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.

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.

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 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 relations, first predicted by Brian David Josephson in 1962.[1] 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 ${\displaystyle eV}$ exceeds twice the value of superconducting energy gap 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 ${\displaystyle f}$, 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 ${\displaystyle nhf/(2e)}$, where ${\displaystyle h}$ is Planck's constant, ${\displaystyle e}$ is the electron charge, and ${\displaystyle n}$ is an integer.[2] Photon-assisted tunneling arises from the response of the quasiparticles and gives rise to steps displaced in voltage by ${\displaystyle nhf/e}$ relative to the gap voltage.[3]

The device is typically 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${\displaystyle _{2}}$O${\displaystyle _{3}}$) with a typical thickness of several nanometers. 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 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 nm) insulating oxide layer with no defects that short-circuit the insulating layer. The superconducting critical temperature of aluminum is approximately 1.2 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 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 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 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 proximitized by the thicker niobium, and the resulting device has a superconducting critical temperature above 4.2 K.[4] Early work used lead-lead oxide-lead tunnel junctions.[5] Lead has a superconducting critical temperature of 7.2 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.

• Superconductivity
• Josephson effect
• Macroscopic quantum phenomena
• Quantum tunneling
• Superconducting quantum interference device (SQUID)
• Superconducting quantum computing
• Rapid single flux quantum (RSFQ)
• Cryogenic particle detectors