This letter presents experiments on junctions fabricated by a technique that enables the use of high-quality aluminum oxide tunnel barriers with normal metal electrodes at low temperatures. Inverse proximity effect is applied to diminish the superconductivity of an aluminum dot through a clean lateral connection to a normal metal electrode. To demonstrate the effectiveness of this method, fully normal-state single electron transistors (SETs) and normal metal-insulator-superconductor (NIS) junctions applying proximized Aljunctions were fabricated. The transport characteristics of the junctions were similar to those obtained from standard theoreticalmodels of regular SETs and NIS junctions.Peer reviewe

Koski, J. V.

Meschke, M.

Pekola, Jukka P.

Peltonen, J. T.

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This reprint may differ from the original in pagination and typographic detail.
Author(s): Koski, J. V. & Peltonen, J. T. & Meschke, M. & Pekola, Jukka
Title: Laterally proximized aluminum tunnel junctions
Year: 2011
Version: Final published version
Please cite the original version:
Koski, J. V. & Peltonen, J. T. & Meschke, M. & Pekola, Jukka. 2011. Laterally proximized
aluminum tunnel junctions. Applied Physics Letters. Volume 98, Issue 20. P. 203501/1-3.
ISSN 0003-6951 (printed). DOI: 10.1063/1.3590922.
Rights: © 2011 American Institute of Physics. This article may be downloaded for personal use only. Any other use
requires prior permission of the author and the American Institute of Physics. The following article appeared
in Applied Physics Letters and may be found at
http://scitation.aip.org/content/aip/journal/apl/98/20/10.1063/1.3590922
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Powered by TCPDF (www.tcpdf.org)
Laterally proximized aluminum tunnel junctions
J. V. Koski, J. T. Peltonen, M. Meschke, and J. P. Pekola 
 
Citation: Applied Physics Letters 98, 203501 (2011); doi: 10.1063/1.3590922 
View online: http://dx.doi.org/10.1063/1.3590922 
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/98/20?ver=pdfcov 
Published by the AIP Publishing 
 
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AIP Conf. Proc. 850, 983 (2006); 10.1063/1.2355035 
 
Aluminum Superconducting Tunnel Junction as X-ray detector: Technological aspects and phonon
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130.233.216.27 On: Tue, 19 May 2015 07:21:48
Laterally proximized aluminum tunnel junctions
J. V. Koski,a J. T. Peltonen, M. Meschke, and J. P. Pekola
Low Temperature Laboratory, Aalto University, P.O. Box 13500, FI-00076 AALTO, Finland
Received 15 April 2011; accepted 20 April 2011; published online 16 May 2011
This letter presents experiments on junctions fabricated by a technique that enables the use of
high-quality aluminum oxide tunnel barriers with normal metal electrodes at low temperatures.
Inverse proximity effect is applied to diminish the superconductivity of an aluminum dot through
a clean lateral connection to a normal metal electrode. To demonstrate the effectiveness of this
method, fully normal-state single electron transistors SETs and normal metal-insulator-
superconductor NIS junctions applying proximized Al junctions were fabricated. The transport
characteristics of the junctions were similar to those obtained from standard theoretical models of
regular SETs and NIS junctions. © 2011 American Institute of Physics. doi:10.1063/1.3590922
An important element in nanoelectronic circuits is a
tunnel barrier between two leads, with applications ranging
from superconducting qubits1 and quantum metrology
devices2 to electronic refrigeration by hybrid normal metal-
insulator-superconductor NIS junctions.3–6 A standard ma-
terial for yielding high-quality junctions is aluminum due to
rapid oxidation of its surface into a thin and stable layer of
insulating aluminum oxide AlOx under oxygen exposure.
Aluminum is superconducting below the critical temperature
TC=1.2 K in bulk and up to 2.7 K in thin films.7 High-
quality nanoscale junctions are typically fabricated in a
single vacuum cycle by shadow evaporation of metals at
multiple angles through a suspended mask,8 combined with
in situ oxidation. Depositing a layer of normal or supercon-
ducting material on top of oxidized Al forms a NIS or a
superconductor-insulator-superconductor SIS junction.
