Josephson coupling in junctions made of monolayer graphene on SiC

2.5 MB·

by  B. Jouault

Noname manuscript No.
(will be inserted by the editor)
Josephson coupling in junctions made of monolayer
graphene grown on SiC
B. Jouault1 · S. Charpentier2 · D. Massarotti3,4 · A. Michon5 · M.
Paillet1 · J.-R. Huntzinger1 · A. Tiberj1 · A. Zahab1 · T. Bauch2 · P.
6 Lucignano4 · A. Tagliacozzo6,4,7 · F. Lombardi2 · F. Tafuri4,3
1
0
2

r
a
M March 16,2016

5
Abstract Chemicalvapordepositionhasprovedtobe Keywords Graphene, Josephson effect, Silicon
1
successfulinproducinggraphenesamplesonsiliconcar- Carbide

] bide(SiC)homogeneousatthecentimeterscaleinterms
n
of Hall conductance quantization. Here we report on
o
c therealizationofco-planardiffusiveAl/monolayergra- 1 Introduction
- phene/ Al junctions on the same graphene sheet, with
r
p separations between the electrodes down to 200 nm. The remarkable electrical properties of graphene, in-
u
Robust Josephson coupling, as the magnetic pattern duced by the chiral nature of its charge carriers, rises
s
. of the critical current, has been measured for separa- big expectations presently, of increasing functionalities
t
a tions not larger than 300 nm. Transport properties are in hybrid superconducting quantum devices. Joseph-
m
reproducible on different junctions and indicate that son junctions in which the supercurrent flows through
- graphene on SiC substrates is a concrete candidate to normal-conducting graphene across closely spaced Su-
d
n providescalabilityofhybridJosephsongraphene/super- perconductors (S), have been realized up to now only
o conductor devices. with exfoliated graphene. Usually the graphene flakes
c
are deposited on a Si/SiO substrate[1,2,3,4,5,6,7,8].
[ 2
Eveniftheachievementsareformanyrespectsspec-

2 1
tacularatpresent,thegoalofrealizingwhateversimple
v Universit´e Montpellier-CNRS, Laboratoire Charles Coulomb
hybrid circuit with a number of these junctions on the
2 UMR5221,F-34095,Montpellier, France
5 same graphene sheet is a long way off, because of the
2
5 severe limits imposed by the present fabrication proto-
Chalmers University of Technology, SE-412 96 Go¨teborg,
4
Sweden cols. Every junction will have its own properties (and
0
story),withnochanceofcontrollingtheelectrodynam-
. 3
1
Dipartimento di Ingegneria Industriale e dell’Informazione, ics and its functionality, when engineering the full cir-
0
SecondaUniversita´ diNapoli, I-81031Aversa(CE),Italy cuit.
6
1 4 In this work we propose to use Chemical Vapor De-
: CNR-SPIN, Monte S. Angelo-Via Cintia, I-80126, Napoli, position (CVD) of graphene on large silicon carbide
v
Italy
i (SiC) areas, which does not require subsequent exfolia-
X 5 tion. We report on the characteristics of the fabricated
r CRHEA - Centre de Recherche sur l’H´et´ero´epitaxie et ses junctions, demonstrating that the high quality of the
a Applications, CNRS, rue Bernard Gr´egory, 06560 Valbonne,
devices,patternedonasinglegraphenesheet,pavesthe
France
way to finally taking on this limit, an utmost priority
6
in view of any future application.
