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Nuclear Instruments and Methods in Physics Research A 370 (1996) 88-90 NUCLEAR INSTRUMENTS

a METHODS IN PHVSICS RESEARCH

Section A

Quasiparticle diffusion and loss processes in superconducting tunnel junctions

J. Martin”‘*, S. Lemke”, R. Grossa, R.P. Huebener”, P. Videlerb, N. Randob, T. Peacockb, P. Verhoeveb, F.A. Jansenb

dPhysikalisches Institut. Lehrstuhl Experirnerltnlphysik II. University of Tiibingen. Morgenstelle 14. D- 72076 Tiibingen, Germane

“Astrophysics Division. Space Science Department of the European Space Agerq. ESTEC, ZOO AC Noordwijk. Netherlands

Abstract

Superconducting Tunnel Junctions (STJ) are promising as X-ray detectors. However, they still do not reach their theoretical energy resolution due to various loss processes such as the diffusion of quasiparticles out of the electrode volume

and trapping in regions of reduced energy gap. Low Temperature Scanning Electron Microscopy (LTSEM) allows the investigation of inhomogeneities in the response of superconducting Nb/AIO,/Nb tunnel junctions with high spatial resolution. In this way diffusion of quasiparticles and local trapping sites can be identified directly. The impact of these processes on the homogeneity of the signal height and the energy resolution can be visualized. Furthermore, the diffusion

length and the lifetime of the quasiparticles is derived. Numerical simulations show that it is necessary to further reduce signal inhomogeneities due to diffusion processes and local traps to ensure a mayor improvement in the energy resolution.

1. Experimental setup

In our experiment a polycrystalline Nb/AlO,/Nb tunnel

.junction is irradiated with electron pulses by means of a LTSEM. The thickness is 250 nm for both Nb electrodes. For further details see Ref. [I]. The junction was irradiated on the top side with 25 l.~s pulses of 5 keV electrons. The energy flux was 90 eV/ns. 5 keV electrons deposit their

energy along a path of about 120 nm, indicating that they only stimulate the counter electrode. The voltage shift due

to the irradiation is detected and averaged by means of a lock-in technique. The experiment was performed at a

temperature of T = 1.6 K. The junction was biased at 0.24 mV

2. Results

Fig. 1 shows the signal distribution from the counter electrode as a three dimensional view. This signal dis- tribution is dominated by a strong decrease of the counter electrode signal towards the counter lead, a signal increase in the edges ,and a signal increase at specific positions.

We first discuss the strong decrease of the signal towards the counter lead. From the signal decay along a single linescan a diffusion length of 7.2 pm is derived,

* Corresponding author. Tel. +49 7071 296317, fax +49 7071

296322, e-mail martin@brahms.pit.physik. uni-tuebingende.

which is shorter than theoretically expected. The diffusion

length is given by A =K. where D is the diffusion

constant and rctt the effective lifetime. The diffusion

constant is given by [2]:

u, A,,RRR . (1)

where k, is the Boltzmann constant, T is the temperature

of the junction, 6 is the energy gap, u, is the Fermi

velocity, A,, is the mean free path at room temperature

Fig. 1. The spatially resolved detector signal of the counter

electrode measured with LTSEM. The signal distribution is shown

in a three dimensional view. The signal is normalized to the

middle of the electrode ( 1007~).

0168-9002/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved

SSDI 016%9002(95)01058-O

J. Martirt rt al. / Nd. Instr. and Meth. in Phys. Res. A 370 (1996) 88-90

and RRR is the residual resistance ratio. For our Nb STJ

we have T = 1.6 K. u, = 5.65 X 10’ m/s [3]. A (300 K) =

2.1 nm [3] and RRR = 4. The diffusion constant is D = 3.8 X IO-” m’/s. The lifetime of the quasiparticles in the counter electrode was measured by LTSEM to be rcrt = 340ns. Therefore one expects a diffusion length of 11.4 km. There is only moderate agreement with the experimental result. The fact that the observed diffusion length is shorter than theoretical expectations, is also

observed in other junctions [5.6].

One can also determine that for this junction the spatial

inhomogeneity of the signal distribution resulting from

diffusion losses into the lead leads to an energy resolution

of AEIE = 0.3%. Therefore, it is important to reduce diffusion losses into the leads using of very narrow current leads or current lead materials with increased energy gap

[71.

Due to diffusion of the quasiparticles the whole signal

distribution in the counter electrode is influenced by

quasiparticle losses into the current lead. The signal

distribution is such that it cannot however be explained by

quasiparticle diffusion into the lead alone (see Fig. 2).