Regrettably, fabrication of low temperature normal
metal-insulator-normal metal NIN junctions or NIS junc-
tions with non-Al superconductor is not straightforward. The
latter would be beneficial in terms of controlling the TC
through the choice of material: a higher TC would benefit for
example a current standard based on the hybrid SINIS
turnstile2 while a lower TC would be efficient for low tem-
perature NIS coolers,3 where the optimal cooling power is at
T0.45TC. In order to use Al for either NIN junctions or
NIS junctions with non-Al superconductor, its superconduc-
tivity must be suppressed. Alternatively, aluminum can be
isotropically deposited as a thin layer on top of the N elec-
trode, after which it is thoroughly oxidized before depositing
the counterelectrode.9 When applied with shadow evapora-
tion the latter technique, however, requires caution to ensure
uniform Al coverage to avoid short circuits.
The aluminum superconductivity may be suppressed by
various means, such as by applying a magnetic field. How-
ever, magnetic fields are not always desirable when combin-
ing NIN and NIS junctions on a single circuit, for example,
when combining normal-state single electron transistors
SETs with NIS thermometers and coolers.10 Although it is
possible to have both NIN and NIS Al-based junctions in a
single circuit by controlling the thicknesses and thus the
critical fields of Al layers,11 in some applications external
fields cannot be tolerated. Another approach is to dope Al
with, e.g., manganese impurities to control or diminish the
TC while retaining the BCS density of states6,12 but the junc-
tion quality could be compromised,13 and it is undesirable to
have both manganese-doped and pure superconducting ma-
terials in the same evaporator due to the risk of contamina-
tion. Also the use of inverse proximity effect arising in a
normal metal-superconductor NS bilayer is a widely used
technique for modifying the superconductor TC:14,15 the nor-
mal metal gains a superconducting character while the super-
conductor gains normal metal features at a short distance
from the interface.14,16
This letter introduces a technique for suppressing the
Al superconductivity in small tunnel junctions by utilizing
the inverse proximity effect in lateral direction. According
to the theory of diffusive, inhomogeneous proximity
superconductivity,17 the superconductivity is fully sup-
pressed in a NSN structure with infinite-size N reservoirs,
when the length of the quasi-one-dimensional S-material
wire L0.
18,19 By symmetry, for a NS structure the sup-
pression is achieved for L0 /2. Utilizing our fabrication
process the Al film superconducting coherence length is typi-
cally 0100 nm,20,21 yielding L150 nm. When taking
into account the nonideal interface, a short Al wire with L
100 nm, proximized by a long normal metal lead, forms a
high-quality NIN or NIS tunnel junction.
The proximized Al junctions were utilized in the fabri-
cation of several normal-state SET samples with multiangle
shadow evaporation, for which the mask layout is shown in
Fig. 1b. Figure 1a sketches a cross-section of a prox-
imized Al junction that is used in the sample. The samples
were fabricated onto an oxidized silicon substrate covered by
a resist mask consisting of 500 nm thick copolymer layer at
bottom and 50–70 nm thick polymethyl methacrylate layer
on top, patterned by electron beam lithography. The fabrica-
tion process had the following order, illustrated in Fig. 1a:
first a 15 nm layer of aluminum at the angle =0° is evapo-
rated to form two small Al dots 400 nm apart, directly fol-
lowed by a thick 45 nm layer of copper at =30° to par-
tially cover the dot with a proximizing normal metal lead.
Third, the remaining uncovered aluminum is oxidized in the
evaporation chamber, after which the SET is formed by de-
positing a 30 nm copper layer at =−30° to contact the two
Al dots.aElectronic mail: jvkoski@cc.hut.fi.
APPLIED PHYSICS LETTERS 98, 203501 2011
0003-6951/2011/9820/203501/3/$30.00 © 2011 American Institute of Physics98, 203501-1
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The suppression of the Al dot superconductivity was ex-
amined by measuring the current ISET as a function of ap-
plied bias and gate voltages, Vbias and Vg, respectively, ac-
cording to the circuit diagram in Fig. 1c. The samples were
measured in a 3He– 4He dilution refrigerator22 with a base
temperature close to 40 mK. The results for the SET of Fig.