Dipartimento di Fisica, Universita` di Napoli ”Federico II”,
Monte S.Angelo-ViaCintia,I-80126Napoli, Italy GraphenemadebyCVDonmetals[9]andgraphene
7 on SiC cover large surfaces and are homogeneous at
INFN, Laboratori Nazionali di Frascati, Via E.Fermi, Fras- the centimetre scale. Thus relatively simple lithogra-
cati,Italy phy processes allow to obtain thousands of devices on
2 B.Jouault1 etal.
thesamewafer[10].Themobilityofgraphenegrownby
CVDonmetalsismainlylimitedbydefectsintroduced
duringthetransferprocess[11],whereasthemobilityof
graphene on SiC is essentially controlled by the car-
rier concentration. In graphene on SiC, the mobility
can reach µ ≈ 45,000 cm2V−1s−1 at T=2 K and at
carrier concentrations 2 × 1010 cm−2[12]. This corre-
sponds to a mean free path l ≈ 80 nm. Additionally,
the quantum Hall effect of graphene on SiC is remark-
ably precise even at magnetic fields as low as B = 3.5
T[13,14], which makes graphene on SiC the material
of choice for the next generation of easy-to-use quan-
tum electrical standards. By combining Josephson ar-
rayssupportingrelativelylargemagneticfieldsandHall
bars made of graphene, complex metrological devices
like quantum current standards could be designed on
thesamewafer[15,16].Besides,theinterfaceand/orthe
surface of graphene on SiC can be modified. Various
atomic species like oxygen, hydrogen, calcium, can be
intercalated or deposited whereas it is well established
that intercalated graphite or few layer graphene be-
come superconductive[17,18,19]. Thus, CVD graphene
onSiCisalsoagoodplatformtostudygraphene-based
superconductivity[20].
2 Sample Characterization and preparation
Fig.1 (a)Ramanspectra(withSiCbackgroundsubtracted)
obtained on a given location (black line) and averaged over
Graphenehasbeengrownbypropane-hydrogenCVD[23] a 24 µm ×24 µm area (red line). (b) Histogram of the inte-
ontheSifaceofasemi-insulating0.24o-off-axis6H-SiC grated intensity AG of the G Raman peak, normalized over
the integrated intensity AHOPG of the G peak of a Highly
substrate purchased from TankeBlue, using conditions G
Oriented Pyrolytic Graphite (HOPG) reference. 98% of the
similartothoseusedinRef.[24].Thegrowthisdonein
surfacegivesavaluecloseto3.2%,whichisclosetoexpecta-
one minute, at a temperature of 1550 ◦C under a 800 tionsforamonolayergraphene(seeRef.[21]).(c)AngleRe-
mbar atmosphere made of argon, propane and hydro- solved Photoemission Spectroscopy (ARPES) Intensity map
of the sample surface. The red dashed line is the theoretical
gen (3 slm argon, 10 slm hydrogen, 5 sccm propane).
dispersion of the valence band of a monolayer graphene, ac-
The substrate is then cleaved to obtain several sam-
cordingtoRef.[22].Theinsetisacolormapofthegraphene
ples of total size 6 mm × 6 mm. Fig. 1 resumes the conduction band in the reciprocal space. The ARPES map
structural characteristics of the graphene samples. Ra- is taken along the direction indicated by a black solid line.
(d)Sketchand(e)scanning-electronmicrographofaL≈200
man spectra, obtained after the growth, are reported
nm short junction with artificial colors. The sample struc-
in panel (a). For 98% of the surface, the integrated in-
ture after lithography is indicated in the inset of panel (b)
tensity of the G peak, normalized with respect to a for completeness (total size 5 mm × 5 mm, green: SiC, blue:
graphite reference, is about 3.2%, see panel (b). This graphene,darkyellow:contacts).
is very close to the experimental value reported for a
graphene monolayer[21]. By contrast, bilayer graphene
wouldyieldabouttwicethisintensity[21].ARPESmea-
presenceoftheso-calledcarbon-richinterface.Here,the
surements,carriedoutontheCassiop´eebeamlineatthe
p-dopingistheexperimentalsignaturethatthiscarbon-
synchrotronradiationfacilitySOLEIL,revealsthatthe
rich interface has been neutralized and replaced by an
sample is a p-doped monolayer graphene, see panel(c).
hydrogenated interface between graphene and SiC[24].
