Reduction of low-frequency 1/f noise in Al-AlOx-Al tunnel junctions ...

Juhani Julin, Panu Koppinen, Ilari Maasilta
juhani.julin@phys.jyu.fi
arXiv:1006.3162
Reduction of low-frequency
1/f noise in Al-AlOx-Al tunnel
junctions by thermal annealing
University of Jyväskylä
Department of physics
Nanoscience Center
N S C
Nanoscience Center

University Of Jyväskylä

Content
n
Al–AlOx–Al tunnel junctions
n Fabrication
n Applications
n Using in quantum computation: Qubit
n
Annealing
n Removes barrier structure defects
n Ceases aging
n
Noise
n Johnson noise, 1/f-noise
n Measuring the noise
n AC Bridge modulation setup
n
n
Results
Conclusion

Al–AlOx–Al tunnel junction
n Insulating layer between conducting
metal (~1-2 nm thick) thus electrons
can tunnel through the barrier
n Fabricated using nanofabrication
methods on Silicon oxide or silicon
nitride substrate
n Line width 200-400 nm
First evaporation at 65 deg
Second evaporation at 0 deg
PMMA
Copolymer
Substrate

Applications
n Low temperature thermometers
n Electron coolers
n Radiation detectors
n SQUIDs
n Single electron transistor
n Memory devices
n Quantum computation

Realization of a qubit
n Coherent state, isolated from the environment
n Absence of dissipation (superconducting)
n ET

Annealing treatment
n
n
After fabrication the barrier structure
is imperfect
Spontaneous relaxation to equilibrium
n Resistance increases in time; aging
n
Annealing: heating sample to 400 C
n Slow cool down; barrier will relax
through the equilibrium states
n Ceases aging (*)
n Reduces the 1/f noise
(*) P. J. Koppinen, L. M. Väisto, and I. J. Maasilta.
Applied Physics Letters, 90:053503 (2007)

Aging

Conductance at 4K

Johnson noise:
n Originates from charge
movements
n Voltage noise
n White noise
n Depends only on
temperature and resistance
Noise
1/f-noise:
n Ensemble of charge traps
or dislocations in barrier
n Resistance fluctuations
while current through the
sample

Measuring the noise
n Must get rid of the noise from the measurement setup
(preamplifiers etc…)
n Low-freq. measurements are difficult, time-consuming,
1/f-noise from preamplifiers dominate.
n Requires a sensitive technique since tunnel junctions are
destroyed by large currents
Noise from the preamplifiers
Noise from the sample

n
n
AC Bridge modulation setup
Solution: shift the noise measurement into the region where
minimal preamplifier noise
Resistance fluctuations modulate the carrier signal

AC Bridge modulation setup
n Phase selectivity, frequency dependent 1/f-noise in
phase with carrier signal
n Out-of phase signal white Johnson noise
In phase
Out-of phase

2kΩ carbon resistor at 4K
Single amplifier measurement
Bridge measurement:
Blue is without and red is with crosscorrelation

Results
Room temperature 1/f noise
spectrum of tunnel junctions
fabricated in HV and UHV
Temperature dependence
of the 1/f noise in nonannealed
tunnel junctions
Noise normalized by tunneling resistance
Noise levels (at 10Hz) as
a function of temperature.
Solid circles represent
non-annealed and open
circles (different)
annealed samples

Conclusion
n Actually not 1/f noise,
SR/R=1.6799.10-8f -1.13 for UHV-samples
n 1/f noise is independent on used substrate
n Samples fabricated in higher vacuum
-> lower noise
n After annealing -> equal 1/f noise levels
n Percentage drop is 80% in UHV samples
and even 90% in HV samples (300K).
q The critical current 1/f noise is related to the
resistance noise

Thank you