1c with highest charging energy EC525 eV are shown
in Fig. 2. All of the measured structures showed similar be-
havior. Figures 2a and 2b show the range over which ISET
varies as Vg is swept, fit by a standard SET model curve
based on the orthodox theory,23 with the inclusion of asym-
metric junction resistances, R10.3R2. The linearity at low
bias voltages for maximum current indicates that the leads in
the SET are normal, for otherwise there would be a gate-
independent zero-current region for eVbias2.2 This is
further supported by Fig. 2c, which shows ISET as a func-
tion of both Vbias and Vg, demonstrating the Coulomb dia-
monds characteristic to a normal SET.
Another more demanding test of the quality of a prox-
imized junction is to utilize them in a NIS junction. The
magnitude of subgap features, such as supercurrent through
the junction, is an excellent indicator for determining the
superconductivity of the Al dot. Also the differential conduc-
tance of the junction at low T is directly proportional to the
superconductor density of states if the counterelectrode is at
normal state. The NIS junction samples are fabricated like
the NIN junctions of the SET described earlier, with Al as the
S lead and copper as the N lead, yet these could be replaced
by other normal and superconducting metals.
Several NIS junctions were fabricated simultaneously
with various L0, the length of Al dot uncovered by the nor-
mal electrode. The NS contact and tunnel junction overlap
areas were kept constant. The inset of Fig. 3a shows a
scanning electron micrograph SEM image of the structure
with the shortest L095 nm. The measured normal-state re-
sistances R of the junctions were in the range of 35–75 k
and the zero temperature superconductor energy gap of the
S–Al was 220 eV. Figure 3 shows the measured
current-voltage IV characteristics at T=65 mK. Addition-
ally, Fig. 4 shows the differential conductance of several
samples obtained by numerical differentiation of the IV data
together with a theoretical fit for a standard NIS junction.
Finally, the IV characteristics of the L095 nm sample in
the temperature range of T=43–675 mK are plotted in Fig.
5, together with theoretical curves using a phenomenological
Dynes parameter24 	1.5
10−4 in the Al quasiparticle den-
sity of states.
In these figures the dependence on the length of the Al
dot is clearly visible: with L0200 nm they appear to have
mixed NIS and SIS junction properties with notable super-
current peaks, but as the dot length approaches 100 nm, both


2nd Cu 4th Cu
Substrate
1st Al
3rd AlOx
200 nm
Cu
Al
Cu
(a) (b)
(c)
ISET
200 nm
FIG. 1. a Sketch of a cross-sectional view and the fabrication order of the
NIN junction with copper leads. The aluminum is proximized via a clean
contact to the left copper electrode. b Sketch of the NININ SET shadow
mask layout, including the positions of evaporated metals at different angles.
c SEM of a normal-state SET sample, integrated to the measurement cir-
cuit diagram.
5 0 0.5 1
0
200
400
600
800
Vbias (mV)
I S
ET
(p
A
)
0.1 0.05 0 0.05 0.1
30
20
10
0
10
20
30
60 40
200
20
0.5
0
0.5
400
200
0
200
400
Vg (mV)
ISET (pA)
Vbias(mV)
(a) (b)
(c)
FIG. 2. Color online a Measured current ISET through the NININ SET
with normal-state resistance R=R1+R21.2 M and charging energy EC
525 eV at T=63 mK. The gate voltage Vg was repeatedly swept over
several gate periods while the bias voltage Vbias was increased slowly. The
boundary lines are theoretical values for normal SET minimum and maxi-
mum currents. b An enlarged view of a. The maximum of ISET has a
linear dependence on Vbias around Vbias=0. c ISET as a function of Vbias and
Vg. The coulomb diamonds of a fully normal SET are clearly visible.
