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The '''superconducting tunnel junction''' ('''STJ''') – also known as a '''superconductor–insulator–superconductor tunnel junction''' ('''SIS''') – is an [[electronics|electronic]] device consisting of two [[superconductors]] separated by a very thin layer of [[Insulator (electrical)|insulating]] material. Current passes through the junction via the process of [[quantum tunneling]]. The STJ is a type of [[Josephson junction]], though not all the properties of the STJ are described by the Josephson effect.

These devices have a wide range of applications, including high-sensitivity [[Cryogenic particle detectors|detectors]] of [[electromagnetic radiation]], [[SQUID|magnetometer]]s, [[RSFQ|high speed digital circuit]] elements, and [[superconducting quantum computing|quantum computing]] circuits.

== Quantum tunneling ==
[[File:Superconducting tunnel junction.svg|270px|thumb|alt=Illustration of a thin-film superconducting tunnel junction.|Illustration of a thin-film superconducting tunnel junction (STJ). The superconducting material is light blue, the insulating tunnel barrier is black, and the substrate is green.]]
[[File:STJ Energy Diagram.svg|alt=Energy diagram of a superconducting tunnel junction.|thumb|Energy diagram of a superconducting tunnel junction. The vertical axis is energy, and the horizontal axis shows the [[density of states]]. [[Cooper pairs]] exist at the [[Fermi energy]], indicated by the dashed lines. A bias voltage V is applied across the junction, shifting the Fermi energies of the two superconductors relative to each other by an energy eV, where e is the [[electron]] charge. [[Quasiparticle]] states exist for energies greater than Δ from the Fermi energy, where Δ is the superconducting energy gap. Green and blue indicate empty and filled quasiparticle states, respectively, at zero temperature.]]
[[File:STJ IV Curve.jpg|thumb|alt=Sketch of the current-voltage curve of a superconducting tunnel junction.|Sketch of the current–voltage (I–V) curve of a superconducting tunnel junction. The [[Cooper pair]] tunneling current is seen at V = 0, while the [[quasiparticle]] tunneling current is seen for V > 2Δ/e and V < -2Δ/e.]] 
All [[Electrical current|currents]] flowing through the STJ pass through the insulating layer via the process of [[quantum tunneling]].  There are two components to the tunneling current. The first is from the tunneling of [[Cooper pairs]]. This supercurrent is described by the ac and dc [[Josephson effect|Josephson relations]], first predicted by [[Brian David Josephson]] in 1962.<ref>{{cite journal | last=Josephson | first=B.D. | title=Possible new effects in superconductive tunnelling | journal=Physics Letters | publisher=Elsevier BV | volume=1 | issue=7 | year=1962 | issn=0031-9163 | doi=10.1016/0031-9163(62)91369-0 | pages=251–253| bibcode=1962PhL.....1..251J }}</ref> For this prediction, Josephson received the [[Nobel prize in physics]] in 1973. The second is the [[quasiparticle]] current, which, in the limit of zero temperature, arises when the energy from the bias voltage <math>eV</math> exceeds twice the value of superconducting energy gap&nbsp;Δ. At finite temperature, a small quasiparticle tunneling current – called the subgap current – is present even for voltages less than twice the energy gap due to the thermal promotion of quasiparticles above the gap.

If the STJ is irradiated with [[photons]] of frequency <math>f</math>, the dc current-voltage curve will exhibit both Shapiro steps and steps due to photon-assisted tunneling. Shapiro steps arise from the response of the supercurrent and occur at voltages equal to <math>nhf/(2e)</math>, where <math>h</math> is the [[Planck constant]], <math>e</math> is the [[electron]] charge, and <math>n</math> is an [[integer]].<ref>{{cite journal | last=Shapiro | first=Sidney | title=Josephson Currents in Superconducting Tunneling: The Effect of Microwaves and Other Observations | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=11 | issue=2 | date=1963-07-15 | issn=0031-9007 | doi=10.1103/physrevlett.11.80 | pages=80–82| bibcode=1963PhRvL..11...80S }}</ref> Photon-assisted tunneling arises from the response of the quasiparticles and gives rise to steps displaced in voltage by <math>nhf/e</math> relative to the gap voltage.<ref>M. Tinkham, ''Introduction to Superconductivity'', 2nd edition, Dover Publications, 1996</ref>

== Device fabrication ==
The device is typically [[Semiconductor device fabrication|fabricated]] by first depositing a thin film of a superconducting metal such as [[aluminum]] on an insulating substrate such as [[silicon]]. The deposition is performed inside a [[vacuum chamber]]. [[Oxygen]] gas is then introduced into the chamber, resulting in the formation of an insulating layer of [[aluminum oxide]] (Al<math>_{2}</math>O<math>_{3}</math>) with a typical thickness of several [[nanometre]]s. After the vacuum is restored, an overlapping layer of superconducting metal is deposited, completing the STJ. To create a well-defined overlap region, a procedure known as the [[Niemeyer-Dolan technique]] is commonly used. This technique uses a suspended bridge of [[Resist (semiconductor fabrication)|resist]] with a double-angle deposition to define the junction.

