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Phonon effects in STJ X-ray detectors
Nuclear Instruments and Methods in Physics Research A 444 (2000) 19}22
Phonon e!ects in STJ X-ray detectors夽 V.A. Andrianov *, P.N. Dmitriev, V.P. Koshelets, M.G. Kozin , I.L. Romashkina , S.A. Sergeev , V.S. Shpinel Institute of Nuclear Physics, Lomonosov Moscow State University, 119899 Moscow, Russia Institute of Radio Engineering and Electronics RAS, 103907 Moscow, Russia
Abstract In#uence of the phonon e!ects on the output signal of superconducting tunnel X-ray detectors was studied for junctions of two types: the standard Nb/Al/AlO /Nb junctions and the multilayer asymmetric Nb/Al/AlO /Al/Nb/NbN V V junctions with the proximity Al trapping layer. It was shown that phonon exchange can change the shape, the amplitude and the polarity of the signal. The most pronounced e!ects were observed in asymmetric junctions for the signals from the electrode with higher gap. 2000 Elsevier Science B.V. All rights reserved.
In the development of superconducting tunnel junction (STJ) detectors the main attention was focused as a rule on the behaviour of the excess quasiparticles (QPs). At the same time phonon processes can be important for the performance of STJ-detectors. In a superconductor QPs are known to be in dynamic equilibrium with 2*-phonons. In thermal equilibrium at low temperature (¹;¹ ) the concentration of 2*-phonons is low. However, after absorption of radiation quantum in STJ-detector the density of the excess QPs can be much higher than the thermal one and the density and role of 2*-phonons increase accordingly. In the present work 2*-phonon exchange, i.e. phonon transfer to the opposite electrode with subsequent pair breaking and the generation of two QPs was studied. The in#uence of the phonon processes on the STJ-detector signal depends on 夽 This work was supported by the Russian Foundation for Basic Research, project no. 99-02-18368. * Corresponding author. Fax: #7-95-939-08-96. E-mail address: [email protected] (V.A. Andrianov).
the particular detector design. STJs of two types were studied: (A) standard tunnel junctions of Nb/Al/AlO /Nb structure (the thickness of layers is V 240/8/2/120 nm correspondingly); (B) multilayer STJs Nb/Al/AlO /Al/Nb/NbN with proximity Al V trapping and NbN re#ecting layers (the thickness of layers is 100/6/2/20/200/30 nm correspondingly). STJs were fabricated by magnetron sputtering in the Ar atmosphere using photolithography, wet and reactive ion etching. Five STJs of rhombic shape with a diagonal ratio 2:1 and areas from 400 to 20 000 lm were patterned on each chip. The peculiarity of the samples (both of A and B type) is their asymmetry (SIS ). Properties of the electrodes (gap parameters, tunnelling probabilities, e!ective QP lifetimes, etc.) are essentially di!erent. The main parameters of the junctions are given in Table 1. The pulse-height spectra arising from irradiation of the junctions with Fe source were measured for various bias voltages < at temperature 1.36 K (Fig. 1). They are composed of 2 subspectra corresponding to absorption in either electrode. The
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 3 1 9 - 4
SECTION I.
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V.A. Andrianov et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 19}22
Table 1 The main parameters of the STJs Type
R S, () cm) ,
* , (meV)
* , (meV)
Q , (10e) *
Q , (10e) &
q , (ls) *
q, (ls) &
q, (ls) &
k (%)
A B
2;10\ 2;6 ) 10\
1.33(1) 1.33(1)
1.48(1) 0.76(1)
30.2(4) 61.0(8)
7.6(4) 39.6(8)
0.78(1) 0.32(1)
0.20(2) 0.37(1)
0.78(4) *
15 59
R S is the normal resistivity of the STJ, * and * are the energy gaps for the base and the counter electrodes respectivly, Q and , @ * Q are the maximum collected charges for low (L) and high (H) gap electrodes correspondingly; q , q and q are time constants of & * & & pulses; k is a fraction of quasiparticles generated in an electrode due to phonon transfer.
Fig. 1. Pulse-height spectra for samples A and B. Subspectra of high gap electrode and low gap electrode are designated by letters H and L correspondingly.
spectra shape can be satisfactorily reproduced by the di!usion model [1]. The spectra were characterised by the maximum collected charge Q and & Q corresponding to absorption of the K -line * ? (5.9 keV) in the centre of the particular electrode. Electrodes are designated according to their gap: high (H) gap (counter electrode for A and base electrode for B) and low (L) gap. Q and Q for the & * spectra measured at < +0.9 meV and pulse rise @ times from the "tting procedure, corresponding to e!ective lifetime of QPs, are represented in Table 1.