0 1 2
0
0.5
1
1.5
eV / Δ
eI
R
/
Δ
0 0.5 1 1.5 2
10
4
10
3
10
2
10
1
10
0
eV /Δ
200 nm
195 nm
145 nm
140 nm
115 nm
105 nm
95 nm
95 nm
(a) (b)
L 0
L =0
FIG. 3. Color online IV characteristics of several of the measured samples
with proximized Al–AlOx–Al NIS junctions on a linear and b semiloga-
rithmic scales. A clear sign of supercurrent is visible in the samples with
longest Al dots. The lengths L0 are indicated in the figure. As L0 decreases,
the supercurrent peak gradually diminishes with only a minor zero-voltage
step remaining for the junctions with the smallest L0. Inset: SEM micro-
graph of the sample with the shortest Al dot, indicating the length L0
95 nm. The copper lead to the left proximizes the Al dot in the center,
which is connected to the Al lead to the right via a tunnel barrier. Due to the
limitations in the mask design, the 20 nm thick Al lead was fabricated first
and oxidized, before depositing the Al dot on top. However, the Al is prox-
imized in the same way as previously.
203501-2 Koski et al. Appl. Phys. Lett. 98, 203501 2011
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130.233.216.27 On: Tue, 19 May 2015 07:21:48
the IV curve and the differential conductance resemble in-
creasingly those of a regular NIS junction. The results sug-
gest that proximized Al tunnel junctions provide a valid tool
for fabricating NIS junctions, where the superconductor is
not necessarily aluminum. Such structures have been a chal-
lenge to fabricate by shadow evaporation till now.
Concluding, the method of forming tunnel junctions with
the help of laterally proximized Al was introduced. These
junctions were implemented in a fully normal SET and indi-
vidual NIS junction configurations. The measured IV char-
acteristics would indicate that the presented method is likely
to allow shadow fabrication of high-quality NIN junctions,
as well as NIS junctions with non-Al superconductor.
We acknowledge financial support from the European
Community’s FP7 Programme under Grant Agreement Nos.
228464 MICROKELVIN, Capacities Specific Programme
and 218783 SCOPE. We thank T. T. Heikkilä and P. Vir-
tanen for discussions.
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0 1 2 3
0
1
2
3
4
eV / Δ
R
d
I/
d
V
200 nm
140 nm
105 nm
model
L0
FIG. 4. Color online Differential conductance of some proximized Al NIS
junctions as a function of voltage, obtained by numerical differentiation of
the IV characteristics. A fit based on the standard theoretical model for a NIS
junction is drawn in black. A transition from SIS-type to a NIS-type dI /dV
dependence occurs with decreasing L0. The samples with the shortest prox-
imized Al dots behave as standard NIS junctions.
0 0.5 1 1.5
10
10
5
10
4
10
3
10
2
10
1
10
0
eV / Δ
1 0 1
1
0.8
0.6
0.4
0.2
0
0.2
0.4
0.6
0.8
1
eV / Δ
eI
R
/
Δ
(a) (b)
FIG. 5. Color online IV characteristics of the NIS junction with L0
=95 nm at various temperatures dashed lines, together with a theoretical
fit of the current solid lines on a linear and b semilogarithmic scales.
There is only a minor deviation at low V and T where the Dynes parameter
does not explain the current step formation at low V and T, however other-
wise the IV characteristics are very similar to standard NIS junctions.
203501-3 Koski et al. Appl. Phys. Lett. 98, 203501 2011
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English

Laterally proximized aluminum tunnel junctions

Aaltodoc Publication Archive

This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Koski, J. V. & Peltonen, J. T. & Meschke, M. & Pekola, JukkaTitle: Laterally proximized aluminum tunnel junctionsYear: 2011Version: Final published versionPlease cite the original version:Koski, J. V. & Peltonen, J. T. & Meschke, M. & Pekola, Jukka. 