[[Aluminum]] is widely used for making superconducting tunnel junctions because of its unique ability to form a very thin (2–3&nbsp;nm) insulating [[Aluminum oxide|oxide]] layer with no defects that [[short-circuit]] the insulating layer. The [[superconducting]] critical temperature of aluminum is approximately 1.2&nbsp;[[kelvin|K]]. For many applications, it is convenient to have a device that is superconducting at a higher temperature, in particular at a temperature above the [[boiling point]] of [[liquid helium]], which is 4.2&nbsp;K at atmospheric pressure. One approach to achieving this is to use [[niobium]], which has a superconducting critical temperature in bulk form of 9.3&nbsp;K. Niobium, however, does not form an oxide that is suitable for making tunnel junctions. To form an insulating oxide, the first layer of niobium can be coated with a very thin layer (approximately 5&nbsp;nm) of aluminum, which is then oxidized to form a high quality aluminum oxide tunnel barrier before the final layer of niobium is deposited. The thin aluminum layer is [[Proximity effect (superconductivity)|proximitized]] by the thicker niobium, and the resulting device has a superconducting critical temperature above 4.2&nbsp;K.<ref>{{cite journal | last1=Joseph | first1=A.A. | last2=Sese | first2=J. | last3=Flokstra | first3=J. | last4=Kerkhoff | first4=H.G. | title=Structural Testing of the HYPRES Niobium Process | journal=IEEE Transactions on Applied Superconductivity | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=15 | issue=2 | year=2005 | issn=1051-8223 | doi=10.1109/tasc.2005.849705 | pages=106–109| bibcode=2005ITAS...15..106J | s2cid=22001764 | url=https://ris.utwente.nl/ws/files/6474965/01439587.pdf }}</ref> Early work used [[lead]]-lead oxide-lead tunnel junctions.<ref>{{cite journal | last1=Dolan | first1=G. J. | last2=Phillips | first2=T. G. | last3=Woody | first3=D. P. | title=Low-noise 115-GHz mixing in superconducting oxide-barrier tunnel junctions | journal=Applied Physics Letters | publisher=AIP Publishing | volume=34 | issue=5 | year=1979 | issn=0003-6951 | doi=10.1063/1.90783 | pages=347–349| bibcode=1979ApPhL..34..347D | doi-access=free }}</ref> [[Lead]] has a superconducting critical temperature of 7.2&nbsp;K in bulk form, but lead oxide tends to develop defects (sometimes called pinhole defects) that short-circuit the tunnel barrier when the device is thermally cycled between [[cryogenic]] temperatures and room temperature, so lead is no longer widely used to make STJs.

== Applications ==

=== Radio astronomy ===
STJs are the most sensitive [[heterodyne]] receivers in the 100&nbsp;GHz to 1000&nbsp;GHz frequency range, and hence are used for [[radio astronomy]] at these frequencies.<ref>{{cite journal | last1=Zmuidzinas | first1=J. | last2=Richards | first2=P.L. | title=Superconducting detectors and mixers for millimeter and submillimeter astrophysics | journal=Proceedings of the IEEE | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=92 | issue=10 | year=2004 | issn=0018-9219 | doi=10.1109/jproc.2004.833670 | pages=1597–1616| s2cid=18546230 }}</ref> In this application, the STJ is [[DC bias|dc biased]] at a voltage just below the gap voltage (<math>|V| = 2\Delta /e</math>). A high frequency signal from an astronomical object of interest is focused onto the STJ, along with a [[local oscillator]] source. Photons absorbed by the STJ allow quasiparticles to tunnel via the process of photon-assisted tunneling. This photon-assisted tunneling changes the current-voltage curve, creating a nonlinearity that produces an output at the difference frequency of the astronomical signal and the local oscillator. This output is a frequency down-converted version of the astronomical signal.<ref>{{cite journal | last=Wengler | first=M.J. | title=Submillimeter-wave detection with superconducting tunnel diodes | journal=Proceedings of the IEEE | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=80 | issue=11 | year=1992 | issn=0018-9219 | doi=10.1109/5.175257 | pages=1810–1826| hdl=2060/19930018580 | s2cid=110082517 | hdl-access=free }}</ref> These receivers are so sensitive that an accurate description of the device performance must take into account the effects of [[quantum noise]].<ref>{{cite journal | last=Tucker | first=J. | title=Quantum limited detection in tunnel junction mixers | journal=IEEE Journal of Quantum Electronics | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=15 | issue=11 | year=1979 | issn=0018-9197 | doi=10.1109/jqe.1979.1069931 | pages=1234–1258| bibcode=1979IJQE...15.1234T }}</ref>

