Abstract:
A photon detector based upon photon-assisted tunneling in superconductor-insulator-superconductor or super-Schottky structures, in which the superconductor is a high transition temperature superconductor. An electrical bias is provided on either side of such structures so that photo-assisted tunneling, in the presence of incident photons on the structure, can occur to thereby permit a tunneling current therebetween.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The invention is directed to exploiting photon-assisted tunneling in high transition temperature superconductor structures for use as an infrared photon detector. One application is an infrared photon detector optimized for the detection of cold bodies in space, i.e., whose operating spectral response is in the wavelengths of about 10 μm to about 100 μm at a temperature of 77° K. or higher. 
     The detection of cold bodies in space is a most difficult technical problem. The wavelengths of interest lie generally in the 10 μm to 100 μm spectral interval. A widely used prior art detector Hg 1-x  Cd x  Te (i.e., Hg.sub..795 Cd.sub..205 Te) operating at 77° K., is limited to wavelengths less than 12 μm as has been shown by Kruse, &#34;The Emergence of Mercury Cadmium Telluride as a Modern Infrared Sensitive Material:&#34; Chapter 1 of Semiconductors and Semimetals 18, Mercury Cadmium Telluride, R. K. Willardson and A. C. Beer, eds. Academic Press, New York (1981). To achieve longer wavelength operation requires a smaller value of &#34;x&#34; in the Hg 1-x  Cd x  Te, such as Hg.sub..82 Cd.sub..18 Te. A strong disadvantage however, is that the lower value of &#34;x&#34; requires cooling to temperatures below 77° K. Another limitation of small bandgap Hg 1-x  Cd x  Te, i.e. (Hg,Cd)Te, is the difficulty in achieving satisfactorily high R o  A products needed for photovoltaic operation. 
     Another prior art detector is extrinsic silicon. This includes silicon with shallow dopants which give rise to spectral responses extending beyond 12 μm, for example, Si:Ga detects radiation out to about 18 μm, Si:As to about 24 μm, and Si:Sb to about 30 μm. Extrinsic Si detectors operating at these longer wavelengths require cooling well below 77° K. Operating temperatures of these photoconductive detectors are below 20° K. requiring liquid He as the cryogenic medium. 
     In the present invention there is disclosed a photon detector that can have a spectral response extending at least to about 100 μm operates at liquid nitrogen temperatures (77° K.) or higher. The present invention utilizes the photoeffects in high transition temperature superconductors such as YBa 2  Cu 3  O 7  and utilizing as the principle of operation photon-assisted tunneling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial representation of a superconductor-insulator-superconductor tunneling infrared detector. 
     FIG. 2 is a diagram of densities of states D(ε) s  for S-I-S tunnel junctions (with the same superconductor on each side) in the neighborhood of the Fermi energy ε F . The energy 2Δ required to break a pair of electrons is shown as a gap in the densities of quasi-particle states for the superconductors. States which are filled at T=0 are shaded. 
     FIG. 3 is a diagram of DC Current-Voltage characteristics of S-I-S structures. 
     FIG. 4 illustrates the effect of electrical bias upon the density of states diagram for S-I-S structures. An absorbed photon of energy  ω breaks a Cooper pair within one superconductor region resulting in an electron tunneling through the insulating barrier into the other superconductor. 
     FIG. 5 is a super-Schottky embodiment of the tunneling IR detector. 
     FIG. 6 is a diagram of densities of states for super Schottky tunnel junctions in the neighborhood of the Fermi level ε F  with the density of states for the superconductor designated D(ε) s  and for the semiconductor designated D(ε) semiconductor . The energy 2Δ required to break a pair of electrons is shown as a gap in the densities of quasi particle states for the semiconductor. The semiconductor is degenerate N-type with the Fermi level in the conduction band. States which are filled at T=0 are shaded. 
     FIG. 7 is a diagram of DC current-voltage characteristics of super-Schottky structures. 
     FIG. 8 illustrates the effect of electrical bias upon the density of states diagram for a super Schottky structure. An absorbed photon of energy nω breaks a Cooper pair within the superconductor resulting in an electron tunneling through the depletion region of the semiconductor. 