2011. Laterally proximizedaluminum tunnel junctions. Applied Physics Letters. Volume 98, Issue 20. P. 203501/1-3.ISSN 0003-6951 (printed). DOI: 10.1063/1.3590922.Rights: © 2011 American Institute of Physics. This article may be downloaded for personal use only. Any other userequires prior permission of the author and the American Institute of Physics. The following article appearedin Applied Physics Letters and may be found athttp://scitation.aip.org/content/aip/journal/apl/98/20/10.1063/1.3590922All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Laterally proximized aluminum tunnel junctionsJ. V. Koski, J. T. Peltonen, M. Meschke, and J. P. Pekola  Citation: Applied Physics Letters 98, 203501 (2011); doi: 10.1063/1.3590922 View online: http://dx.doi.org/10.1063/1.3590922 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/98/20?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Planar S-(S/F)-S Josephson junctions induced by the inverse proximity effect Appl. Phys. Lett. 95, 062501 (2009); 10.1063/1.3200226  Direct Observation Of The Proximity Effect In The N Layer In A Multi‐Terminal SINIS Josephson Junction AIP Conf. Proc. 850, 983 (2006); 10.1063/1.2355035  Aluminum Superconducting Tunnel Junction as X-ray detector: Technological aspects and phonondecoupling from the substrate AIP Conf. Proc. 605, 157 (2002); 10.1063/1.1457618  Strong quasiparticle trapping In A 6×6 array of vanadium-aluminum superconducting tunnel junctions AIP Conf. Proc. 605, 59 (2002); 10.1063/1.1457595  The effect of thermal annealing on the properties of Al–AlO x – Al single electron tunneling transistors J. Appl. Phys. 90, 2528 (2001); 10.1063/1.1389077    This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Tue, 19 May 2015 07:21:48Laterally proximized aluminum tunnel junctionsJ. V. Koski,a J. T. Peltonen, M. Meschke, and J. P. PekolaLow Temperature Laboratory, Aalto University, P.O. Box 13500, FI-00076 AALTO, FinlandReceived 15 April 2011; accepted 20 April 2011; published online 16 May 2011This letter presents experiments on junctions fabricated by a technique that enables the use ofhigh-quality aluminum oxide tunnel barriers with normal metal electrodes at low temperatures.Inverse proximity effect is applied to diminish the superconductivity of an aluminum dot througha clean lateral connection to a normal metal electrode. To demonstrate the effectiveness of thismethod, fully normal-state single electron transistors SETs and normal metal-insulator-superconductor NIS junctions applying proximized Al junctions were fabricated. The transportcharacteristics of the junctions were similar to those obtained from standard theoretical models ofregular SETs and NIS junctions. © 2011 American Institute of Physics. doi:10.1063/1.3590922An important element in nanoelectronic circuits is atunnel barrier between two leads, with applications rangingfrom superconducting qubits1 and quantum metrologydevices2 to electronic refrigeration by hybrid normal metal-insulator-superconductor NIS junctions.3–6 A standard ma-terial for yielding high-quality junctions is aluminum due torapid oxidation of its surface into a thin and stable layer ofinsulating aluminum oxide AlOx under oxygen exposure.Aluminum is superconducting below the critical temperatureTC=1.2 K in bulk and up to 2.7 K in thin films.7 High-quality nanoscale junctions are typically fabricated in asingle vacuum cycle by shadow evaporation of metals atmultiple angles through a suspended mask,8 combined within situ oxidation. Depositing a layer of normal or supercon-ducting material on top of oxidized Al forms a NIS or asuperconductor-insulator-superconductor SIS junction.Regrettably, fabrication of low temperature normalmetal-insulator-normal metal NIN junctions or NIS junc-tions with non-Al superconductor is not straightforward. Thelatter would be beneficial in terms of controlling the TCthrough the choice of material: a higher TC would benefit forexample a current standard based on the hybrid SINISturnstile2 while a lower TC would be efficient for low tem-perature NIS coolers,3 where the optimal cooling power is atT0.45TC. In order to use Al for either NIN junctions orNIS junctions with non-Al superconductor, its superconduc-tivity must be suppressed. Alternatively, aluminum can beisotropically deposited as a thin layer on top of the N elec-trode, after which it is thoroughly oxidized before depositingthe counterelectrode.9 When applied with shadow evapora-tion the latter technique, however, requires caution to ensureuniform Al coverage to avoid short circuits.The aluminum