R. Moerman

Nucleation and growth of PrBa2Cu3O7−δ barrier layers on ramps in DyBa2Cu3O7−δ studied by atomic force microscopy

We studied the microstructure of Ar ion-beam etched ramps in epitaxial DyBa2Cu3O7-d films by atomic force microscopy. Generally, ramps were well aligned with one of the crystal axes of the (001) SrTiO3 substrate. In those cases we observed a surface reconstruction into a regular pattern of very long (up to 0.5 mm), about 20 nm wide facets, parallel to the ramp edge. For high misorientation angles, the reconstruction appeared much more complex and less regular. Furthermore, we investigated the nucleation and growth of very thin barrier layers of PrBa2Cu3O7-d on well aligned ramps. We found that nucleation takes place in the shallow trenches in the ramp, where facets meet. When more material is deposited, islands coalesce to form closed domains parallel to the trenches. This causes a thickness modulation close to 100% for barrier layer thickness up to 6 nm. The conclusions drawn from this research allow for an important improvement of the technology for the fabrication of ramp-type junctions.

Material aspects for preparing HTS quasiparticle injection devices

Quasiparticle (QP) injection devices based on HTS could play an important role in future superconducting applications if material aspects can be better controlled. One reason why this kind of device received little attention in the past is the lack of an appropriate barrier for QP tunnelling. In a series of experiments, we used different barriers to test if they are suitable, i.e. if a current and possibly a voltage gain can be achieved. We improved the performance of planar YBCO/natural barrier/Au devices and a current gain of more than 6 at 40 K was observed. Most devices, however, showed signs of heating effects. Another barrier material was SrTiO/sub 3/ with layers of 5-6 nm thickness. Current-voltage characteristics showed that the barriers were continuous and we observed current gains of up to 3 at 60 K. PrBa/sub 2/Cu/sub 3/O/sub 7-x/ is an interesting candidate if one could overcome the problem of resonant inelastic tunnelling for QP. In a series of experiments we demonstrated that, even for 3 Mn thin PBCO barriers on a- and c-axis oriented YBa/sub 2/Cu/sub 3/O/sub 7-x/, most devices showed at best a current gain of 1. However, we have indications that a current gain of 10 could be possible with unity voltage gain.

HTS quasiparticle injection devices with large current gain at 77 K

Recent progress on the development of planar QP-injection devices using YBCO and STO as an epitaxial injection barrier will be discussed. The main problem for HTS injection devices is to grow reliably a well defined, ultra-thin tunneling barrier suitable for QP tunneling. For this purpose, we used inverted cylindrical magnetron sputtering to first optimize the smoothness of our YBCO films by controlling tightly an relevant sputtering conditions. We are able to prepare smooth [001] YBCO films on [001] STO substrates on a routine basis with an average roughness varying between 1 and 2 nm. With these flat YBCO films both planar as well as grain boundary junctions were fabricated using epitaxial STO barriers between 2 and 8 nm thick and a 50 nm of Au counter electrode. Planar junctions with 6 nm STO barriers were in most cases fully insulating, in some cases, a current gain of up to 7.4 at 77 K was obtained. For 3 nm STO barriers, the highest current gain was 15 at 81 K. The injection results also show a scaling behavior with junction size. Based on the present materials development and device understanding, we consider a current gain of up to 20 at 77 K possible.

Surface roughness and height-height correlations dependence on thickness of YBaCuO thin films

For high Tc superconducting multilayer applications, smooth interfaces between the individual layers are required. However, in general, e.g., YBaCuO grows in a 3D screw-dislocation or island nucleation growth mode, introducing a surface roughness. In this contribution we study the surface layer roughness as a function of different deposition techniques as well as deposition parameters. Special attention will be paid to the increase in film roughness with increasing film thickness. For these studies we used scanning probe microscopy. From these experiments, we obtained an island density decreasing with a square root dependence on the film thickness. Furthermore, height-height correlations indicate that the film growth can be described by a ballistic growth process, with very limited effective surface diffusion. The correlation lengths ? are on the order of the island size, inferring that the island size forms the mean diffusion barrier. This results in a representation of non-correlated islands, which can be considered as autonomous systems.

Tunnelling barriers for high temperature superconductor devices

In this contribution we will discuss the requirements to be imposed on the barriers of HTS Josephson junctions as well as quasi particle injection devices. Factors that play a role are noise immunity, operating margins, injection efficiency etc., leading to the need for barriers in which the charge transport is dominated by direct tunnelling. So far, it is unclear if this has ever been observed in any practical device. We will demonstrate that localised states, that are probably ubiquitous in the materials used, are of prime importance in understanding device properties. In the model system we investigated, i.e. DyBCO/PrBCO/DyBCO, gallium-doping on the copper chain sites reduces the density of localised states appreciably without affecting the barrier height. Furthermore, it seems that due to an on-site Coulomb repulsion, pair transport proceeds via direct tunnelling, at least in the temperature region T<Tc/2. Quasi particles, in contrast, tunnel resonantly via one or more localised states. At higher temperatures and bias voltages inelastic processes may dominate. These resonant tunnelling processes reduce the normal state resistance, and hence the IcRn-product. The consequences of these findings will be discussed and directions for future work set.

Introduction of ramp-type technology in HTS quasiparticle injection devices

Injection of quasiparticles with an energy larger than the superconducting gap into a superconducting strip results in breaking of Cooper-pairs and hence the suppression of the superconducting properties. Experiments using planar injection devices made of HTS materials with various barrier materials showed current gains varying from 2 up to 15 at 77 K. By changing the junction size and therefore the superconducting volume the current gain could be increased. A further reduction of the junction volume is very difficult using the planar device geometry. However, by applying the ramp-type technology it is possible to reduce the junction volume by at least one order of magnitude and a further increase in current gain is expected. Another advantage of this technology is the formation of in-situ barriers and electrodes and hence a better control of the junction characteristics should be possible, also the compatibility with the processes involved making RSFQ devices can be interesting for later applications. We have fabricated ramp-type injection devices, using various types of barriers. Characterization of these devices has been performed and the results of these experiments will be presented and discussed.

Growth and Transport Properties of Tunneling Barriers in HTS devices

In this contribution we will discuss the charge transport of ramp-type HTS Josephson junctions with a Ga-doped PBCO barrier layer. It will be demonstrated that in these junctions charge transport takes place via tunneling processes. The Cooper pairs tunnel directly, at least for T ≤ Tc/2, whereas the quasiparticles tunnel indirectly via localized states. By substituting Cu-chain atoms with Ga-atoms the density of localized states appear to be reduced, resulting in an increase in IcRn-product. Another way to increase this product is a reduction in barrier thickness. Growth studies by AFM of PBCO barriers on ramps indicate that below about 10 nm barriers become increasingly less homogeneous, and below about 6 nm pin holes are very likely to occur. This sets a lower limit on the useful barrier thickness. Presently critical-current densities up to 104 A/cm2 at 40 K, and IlR,-products up to 10 mV at 4.2 K are easily obtained.