Superconducting tunnel junction
The superconducting tunnel junction – also known as a superconductor–insulator–superconductor tunnel junction – is an electronic device consisting of two superconductors separated by a very thin layer of 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 detectors of electromagnetic radiation, magnetometers, high speed digital circuit elements, and quantum computing circuits.
Quantum tunneling
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. 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 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, 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, where is the Planck constant, is the electron charge, and is an integer. Photon-assisted tunneling arises from the response of the quasiparticles and gives rise to steps displaced in voltage by relative to the gap voltage.
Device fabrication
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 with a typical thickness of several nanometres. 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 insulating oxide layer with no defects that short-circuit the insulating layer. The superconducting critical temperature of aluminum is approximately 1.2 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 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. Early work used lead-lead oxide-lead tunnel junctions. Lead has a superconducting critical temperature of 7.2 K in bulk form, but lead oxide tends to develop 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.