When irradiating the current lead directly, the signal is significantly lower than for the identical energy deposition

in the counter electrode. Therefore, we conclude that the Nb/Nb interface between the counter electrode and the wiring layer may be of poor quality. Either the Nb close to

the interface may have a reduced band gap, so that

quasiparticles are trapped and lost in these areas, or the interface may form a thin barrier itself. In this case

quasiparticles in the leads are hampered in flowing to the tunnel area. To improve the present understanding of the influence of local sample regions causing quasiparticle

losses numerical simulations were performed. Based on the

reduced Rothwarf-Taylor-equations [4] the density of the

quasiparticles is calculated in two dimensions. Using this method any junction geometry can be simulated and diffusion effects can be investigated.

Next, we discuss the slight signal increase at the edges of the counter electrode (Fig. 1). The phenomenon can be

explained by phonon coupling between the two electrodes. While for 5 keV electrons a direct stimulation of the base

electrode when irradiating the counter electrode is ex-

cluded, the quasiparticles created in the counter electrode

may recombine into 24 phonons. These recombination phonons can enter the base electrode, however since they

can cross the barrier much easier than the quasiparticles,

and can break up Cooper pairs in the base electrode. Thus, by the process quasiparticles are transported from the counter electrode to the base electrode. When the junction is irradiated at its edges the diffusion of the created

quasiparticles is limited to about 180”. which results in an

increased local quasiparticle density. Because the recombi- nation rate of quasiparticles is proportional to the quasi-

particle density, the phonon coupling between the two electrodes is strongest at the electrode edges. In all

LTSEM measurements on this junction the base electrode

signal was significantly higher than from the counter, indicating that quasiparticles have a larger tunnel prob-

ability when they are in the base electrode. Therefore increased self-recombination and phonon coupling is the

most likely cause of the signal increase towards the edges. The numerical simulations confirm the assumption that

the Nb/Nb interface between the counter electrode and lead is of poor quality. In Fig. 2 two linescans from the model are shown in comparison with an experimental one. Model scan c) provides a reasonable fit to the LTSEM experimental data.

Finally, we consider the signal increase at the specific positions which disturb the signal distribution (Fig. 1). These dots are regularly distributed. One explanation of

the dots may be that they are caused by Abricosov vortices. Around Abricosov vortices the energy gap is reduced. By a reduction of the energy gap the tunnelling

characteristics is influenced. Moreover the regular dis-

tribution of dots is well explained by a regular distribution

of flux in the junction. When the junction was cooled through its superconducting transition temperature in a perpendicular magnetic field, it was expected that the

density of dots would greatly increased. This was however not confirmed by experiment. No other LTSEM experi- ments with trapped flux in junctions show the appearance of dots [8]. A local modulation of the signal could be

caused by variations in the surface morphology (material or topography contrast). However such variations do not explain the regular distribution of the dots. Moreover, the distribution of the dots changed after heating the sample above its critical temperature.

0 IO x-position [nm]

20

Fig. 2. Detector response signal along a single linescan a). Also

shown are calculated linescans: b) the tunnelling rate at the

interface between lead and electrode is reduced (50%) due to the

larger layer thickness, c) additionally it is assumed that the

lifetime of the quasiparticles at the interface is reduced by a factor

of 10 and that the diffusion length in the lead is A = 3.5 pm.

89

3. Conclusions

The LTSEM measurements confirm that diffusion and local losses of quasiparticles affect the homogeneity of the

III. TUNNEL JUNCTIONS

90 J. Martin et al. I Nucl. Instr. and Meth. in Phys. Rex. A .Z70 (1996) 88-90

junction response in various ways: Firstly, the detector performance is reduced by quasiparticle losses in the counter electrode lead. Secondly, phonon coupling causes a signal increase towards the edges. Thirdly, positions of increased signal response are observed, which must also degrade the response. Currently their origin is not yet

satisfactorily explained.

Acknowledgement

We are indebted to the Technical Research Centre of Finland, Espoo for fabrication of the sample.

References

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[4] J.B. le Grand, X.-ray Response of Superconducting Tunnel

Junctions with Trapping Layers, Ph.D. thesis. Space Research

Organisation of the Netherlands (SRON).

[5] S.P. Lemke, RGntgenspektroskopie und Mikromechanik mit

supraleitenden Tunnelkontakten, Ph.D. Thesis, Universitit

Tiibingen ( 1995 ).

[6] F. Hebrank et al., IEEE Trans. Applied Superconductivity 3

(1993) 2084.

[7] P. Videler et al.. EUV. X-Ray and Gamma-Ray Instrumen-

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[8] J.B. le Grand et al., these Proceedings (Workshop on Low

Temperature Detectors (LTD6). Beatenberg/Interlaken, Swit-

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