Harry Kroger

Selective niobium anodization process for fabricating Josephson tunnel junctions

A novel process for fabricating refractory sperconducting tunnel junctions is described, which is useful with both deposited and native oxide barriers. The distinguishing feature of the method is that the entire superconductor‐barrier‐superconductor sandwich is formed before the patterning of any layer. Isolated Josephson junctions are then formed by anodizing through the upper electrode, while the devices themselves are protected by a photoresist mask. Using this process, Nb‐Si:H‐Nb junctions have been fabricated, whose product of critical current and subgap resistance exceeds 10 mV and whose critical current density varies by about 50% over a 2‐in. diameter wafer.

Steady-state characteristics of two terminal inversion-controlled switches

Abstract The direct current-voltage (I–V) characteristics of three terminal inversion controlled switches are described. These devices are layered sequences of metal/conducting “insulator”/semiconductor junction with electrical terminals at the metal and both sides of the junction. If a bias is applied between the metal and the far side of the junction, in the sense which tends to deplete the surface and forward bias the junction, the device shows bistable impedance states similar to the current-voltage characteristics of a silicon-controlled rectifier. The intermediate terminal which contacts the semiconductor region between the insulator and the junction, can be used, in proper circuit and biasing arrangements, to switch the device both into and out of its low impedance state without varying the voltage supplied to the outer terminals of the device and a series-connected resistor. The I–V characteristics of these three terminal devices support the inversion-controlled conduction model of device behavior which permits high conductivity of the device only when inversion of the semi-conductor surface occurs. The high and low impedance states and the pulses required to induce transistions between states are contained entirely within the “active” bias configuration for these devices, which is defined by analogy with the active bias region of conventional bipolar transistors.

Niobium nitride‐niobium Josephson tunnel junctions with sputtered amorphous silicon barriers

Niobium nitride‐niobium Josephson tunnel junctions with sputtered amorphous silicon barriers (NbN‐αSi‐Nb) have been prepared using processing that is fully compatible with integrated circuit fabrication. These junctions are of suitable quality and uniformity for digital circuit and S‐I‐S detector applications. The junction quality depends critically upon the properties of the NbN surface, and seems to correlate well with the UV/visible reflectivity of this surface.

Superconductor-semiconductor hybrid devices, circuits, and systems

The discovery of superconductors whose critical temperatures are above liquid nitrogen temperature has prompted considerable interest in hybrid superconducting-semiconducting electronics applications. The authors review the efforts to hybridize these technologies. Some of these efforts have already been demonstrated on a laboratory scale; others are at present just theoretical proposals. Hybridization is possible on the system, circuit, and device levels. The authors review studies of the applications of superconductors for interconnecting semiconductor systems and combining semiconductor and superconductor devices to enhance the performance of both digital and analog systems. Novel circuit combinations of superconducting and semiconducting devices are mentioned, as are proposals to combine these materials on the device level. It is noted that the use of hybrid combinations may permit some electronic functions to be performed better than either technology could perform separately. >

Hysteretic Josephson junctions from YBa2Cu3O7−x/SrTiO3/Ba1−xKxBiO3 trilayer films

We have fabricated hysteretic Josephson junctions from YBCO/SrTiO3/BKBO trilayer films. These large area rf‐sputtered junctions are reproducible, show Shapiro steps in response to microwave radiation, and their Josephson current can be modulated with a small magnetic field up to about 20 K. In addition, geometrical and magnetic field dependent resonances were observed for the first time in cuprate superconductor junctions. The devices appear to be S‐N‐S like without any gap structures discernable in the electrical characteristics. The decay length of critical current with SrTiO3 layer thickness indicates a normal‐state coherence length in the SrTiO3 of about 5 A.

YBa2Cu3O7−x‐Y2O3 system and in situ deposition of trilayer heterostructures by coevaporation

We have deposited YBa2Cu3O7−x‐Y2O3‐YBa2Cu3O7−x heterostuctures by an in situ electron‐beam coevaporation technique. Physical and chemical properties of Y2O3 were studied. The deposition conditions for Y2O3 and YBa2Cu3O7−x are completely compatible. The crystal structure of Y2O3 provides a close lattice match with YBa2Cu3O7−x and allows oxygen diffusion through Y2O3, so that heteroepitaxy and the tetragonal‐orthorhombic structural transformations of YBa2Cu3O7−x can be achieved. The heterostructures are therefore of high quality. Both the top and the bottom YBa2Cu3O7−x layers are superconducting above 85 K. Tunneling phenomena on junctions fabricated from these trilayers were observed.

Low‐leakage thin‐film superconductor‐insulator‐normal metal tunnel junctions on co‐evaporated Ba1−xRbxBiO3 and rf‐sputtered Ba1−xKxBiO3

The oxide superconductors Ba1−xKxBiO3 and Ba1−xRbxBiO3 exhibit ideal BCS‐like characteristics with Tc∼30 K. We report the deposition of thin films with zero‐resistance temperatures up to 25 K and transition widths less than 1 K by thermal co‐evaporation and rf sputtering. Thin‐film S‐I‐N tunnel junctions fabricated by evaporating Ag or Au onto a native barrier exhibit low leakage over 1 mm2 junctions and near‐ideal BCS‐like behavior with a value of 2Δ(0)/kBTco=3.5±0.2.

Josephson tunnel junctions with chemically vapor deposited polycrystalline germanium barriers

High quality Josephson tunnel junctions have been fabricated whose tunneling barrier is polycrystalline germanium chemically vapor deposited on a NbN base electrode and covered by a Nb counterelectrode. These junctions have excellent characteristics for device applications: values of Vm (the product of the critical current and the subgap resistance measured at 2 mV and 4.2 K) ranging between 35–48 mV, ideal threshold curves, a steep current rise at the gap voltage, and Josephson current densities from 100 to 1100 A/cm2.

Memory switching in polycrystalline silicon films

Abstract Non-volatile memory switching has been observed in polycrystalline silicon layers produced by chemical vapor deposition. Evidence for filamentary conduction is found for devices which are in their low impedance state. Devices have been cycled through high and low impedance states up to a maximum of 2x10 4 times. Exposure to transient ionizing electron radiation caused the devices to switch to their low impedance state.

Ba1−xKxBiO3 thin film Josephson tunnel junctions

We have fabricated sandwich‐type Josephson tunnel junction devices from Ba1−xKxBiO3 electrodes and KNbO3 insulating barriers. Near ideal superconductor‐insulator‐superconductor tunnel junction gap structure was seen in high resistance devices at 4.2 K. Lower resistance devices with similar gap structure but much higher subgap leakage were hysteretic with switching voltages up to 5 mV. Hysteresis and well‐defined gap structure persisted past 12 K. Magnetic field modulation of the zero‐bias pair current showed Fraunhofer‐like behavior with almost complete quenching at the nodes.