superconductivity may be suppressed byvarious means, such as by applying a magnetic field. How-ever, magnetic fields are not always desirable when combin-ing NIN and NIS junctions on a single circuit, for example,when combining normal-state single electron transistorsSETs with NIS thermometers and coolers.10 Although it ispossible to have both NIN and NIS Al-based junctions in asingle circuit by controlling the thicknesses and thus thecritical fields of Al layers,11 in some applications externalfields cannot be tolerated. Another approach is to dope Alwith, e.g., manganese impurities to control or diminish theTC while retaining the BCS density of states6,12 but the junc-tion quality could be compromised,13 and it is undesirable tohave both manganese-doped and pure superconducting ma-terials in the same evaporator due to the risk of contamina-tion. Also the use of inverse proximity effect arising in anormal metal-superconductor NS bilayer is a widely usedtechnique for modifying the superconductor TC:14,15 the nor-mal metal gains a superconducting character while the super-conductor gains normal metal features at a short distancefrom the interface.14,16This letter introduces a technique for suppressing theAl superconductivity in small tunnel junctions by utilizingthe inverse proximity effect in lateral direction. Accordingto the theory of diffusive, inhomogeneous proximitysuperconductivity,17 the superconductivity is fully sup-pressed in a NSN structure with infinite-size N reservoirs,when the length of the quasi-one-dimensional S-materialwire L0.18,19 By symmetry, for a NS structure the sup-pression is achieved for L0 /2. Utilizing our fabricationprocess the Al film superconducting coherence length is typi-cally 0100 nm,20,21 yielding L150 nm. When takinginto account the nonideal interface, a short Al wire with L100 nm, proximized by a long normal metal lead, forms ahigh-quality NIN or NIS tunnel junction.The proximized Al junctions were utilized in the fabri-cation of several normal-state SET samples with multiangleshadow evaporation, for which the mask layout is shown inFig. 1b. Figure 1a sketches a cross-section of a prox-imized Al junction that is used in the sample. The sampleswere fabricated onto an oxidized silicon substrate covered bya resist mask consisting of 500 nm thick copolymer layer atbottom and 50–70 nm thick polymethyl methacrylate layeron top, patterned by electron beam lithography. The fabrica-tion process had the following order, illustrated in Fig. 1a:first a 15 nm layer of aluminum at the angle =0° is evapo-rated to form two small Al dots 400 nm apart, directly fol-lowed by a thick 45 nm layer of copper at =30° to par-tially cover the dot with a proximizing normal metal lead.Third, the remaining uncovered aluminum is oxidized in theevaporation chamber, after which the SET is formed by de-positing a 30 nm copper layer at =−30° to contact the twoAl dots.aElectronic mail: jvkoski@cc.hut.fi.APPLIED PHYSICS LETTERS 98, 203501 20110003-6951/2011/9820/203501/3/$30.00 © 2011 American Institute of Physics98, 203501-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Tue, 19 May 2015 07:21:48The suppression of the Al dot superconductivity was ex-amined by measuring the current ISET as a function of ap-plied bias and gate voltages, Vbias and Vg, respectively, ac-cording to the circuit diagram in Fig. 1c. The samples weremeasured in a 3He– 4He dilution refrigerator22 with a basetemperature close to 40 mK. The results for the SET of Fig.1c with highest charging energy EC525 eV are shownin Fig. 2. All of the measured structures showed similar be-havior. Figures 2a and 2b show the range over which ISETvaries as Vg is swept, fit by a standard SET model curvebased on the orthodox theory,23 with the inclusion of asym-metric junction resistances, R10.3R2. The linearity at lowbias voltages for maximum current indicates that the leads inthe SET are normal, for otherwise there would be a gate-independent zero-current region for eVbias2.2 This isfurther supported by Fig. 2c, which shows ISET as a func-tion of both Vbias and Vg, demonstrating the Coulomb dia-monds characteristic to a normal SET.Another more