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Nuclear Instruments and Methods in Physics Research A 520 (2004) 519–522

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0168-9002/$

doi:10.1016

Responsivity of Ta-based STJ’s to visible light as a functionof Al trapping layer thickness

I. Jerjena,*, E.C. Kirka, Ph. Lercha, A. Mchedlischivilia, M. Furlana,D. Twerenbolda, A. Zehndera, H.R. Ottb

aPaul Scherrer Institute, Villigen CH-5232, SwitzerlandbETH-H .onggerberg, Z .urich CH-8093, Switzerland

Abstract

Superconducting tunneling junction devices are being developed as energy resolving photon counting imagers. We

have fabricated a series of Ta/Al/AlOx/Al/Ta devices with variable Al layer thickness, dAl; and measured their response

to a weak visible photon source. Single photon sensitivity is observed. We model our system with a set of modified

Rothwarf–Taylor equations including quasiparticle losses, trapping, (back)tunneling and calculate the charge

responsivity as a function of dAl with one fitting parameter. The quasiparticle loss time for Al and Ta is found to be

tlðAlÞ ¼ 35 ms and tlðTaÞ ¼ 0:14 ms in our devices, respectively.

r 2003 Elsevier B.V. All rights reserved.

PACS: 85.25.Oj

Keywords: STJ; Responsivity; Trapping; Optical; Tantalum

1. Introduction

Tantalum based superconducting tunnelingjunctions (STJ) have the sensitivity to detect singlephotons in the visible and UV energy range [1,2].The responsivity measures the charge collected perunit of deposited energy. Signal enhancement inSTJs is obtained by combining quasiparticle (QP)trapping and multiplication [3] and backtunneling[4] mechanisms at the expense of enhanced noise[5,6]. QP loss processes due to diffusion as well astrapping in unwanted regions within the STJ needto be minimized. The increase of the trapping rate

onding author.

ddress: iwan.jerjen@psi.ch (I. Jerjen).

- see front matter r 2003 Elsevier B.V. All rights reserve

/j.nima.2003.11.303

Gtr with an increasing trapping volume has beenobserved in a Cu trap in good electrical contact toa superconductor [7] as well as in Nb/Al devices[8]. The trapping rate depends on the QP scatteringtime [9] which in turn is a strong function of thenormalized QP energy E=D: We investigate thesituation in Ta/Al devices.

2. Model

We divide our device into four regions (Fig. 1)and apply the Rothwarf–Taylor equations [10] todescribe changes in the QP populations. Region 1is formed by the volume of the entire left-hand side(LHS) electrode. Region 2 is the LHS Al trap.

d.

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Fig. 1. Schematical representation of the quasiparticle popula-

tions considered in the model.

I. Jerjen et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 519–522520

Region 3 is the right-hand side (RHS) trap, andregion 4 is the whole RHS electrode. Region 1contains n1ðtÞ QPs whose energies are larger thanthe value of energy gap, D1; of region 1. The n1ðtÞQPs can tunnel through the barrier with a rateGt;n14 ; get trapped into region 2 with a rate Gtr;n12 orsimply get lost with a rate Gl;n1 : The n2ðtÞ QPs inregion 2 can tunnel into region 3 with a rate Gt;n23 ;or get lost with a rate Gl;n2 : Excitation out of thetraps is neglected. The functions n3ðtÞ and n4ðtÞdescribe the QP populations on the RHS. Weneglect trapping multiplication because the gapdifference between Ta and Al is smaller than 3DAlðDTa ¼ 0:7 meV;DAl ¼ 0:27520:51 meVÞ:

dn1ðtÞdt

¼ � Gtr;n12 þ Gl;n1 þ Gt;n14

� �n1ðtÞ

þ Gt;n41n4ðtÞ ð1Þ

dn2ðtÞdt

¼Gtr;n12n1ðtÞ � Gl;n2 þ Gt;n23

� �n2ðtÞ

þ Gt;n32n3ðtÞ ð2Þ

dn3ðtÞdt

¼Gtr;n43n4ðtÞ � Gl;n3 þ Gt;n32

� �n3ðtÞ

þ Gt;n23n2ðtÞ ð3Þ

dn4ðtÞdt

¼ � Gtr;n43 þ Gl;n4 þ Gt;n41

� �n4ðtÞ

þ Gt;n14n1ðtÞ ð4Þ

Eqs. (1) and (2) describe the LHS electrode. Therate terms in parentheses in Eq. (1) account for areduction in the QP population in region 1 by