On a similar growth, magnetotransport measurements
also revealed the half-integer quantum Hall effect spe- Van der Pauw and Hall measurements, done before
cific to monolayer graphene[24]. The ARPES measure- lithography on the 6 mm × 6 mm sample, give a car-
ments are done under ultra-high vacuum, after an ini- rier concentration p ≈ 6.2×1012 cm−2 and a mobility
tial degassing performed at 500 oC. On the silicon-face µ ≈ 1,100 cm2·V−1·s−1 at room temperature under
of SiC, the graphene is usually n-doped, because of the ambient atmosphere. This corresponds to a sheet resis-
Josephsoncouplinginjunctions madeofmonolayergraphenegrown onSiC 3
tivity ρ = 920 Ω, with a form factor f = 0.9 which
evidences a small anisotropy of conductivity linked to
the SiC steps. The low mobility is typical for graphene
on SiC which is not compensated. In principle, mobil-
ity higher by one order of magnitude can be obtained
by an appropriate gating which would reduce the car-
rier concentration[12]. As both ARPES and Hall mea-
surements give similar carrier concentration, the atmo-
sphericcontaminationplaysaminorroleandisnotthe
main source of doping, which is likely to be due to the
interface.
The Josephson devices are fabricated by conven-
tional e-beam lithography. The graphene junctions are
patterned by oxygen plasma. Then, the contacts are
deposited,seeFig.1(d,e).Theyconsistofa5nminter-
facial layer of titanium, a 80 nm thick aluminium layer
and a 3 nm thick gold layer. The gaps L between the
electrodes ranges from 200 nm to 600 nm. The width
is fixed at W =4 µm. A total of twelve junctions have
been defined, with two orientations with respect to the
substrate. The samples have been thermally anchored
to the 3He pot of a 3He cryostat (Heliox VL Oxford Fig. 2 (a) Temperature dependence of the resistance R(T)
for four junctions of lengths L ≈ 200,300,400 and 600 nm,
Instruments) and four probes electrical measurements
normalizedbytheirresistanceatT =1.2K.Theinsetrepre-
have been performed. The cryostat is equipped with sents the resistance at T =1.2 K of the same four junctions,
a room temperature electromagnetic interference filter plottedasafunctionoftheirwidthasfilledsquareswiththe
samecolourcode.TwoadditionaljunctionsofwidthL≈200
stage followed by low pass RC filters anchored at 1.5
nm are also plotted as open squares. (b) V(I) characteris-
K and by copper powder filters stage anchored at the
tics of the four junctions presented in panel (a), at the base
sample stage[25]. Standard measurements of current- temperatureofT =280mK.
voltage V(I) characteristics as a function of tempera-
ture and magnetic field along with R(T) curves with a
biascurrentofabout5nAhavebeenperformed.Inad-
dition, conductance spectra dI/dV(V) have been mea-
corresponding V(I) characteristics taken at T = 280
sured by superimposing a low amplitude (a few nA)
mK for the four junctions presented in panel (a). The
sinusoidal signal with frequency of about 30 Hz to a
V(I)characteristicispracticallyohmicforthejunction
triangularslowramp,withafrequencyofabout1mHz,
with L≈600 nm. A Josephson critical current I ≈40
and by reading the response from the sample by using c
and10nAcanbereadoffforthejunctionswithL≈200
the lock-in technique.
nm and 300 nm respectively. The small steps at finite
voltagebelowtheTi/Algapcanbeattributedtolocal-
3 Results ized subgap states (see below). The mean free path of
grapheneonhydrogenatedSiCisestimatedtobe(cid:96)∼50
The temperature dependence of the resistance of four nmfromroomtemperaturemeasurementsandthecar-
junctions is presented in Fig. 2(a). There is a kink at rierdensityandmobilityareonlyweaklymodifiedfrom
about1Kmonitoringthetransitiontosuperconductiv- roomtemperaturetoliquidHeliumtemperature[26,24].
ity of the Ti/Al contacts. The junction with L ≈ 600 Thus even the smallest of the junctions should be dif-
nm (green curve), appears to be going insulating. Not fusive. Estimating the aluminium gap ∆ ≈ 100 µeV
even in the smallest junctions with L≈200 nm (black from conductance dI/dV (V) measurements (see Fig.