demanding test of the quality of a prox-imized junction is to utilize them in a NIS junction. Themagnitude of subgap features, such as supercurrent throughthe junction, is an excellent indicator for determining thesuperconductivity of the Al dot. Also the differential conduc-tance of the junction at low T is directly proportional to thesuperconductor density of states if the counterelectrode is atnormal state. The NIS junction samples are fabricated likethe NIN junctions of the SET described earlier, with Al as theS lead and copper as the N lead, yet these could be replacedby other normal and superconducting metals.Several NIS junctions were fabricated simultaneouslywith various L0, the length of Al dot uncovered by the nor-mal electrode. The NS contact and tunnel junction overlapareas were kept constant. The inset of Fig. 3a shows ascanning electron micrograph SEM image of the structurewith the shortest L095 nm. The measured normal-state re-sistances R of the junctions were in the range of 35–75 kand the zero temperature superconductor energy gap of theS–Al was 220 eV. Figure 3 shows the measuredcurrent-voltage IV characteristics at T=65 mK. Addition-ally, Fig. 4 shows the differential conductance of severalsamples obtained by numerical differentiation of the IV datatogether with a theoretical fit for a standard NIS junction.Finally, the IV characteristics of the L095 nm sample inthe temperature range of T=43–675 mK are plotted in Fig.5, together with theoretical curves using a phenomenologicalDynes parameter24 	1.510−4 in the Al quasiparticle den-sity of states.In these figures the dependence on the length of the Aldot is clearly visible: with L0200 nm they appear to havemixed NIS and SIS junction properties with notable super-current peaks, but as the dot length approaches 100 nm, both2nd Cu 4th CuSubstrate1st Al3rd AlOx200 nmCuAlCu(a) (b)(c)ISET200 nmFIG. 1. a Sketch of a cross-sectional view and the fabrication order of theNIN junction with copper leads. The aluminum is proximized via a cleancontact to the left copper electrode. b Sketch of the NININ SET shadowmask layout, including the positions of evaporated metals at different angles.c SEM of a normal-state SET sample, integrated to the measurement cir-cuit diagram.5 0 0.5 10200400600800Vbias (mV)I SET(pA)0.1 0.05 0 0.05 0.1302010010203060 40200200.500.54002000200400Vg (mV)ISET (pA)Vbias(mV)(a) (b)(c)FIG. 2. Color online a Measured current ISET through the NININ SETwith normal-state resistance R=R1+R21.2 M and charging energy EC525 eV at T=63 mK. The gate voltage Vg was repeatedly swept overseveral gate periods while the bias voltage Vbias was increased slowly. Theboundary lines are theoretical values for normal SET minimum and maxi-mum currents. b An enlarged view of a. The maximum of ISET has alinear dependence on Vbias around Vbias=0. c ISET as a function of Vbias andVg. The coulomb diamonds of a fully normal SET are clearly visible.0 1 200.511.5eV / ΔeIR/Δ0 0.5 1 1.5 2104103102101100eV /Δ200 nm195 nm145 nm140 nm115 nm105 nm95 nm95 nm(a) (b)L 0L =0FIG. 3. Color online IV characteristics of several of the measured sampleswith proximized Al–AlOx–Al NIS junctions on a linear and b semiloga-rithmic scales. A clear sign of supercurrent is visible in the samples withlongest Al dots. The lengths L0 are indicated in the figure. As L0 decreases,the supercurrent peak gradually diminishes with only a minor zero-voltagestep remaining for the junctions with the smallest L0. Inset: SEM micro-graph of the sample with the shortest Al dot, indicating the length L095 nm. The copper lead to the left proximizes the Al dot in the center,which is connected to the Al lead to the right via a tunnel barrier. Due to thelimitations in the mask design, the 20 nm thick Al lead was fabricated firstand oxidized, before depositing the Al dot on top. However, the Al is prox-imized in the same way as previously.203501-2 Koski et al. Appl. Phys. Lett. 98, 203501 2011 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Tue, 19 May 2015 07:21:48the IV curve and the differential conductance resemble in-creasingly those of a regular NIS junction. The results sug-gest that proximized