Fig. 2. Bias voltage dependencies of the maximum collected charge for samples A and B. Data for low (L) and high (H) gap electrodes are indicated by corresponding letters. P is the & calculated tunnelling probability.
Dependencies of Q and Q on the bias voltage & * for samples A and B are shown in Fig. 2. For both type samples the signals from the L electrodes have higher amplitudes than signals of opposite H electrodes and have characteristic maximums near < ""* !* "/e. This behaviour is in accordance with the expected dependence of the tunnelling
V.A. Andrianov et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 19}22
21
probability on the bias voltage for low gap electrode in an SIS junction. However bias voltage dependencies for H electrodes are quite di!erent. According to Ref. [2] in asymmetric junctions at < 4"* !* "/e pulses of anomalous polarity are expected. Instead, monotonic signal increase for sample A and dependence with the maximum similar to that of the opposite electrode for sample B are observed. In order to understand this behaviour the pulse shapes were considered. For this purpose pulses were digitised, sorted according to their origin from either electrode, averaged to reduce random noise and analysed according to: (1) Q (t)"eN [C(1!e\R O )#D(1!e\R O )], where Q (t) is the dependence of the collected charge on time. Subscript 1 refers to the electrode in which the absorption occurred, subscript 2 refers to the opposite electrode, e is the electron charge, N is the initial number of QPs (N "2.3;10 for absorption of a 6 keV X-ray in Nb), q and q are the e!ective life times of QPs. Coe$cients C and D are given by the formulae: C"P [(1!k)#P q /(q !q )] and D"P [k!P q /(q !q )], (2) where k is the fraction of QPs generated in the electrode 2 due to phonon transfer from the electrode 1; P "q /q and P "q /q , where 2 2 q is the e!ective tunnelling time. Expressions (1) 2G and (2) were obtained from Rothwarf}Taylor equations under the assumption that the redistribution of QPs between the electrodes due to phonon exchange takes place in the initial moment. Back tunnelling was accounted for in the "rst-order approximation on the parameters P and P . Ap plicability of this approximation is justi"ed by low barrier transparency of investigated samples. The exact solution of Rothwarf}Taylor equations for QP multitunneling without considering phonon exchange is given in Ref. [6]. For A-type STJs it was found that pulses of L electrode could be "tted with one rise time exponent only (Fig. 3). Pulses of the H electrode comprised two exponents, the contribution of the second exponent being unexpectedly high (up to 70%).
Fig. 3. Shape of the pulses for A-type STJs. Upper plot } for L electrode and lower plot } for H electrode. The red lines are results of two exponent "tting procedure.
From Eq. (2) it follows that when absorption occurs in H electrode about 15% of QPs appear in L electrode due to 2*-phonon transfer and they contribute up to 70% of the signal. For low bias voltages < 4"* !* "/e bipolar pulses of H elec trode were observed (Fig. 4). They originate from the negative current from the H electrode superimposed on the more prolonged positive current from the L electrode. This latter current is produced by QPs, which have arisen from the phonon transfer. These results are considered in more detail in Ref. [3]. Pulse shape analysis for B-type STJs revealed that signals from both electrodes could be "tted with only one exponent. Rise times are almost the same (q "0.32 ls and q "0.37 ls). Absence of * & di!erence between rise time of the pulses, similarity of pulse-height spectra and similarity of Q and * Q dependencies on bias voltage were rather & SECTION I.
22
V.A. Andrianov et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 19}22
Fig. 4. The anomalous bipolar pulse from H electrode of A-type STJ (< "0.09 mV). The black line is the "tting with Eq. (1), red lines are contributions from each electrode.
surprising because the structure and properties of B-sample electrodes are very di!erent. In order to analyse the signal dependence on bias voltage < the expression that takes into account multiple tunnelling [4] and phonon exchange was used: Q "eN (P #R P )/(1!R R ) #keN (P #R P )/(1!R R ), (3) where P and P are reduced charge tunnelling probabilities, R and R are reduced QP tunnelling probabilities. QP transfer is determined by the sum of two channels of tunnelling: electron and hole one; charge transfer is determined by the di!erence of these two channels; k accounts for QPs transfer to the opposite electrode due to phonon exchange. For low gap proximity electrode of B-type junctions k"0 because the energy of 2* -phonons is * insu$cient for pair breaking in the H electrode. Multiple tunnelling is also weak because tunnelling probability from the H electrode P is close to zero & (see below). So the charge from the L electrode is proportional to the tunneling probability Q "eN P in accordance with the experimental * * data (Fig. 2B, curve L).