    
    
     DESCRIPTION 
     In this description there is described two embodiments of an IR photon detector in a high transition temperature superconductor whose transition temperature lies at or above 77° K. Such a superconductor is YBa 2  Cu 3  O 7  whose transition temperature is in the range of 89°-94° K. One of these embodiments is a superconductor-insulator-superconductor structure and the other a super-Schottky structure. Referring now to FIG. 1 there is shown a sketch of a superconductor-insulator-superconductor (S-I-S) tunneling infrared detector 10. The S-I-S device is fabricated by depositing a layer 11 of a high transition temperature superconductor such as YBa 2  Cu 3  O 7  on an insulating substrate 12, e.g. by sputtering such a material. Layer 11 may be thin-film or a much thicker structure without any significant difference in electrical behavior. The fabrication of layer 11 on a substrate 12 is not a part of the invention as layer 11 may instead be a self supporting member if desired. Over the layer 11 is deposited a thin film insulator 13 (a tunneling barrier) such as about 50 angstroms of SiO 2 . Over that tunneling barrier insulator film 13 is deposited a second thin film layer 14 of a high transition temperature superconductor. Ohmic contacts 15 and 16 are made to both superconducting layers 11 and 14, respectively. The structure 10 can be mounted on the cold finger of a Dewar. A cold shield limits the field of view of the detector through a long wavelength infrared transmitting window such as C s  I. 
     When two superconductors 11 and 14 are separated by a thin insulator 13, the composite structure 10 is known as a S-I-S structure. Below the superconducting transition temperature the free electrons in the superconductor are bound in pairs known as Cooper pairs. In FIG. 2 there is shown a density of states diagram of an S-I-S structure giving the superconductor density of states, D s , as a function of energy. Here the forbidden energy gap, whose value in electron volts is given by 2Δ/e, where e is the free electron charge, and Δ is the superconductor energy gap parameter represents the energy to break a Cooper pair. 
     The dc current-voltage characteristic of an S-I-S structure is illustrated in FIG. 3 giving the current through the structure, plotted on the ordinate axis, as a function of the voltage thereacross plotted on the abscissa. There are two branches to the current. At zero bias a current (curve a) flows due to Cooper pair tunneling through the oxide separating the superconductors. This is known as the Josephson, or electron pair, current. When a bias is applied across the junction the other branch of the current (curve b) appears, known as quasi-particle or single electron tunneling. This represents the condition under which a single electron from a filled state on one side of the junction can tunnel into an empty state on the other side. The applied voltage at which this occurs is therefore 2Δ/e. At voltages exceeding this value, the current rises with a slope given by the normal resistance of the junction. 
     Radiation incident upon an S-I-S structure affects both branches of the current-voltage characteristic. This discussion however concerns only the photon-assisted quasi-particle tunneling current. 
     The density of states diagram of FIG. 2 is redrawn in FIG. 4 with a bias voltage applied to the structure of FIG. 1 for illustrating photon-assisted quasi-particle tunneling. Under the action of an electrical bias, the Fermi levels are offset as shown. Thus a photon of energy  ω (where   is Planck&#39;s constant divided by 2π and ω is the angular frequency of the radiation) can excite single-particle tunneling from a filled state on one side of the junction to an empty state on the other side. Similarly, at lower bias, two photons, each of energy  ω, can supply the requisite energy. At even lower biases, three, four, or more photons can supply the energy. Thus the current-voltage characteristic for photon-assisted quasi-particle tunneling is a series of steps. In this invention therefore the principle of operation is based upon photon-assisted tunneling in an S-I-S structure. Infrared photons incident upon the structure assist electrons to tunnel from the filled states on one side of the S-I-S structure to the empty states on the other side as shown in FIG. 4. A positive electrical bias V o  on one side with respect to the other provides additional energy eV o  such that the sum of the photon energy  ω and electrical energy eV equals or exceeds the forbidden gap energy 2Δ of the superconductor. Because the forbidden gap energies of high transition temperature superconductor, such as YBa 2  Cu 3  O 7 , are far larger than those of conventional superconductors, the spectral response of this S-I-S structure lies in the infrared rather than in the microwave or millimeter-wave region. Wavelength tunability is achieved by varying the electrical bias; increasing eV o  reduces  ω in order that the sum equals or exceeds the gap energy 2Δ. Thus in FIG. 1 when a bias is applied to contacts 15 and 16, and photons impinge on the detector superconductor layer 14, tunneling electrons occur through the insulator layer 13 to the superconductor region 11 which receives the tunneling electrons. 