Al tunnel junctions provide a valid toolfor fabricating NIS junctions, where the superconductor isnot necessarily aluminum. Such structures have been a chal-lenge to fabricate by shadow evaporation till now.Concluding, the method of forming tunnel junctions withthe help of laterally proximized Al was introduced. Thesejunctions were implemented in a fully normal SET and indi-vidual NIS junction configurations. The measured IV char-acteristics would indicate that the presented method is likelyto allow shadow fabrication of high-quality NIN junctions,as well as NIS junctions with non-Al superconductor.We acknowledge financial support from the EuropeanCommunity’s FP7 Programme under Grant Agreement Nos.228464 MICROKELVIN, Capacities Specific Programmeand 218783 SCOPE. We thank T. T. Heikkilä and P. Vir-tanen for discussions.1Yu. Makhlin, G. Schön, and A. Shnirman, Rev. Mod. Phys. 73, 3572001.2J. P. Pekola, J. J. Vartiainen, M. Möttönen, O.-P. Saira, M. Meschke, andD. V. Averin, Nat. Phys. 4, 120 2008.3F. Giazotto, T. T. Heikkilä, A. Luukanen, A. M. Savin, and J. P. Pekola,Rev. Mod. Phys. 78, 217 2006.4M. Nahum, T. M. Eiles, and J. M. Martinis, Appl. Phys. Lett. 65, 31231994.5M. M. Leivo, J. P. Pekola, and D. V. Averin, Appl. Phys. Lett. 68, 19961996.6A. M. Clark, A. Williams, S. T. Ruggiero, M. L. van den Berg, and J. N.Ullom, Appl. Phys. Lett. 84, 625 2004.7P. W. Adams, Phys. Rev. Lett. 92, 067003 2004.8G. J. Dolan, Appl. Phys. Lett. 31, 337 1977.9S. Guéron, H. Pothier, N. O. Birge, D. Esteve, and M. H. Devoret, Phys.Rev. Lett. 77, 3025 1996.10B. Kubala, J. König, and J. P. Pekola, Phys. Rev. Lett. 100, 0668012008.11T. Greibe, M. P. V. Stenberg, C. M. Wilson, T. Bauch, V. S. Shumeiko, andP. Delsing, Phys. Rev. Lett. 106, 097001 2011.12G. C. O’Neil, D. Schmidt, N. A. Miller, J. N. Ullom, A. Williams, G. B.Arnold, and S. T. Ruggiero, Phys. Rev. Lett. 100, 056804 2008.13G. C. O’Neil, D. R. Schmidt, N. A. Tomlin, and J. N. Ullom, J. Appl.Phys. 107, 093903 2010.14A. I. Buzdin, Rev. Mod. Phys. 77, 935 2005.15A. K. Gupta, L. Crétinon, N. Moussy, B. Pannetier, and H. Courtois, Phys.Rev. B 69, 104514 2004.16B. Pannetier and H. Courtois, J. Low Temp. Phys. 118, 599 2000.17W. Belzig, F. K. Wilhelm, C. Bruder, and G. Schön, Superlattices Micro-struct. 25, 1251 1999.18Z. Radović, M. Ledvij, L. Dobrosavljević-Grujić, A. I. Buzdin, and J. R.Clem, Phys. Rev. B 44, 759 1991.19W. Liniger, J. Low Temp. Phys. 93, 1 1993.20H. Courtois, M. Meschke, J. T. Peltonen, and J. P. Pekola, Phys. Rev. Lett.101, 067002 2008.21J. T. Peltonen, P. Virtanen, M. Meschke, J. V. Koski, T. T. Heikkilä, and J.P. Pekola, Phys. Rev. Lett. 105, 097004 2010.22J. P. Pekola and J. P. Kauppinen, Cryogenics 34, 843 1994.23D. V. Averin and K. K. Likharev, J. Low Temp. Phys. 62, 345 1986.24R. C. Dynes, J. P. Garno, G. B. Hertel, and T. P. Orlando, Phys. Rev. Lett.53, 2437 1984.0 1 2 301234eV / ΔRdI/dV200 nm140 nm105 nmmodelL0FIG. 4. Color online Differential conductance of some proximized Al NISjunctions as a function of voltage, obtained by numerical differentiation ofthe IV characteristics. A fit based on the standard theoretical model for a NISjunction is drawn in black. A transition from SIS-type to a NIS-type dI /dVdependence occurs with decreasing L0. The samples with the shortest prox-imized Al dots behave as standard NIS junctions.0 0.5 1 1.510105104103102101100eV / Δ1 0 110.80.60.40.200.20.40.60.81eV / ΔeIR/Δ(a) (b)FIG. 5. Color online IV characteristics of the NIS junction with L0=95 nm at various temperatures dashed lines, together with a theoreticalfit of the current solid lines on a linear and b semilogarithmic scales.There is only a minor deviation at low V and T where the Dynes parameterdoes not explain the current step formation at low V and T, however other-wise the IV characteristics are very similar to standard NIS junctions.203501-3 Koski et al. Appl. Phys. Lett. 98, 203501 2011 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:130.233.216.27 On: Tue, 19 May 2015 07:21:48

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Laterally proximized aluminum tunnel junctions

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