The contribution of the phonon exchange to the signal from the H electrode of B-type STJs is essential. Phonons from this electrode with an energy 2* and subgap phonons with an energy exceeding & 2* can produce pair breaking in the proximity (L) * electrode. Analysis of experimental dependence Q on the basis of (3) showed that the phonon & transfer coe$cient k"0.59. In the calculation the dependence of the P/R ratio on < from Ref. [2] was used. Calculated tunnelling probability dependence for the H electrode P (< ) is described by & bipolar curve in accordance with theoretical prediction but its contribution to the signal does not exceed 10% (Fig. 2B, curve P ). The main contri& bution to the signal is due to tunnelling of QPs produced in the proximity ¸ electrode because of the transfer of 2* and subgap phonons. Taking & this into account one can easily understand the similarity of signals from both electrodes (Fig. 2B) and similarity of their pulse-height spectra (Fig. 1B). It should be noted that qualitatively similar e!ects were observed for Nb/AlO /Al STJs in Ref. [5]. V Our results show that phonon exchange can lead to a substantial redistribution of QPs between electrodes. The most pronounced e!ects are expected for asymmetric STJs for signals from the higher gap electrodes. In this case, the 2* and subgap phonons are involved in phonon exchange process.
References [1] O.J. Luiten et al., Quasiparticle di!usion and losses in Nb/AlO and Ta/AlO superconducting tunnel junction V V photon detectors, in: S. Cooper (Ed.), Proceedings of the 7th International Workshop Low Temp. Detectors (LTD-7), Munich, 1997, pp. 25}27. [2] A.A. Golubov et al., Phys. Rev. B 49 (18) (1994) 12 953}12 968. [3] V.A. Andrianov et al., Phys. Solid State 41 (7) (1999) 1063}1069. [4] D.J. Goldie et al., Appl. Phys. Lett. 64 (23) (1994) 3169}3171. [5] D.J. Goldie et al., Supercond. Sci. Technol. 6 (1993) 203}208. [6] A. Steele et al., Nucl. Instr. and Meth. A 444 (2000) 8. These Proceedings.
Phonon e!ects in STJ X-ray detectors夽 V.A. Andrianov *, P.N. Dmitriev, V.P. Koshelets, M.G. Kozin , I.L. Romashkina , S.A. Sergeev , V.S. Shpinel Institute of Nuclear Physics, Lomonosov Moscow State University, 119899 Moscow, Russia Institute of Radio Engineering and Electronics RAS, 103907 Moscow, Russia
Abstract In#uence of the phonon e!ects on the output signal of superconducting tunnel X-ray detectors was studied for junctions of two types: the standard Nb/Al/AlO /Nb junctions and the multilayer asymmetric Nb/Al/AlO /Al/Nb/NbN V V junctions with the proximity Al trapping layer. It was shown that phonon exchange can change the shape, the amplitude and the polarity of the signal. The most pronounced e!ects were observed in asymmetric junctions for the signals from the electrode with higher gap. 2000 Elsevier Science B.V. All rights reserved.
In the development of superconducting tunnel junction (STJ) detectors the main attention was focused as a rule on the behaviour of the excess quasiparticles (QPs). At the same time phonon processes can be important for the performance of STJ-detectors. In a superconductor QPs are known to be in dynamic equilibrium with 2*-phonons. In thermal equilibrium at low temperature (¹;¹ ) the concentration of 2*-phonons is low. However, after absorption of radiation quantum in STJ-detector the density of the excess QPs can be much higher than the thermal one and the density and role of 2*-phonons increase accordingly. In the present work 2*-phonon exchange, i.e. phonon transfer to the opposite electrode with subsequent pair breaking and the generation of two QPs was studied. The in#uence of the phonon processes on the STJ-detector signal depends on 夽 This work was supported by the Russian Foundation for Basic Research, project no. 99-02-18368. * Corresponding author. Fax: #7-95-939-08-96. E-mail address: [email protected] (V.A. Andrianov).
