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Molecular Beam Epitaxy of High TC Superconducting Oxides

IP.com Disclosure Number: IPCOM000103738D
Original Publication Date: 1993-Jan-01
Included in the Prior Art Database: 2005-Mar-18
Document File: 2 page(s) / 122K

Publishing Venue

IBM

Related People

Delage, SL: AUTHOR [+3]

Abstract

Previous discoveries of superconductivity above liquid nitrogen temperatures open a way for unprecedented practical applications. It may become of extreme importance in the wiring in VLSI circuits as well as in the semiconducting and superconducting devices. Si-based FETs with fine line dimensions below 0.5 mm are designed to be operated at liquid nitrogen, and a CMOS process for 77K operation has been tested already. Heterojunction bipolar transistors for low temperature operation have also been proposed. The lowering of the temperature is essential, as it significantly reduces RC losses in the interconnects, and if the interconnect metal undergoes a resistive transition above 77oK, the delays due to the resistive voltage drop in the transmission lines would be zero.

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Molecular Beam Epitaxy of High TC Superconducting Oxides

       Previous discoveries of superconductivity above liquid
nitrogen temperatures open a way for unprecedented practical
applications.  It may become of extreme importance in the wiring in
VLSI circuits as well as in the semiconducting and superconducting
devices.  Si-based FETs with fine line dimensions below 0.5 mm are
designed to be operated at liquid nitrogen, and a CMOS process for
77K operation has been tested already.  Heterojunction bipolar
transistors for low temperature operation have also been proposed.
The lowering of the temperature is essential, as it significantly
reduces RC losses in the interconnects, and if the interconnect metal
undergoes a resistive transition above 77oK, the delays due to the
resistive voltage drop in the transmission lines would be zero.

      The most studied high Tc superconductor so far may be the
Y-Ba-Cu-O system.  Transition temperatures as high as 98oK were
reported for this material with the transition widths of only few
degrees.  This Tc can be maintained if yttrium is replaced by a
variety of rare earths, such as europium, gadolinium, neodymium, etc.
The materials with sharp resistive transitions are mostly prepared as
ceramics.  Recently, however, films of Y1Ba2Cu3O9-x (1-2-3 compound)
were prepared by triple gun evaporation and by sputtering from a
single hot pressed target.  Good quality films are, of course,
essential for VLSI applications.  At the present time, the onset of
resistive transition in the films prepared so far is also above 90oK,
but the transition is broad, often completed around 70oK.  This is
due to the fact that the films are inhomogeneous, since they comprise
a mixture of two or more phases, not all superconducting.  The films
are polycrystalline after a complex high temperature annealing
(800oC-900oC) schedule in oxygen or vacuum, and it appears that
non-superconducting phase is present between the grains.  This, in
turn, assures that materials prepared so far cannot support high
current densities.

      The structure of the superconducting phase was previously
identified by an IBM group, AT&T Bell Labs and University of Houston
group.  There is a consensus that it is a distorted oxygen-deficient
perovskite.  The unit cell is layered, i.e., yttrium and barium ions
are ordered so that two barium planes are interrupted by smaller
yttrium.  There are 3 copper planes per unit cell, each copper ion
surrounded by the octahedra of oxygen.  There is a consensus as to
the importance of ordering and actual position of oxygen vacancies on
the value of Tc .  There is no consensus as to where these vacancies
occur, since the methods of preparation so far cannot control vacancy
positions.

      In this article, it is proposed to prepare thin films of high
Tc ternary oxides by molecular beam epitaxy (MBE).  There are
numerous advantages to this method of film growth.  A wide variety of
semi...