curve) and L≈300 nm (red curve) is the transition to 4(a)), we obtain a superconducting coherence length
(cid:112)
thesuperconductingstatecompleteabove280mK.The ξ = ¯hD/∆ ≈ 300–400 nm, where D is the diffusion
0
precursory effects of the dissipationless conductance in constant. Hence our junctions with L ≤ 300 nm are in
these junctions will be discussed elsewhere. Here we the crossover between the short and the long junction
show that, no matter that some residual resistance is limit. A reduction of the Josephson critical current is
presentabove280mK,thesejunctionsattainphaseco- expectedinthelongjunctionlimit,asconfirmedbythe
herenceandJosephsonconduction.Fig.2(b)showsthe experiment.
4 B.Jouault1 etal.
Fig.4 (coloronline)(a)Measurementsofconductancespec-
tra dI/dV(V) as a function of temperature on the L ≈ 200
nm junction. On a large scale the temperature modulation
of the Al gap clearly appears, as indicated by the black ar-
rows.Panel(b)isazoomaroundzerovoltageandshowsthat
subgappeaksdonotshiftwithtemperature.
sistance.Consequently,asimilarmagneticpatternisre-
trievedfromthemagneticfielddependenceofdV/dI at
I =0, see panel (c). In panels (a,c), we have added for
reference a conventional Fraunhofer interference curve:
I ∝|sin(πBS /Φ )/(πBS /Φ )|
c eff 0 eff 0
where Φ = hc/2e is the flux quantum. The one dis-
Fig. 3 (color online) (a) Colormap of the differential resis- 0
played here corresponds to an effective area S = 1.6
tance dV/dI(B,I) for the junction of width L≈300 nm pre- eff
sented in Fig. 2. The data have been collected by decreasing µm2. As the width of the junction is 4 µm and the
the absolute value of the applied magnetic field. The dark length L ≈ 300 nm one could infer that the critical
blueareacorrespondstothesuperconductiveregion.Thesu-
currentdensityishomogeneousacrossthejunctionand
perposedbluecurveisaFraunhoferinterferencepatterngiven
as a reference, corresponding to a total area S =1.6 µm2. that the physical area of the junction is involved, with
eff
(b) Enlargement of a region (white rectangle) of panel (a), the addition of some small penetration of the magnetic
evidencing additional fast oscillating patterns. (c) Residual fieldintheAl/Ticontacts.Howeverthesample,includ-
resistance at I= 0 nA. The data are shown as open red cir-
ingtheAl/Ticontacts,isaplanarstructureinthethin
cles. The blue solid line appearing in panel (a) is also shown
film limit and the field penetration in the Al/Ti con-
here,asa guide fortheeye.
tacts is expected to be large. In fact, a conventional
Fraunhofer pattern only gives a qualitative envelope of
a strongly oscillating pattern that is observed.
For the L ≈ 200 nm junction, the product eIcRN Fig.3,panel(b)isanenlargementoftheselowfields
/∆ ≈ 0.09 is smaller than the theoretical estimate[27] oscillations. They are quasi periodic, with a pseudo-
eIcRN/ ∆ ≈ 0.66 derived for junctions in which the periodof1–3Gausswhichcorrespondstoamuchlarger
Thouless energyis ¯hD/L2 ∼∆. Thisis the upper limit effective area of ≈ 32µm2. To account for a more ex-
of the difference, since it is calculated for Ic at T ≈0.3 tended flux penetration in the contacts, we have nu-
TcAl (TcAl is the critical temperature of the electrodes). merically calculated the macroscopic magnetic vector
The measured critical current density Jc =Ic/W ≈ 10 potential profile by solving the London equation, fol-
nAµm−1 atL≈200nmfallsintherangeofthevalues lowing a method originally employed by Rosenthal et
reported in the literature, i.e. between 1 and 100 nA al.[30,31,32] for planar junctions. The I oscillations
c
µm−1,whenthetemperatureisnormalizedtoTc ofthe with the period of 3 Gauss and their enveloppe can
electrodes[28,7,29,2]. be reproduced in this way by choosing a Pearl pen-
OneofthemostdirectevidenceofagenuineJoseph- etration length within the contacts λ = 1 µm and a
son coupling is provided by a modulation of I with a non uniform critical current density across the junc-