the particular detector design. STJs of two types were studied: (A) standard tunnel junctions of Nb/Al/AlO /Nb structure (the thickness of layers is V 240/8/2/120 nm correspondingly); (B) multilayer STJs Nb/Al/AlO /Al/Nb/NbN with proximity Al V trapping and NbN re#ecting layers (the thickness of layers is 100/6/2/20/200/30 nm correspondingly). STJs were fabricated by magnetron sputtering in the Ar atmosphere using photolithography, wet and reactive ion etching. Five STJs of rhombic shape with a diagonal ratio 2:1 and areas from 400 to 20 000 lm were patterned on each chip. The peculiarity of the samples (both of A and B type) is their asymmetry (SIS ). Properties of the electrodes (gap parameters, tunnelling probabilities, e!ective QP lifetimes, etc.) are essentially di!erent. The main parameters of the junctions are given in Table 1. The pulse-height spectra arising from irradiation of the junctions with Fe source were measured for various bias voltages < at temperature 1.36 K (Fig. 1). They are composed of 2 subspectra corresponding to absorption in either electrode. The
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 3 1 9 - 4
SECTION I.
20
V.A. Andrianov et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 19}22
Table 1 The main parameters of the STJs Type
R S, () cm) ,
* , (meV)
* , (meV)
Q , (10e) *
Q , (10e) &
q , (ls) *
q, (ls) &
q, (ls) &
k (%)
A B
2;10\ 2;6 ) 10\
1.33(1) 1.33(1)
1.48(1) 0.76(1)
30.2(4) 61.0(8)
7.6(4) 39.6(8)
0.78(1) 0.32(1)
0.20(2) 0.37(1)
0.78(4) *
15 59
R S is the normal resistivity of the STJ, * and * are the energy gaps for the base and the counter electrodes respectivly, Q and , @ * Q are the maximum collected charges for low (L) and high (H) gap electrodes correspondingly; q , q and q are time constants of & * & & pulses; k is a fraction of quasiparticles generated in an electrode due to phonon transfer.
Fig. 1. Pulse-height spectra for samples A and B. Subspectra of high gap electrode and low gap electrode are designated by letters H and L correspondingly.
spectra shape can be satisfactorily reproduced by the di!usion model [1]. The spectra were characterised by the maximum collected charge Q and & Q corresponding to absorption of the K -line * ? (5.9 keV) in the centre of the particular electrode. Electrodes are designated according to their gap: high (H) gap (counter electrode for A and base electrode for B) and low (L) gap. Q and Q for the & * spectra measured at < +0.9 meV and pulse rise @ times from the "tting procedure, corresponding to e!ective lifetime of QPs, are represented in Table 1.
Fig. 2. Bias voltage dependencies of the maximum collected charge for samples A and B. Data for low (L) and high (H) gap electrodes are indicated by corresponding letters. P is the & calculated tunnelling probability.
Dependencies of Q and Q on the bias voltage & * for samples A and B are shown in Fig. 2. For both type samples the signals from the L electrodes have higher amplitudes than signals of opposite H electrodes and have characteristic maximums near < ""* !* "/e. This behaviour is in accordance with the expected dependence of the tunnelling
V.A. Andrianov et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 19}22
21
probability on the bias voltage for low gap electrode in an SIS junction. However bias voltage dependencies for H electrodes are quite di!erent. According to Ref. [2] in asymmetric junctions at < 4"* !* "/e pulses of anomalous polarity are expected. Instead, monotonic signal increase for sample A and dependence with the maximum similar to that of the opposite electrode for sample B are observed. In order to understand this behaviour the pulse shapes were considered. For this purpose pulses were digitised, sorted according to their origin from either electrode, averaged to reduce random noise and analysed according to: (1) Q (t)"eN [C(1!e\R O )#D(1!e\R O )], where Q (t) is the dependence of the collected charge on time. Subscript 1 refers to the electrode in which the absorption occurred, subscript 2 refers to the opposite electrode, e is the electron charge, N is the initial number of QPs (N "2.3;10 for absorption of a 6 keV X-ray in Nb), q and q are the e!ective life times of QPs. Coe$cients C and D are given by the formulae: C"P [(1!k)#P q /(q !q )] and D"P [k!P q /(q !q )], (2) where k is the fraction of QPs generated in the electrode 2 due to phonon transfer from the electrode 1; P "q /q and P "q /q , where 2 2 q is the e!ective tunnelling time. Expressions (1) 2G and (2) were obtained from Rothwarf}Taylor equations under the assumption that the redistribution of QPs between the electrodes due to phonon exchange takes place in the initial moment. Back tunnelling was accounted for in the "rst-order approximation on the parameters P and P . Ap plicability of this approximation is justi"ed by low barrier transparency of investigated samples. The exact solution of Rothwarf}Taylor equations for QP multitunneling without considering phonon exchange is given in Ref. [6]. For A-type STJs it was found that pulses of L electrode could be "tted with one rise time exponent only (Fig. 3). Pulses of the H electrode comprised two exponents, the contribution of the second exponent being unexpectedly high (up to 70%).