trapping, losses, and tunneling, respectively. Thesecond term is the positive contribution due toback-tunneling from high energy QPs out ofregion 4. The first and last term of Eq. (2) describethe positive contribution due to trapping fromregion 1 and back-tunneling from region 3,respectively. The negative contribution is due tolosses and tunneling. Eqs. (3) and (4) describe QPpopulations in the RHS electrode.

The trapping rates are computed [9] assumingGtr;n12 ¼ Gtr;n43 ; where.

Gtr;n12 ¼1

tsðDTa=DAl; t0ðAlÞÞdAl

dAl þ dTa: ð5Þ

In Eq. (5) we use DTa ¼ 0:7 meV; measure DAlðdAlÞfrom the IV curves of the devices and taket0ðAlÞ ¼ 438 ns: The scattering lifetime ts is afunction of the characteristic time t0 [9]. The Aland Ta film thicknesses dAl and dTa are measuredduring fabrication.

The tunneling rates [11] depend on the appliedbias, Vb; and are computed assuming Gt;n14 ¼ Gt;n41

and Gt;n23 ¼ Gt;n32

Gt;n14 ¼1

4e2Nð0ÞRnAdTaþAlf ðDTa;VbÞ

Gt;n23 ¼1

4e2Nð0ÞRnAdAlf ðDAl;VbÞ ð6Þ

where e is the electron charge, Rn are the measuredtunnel barrier resistances, Nð0Þ the single spindensity of states, A the device area measured afterfabrication, neglecting a small Ta rim underneathand around the perimeter of the Al layer. In ourdevices RnA ¼ 5 mO cm2: The function f ðD;VbÞtakes into account the particular shape of thedensity of states in a superconductor.

We consider symmetrical loss rates, Gl;n1 ¼ Gl;n4 ;and Gl;n2 ¼ Gl;n3 ¼ Gl;Al; hence

Gl;n1 ¼Gl;AldAl þ Gl;TadTa

dTa þ dAl: ð7Þ

Several factors affect QP losses. We propose totake losses into account in the most simplemanner, Gl;Al ¼ 1=at0ðAlÞ; and Gl;Ta ¼ 1=at0ðTaÞ;a being the same fitting parameter for both Taand Al. This crude approach takes into accountthe fact that losses are material dependent and

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50

40

30

onsi

vity

(ke

/eV

)

7

8

9

1

2∆ (m

eV)

2∆ ==>

exp model

I. Jerjen et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 519–522 521

includes the volume ratio of the regions formingthe devices.

The system of ordinary differential equations(1–4) has been solved numerically, and the totalcurrent integrated over time gives the collectedcharge. We assume that 550 e=eV are created inour Ta absorber [12].

20

10

Res

p

1.00.80.60.40.2

Al trap thickness (x 100 nm)

4

5

6

ESTEC

Fig. 3. Left: measured and calculated responsivity as a function

of Al trap thickness. Right: Measured gap parameter values.

The lines are guide to the eye.

3. Experiment

Our standard STJ fabrication process [13] forTa/Al/AlOx/Al/Nb devices has been modified inorder to produce devices with Ta layers (with athin Nb seed layer) on both sides of the tunnelbarrier. The important interfaces are ion beamcleaned before the deposition of the next film. TheIV curves reveal symmetrical gap devices [14].Light is coupled to the STJ mounted in a 3Hecryostat through an optical fiber. The responsivityis measured with a charge sensitive amplifieroperating at room temperature. The STJs arecurrent biased with a battery via a cold currentlimiting resistor.