c
magneticfieldBwhichisappliedperpendicularlytothe tion.However,amacroscopicLondonpictureisstillnot
substrate. The magnetic field dependence of the mea- fully convincing as the aperiodicity of the fast oscillat-
sureddifferentialresistancefortheL≈300nmjunction ing pattern, the residual resistance (see Fig. 3(c)) and
is reported in Fig. 3(a). Experimentally, there is also a some (fully reproducible) dependence on the sweeping
one to one correspondence between the magnitude of of the applied magnetic field imply additional phenom-
theJosephsoncriticalcurrentandthesmallresidualre- ena which cannot be accounted for within the macro-
Josephsoncouplinginjunctions madeofmonolayergraphenegrown onSiC 5
scopic Rosenthal model. Most likely, vortex pinning in 43]. The possible use of graphene junctions in real cir-
the Al/Ti contacts takes also place, as many impuri- cuits is hindered, however, by the substantial limit of a
ties and pinning centers are expected to diffuse in the technologythatcannotproducealargenumberofjunc-
contacts during the deposition process. The fast oscil- tions patterned on a single graphene flake with simi-
lations in the pattern could be due to a non-uniform larproperties,thuscontrollingtheelectrodynamicsand
critical current density distribution[33,34]. A more de- functionality of each of them, as well as of the global
tailed account on the magnetic field dependence of I circuit.
c
will be given elsewhere[35]. This work demonstrates the Josephson conduction
When investigating the behavior of the Al/Ti con- at subKelvin temperature, in various co-planar struc-
tacts further, by monitoring the conductance measure- tures on one single monolayer of graphene and pat-
mentsdI/dV(V)fordifferenttemperaturesreportedin ternedAlsuperconductingcontacts.Grapheneisgrown
Fig. 4, we can identify the Al/Ti gap, with a marked by CVD on SiC. The Josephson coherence appears to
temperature dependence (panel (a)), and few subgap be highly reproducible, isotropic with respect to the
structureswhichgiverisetothestepsintheV(I)char- substrate orientation and easily scalable with present
acteristics at finite voltage, see Fig. 2(b). Temperature standardlithographictechniques.Theseresultshavean
seems not to have any effect on these subgap struc- immediate applicative impact opening up to a system-
tures, as shown in Fig. 4(b). This rules out the possi- atic comparative study where single constructive pa-
bility that they originate from multiple Andreev reflec- rameters of the graphene junctions can be selectively
tion (MAR)[3]. This can be explained by considering changed and barriers can be appropriately engineered.
thattheinterfaceswiththecontactsareexpectedtobe
rather rough. Poor transmission between the metallic
aluminiumcontactsandtheunderlyinggraphenelayer, 5 Acknowledgements
responsible for a sizeable contact resistance, has also
been found in the case of diffusive graphene transis- DiscussionswithPietBrouwerandVictorRoucoGomez
tors[36]. Possible origin of the subgap resonances could are gratefully acknowledged. Work supported by PICS
be resonant tunneling via localized states or even edge CNRS-CNR2014-2016”TransportphenomenaandPro-
states in the graphene barrier. This feature will be in- ximity-induced Superconductivity in Graphene junc-
vestigated further in the future. tions”,SwedishFoundationforStrategicResearch(SSF)
Uncovering the structure of the Al/Ti contacts in undertheproject”Graphenebasedhighfrequencyelec-
these devices is of the utmost relevance. Graphene and tronics”,FIRB”HybridNanoDevRBFR1236VV”(Italy)
theTi/Alcontactshavealargeworkfunctiondifference and by EU FP7, under grant agreement no 604391
and graphene may become n-doped by charge transfer Graphene Flagship.
from the Al/Ti contact[37]. As graphene on the hy-
drogenated SiC interface is intrinsically p-doped, de-
vices could be engineered in the form of n-p-n junc-
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