Gregory John Clark

Quantum interference devices made from superconducting oxide thin films

We have fabricated superconducting quantum interference devices (dc SQUID’s) from thin films of the superconducting oxide YBa2Cu3Oy. The devices were made by first lithographically patterning an ion implant mask containing a 40 by 40 μm loop and two 17‐μm‐wide weak links over a ∼1‐μm‐thick oxide film. Ion implantation was then used to destroy the superconductivity in the film surrounding the device without actually removing material, resulting in a completely planar structure for the SQUID’s. The SQUID’s were operated in the temperature range from 4.2 to 68 K. The superconducting flux quantum was measured to be h/2e in these materials.

Effects of radiation damage in ion‐implanted thin films of metal‐oxide superconductors

The effects of ion implantation into thin films of the superconductor YBa2Cu3Ox have been studied. Using oxygen and arsenic ions, the superconducting transition temperature Tc, the change in room‐temperature electrical properties from conducting to insulating, and the crystalline to amorphous structural transition in the films were studied as a function of ion dose. The deposited energy required to change Tc was found to be 0.2 eV/atom, while 1–2 eV/atom was required to affect the room‐temperature conductivity, and 4 eV/atom to render the film amorphous. This hierarchy of effects is discussed in terms of the damage mechanisms involved.

Ion beam amorphization of YBa2Cu3Ox

The microstructure of ion‐implanted thin films of the superconductor YBa2Cu3Ox has been investigated by transmission electron microscopy. The superconducting properties of the films were dominated by large pancake‐shaped grains of YBa2Cu3Ox with their c axis perpendicular to the substrate. Other grains of YBa2Cu3Ox whose c axis was parallel to the substrate formed spherulites. Irradiation with 500 keV O+ ions caused amorphous zones to appear on the grain boundaries between the pancake grains, which initially were free of amorphous or second phases. At higher dose a continuous amorphous layer 150 A thick was formed. However, the interior of the grains showed no irradiation‐induced microstructural features until they became amorphous at a dose of 3×1014 ions/cm2. The appearance of the amorphous layer on the grain boundaries at low doses accounts for the reduction in the superconducting transition temperature observed in these films.The microstructure of ion-implanted thin films of the superconductor YBa/sub 2/Cu/sub 3/O/sub x/ has been investigated by transmission electron microscopy. The superconducting properties of the films were dominated by large pancake-shaped grains of YBa/sub 2/Cu/sub 3/O/sub x/ with their c axis perpendicular to the substrate. Other grains of YBa/sub 2/Cu/sub 3/O/sub x/ whose c axis was parallel to the substrate formed spherulites. Irradiation with 500 keV O/sup +/ ions caused amorphous zones to appear on the grain boundaries between the pancake grains, which initially were free of amorphous or second phases. At higher dose a continuous amorphous layer 150 A thick was formed. However, the interior of the grains showed no irradiation-induced microstructural features until they became amorphous at a dose of 3 x 10/sup 14/ ions/cm/sup 2/. The appearance of the amorphous layer on the grain boundaries at low doses accounts for the reduction in the superconducting transition temperature observed in these films.

Radiation effects in thin films of high Tc superconductors

Abstract The discovery of a class of oxides showing superconducting properties at temperatures of 38 K. and above has generated intense interest. We have studied the effects of ion implantation into thin films consisting largely of one of these oxides, YBa 2 Cu 3 O x , which typically has a superconducting transition temperature, T c of greater than 90 K. Ion implantation with 500 keV oxygen ions causes degradation of the completion temperature of the superconducting transition, and changes the room temperature electrical properties from conducting to insulating. Further irradiation causes a sudden crystalline to amorphous structural transition. In the case of arsenic ion bombardment the c-a transition was more gradual. These effects were studied using TEM and X-ray diffraction to characterize the structure of the films. The deposited energy required to change T c was found to be 0.2 eV/atom, and the change was ascribed to the formation of an amorphous layer on the grain boundaries. 1 to 2 eV/atom was required to affect the room temperature conductivity, and 4 eV/atom to render the film amorphous. The damage mechanisms responsible for these effects are discussed.

Ion implantation in high-Tc superconductors

Abstract We describe effects that occur when high- T c oxide superconductors are ion implanted or irradiated. Most of the information has come from studies done with MeV ions and thin films of YBa 2 Cu 3 O 7 , and many of the changes are due to radiation damage. Small ion doses introduce defects which are mobile, and anneal at room temperature. At high dose the materials are no longer superconductors, and the effects seen with increasing ion fluence are a reduction in the normal-state conductivity, which falls exponentially with dose, and a metal to semiconductor transition. These can be viewed as consequences of increasing localization of the charge carriers. At still higher doses a crystalline-to-amorphous transition occurs. Some progress in ion-implantation processing of superconducting thin films has been made, in particular in patterning the films with high implant doses.