Traditionally in Japan there has been a greater dependence on industry for long-term research motivated by applications than in the United States, where universities have been more deeply involved. Also, Japan historically has tended to emphasize the materials aspects of advanced electronics, whereas the United States has tended to emphasize the relevant device physics. These cultural differences are reflected clearly in the relative strengths of Japan and the United States in HTS materials research today.

The panel found that Japan is more active and stronger in the identification and synthesis of known classes of materials with structures and chemistries of particular scientific interest in the context of high temperature superconductivity. The proliferation of interesting ladder compounds from Japan is a good example. On the other hand, the search for entirely new classes of superconducting materials is probably still stronger in the United States and in Europe.

Table 3.1
Critical Materials Research Needs for HTS Superconducting Electronics
I. Focus of Current Industrial Materials Research
Identification and development of deposition techniques that will allow economical manufacture of large area films (e.g., 2", 4" diameter)
Increased reproducibility of film properties and device performance
Improvement in rf and microwave power handling capability and reduction of high-power rf non-linearities
Improvement in barrier uniformity of current SNS junctions
Increased IcRn and RnA products of SNS junctions
Search for more satisfactory normal barrier materials for SNS junctions
Development of a multilayer fabrication process, including ground planes, compatible dielectric layers, and growth over steps
II. Electrical Characteristics of Importance to Industry for Passive RF Applications
Ultimate physical limits of the materials
Rs(T) in small J limit
Rs(T) vs. J in dc magnetic field (0 to 20 T in parallel and perpendicular directions)
Nonlinearities in the surface impedance
Mechanism and dynamics of vortex entry at film edges
Understanding of electrical design vs. materials tradeoff
III. Key Materials Research Issues for Passive RF Applications
  • Better substrates (non-twinned, cheaper, larger)
  • Surface preparation
  • Surface characterization
Initial growth
  • mechanism of nucleation and initial growth
  • effect of buffer layers
  • effect of vicinal substrates
Factors governing microstructure during growth
  • Growth mechanisms (macroscopic holes and boulders)
  • Effects of composition and temperature
  • Second phases
  • Cation disorder
Oxygen content and disorder
Nanometer-scale defects
Grain boundaries
Role of pinning centers and weak links (vortex motion) on rf properties
Mechanical properties of the film (e.g., effects of stress, microcracks)
Process damage on surfaces and edges
Passivation and normal metal contacts
Aging (oxygen stability, passivation)
IV. Electrical Characteristics of Importance to Industry for Active (Josephson Junction) Applications
1/f noise in films, junctions, multilayers, and fully processed devices
Predictability and uniformity of Ic and Rn in junctions, initially bicrystals and large IcRn-product SNS
Inductance of films over ground planes
Temperature dependence of Ic
Jc in non-planar films, e.g., over steps and in vias
V. Materials Research for Active Devices ( Josephson Junction and Multilayer Circuits
Doped YBCO thin films
  • Less oxygen diffusivity
  • Better chemistry
  • Modified structure and electronic properties
  • Processed surface
  • Cation diffusion
  • Oxygen disorder
  • Nanometer-scale inhomogeneity
  • Thermal boundary resistances and electrical interface resistances
Growth on steps (slopes)
  • Nucleation
  • Jc and 1/f noise
Dielectric materials
  • Pinhole-free epitaxial growth
  • Oxygen diffusion
Barrier materials
  • Transport mechanisms, physical models
  • Initial nucleation mechanisms
  • Interfaces with superconductors
Ferroelectric thin films
  • Dielectric constant (real and imaginary) vs. microstructure
Magnetic oxide thin films
 Source: Beasley, Char, and Rowell n.d.

In keeping with historical and cultural differences between Japan and the United States, there is in Japan more emphasis at the basic level on systematic physical property determination of interesting high Tc and related superconducting materials, mainly on bulk single crystals. In the United States, on the other hand, there is greater emphasis on theory and experiment focused on critical fundamental physical issues, such as efforts to determine the pairing symmetry of the cuprate superconductors. In addition, as a consequence of its emphasis on device physics, the United States is stronger in the fundamental study and modeling of the microwave properties (linear and nonlinear) of the HTS materials, and of the transport and coupling mechanisms in HTS Josephson junctions relevant to applications. Looking ahead, these historical differences may even out somewhat. The Science and Technology Basic Law in Japan is likely to lead to a substantial increase in university research, while industry shows some signs of contracting its long-term research. There is increasing emphasis in the United States on the growth and systematic physical study of HTS single crystals, and there are signs of more device physics studies in Japan. In any event, as discussed in more detail below, Japan appears destined to continue to lead in the materials science (both bulk and thin film) of HTS materials.

Published: July 1998; WTEC Hyper-Librarian