Fig. 2 shows a pulse height histogram of onedevice illuminated with a weak source of ð2:5 eVÞgreen light. Single photon sensitivity is evident

350

300

250

200

150

100

50

0

Cou

nts

5004003002001000Collected charge number x103

illumination with 2.5 eV photons responsivity 21,500 e / eV

Fig. 2. Pulse height spectrum measured with a Ta STJ

illuminated with a weak green ð2:5 eVÞ photon flux. The peak

at 400,000 is the test pulser, noise ¼ 4400 e FWHM. The

smooth curve is a fit obtained by a superposition of Poisson and

Gaussian distributions.

from the Poisson distribution. The LED is pulsedin order to reduce the photon flux reaching thedevice. The readout electronics is not triggered.The measured and calculated responsivities areshown on the left-hand scale of Fig. 3 as a functionof dAl: Also shown are Ta data measured by theESTEC team [15,16]. Clearly, the responsivityincreases with an increase of dAl: On the right-hand scale we plot 2DðAlÞ values measured fromIV data. From the unique fitting parameter used inthe calculation we obtain tlðAlÞ ¼ 35 ms andtlðTaÞ ¼ 0:14 ms: The present value for Ta issurprisingly small and contrasts with the valueobtained previously [17,18] in a situation involvingQP transport over longer distances and probingwith 6 keV photons rather than with visible light.The puzzling low value of tlðTaÞ implies thattrapping has to be a fast process to allow for highresponsivity.

References

[1] P. Verhoeve, N. Rando, A. Peacock, A. van Dordrecht,

A. Poelaert, D.J. Goldie, IEEE Trans. Appl. Supercond. 7

(1997) 3359.

[2] C.M. Wilson, L. Frunzio, D. Prober, IEEE Trans. Appl.

Supercond. 13 (2003) in press.

[3] N.E. Booth, Appl. Phys. Lett. 50 (1987) 293.

[4] K.E. Gray, Appl. Phys. Lett. 32 (1978) 392.

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I. Jerjen et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 519–522522

[5] C.A. Mears, S.E. Labov, A. Barfknecht, Appl. Phys. Lett.

63 (1993) 2961.

[6] D.J. Goldie, P.L. Brink, C. Patel, N.E. Booth,

G.L. Salmon, Appl. Phys. Lett. 64 (1994) 3169.

[7] D.J. Goldie, N.E. Booth, C. Patel, G.L. Salmon, Phys.

Rev. Lett. 64 (1990) 954.

[8] A. Poelaert, P. Verhoeve, N. Rando, A. Peacock,

D.J. Goldie, R. Vens, SPIE 2808 (1996) 523.

[9] S.B. Kaplan, C.C. Chi, D.L. Langenberger, Phys. B 14

(1976) 4854.

[10] A. Rothwarf, B.N. Taylor, Phys. Rev. Lett. 19 (1967)

27.

[11] A.F. Cattell, et al., J. Phys. 13 (1983) 855.

[12] A. Zehnder, Phys. Rev. B 52 (1995) 12858.

[13] E.C. Kirk, Ph. Lerch, J. Olsen, A. Zehnder, H.R. Ott,

Nucl. Instr. and Meth. A 444 (2000) 201.

[14] A. Zehnder, Ph. Lerch, S.P. Zhao, Th. Nussbaumer,

E. Kirk, H.R. Ott, Phys. Rev. B 59 (1999) 8875.

[15] P. Verhoeve, N. Rando, A. Peacock, A. van Dordrecht,

M. Bavdaz, J. Verveer, D.J. Goldie, M. Richter, G. Ulm,

Proc. LTD-7, 97, M .unchen, 1997.

[16] P. Verhoeve, R. den Hartog, D. Martin, N. Rando,

A. Peacock, D. Goldie, SPIE 4008 (2000) 683.

[17] S. Friedrich, K. Segall, M.C. Gaidis, C.M. Wilson,

D.E. Prober, Appl. Phys. Lett. 71 (1998) 26.

[18] Th. Nussbaumer, Ph. Lerch, E. Kirk, A. Zehnder,

R. F .uchslin, P.F. Meier, H.R. Ott, Phys. Rev. B. 61

(2000) 9719.