometers.
In an effort to determine the suitability of Josephson junctions for applications in quantum computation, we measured low-noise dc current-voltage characteristics of Nb/AlO/sub x//Nb and Al/AlO/sub x//Al junctions in the temperature range from 90 mK to 1 K. Nb-based samples were obtained from several different facilities with critical currents of a few /spl mu/A and critical current densities between 100 and 3000 A/cm/sup 2/. We fabricated the Al-based samples using double-angle evaporation and obtained critical currents of a few /spl mu/A with critical current densities of about 30 A/cm/sup 2/. We found that the sub-gap leakage current in the Nb-based samples does not depend on temperature in the range 90 mK to 1 K, whereas that for the Al-based samples follows the expected BCS behavior to about 150 mK. Our Al-based samples have a lower level of dissipation than Nb-based devices; however, both Al- and Nb-based samples achieved dissipation levels sufficiently low for some quantum computing applications.
We have measured Current-Voltage ( I-V ) characteristics of a large number of two-dimensional arrays of small area Al-AlOx-Al junctions down to temperatures of 40 mK, recording the temperature dependence of the zero bias resistance R 0 ( T ). The normal state resistance of the junctions R N varies from 2 kΩ to 270 kΩ and the junction capacitance C from 0.2 f F to 2.2 f F. The arrays are classified into different types according to the E J / E c and the R Q / R N ratios, and mapped onto a parameter diagram. E J is the Josephson coupling energy, E c = e 2 /2 C is the charging energy, and R Q = h /4 e 2 = 6.5 kΩ is the quantum resistance. For low resistance arrays, R Q / R N >> 1 with E J / E c > 1, the Josephson current prevails and the arrays are superconducting. When R Q / R N << 1 and E J / E c < 1, the I-V characteristics show typical Coulomb blockade features. In an intermediate region where R Q / R N similar 0.4, E J / E c similar 0.2, we find a "back-bending" I-V curve with negative differential resistance. The behavior of R 0 ( T ) also falls into three categories which can be correlated with the type of I-V characteristic. At the lowest temperature the arrays appear to be either insulating (d R 0 /d T < 0) or superconducting (d R 0 /d T > 0) depending on whether R N is greater than R Q or less than R Q respectively.
This thesis presents results focused on fabrication and electrical characterization of superconducting tunnel junctions and Josephson junctions, e.g. measuring the current–voltage response and 1/f noise from different samples at different temperatures. Junctions were fabricated inside a vacuum chamber where metal evaporation through a resist mask was used to create submicron-sized junctions. The tunneling barrier was common between all different samples, i.e. in-situ oxidized aluminium between the metal depositions. Different material combinations were used, e.g. Al–AlOₓ–Al, Al–AlOₓ–Al:Mn and Al–AlOₓ–Nb. The manganese doping in aluminium (Al:Mn) suppresses the superconductivity making the Al:Mn parts normal, while pure aluminium is superconducting, creating a normal metal–insulator–superconductor (NIS) device. The qualities of junctions were improved, if possible, by vacuum thermal annealing at 400 °C, which was found both to stabilize aluminium and aluminium–manganese based junctions and to reduce the characteristic 1/f noise in them. These results are desired, since for most applications it is important to have a junction with stable parameters over long time and 1/f noise is limiting the sensitivity of many devices. However, the procedure is not for granted, since most other NIS material combinations didn't survive the annealing treatment. We studied Al–AlOₓ–Nb junctions at cryogenic temperatures (≈ 100 mK) either as an SIS' device, or as an NIS device by suppressing the superconductivity of aluminium with magnetic field, and observed excess sub-gap current, which could not be explained by standard tunneling theory with the Dynes model.