Fig. 3. Shape of the pulses for A-type STJs. Upper plot } for L electrode and lower plot } for H electrode. The red lines are results of two exponent "tting procedure.
From Eq. (2) it follows that when absorption occurs in H electrode about 15% of QPs appear in L electrode due to 2*-phonon transfer and they contribute up to 70% of the signal. For low bias voltages < 4"* !* "/e bipolar pulses of H elec trode were observed (Fig. 4). They originate from the negative current from the H electrode superimposed on the more prolonged positive current from the L electrode. This latter current is produced by QPs, which have arisen from the phonon transfer. These results are considered in more detail in Ref. [3]. Pulse shape analysis for B-type STJs revealed that signals from both electrodes could be "tted with only one exponent. Rise times are almost the same (q "0.32 ls and q "0.37 ls). Absence of * & di!erence between rise time of the pulses, similarity of pulse-height spectra and similarity of Q and * Q dependencies on bias voltage were rather & SECTION I.
22
V.A. Andrianov et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 19}22
Fig. 4. The anomalous bipolar pulse from H electrode of A-type STJ (< "0.09 mV). The black line is the "tting with Eq. (1), red lines are contributions from each electrode.
surprising because the structure and properties of B-sample electrodes are very di!erent. In order to analyse the signal dependence on bias voltage < the expression that takes into account multiple tunnelling [4] and phonon exchange was used: Q "eN (P #R P )/(1!R R ) #keN (P #R P )/(1!R R ), (3) where P and P are reduced charge tunnelling probabilities, R and R are reduced QP tunnelling probabilities. QP transfer is determined by the sum of two channels of tunnelling: electron and hole one; charge transfer is determined by the di!erence of these two channels; k accounts for QPs transfer to the opposite electrode due to phonon exchange. For low gap proximity electrode of B-type junctions k"0 because the energy of 2* -phonons is * insu$cient for pair breaking in the H electrode. Multiple tunnelling is also weak because tunnelling probability from the H electrode P is close to zero & (see below). So the charge from the L electrode is proportional to the tunneling probability Q "eN P in accordance with the experimental * * data (Fig. 2B, curve L).
The contribution of the phonon exchange to the signal from the H electrode of B-type STJs is essential. Phonons from this electrode with an energy 2* and subgap phonons with an energy exceeding & 2* can produce pair breaking in the proximity (L) * electrode. Analysis of experimental dependence Q on the basis of (3) showed that the phonon & transfer coe$cient k"0.59. In the calculation the dependence of the P/R ratio on < from Ref. [2] was used. Calculated tunnelling probability dependence for the H electrode P (< ) is described by & bipolar curve in accordance with theoretical prediction but its contribution to the signal does not exceed 10% (Fig. 2B, curve P ). The main contri& bution to the signal is due to tunnelling of QPs produced in the proximity ¸ electrode because of the transfer of 2* and subgap phonons. Taking & this into account one can easily understand the similarity of signals from both electrodes (Fig. 2B) and similarity of their pulse-height spectra (Fig. 1B). It should be noted that qualitatively similar e!ects were observed for Nb/AlO /Al STJs in Ref. [5]. V Our results show that phonon exchange can lead to a substantial redistribution of QPs between electrodes. The most pronounced e!ects are expected for asymmetric STJs for signals from the higher gap electrodes. In this case, the 2* and subgap phonons are involved in phonon exchange process.
References [1] O.J. Luiten et al., Quasiparticle di!usion and losses in Nb/AlO and Ta/AlO superconducting tunnel junction V V photon detectors, in: S. Cooper (Ed.), Proceedings of the 7th International Workshop Low Temp. Detectors (LTD-7), Munich, 1997, pp. 25}27. [2] A.A. Golubov et al., Phys. Rev. B 49 (18) (1994) 12 953}12 968. [3] V.A. Andrianov et al., Phys. Solid State 41 (7) (1999) 1063}1069. [4] D.J. Goldie et al., Appl. Phys. Lett. 64 (23) (1994) 3169}3171. [5] D.J. Goldie et al., Supercond. Sci. Technol. 6 (1993) 203}208. [6] A. Steele et al., Nucl. Instr. and Meth. A 444 (2000) 8. These Proceedings.