     Another embodiment of the invention is shown in FIG. 5. Briefly, this embodiment employs a super-Schottky structure using a high temperature superconductor such as YBa 2  Cu 3  O 7  and a heavily doped degenerate semiconductor such as GaAs or Si. The principle of operation is based on photon-assisted tunneling. Infrared photons incident on the structure assist electrons to tunnel from the filled states of the superconductor to the empty states of the semiconductor. 
     The super-Schottky diode 30 of Figures consists of a thin film high transition temperature superconductor 31 in intimate contact with the surface of a heavily doped, degenerate semiconductor 32 (e.g. GaAs or Si) which is on a semiconductor substrate 33. In another embodiment the heavily doped semiconductor forms its own substrate. Rather than an oxide layer forming the tunneling barrier, as in the S-I-S embodiment described above, the tunneling barrier is the depletion region 34 within the heavily doped semiconductor adjacent the interface, the remaining portion of the layer 32 being the region which receives the tunneling electrons. FIG. 6 shows the density of states diagram of an unbiased super-Schottky structure giving the superconductor and semiconductor density of states, D s  and D semiconductor , respectively, as a function of energy ε, and FIG. 8 shows the density of states diagram of an electrically biased super-Schottky structure showing the photon-assisted tunneling between the filled states in the superconductor 31 and the empty states in the semiconductor 32. The semiconductor 32 is heavily doped to reduce the depletion width 34, thereby increasing the tunneling probability. When sufficient bias is applied, electrons tunnel from the superconductor 31, to the semiconductor 32. From FIG. 6 it can be seen that at absolute zero, in the absence of incident photons, no current can flow until the applied bias V reaches Δ/e, corresponding to half the energy gap of the superconductor since there will be no electrons thermally excited across the energy gap to the empty states of the superconductor to then pass directly through the semiconductor depletion region to the empty states of the semiconductor bulk. After the applied bias voltage reaches Δ/e, current flow rapidly increases because direct tunneling becomes possible. At temperatures above T=0 but below the transition temperature of the superconductor, a small current will flow even for V less that Δ/e due to some such thermally excited electrons crossing the energy gap to the empty superconductor states. This current is proportional to exp (eV/kT), where k is Boltzmann&#39;s constant. Thus the corresponding current-voltage characteristic shown in FIG. 7, giving the current through the structure of FIG. 5, plotted on the ordinate axis, as a function of the voltage thereacross, plotted on the abscissa, and is seen to be highly nonlinear. 
     When the semiconductor is biased with respect to the superconductor, absorbed photons allow electrons to tunnel from the filled states of the superconductor through the depletion region into empty states within the semiconductor. Since availability of empty states within the semiconductor is a function of kT, it is clear that photon-assisted tunneling is a function of  ω/kT. The implication of this to high transition temperature superconductors is that the minimum energy for photon detection increases as the operating temperature is raised. In other words, the long wavelength limit of a super-Schottky photon detector decreases as the temperature of operation increases. Restated, an electrical bias on the semiconductor with respect to the superconductor (e.g., positive bias on an n-type semiconductor) provides additional energy such that the sum of the photon energy  ω and the electrical energy eV o  equals or exceeds half the forbidden energy gap (half of 2Δ) of the superconductor. Because the forbidden gap energies of the high transition temperature superconductors are far larger than those of conventional superconductors, the spectral response of this super-Schottky structure lies in the infrared rather than the microwave or millimeter wave region. Wavelength tunability is achieved by varying the electrical bias; increasing eV o  reduces  ω in order that the sum equals Δ.