|
Efforts to grow high quality films of YBCO on Si have been complicated by factors discussed in Ref. 1, chief among
them being the reaction between YBCO and Si, which is damaging even at 550 C. This is well below the customary
temperatures for YBCO film growth. To avoid the reaction problem, epitaxial YBCO films were grown on Si (100) using an
intermediate buffer layer of yttria-stabilized zirconia (YSZ).2 Both layers are grown via an entirely in situ process by pulsed
laser deposition (PLD). Although the buffer layer prevents reaction, another problem arises; the large difference in thermal
expansion coefficients between silicon and YBCO causes strain at room temperature. Thin (<500 A) YBCO films are unrelaxed
and under tensile strain with a distorted unit cell. Thicker films are cracked and have poorer electrical properties. The thermal
strain may be reduced by growing on silicon-on-sapphire (SOS) rather than silicon.3 This allows the growth of films of
arbitrary thickness. Ion channeling reveals a high degree of crystalline perfection with a channeling minimum yield for Ba as
low as 12% on either silicon or SOS. The normal state resistivity is 250-300 i-cm at 300 K; the critical temperature, Tc
(R=0), is 86-88 K with a transition width (ATc) of I K. Critical current densities (J)°f 2x107 A/cm2 at 4.2 K and >2x106
A/cm2 at 77 K have been achieved. In addition, the surface resistance of a YBCO film on SOS was measured against Nb at 4.2
K. At 10 GHz, a value of 45 was obtained. This compares favorably to values reported for LaAlO3.
Application of this technology to produce reaction patterned microstrip lines has been tested.4 This was done by ion
milling away portions of the YSZ buffer layer prior to the YBCO deposition. YBCO landing on regions of exposed Si reacts
to form an insulator. This technique was used to make 3 micron lines 1.5 mm long. The resulting structure had a Jc of
l.6xl06 A/cm2 at 77 K. Isolation of separate structures exceeded 20 M. Several advantages of this technique are that no
solvents, etchants or photoresist come into contact with the YBCO, hence this technique has a potential for operational-asgrown
devices.
In summary, it is now possible to produce YBCO films with structural and DC electrical properties which rival the
most optimized c-axis epitaxial YBCO films on MgO, SrTiO3 and LaAlO3. Preliminary measurements of microwave
properties appear promising.
We thank Bruce Lairson for help obtaining magnetization data and Richard Johnson, Steve Ready and Lars-Erik Swartz
for technical assistance. This work benefits from AFOSR (F49620-89-C-0017). DBF received support from NSF (DMR-
8822353). DKF acknowledges the AT&T scholarship.
|
