Appl. Phys. Lett., Vol. 79, No. 1, 2 July 2001
Fan et al.
89
ichiometry and high superconducting transition tempera-
tures. The Twente group was able to attain improved transi-
tion temperatures with somewhat rapid thermal anneals at
600 °C of a film deposited at lower temperatures. Thus, it
appears that epitaxy might be possible for substrate tempera-
tures in the range of 600–850 °C, if conditions to achieve
stoichiometric films could be defined.
Reactive synthesis using vacuum process techniques
͑e.g., sputtering, and pulsed laser deposition.͒ can readily
drive the forward synthesis reaction at a few monolayers per
second. The heat of Mg and/or B condensation, or the use of
activated reactant species, could provide sufficient energy
required to form the desired MgB2 phase under nonequilib-
rium conditions.
At a temperature of 650 °C, the Mg desorption rate from
the surface of the film will be nearly one monolayer per
second. If the Mg sticking coefficient is on the order of unity
at this substrate temperature, successful synthesis at a rate on
the order of a monolayer per second could be achieved with
Mg fluxes of a few 1015 cmϪ2 sϪ1. Even higher substrate
temperatures would be possible if larger Mg fluxes are used.
However, if the sticking coefficient is significantly less than
0.1 at these temperatures, much larger Mg fluxes would be
necessary. Several other options could also be tried. One
would be to deposit the reactants at a reduced substrate tem-
perature where the sticking coefficient is much higher ͑i.e.,
order of unity͒. The temperature of the thin film could then
be increased to higher temperatures for time periods suffi-
ciently short to result in small levels of Mg loss. These tem-
perature excursions could be executed cyclically during
growth, or at the end of the process, as was done by the
Twente group. Another alternative would be to increase the
sticking coefficient by using high kinetic energy Mg bom-
bardment ͑Ͼϳ30 eV͒ so that this reactant is buried below
the surface. This process would improve the sticking coeffi-
cient, but might also give other deleterious effects including
an enhanced decomposition rate and/or creation of subsur-
face defects.
composes and magnesium is desorbed. At a temperature of
650 °C, the desorption rate of MgB2 is nearly one monolayer
per second. The MgB2 decomposition rate is found to have
an activation energy of 2.0 eV. The evaporation coefficient is
inferred to be ϳ10Ϫ4, indicating that this process is kineti-
cally limited. The presence of the large kinetic barrier to
decomposition indicates that the synthesis of MgB2 thin
films will be possible with in situ vacuum processing meth-
ods, albeit within a narrow window in reactive growth con-
ditions.
The work at ASU was supported by the Office of Naval
Research ͑Contract Nos. N00014-00-1-0783 and N00014-
01-1-0047͒. The work at Argonne was supported by U.S.
Department of Energy, Division of Science under Contract
No. W-31-ENG-38. One of the authors ͑J.M.R.͒ gratefully
acknowledges the support of ASU and NU. The authors
would also like to thank Ken Gray for facilitating this col-
laboration and Barry Wilkens for performing the RBS mea-
surements.
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