=== Single-photon detection ===
In addition to [[heterodyne]] detection, STJs can also be used as direct detectors. In this application, the STJ is biased with a dc voltage less than the gap voltage. A [[photon]] absorbed in the superconductor breaks [[Cooper pairs]] and creates [[quasiparticles]]. The quasiparticles tunnel across the junction in the direction of the applied voltage, and the resulting tunneling current is proportional to the photon energy. STJ devices have been employed as single-photon detectors for photon frequencies ranging from [[X-rays]] to the [[infrared]].<ref>[http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=33525 STJ detectors from the European Space Agency], accessed 8-17-11</ref>

=== SQUIDs ===
The [[superconducting quantum interference device]] or [[SQUID]] is based on a superconducting loop containing Josephson junctions. SQUIDs are the world's most sensitive [[magnetometer]]s, capable of measuring a single [[magnetic flux quantum]].

=== Quantum computing ===
[[Superconducting quantum computing]] utilizes STJ-based circuits, including [[charge qubit]]s, [[flux qubit]]s and [[phase qubit]]s.

=== RSFQ ===
The STJ is the primary active element in [[rapid single flux quantum]] or [[RSFQ]] fast logic circuits.<ref>{{cite journal | last1=Likharev | first1=K.K. | last2=Semenov | first2=V.K. | title=RSFQ logic/memory family: a new Josephson-junction technology for sub-terahertz-clock-frequency digital systems | journal=IEEE Transactions on Applied Superconductivity | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=1 | issue=1 | year=1991 | issn=1051-8223 | doi=10.1109/77.80745 | pages=3–28| bibcode=1991ITAS....1....3L | s2cid=21221319 }}</ref>

=== Josephson voltage standard ===
When a high frequency current is applied to a Josephson junction, the ac Josephson current will synchronize with the applied frequency giving rise to regions of constant voltage in the I–V curve of the device (Shapiro steps).  For the purpose of voltage standards, these steps occur at the voltages <math> nf/K_\text{J} </math> where <math> n </math> is an integer, <math> f </math> is the applied frequency and the Josephson constant <math> K_\text{J} </math> = {{physconst|KJ}} is a constant that is equal to <math> 2e/h </math>.  These steps provide an exact conversion from frequency to voltage. Because frequency can be measured with very high precision, this effect is used as the basis of the Josephson voltage standard, which implements the [[SI]] definition of the [[volt]].<ref>{{cite journal | last1=Hamilton | first1=C.A. | last2=Kautz | first2=R.L. | last3=Steiner | first3=R.L. | last4=Lloyd | first4=F.L. | title=A practical Josephson voltage standard at 1 V | journal=IEEE Electron Device Letters | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=6 | issue=12 | year=1985 | issn=0741-3106 | doi=10.1109/edl.1985.26253 | pages=623–625| bibcode=1985IEDL....6..623H | s2cid=19200552 | url=https://zenodo.org/record/1232287 }}</ref><ref>[https://www.nist.gov/pml/div686/devices/voltage.cfm Quantum voltage metrology at NIST], accessed 11-5-11</ref>

=== Josephson diode ===
In the case that the STJ shows asymmetric Josephson tunneling, the junction can become a [[Josephson diode]]. <ref>{{Cite journal |last1=Wu |first1=Heng |last2=Wang |first2=Yaojia |last3=Xu |first3=Yuanfeng |last4=Sivakumar |first4=Pranava K. |last5=Pasco |first5=Chris |last6=Filippozzi |first6=Ulderico |last7=Parkin |first7=Stuart S. P. |last8=Zeng |first8=Yu-Jia |last9=McQueen |first9=Tyrel |last10=Ali |first10=Mazhar N. |date=2022-04-27 |title=The field-free Josephson diode in a van der Waals heterostructure |url=http://dx.doi.org/10.1038/s41586-022-04504-8 |journal=Nature |volume=604 |issue=7907 |pages=653–656 |doi=10.1038/s41586-022-04504-8 |pmid=35478238 |bibcode=2022Natur.604..653W |issn=0028-0836|arxiv=2103.15809 |s2cid=248414862 }}</ref>

== See also ==
* [[Superconductivity]]
* [[Josephson effect]]
* [[Macroscopic quantum phenomena]]
* [[Quantum tunneling]]
* [[Superconducting quantum interference device]] (SQUID)
* [[Superconducting quantum computing]]
* [[Rapid single flux quantum]] (RSFQ)
* [[Cryogenic particle detectors]]

== References ==
{{reflist}}

[[Category:Superconductivity]]
[[Category:Josephson effect]]
[[Category:Quantum electronics]]
[[Category:Superconducting detectors]]
[[Category:Sensors]]
[[Category:Radio astronomy]]
[[Category:Mesoscopic physics]]