APPLIED PHYSICS LETTERS
VOLUME 80, NUMBER 2
14 JANUARY 2002
Reactivity of MgB2 with common substrate and electronic materials
T. Hea) and R. J. Cava
Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton,
New Jersey 08544
John M. Rowell
Materials Research Institute, Northwestern University, Evanston, Illinois 60208
͑Received 24 September 2001; accepted for publication 15 November 2001͒
The reactivity of MgB2 with powdered forms of common substrate and electronic materials is
reported. Reaction temperatures between 600 and 800 °C, encompassing the range commonly
employed in thin-film fabrication, were studied. The materials tested for reactivity were ZrO2 , yttria
stabilized zirconia, MgO, Al2O3 , SiO2 , SrTiO3 , TiN, TaN, AlN, Si, and SiC. At 600 °C, MgB2
reacted only with SiO2 and Si. At 800 °C, however, reactions were observed for MgB2 with Al2O3 ,
SiO2 , Si, SiC, and SrTiO3 . The Tc of MgB2 decreased in the reactions with SiC and Al2O3 .
© 2002 American Institute of Physics. ͓DOI: 10.1063/1.1433915͔
MgB2 , with a Tc of 39 K,1 may offer higher operating
temperatures and device speeds than today’s Nb-based tech-
nologies for potential applications in thin-film electronic de-
vices, and may have a simpler multilayer film fabrication
process than high temperature superconductors. An extensive
review of the progress in thin-film fabrication of MgB2 has
been presented.2 For electronics applications, it is desirable
that films with Tcs close to 39 K be made by a ‘‘single-step
in situ’’ process, in which MgB2 is formed directly on sub-
strates. The majority of films to date have been made by a
two-step process, in which a precursor film of either B or
MgϩB is annealed, typically either in Mg vapor at 900 °C,
or in inert gas at about 600 °C, though some single-step pro-
cesses have also been reported. In all cases, the reactivity of
MgB2 with substrate materials or insulating or metallic lay-
ers in multilayer circuits is an important factor in determin-
ing both the conditions of film fabrication and the operating
characteristics of the resulting device. Here we describe the
results of experiments in which fine powders of different
materials were reacted with MgϩB in the 600–800 °C tem-
perature range relevant to device fabrication. It was found
that MgB2 is inert toward many common electronic and sub-
strate materials at 600 °C ͑with the notable exceptions of
SiO2 and Si͒, but is highly reactive toward some of the most
common substrate materials ͑i.e., Al2O3 and SrTiO3͒ by
800 °C, suggesting their limited usefulness if high fabrication
temperatures must ultimately be used during film deposition.
Polycrystalline samples of MgB2 plus various materials
commonly used in thin-film devices were made by solid state
reaction. Starting materials were bright Mg flakes, submicron
amorphous B powder, and prereacted electronic materials
obtained commercially. They were ZrO2 , yttria stabilized
zirconia ͑YSZ͒, MgO, Al2O3 , SrTiO3 , TiN, TaN, AlN, SiO2
͑crystalline͒, Si, and SiC. Elemental MgϩB were employed
in the reactions, rather than preformed MgB2 , to better
model film fabrication processes. The ratio of MgB2 to reac-
tant material was set to a 9:1 mole ratio. Starting materials
were mixed thoroughly in quarter-gram batches and pressed
into pellets to enhance reaction rates. The pellets were placed
on Ta foil, which was in turn placed on a dense Al2O3 boat,
and heated in a quartz tube furnace under a mixed gas of
95% Ar 5% H2 . Each set of pellets was heated for 24 h at
one of three temperatures: 600, 700, or 800 °C. Due to Mg
volatility and the relatively long heating time, 20% excess
Mg was employed in these reactions. To get a clear test of
the reactivity at 800 °C, where the volatility of Mg in open
systems at long heating times is significant,3 those tests were
repeated by sealing the starting materials in stoichiometric
proportions in Ta tubes backfilled with Ar. Those tubes were
then sealed in evacuated quartz tubes and heated in a box
furnace at 800 °C for 10 h. Reported results for stability at
800 °C are taken from those samples.
The phases present after annealing were determined by
powder x-ray diffraction ͑XRD͒ with Cu K␣ radiation at
room temperature. The results of all the reactivity studies are
summarized in Table I and Figs. 1–3. MgB2 was found to be
inert with respect to ZrO2 , YSZ, MgO, TiN, and AlN up to
700 °C ͑only MgB2 plus the reactant compound starting ma-
terial was observed in the XRD patterns͒. In an earlier study4
it was shown that bulk MgB2 prepared at temperatures as
TABLE I. Reactivity of MgB2 with various electronic materials.
Electronic material
600 °C anneal
800 °C anneal
ZrO2
YSZa
MgO
Al2O3
No reaction
No reaction
No reaction
No reaction
No reaction
MgB2 , small amount of MgO
No reaction
MgB2 with altered cell size,
MgO, unknown
SiO2
MgB2 , MgO, Si
No reaction
MgB2 , MgB4
MgO, Mg2Si, Si
MgB2 , SrTiO3 , MgO
SrB6 , TiB2
SrTiO3
Si
MgB2 , Mg2Si
No reaction
No reaction
No reaction
No reaction
MgB2 , Mg2Si, MgB4
No reaction
No reaction
No reaction
MgB2 with altered cell size
TiN
TaNb
AlN
SiC
aZrO2 is present the YSZ before reaction.
bTaN0.8 is present in the TaN before reaction.
a͒
Electronic mail: taohe@princeton.edu
0003-6951/2002/80(2)/291/3/$19.00
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© 2002 American Institute of Physics